Marshall JCM 900 fried! Need help :)

[g1]

You can discharge the caps like that but it is hard on them and may create sparks/arcing. Better to use a lower value power resistor, I usually use something like a 100ohm 5 watt.
Also, switching the standby switch to the "run" position will help discharge the caps and keep them discharged. Usually after I disconnect the AC cord, I switch the standby to the run postition, then check with a meter for DC on the main filter caps. If there is still DC there, I discharge through the 100ohm 5watt resistor.

[sibo]

Thanks, didn't have a spare resistor but it worked out fine.

News are that I found some overheated spots at the back of the board, CAN BE SEEN HERE



I scraped them clean with a screwdriver and applied some 5-56. I also took the time to lube all pots, buttons and jacks with 5-56. I will start soldering when it's dry and when I feel more focused..

[sibo]

Done and done.
Components changed and installed new tubes after using 5-56 to lube the slots (using old tubes to get it in there). However I didn't have the required tool to set bias. I also changed the speaker cable to a new, beefier one.

The outcome?
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. Settings were: both gains off, eq=5(all around), presence off, master vol 2, reverb 2.

Is that buzz normal after tube change?

Pic of how it looks now:

[g1]

How loud is the buzzing? Remove the preamp tubes and see if you still get the buzz.

[sibo]

Ok I'll try that tomorrow! Thanks

[Audiotexan]

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:


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!

[Audiotexan]

And to answer your question in post 18..

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.

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

[sibo]

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..

[g1]

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.

[sibo]

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?

[Audiotexan]

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.
2100,2101,4100,4101,4102-schem-power-el34.pdf

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

[g1]

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.

[Audiotexan]

I know, genious - right?

You laugh, but you'd be surprised how many folks would let it sit that way... lol

I will add new high-res pictures !HERE! if u wanna see more.

Thanks!! Will take a look at that shortly...

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

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!!

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...

[Audiotexan]

Another quick item of interest is R27.

In the pic at

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:

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

[Audiotexan]

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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