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Thread: Marshall JCM 900 fried! Need help :)

  1. #36
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    Quote Originally Posted by sibo View Post
    When I switched bypass to ON there was some ticking coming from the tubes. I've never played brand new tubes before so I guess that's normal.. But then, after like 5 seconds it started buzzing so I turned it off instantly.
    Definitely not normal for 'ticking'....and *great* call instantly shutting it off.
    I'm still looking over your new pic, but right off the bat, it appears there's a burnt resistor on the output pcb just to the left of the point that's labelled "V12" -looks like a 2w...but still trying to check your other pics for a location number (and then I'll check the schem for specs).

    But here's a pic in the mean time:
    Click image for larger version. 

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    Consulting "http://i.imgur.com/KNWJJjVh.jpg" it looks like it should be R30 (a 56k resistor). Which is tied directly to your bias network.

    Might also check everything else 'around' the bias circuit: D1, r31, pr1 (check for resistance as you turn the pot to make sure the value changes) r28, r29, r7) And double check polarity on C13, C14 as I can't tell orientation.

    While it might've been good to wait for help on how to check bias before firing it up, it does look like it was blown prior to the repair though. Only sadly, no one saw it.

    Either way though, hopefully no damage was done to the new tubes! I'd pull the outputs at this point just to be safe anyways, and leave them out, until we can guide you along, and hopefully to get some safe readings from you via DMM.

    For the moment though, I'm gonna hold off on that, and ask if you can get as good a picture as you had done before with the other pics!
    The lighting's pretty bad on this one, but the closeness is good!
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Last edited by Audiotexan; 09-01-2014 at 08:35 AM.
    Start simple...then go deep!

    "EL84's are the bitches of guitar amp design." Chuck H

    "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

  2. #37
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    And to answer your question in post 18..

    Quote Originally Posted by sibo View Post
    There's a capacitor missing in C5 power section, is that alright?
    Now that we have a model (4102) looking on the schem C5 seems to be on some versions and not others.

    Quote Originally Posted by the schematic notes:
    MVs - 0.1u
    DRs - NOT FITTED
    So I'll assume yours is a DR... whatever that may be. I can't place it at the moment, but I know it's not "Deluxe Reverb" :P lol

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  3. #38
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    Quote Originally Posted by Audiotexan View Post
    So I'll assume yours is a DR... whatever that may be. I can't place it at the moment, but I know it's not "Deluxe Reverb" :P lol
    Thanks for that!
    Actually I think that may refer to "Dual Reverb" as the model is aka "Hi gain dual reverb".
    Just out of curiosity, is that metallic box mounted on the back of the amp the reverb unit? There are two cables from the amp to that unit anyways. And since all of them don't come with reverb that would make sense..

    On a sidenote, I remember the reverb being intermittent. When I tapped the pot it started working again but that may be solved now due to all the contact spraying..

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  4. #39
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    Quote Originally Posted by Audiotexan View Post
    Consulting "http://i.imgur.com/KNWJJjVh.jpg" it looks like it should be R30 (a 56k resistor). Which is tied directly to your bias network.
    Which schematic are you looking at? The one here: http://www.drtube.com/schematics/mar...0192-iss10.gif
    shows R30 to be 22K 1W, voltage dropper for B+ to preamp.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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    Quote Originally Posted by Audiotexan View Post
    Definitely not normal for 'ticking'....and *great* call instantly shutting it off.
    I know, genious - right?

    I will add new high-res pictures !HERE! if u wanna see more. I did some more measuring and this is what I found:
    Power section:
    D1: 147k
    R7: 56k
    R25: 1.5k
    R27: 150k (look at the close-up pic, I think it looks dark)
    R28: 56k
    R29: 14.7k
    R30: 38.5k
    R31(already changed it): 2.3k
    R35: 102 Ohms
    PR1: 21.2k - This pot seems ok, it changes value when I turn it. It was set to maximum resistance which is 21.2k so I turned it back there. Now I'm no pro but when something is set to maximum eighter way it seems like a compromise to me. I just got a bad feeling about that being at max res - hopefully I'm very wrong..
    C13 & 14 polarity should be right.

    In pre section I think IC3 looked a little weird so I scraped something off it, seems like dirt but who knows. What does that handle? Gain stage?
    Also there's a brownish spot on that big 33uF capacitor which you can see in the pics, what do you think?

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  6. #41
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    Quote Originally Posted by g-one View Post
    Which schematic are you looking at? The one here: http://www.drtube.com/schematics/mar...0192-iss10.gif
    shows R30 to be 22K 1W, voltage dropper for B+ to preamp.
    One on file...I believe it was from here some time back.. but could be mistaken. Anyways, it shows that it's supposed to be correct for a 4102, but it's a 'series' style, that covers several models output section.
    Click image for larger version. 

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    EDIT: Just A/B'd them...and interestingly enough, appear's to be quite similar, but strikingly different.

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    Interesting. In your schem. that power resistor is shown as R32, 10K ohm.
    Sibo please measure R30, I think it will be close to either 22K or 10K.
    The picture shows it close to the HI/Lo switch so I think it is the voltage dropper, not a bias cct. resistor. Also I've not seen 1W resistors used in the bias cct.

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    "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

  8. #43
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    Quote Originally Posted by sibo View Post
    I know, genious - right?
    You laugh, but you'd be surprised how many folks would let it sit that way... lol

    Quote Originally Posted by sibo View Post
    I will add new high-res pictures !HERE! if u wanna see more.
    Thanks!! Will take a look at that shortly...

    Quote Originally Posted by sibo View Post
    I did some more measuring and this is what I found:
    Power section:
    D1: 147k
    Diode tests should be done in 'diode mode' on the meter (the symbol for a diode >| ), and you'll have to flip the leads (pos. and neg.) as you do it. If it's a good diode, it should read approx .7v when forward biased (pos. red-lead on anode, neg. black-lead on cathode [striped end]), and it should read 'infinite' when reverse biased.

    Observation notes in colors:
    green = g-one's reference schem
    red = schem I have on file

    Quote Originally Posted by sibo View Post
    R7: 56k good, same on both schems.
    R25: 1.5k good, 1.5k accord. to schem g1 posted / same value, but diff. locations according to mine/2nd schem
    R27: 150k good, 150k accord. to schem g1 posted / should be 1.5k according to mine/2nd schem
    R28: 56k could not find on schem g1 posted / should be 220k according to mine/2nd schem
    R29: 14.7k good, 15k accord. to schem g1 posted / should be 220k according to mine/2nd schem
    R30: 38.5k RECHECK, should be 22k/1w accord. to schem g1 posted / should be 56k according to mine/2nd schem
    R31(already changed it): 2.3k good, 2.2k accord. to schem g1 posted / should be 15k according to mine/2nd schem
    R35: 102 Ohms good, 100ohm accord. to schem g1 posted / should be 2.2k according to mine/2nd schem
    So it would seem to appear that your amp leans heavily towards the theory that the file I have is for a different 'batch', or something else is at hand. Not sure what to make there yet...
    But regardless, I'll trust g-one's reference for the remainder, and look into that matter later. BTW, Thanks for asking g-one!!

    Quote Originally Posted by sibo View Post
    PR1: 21.2k - This pot seems ok, it changes value when I turn it. It was set to maximum resistance which is 21.2k so I turned it back there. Now I'm no pro but when something is set to maximum eighter way it seems like a compromise to me. I just got a bad feeling about that being at max res - hopefully I'm very wrong..
    C13 & 14 polarity should be right.
    Thanks for double-checking C13/14!

    Going back to look at pics now...

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    Start simple...then go deep!

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    "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

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    Another quick item of interest is R27.

    In the pic at
    Click image for larger version. 

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    it looks like a 100-ohm that's gotten a bit of a heat mark to it. Maybe I'm not making out that it's a 5-band resistor, but I'm only seeing 4 bands in the pics I've viewed so far. But either way, I'm clearly not seeing the green stripe (denoting the '5' in the 150k), so this has me curious.

    Previously we said:
    R27: 150k (good, 150k accord. to schem g1 posted / should be 1.5k according to mine/2nd schem)

    EDIT:
    Click image for larger version. 

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    R27 is *clearly* 'smoked', and was either mis-measured, or read 'as expected' strictly by chance. Replace.

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    Last edited by Audiotexan; 09-01-2014 at 10:31 PM.
    Start simple...then go deep!

    "EL84's are the bitches of guitar amp design." Chuck H

    "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

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    As a note to the difference between schems, I'm looking at the lower right, and it seems mine's for earlier revisions as mine stops at 7 (2-7-90), and the one that g-one linked is rev.10 (10-3-92).

    What also strikes me odd is that they couldn't even keep the revision difference dates the same. As in the later rev.10 notes, they show that rev.7 was dated 4-9-90.
    /

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    Start simple...then go deep!

    "EL84's are the bitches of guitar amp design." Chuck H

    "How could they know back in 1980-whatever that there'd come a time when it was easier to find the wreck of the Titanic than find another SAD1024?" -Mark Hammer

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    Again, thanks!
    I'll look into the diode tomorrow but I did a quick recheck on R30 and it shows 38.5k again. What's a voltage dropper? Might this be part of the problem?

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    Quote Originally Posted by Audiotexan View Post
    Another quick item of interest is R27.
    ...
    Previously we said:
    R27: 150k (good, 150k accord. to schem g1 posted / should be 1.5k according to mine/2nd schem)
    R27 is *clearly* 'smoked', and was either mis-measured, or read 'as expected' strictly by chance. Replace.
    Good eye! Yes it should be replaced with 150K, even if it still reads 150K it has been overheated. Makes me worry about C11 a little.

    Recheck R30 with one end disconnected. It should measure 22K, if not, replace it. The voltage dropper reduces the supply voltage that is going to the preamp tubes. I don't think it would cause your problem but it is possible.

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    Quote Originally Posted by Audiotexan View Post
    Diode tests should be done in 'diode mode' on the meter (the symbol for a diode >| ), and you'll have to flip the leads (pos. and neg.) as you do it. If it's a good diode, it should read approx .7v when forward biased (pos. red-lead on anode, neg. black-lead on cathode [striped end]), and it should read 'infinite' when reverse bias
    Ok so I did that and the digits shown on my display are 616 with right polarity and infinite the other way around. What does that mean and what exactly am I measuring?

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    Quote Originally Posted by sibo View Post
    Ok so I did that and the digits shown on my display are 616 with right polarity and infinite the other way around. What does that mean and what exactly am I measuring?
    What that shows is that the diode is testing correctly.

    By swapping the leads around, it shows that it conducts in one direction only, which is what makes it a diode.

    The 616 display represents 0.616 volts dc. This is the voltage that it takes for the diode to start conducting or to turn on.

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    Here we go again, part two of my component-changing list. I will "over-do" it this time to minimize the risk of something else being bad. I found nothing on the +- tolerances, what should I aim for?
    I would love some help on the missing data.

    Power section:
    R27: 150k, Effect?, Wirewound resistor with silicon coating, Axial connection
    R30: 22k, 1W, Ceramic resistor, Axial connection
    R35: 100Ohms, Effect?, Ceramic resistor, Axial connection
    C11: 0,047uF, 250V (is this ac?), Connection type?
    C12: 0,047uF, 250V (is this ac?), Connection type?
    C15: Unknown

    Pre section:
    IC3: Does this need changing? I scraped some goo off it.
    C3X: That big capacitor next to TR2, 33uF, 450V, Type?, Axial connection
    C11: 0,1uF, 250V AC, Connection type?
    C13: Capacitance?, 63V, Electrolytic, Axial connection
    C18: 100uF, voltage?, Electrolytic, Axial connection
    C31: Capacitance?, Voltage?, Type?, Radial connection
    C32: Capacitance?, 35V, Radial connection
    C45: 2,2uF, 63V, Electrolytic, Axial connection
    R47: Unknown

    If this doesn't help I guess I'll have to hack my axe through it...

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    Quote Originally Posted by sibo View Post
    Here we go again, part two of my component-changing list. I will "over-do" it this time to minimize the risk of something else being bad. I found nothing on the +- tolerances, what should I aim for?
    Shotgunning parts at it will only cost you money and time. So hold off just yet. But I feel your frustration. We've all been there. Just breath a moment, and it'll be alright! lol

    Quote Originally Posted by sibo View Post
    I would love some help on the missing data.
    Will try to help where we can!

    As for the list above that you drew up...The only things that are needed thus far are:

    Quote Originally Posted by sibo View Post
    Power section:
    R27: 150k, Wirewound resistor
    and if you did as g-one said here:
    Quote Originally Posted by g-one
    Recheck R30 with one end disconnected. It should measure 22K, if not, replace it.
    As, while in circuit, it could measure differently simply due to other components affecting the reading.

    So possibly adding R30 to the needed parts list:
    R30: 22k, 1W, Ceramic resistor, Axial connection

    Quote Originally Posted by sibo View Post
    Pre section:
    IC3: Does this need changing? I scraped some goo off it.
    No. Not unless it's been verified that it's bad.

    Quote Originally Posted by sibo View Post
    If this doesn't help I guess I'll have to hack my axe through it...
    Again, I feel your pain. Patience is a #$^$@ sometimes.

    Once you get R27 changed out (which was already confirmed as needing replacement), and confirm verification that you used g-one's procedure, and possibly replacing R30 as well, then we'll have you get some voltage readings.

    But we need to know where things stand once R27 has been replaced and R30 has been addressed/confirmed/replaced (if warranted) before we move to voltage readings. =)

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  17. #52
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    Quote Originally Posted by sibo View Post
    I would love some help on the missing data.
    Again, this is just to cover your questions...but not needed unless mentioned in the previous post.

    Quote Originally Posted by sibo View Post
    Power section:
    R27: 150k, Effect?, Wirewound resistor (part of voltage divider network for bias)
    R35: 100Ohms, Effect? (think of it as an extra fuse)
    C11: 0,047uF, 250V (is this ac?), Connection type? (non-polarized coupling cap to pass signal, rating is DC)
    C12: 0,047uF, 250V (is this ac?), Connection type? (non-polarized coupling cap to pass signal, rating is DC)
    C15: Unknown (47nf -not sure what "Class X" is)

    Pre section:
    C3X: That big capacitor next to TR2, 33uF, 450V, Type?, Axial connection (standard electrolytic polarized PSU cap)
    C11: 0,1uF, 250V AC, Connection type?
    C13: Capacitance?, 63V, Electrolytic, Axial connection
    C18: 100uF, voltage?, Electrolytic, Axial connection
    C31: Capacitance?, Voltage?, Type?, Radial connection
    C32: Capacitance?, 35V, Radial connection
    C45: 2,2uF, 63V, Electrolytic, Axial connection
    R47: Unknown
    R47 pertains to the preamp circuit if I'm not mistaken, so I'll not make any further assumptions due to we've already made established there are revision differences and a preamp schem hasn't been posted and (more importantly confirmed) as matching your amp.

    Basically though, most of your questions about things like radial vs axial, polarized vs non-polarized, are basics that need to be learned by reading some basic sites. Not meant to be rude/snarky in any way whatsoever, moreso that, some folks have taken a lot of time to do those types of wonderfully informative sites, and it'd be far easier than explaining it all here.

    But the extreme basics are:
    Radial: both leads on one end
    Axial: one lead on each end

    Polarized: has a side for +v, and a side for "ground" (can also be "less-positive" voltage: meaning not 'true' ground, but 'ground' with respect to the circuit, or what we refer to as a 'floating ground')
    Non-polarized: typically used for passing AC signals (at least in the majority of our applications)

    Ratings: WVDC (Working Voltage, DC)

    Capacitor - Wikipedia, the free encyclopedia

    Introduction to Capacitors, Capacitance and Charge

    https://www.youtube.com/watch?v=CmII_BmOf0I

    Hopefully that'll help get you off on the right foot

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    Agree with Audiotexan about resisting the urge to shotgun replacing components not proven bad. For starters, you won't learn anything that way . But the bigger problem is you run the risk of introducing new problems which will interfere with the troubleshooting process.

    "C15: Unknown (47nf -not sure what "Class X" is)" Class X & class Y caps are special safety rated caps that are approved to run "line to neutral" (class X) or "line to ground"-AKA "death cap" (class Y).
    C15 is the bias feed cap, between the HV winding and the bias diode. I'm not sure if they are using class X here because of legal requirements, or because of how much damage can occur if that cap fails (loss of bias). In any case, it is quite a critical component.

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    I guess they use it for the guaranteed reliability.
    And even so they are using it beyond its rating: itīs meant for "up to" 250VAC lines, they are using it with some 350VAC .

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    Juan Manuel Fahey

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    Big thanks again. I took your advice and calmed down. And you're right, I do this because it's interesting. Otherwise I could've just turned it in at a tech shop and got it fixed! Also you're most definitely not being rude, Audiotexan. I have no prestige in this whatsoever, I'm just thankful for all the help you give me and I will start reading those links right now. Also, I will order R27 & R30 and after they're changed I will get back (I'll measure R30 with one end loosened first).

    I'll keep you updated in a couple of days when the components have arrived and gotten changed <3

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    On a sidenote!
    I actually have to admit smelling solder getting heated up turns me on I will be needing my own multimeter (the one I'm using is borrowed); would this be a good first invest? https://www.elfa.se/elfa3~se_en/elfa...%2C+AM-510-EUR

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  22. #57
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    Quote Originally Posted by sibo View Post
    ...I will be needing my own multimeter (the one I'm using is borrowed); would this be a good first invest? https://www.elfa.se/elfa3~se_en/elfa...%2C+AM-510-EUR
    At first I panicked, thinking holy crap if you're going to spend 379.00 might as well get a Fluke 87...then common sense kicked me in the teeth again, and I noticed the Zurich IP. (Thank you again Flagfox!)

    At current conversion rates, I'm showing 1.00 SEK = 0.141336 USD, so that puts that meter around $54 USD. Modestly priced, and some nice features! True RMS, .1ohm accuracy, does small caps (up to 100uf) does frequency, and a count up to 3999. Not bad at all!

    However, it all depends on how 'serious' (read as: how often) you'll be needing a DMM, as well as what you're working on. If you plan on doing a lot more in tube amps, I'd then suggest going '1 step up' and considering the 1000v model. Bear in mind, it's triple the price, but, considering that many tube amps operate with B+ in the range of 500+ volts, you're not leaving much room for 'disaster' to strike with the 600v model. [A guy I worked with cooked my 1000v Fluke 87 once on a TV flyback by not paying complete attn when he should have (hey, we all have off days!)] Regardless, if you're only going to be working on 'small' (25-50w) tube amps, and stuff around the house, then the meter you linked should be more than adequate.
    Given you're already into a 100w amp that runs at 500v...

    I look at it like this. IF you cook that meter even once, and replace it with the same, then you're already 2/3 of the way to the price of the 1000v model. The question lies with your outlook. Is it worth spending 3x as much for 50% more "insurance" that you won't even come close to cooking it (at least by raw B+)?

    On a slightly different note, the single most important thing in a meter to me (aside from accuracy), is that it's True RMS.

    A perk of the 1000v model I linked is that it's 10x more accurate in VDC readings, which is the majority of what you'll use. (Accuracy to within 100uv on the 1kv model, vs 1000uv accuracy on the 600v model) But again, that's a *perk*, the 1000uv reading is still quite accurate! Let your needs be your guide.

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    Quote Originally Posted by g-one View Post
    "C15: Unknown (47nf -not sure what "Class X" is)" Class X & class Y caps are special safety rated caps that are approved to run "line to neutral" (class X) or "line to ground"-AKA "death cap" (class Y).
    Ahhhhh! TY g-one!

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    Quote Originally Posted by J M Fahey View Post
    I guess they use it for the guaranteed reliability.
    And even so they are using it beyond its rating: itīs meant for "up to" 250VAC lines, they are using it with some 350VAC .
    Yeah, I was wondering about that as well...

    I wish all manufacturers would put the damned voltages down.
    /minor annoyance rant off

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    Yeah, I ate too many $15.00 sense resistors on my Fluke 600 volters.

    I have sinse gone to the 1000 volt model.

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    Quote Originally Posted by Audiotexan View Post
    Yeah, I was wondering about that as well...

    I wish all manufacturers would put the damned voltages down.
    /minor annoyance rant off
    Well, in that particular case actual voltage is much higher than apparently rated .

    Literally, something rated "250VAC" should be rated for 1.4142 times that in DC voltage, or somewhat over 300VDC ... which no sensible company would do, if you read the datasheet small print, youīll find the "400V DC" rated caps are "100% tested with 600V DC " and so on.

    BUT this is not a standard "in circuit" cap like all we use, but a safety product rated to meet or exceed official Safety rules covering 250VAC lines or apparatus (as they call them) connected to them.

    And although 250VAC has the peak value I mentioned, fact is that they can often and easily have peaks much higher (rays hitting a power line anywhere along the grid) so AFAIK 120V rated stuff must at least stand "1500V DC for 1 second" and 240V one "3000VDC for at least a second" .

    Quoting from memory but basically that.

    So I guess an X or Y rated "250V" cap must probably be a 2000VDC rated one, "tested at 3000VDC" .

    So, inside a tube amp connected to some 350VAC it must be well within ratings.

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    Would this be a better choice then? USB UT61D Multimeter - UNI-T | Clas Ohlson Says Cat4 600V, Cat3 1000V so it should work up to 1kV then.

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    Update:

    I now changed R27 and R30 (R30 measured faulty even with one and two ends disconnected).
    Fired her up and started playing. After like 30 seconds there's a vague but increasing sound like the one when you tap a tele jack that is connected to your amp with your finger. The overall hum starts to increase and something smells burnt so I turned it off again. I tried opening it fast to locate the source of the heat but couldn't really manage. There was most heat coming from the two first tubes or perhaps the blue "cups" beside them, whatever is located inside those..

    This all happened at the same time as I was fiddling with eq, presence and reverb but that might just be a coincidence.
    Also I might recall it being louder at lower volume settings than it was now that I tested but I may be wrong as it was a while ago I used this amp..

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    Quote Originally Posted by sibo View Post
    Update:

    I now changed R27 and R30 (R30 measured faulty even with one and two ends disconnected).
    Ok, that would have been a good place for us all to resume with you, however...

    Quote Originally Posted by sibo View Post
    Fired her up and started playing.
    I know it's hard to resist, but in order to be able to have consistency in troubleshooting, *we all have to be on the same page at the same time*. That way neither you nor us (anyone following in hopes of helping you out) get lost in what's going on, as we have to know the current status of the amp at each stage in relation to the progressive steps of troubleshooting.

    If that status is constantly changing in between being able to chat with you here, there's no way for any of us to know 'where it's at now'.

    So if you would, help us out on this, and please try and resist the urge to fire it up and "see if things are fixed" at each replacement of parts, unless requested to do so...
    If we verify through proper steps, and see that a voltage is too high, or too low, before you plug the output tubes back in, hopefully we can save you from blowing that new set of tubes (if they weren't already damaged from before, which again, hopefully they were not!)

    Quote Originally Posted by sibo View Post
    After like 30 seconds there's a vague but increasing sound like the one when you tap a tele jack that is connected to your amp with your finger.
    While this is also good for us to know, we know it still has problems, but there are some more basic things we can do, and ideally, in a manner that doesn't put additional (or costlier) parts at risk. Which is always a side-goal as we go (to minimize the expense of the repair).

    Quote Originally Posted by sibo View Post
    The overall hum starts to increase and something smells burnt so I turned it off again.
    Ok, so we'll pick up here.

    - Make sure power's off, and unplugged
    - pull the output tubes (and leave them out)
    - drain the caps
    - crack her open and see if you can see anything that's obviously burnt.

    Report back with your findings!

    Quote Originally Posted by sibo View Post
    I tried opening it fast to locate the source of the heat but couldn't really manage. There was most heat coming from the two first tubes or perhaps the blue "cups" beside them, whatever is located inside those..
    Also good to know! Since you say 'blue cups', I'm assuming that you mean the two nearest the mains power supply (PSU) caps. So those two tubes were /noticeably/ hotter than the other pair???

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    Quote Originally Posted by Audiotexan View Post
    I know it's hard to resist, but in order to be able to have consistency in troubleshooting, *we all have to be on the same page at the same time*.
    Totally with you on that. Will not start until we reach the point if it being safe.

    Quote Originally Posted by Audiotexan View Post
    - Make sure power's off, and unplugged
    - pull the output tubes (and leave them out)
    - drain the caps
    - crack her open and see if you can see anything that's obviously burnt.
    Nothing's obviously burnt.


    Quote Originally Posted by Audiotexan View Post
    Also good to know! Since you say 'blue cups', I'm assuming that you mean the two nearest the mains power supply (PSU) caps. So those two tubes were /noticeably/ hotter than the other pair???
    Yeah it felt that way but the heat may be coming from elsewhere but at least that area seemed warmer to me. Not like-crazy-warmer, just warmer.

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    Quote Originally Posted by sibo View Post
    Would this be a better choice then? USB UT61D Multimeter - UNI-T | Clas Ohlson Says Cat4 600V, Cat3 1000V so it should work up to 1kV then.
    Ok, the important part of those specs are that it's DC rated for 1000v.
    Looks like it should be fine! And as a bonus (compared to the first one) it's capacitance is rated to 4000uf, which will be much more useful than the 100uf limit! So IMO yes, good pick on that one for a starter!

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    Quote Originally Posted by sibo View Post
    Totally with you on that. Will not start until we reach the point if it being safe.
    Thank you!

    Quote Originally Posted by sibo View Post
    Nothing's obviously burnt.
    Ok.

    Quote Originally Posted by sibo View Post
    Yeah it felt that way but the heat may be coming from elsewhere but at least that area seemed warmer to me. Not like-crazy-warmer, just warmer.
    I know you mentioned before that you were near having to return the meter that was borrowed....do you still have it ATM? If not, let us know when you have recieved your new meter, and then we'll look at getting some voltage readings from there!

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    Quote Originally Posted by Audiotexan View Post
    I know you mentioned before that you were near having to return the meter that was borrowed....do you still have it ATM? If not, let us know when you have recieved your new meter, and then we'll look at getting some voltage readings from there!
    I still got it. How and what do you want me to read?

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    Ok, given that you've mentioned that it develops a hum (and then a burning smell) the longer it's left on, I'm going to strongly recommend clear eye protection at this point. As given the amount of time that it will take getting basic readings of the mains/secondary voltages...it's going to have been longer than your 30 seconds reference point. If a drifting cap explodes while you're checking other things, it can potentially send 'shrapnel' everywhere. Not to mention scare the crap out of you, and risk jerking your leads/hands into somewhere you didn't intend them to be....so please, OBSERVE SAFETY BEFORE ALL ELSE! Think: one hand behind your back rule while taking readings in a live amp! If that means you have to alligator clip your ground lead in place so you hold the red lead and only use one hand, DO IT! We want you around to be able to enjoy life, regardless of if an amp gets fixed!!

    Quote Originally Posted by sibo View Post
    I still got it. How and what do you want me to read?
    Cool! Just wanted to take some basic measurements to start with...
    Make sure that you keep the output tubes out of the circuit while you measure. Turn all control knobs to '0'. We do expect that some readings will be slightly high, as those tubes are not there to 'weigh things down' so to speak. This is normal.

    Checking the power supply:
    _______________________
    (looks like it should be under (but near) 500VDC, as the mains caps are rated 50uf+50uf @ 500vdc.)
    You should be able to take readings at the caps (the blue 'cups' you referred to earlier).

    Mains voltage:
    *Set meter to DC (highest voltage if there are selectable options)*

    1 - locate the blue PSU cap that has blue/yellow/green wires.
    1a. Put your black lead on the lug of the cap that has the green wire soldered to it, as that should be the grounded lug.
    [ref:http://i.imgur.com/ISUg51Xh.jpg]
    1b. Now put your red lead on the lug that has the blue wire soldered to it.
    1c. Take reading
    1d. Now put your red lead on the lug that has the yellow wire soldered to it.
    1e. Take reading


    2a. Repeat 1a. (Leave your meter 'ground' where it is)
    2b. Now, on the other adjacent blue cap, put your red lead on the lug that has the /red dot/ inked on it (looks like it should be a brown wire soldered to it that feeds the center tap of the OT -but I can't confirm this from the pic. I can only assume it from the schematic.)
    *note* both parts of this cap are tied together with a jumper, effectively making it a 100uf/500v cap.
    2c. Take reading



    Heater voltages:
    Set meter to AC voltage 50-100v setting should be fine.

    [ref: the long twisted pair of red and black wires that connect all of the tubes (both preamps and outputs) http://i.imgur.com/KNWJJjVh.jpg]
    *note* The filaments (heaters) are going to be pins 2+7 on the EL34's, and on the 12ax7's they're pins 4+9, and pins 5+9. Pin 9 is common to both halves of the ax7, and that's why you see the preamp tubes tied together at pins 4+5 by the heater wire.*

    Hopefully you've got a rock solid 6.3vac here.



    3. There's also a 20vac line that feeds the preamp board. (There should be a black and white wire from the primary transformer, and you need to follow those leads. Check again that your meter is on AC mode, and then put your black lead on the Black wire, and your red lead on the White wire)

    Again, this 20vac should not fluctuate.


    4. Bias supply. We're hoping for -38 to -40v (only as a reference)
    I'm almost expecting that this line will have some drift in it, so take a bit longer to measure it (again, as you say, it takes up to 30 seconds for it to hum).
    Watch closely and see what it does when taking this reading...if it drifts or bounces around (doesn't stay a steady voltage within a volt or two) then that's a sign of the bias circuit still having problems. This is where I'm thinking your comment of "After like 30 seconds there's a vague but increasing sound...." and "The overall hum starts to increase and something smells burnt..." comes in. However, by the time you've taken your other readings, it may have already well drifted up into enough to give us a bad reading flat out. Either that, or something that has been trying to smoke WILL while you're trying to get your readings. At least the output tubes won't suffer the wrath since they're not in there!

    set meter to DC voltage (if selectable: around 100v should be fine, we're looking for a NEGative voltage here anyways.)

    4a. black meter lead to chassis ground
    4b. red meter lead to *the common tie point between* R26 and R27. From the looks of one of the pics, that common point should be the side of those two resistors that are *closest to* the 'D' in the small sticker that says "Dupe 4102". To verify that those resistors are tied together on that end, you should get the same reading on either of those resistors AT THAT END.
    4c. take reading.


    I could be wrong, but, that's my hunch offhand.
    It could also quite easily be a signal/blocking cap that's gotten weak, and is allowing dc into where there should only be ac (signal) passing or vice-versa. Hence the hum. But since you don't have a scope (or at least that I recall offhand) it's going to be a bit harder to tell/see that on a DMM then it would be on a scope.

    Again, If you're not comfortable for ANY reason, or have ANY doubts about your ability to safely do any of the above, please DON'T. Just take it to a pro.
    I'd rather risk hurting your feelings, or have you mad at me, but this is a whole different level of safety precautions than simply changing components in circuit that's been properly drained. These will be LIVE voltages that can kill you if not given the proper respect they're due. I can't emphasize that strong enough. Know your limits, and respect them. I have FAR more respect for those who admit when something's too much than I do for those that barrel in without sufficient safety/knowledge.

    Forgive the 'lecture' if you already know all of this. I'd rather say it than regret not having said it though. :x
    -AT

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    Wow! Thanks for taking your time writing all that down for me <3
    A lecture is always good when it comes to dangerous environments, I worked a couple of years as an electrician so I've seen what happens to those who disrespect directions - Einstein hairdos.

    I did all those readings and they are as follows:
    Mains voltage
    1c: 368VDC
    1e: 538VDC
    2c: 538VDC
    Heater voltage: 7.2-7.3VAC (measured every tube socket).
    Preamp feed: 23.7VAC
    Bias supply: -58.7VDC

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