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Thread: Crate Blue Voodoo 2612 hum

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    Crate Blue Voodoo 2612 hum

    Hello all...

    I have a Crate BV-2612 that I am tracking down a nasty hum problem. I found some old posts where Enzo (and other forum members) had some wonderful feedback/suggestions specifically around removing the hum balance pot (which if memory serves, corrected the problem i am going to be describing below). I really did not understand how hum balance for the entire amp would have an impact to just the clean channel, but in any case, I did speak with Enzo outside of this forum and he recommended I post this to rekindle the conversation. So bear with me as i try to be as complete as i can in conveying the full history i have with this amp.

    First...some background history on this unit. A Customer of mine brought this amp to me a little over a year ago saying that they are getting no sound out of it. One of the two output tubes were shot and the 12AU7 tube was gone. The remaining 12AX7's were also very weak. Opening up the amp, I found that the unit had already been worked on by someone else prior to it getting to me. I'll get to that part later. I found both fuses (F2 and F3) were blown, and also found R90, D9 and D10 were fried. Research at the time indicated that these amps were somewhat known for having cold solder joints. After testing the rest of the board and performing a close inspection of the solder pads, i found that the daughter board for the output tube have a number of loose solder joints. I went through and repaired all the bad parts and re-flowed the solder pads on the daughter and main boards....basically anything that looked suspicious....which was pretty much everything . After putting it all back together with a new set of JJ tubes, all was well with the unit. I will be honest, i recall that this amp did have some ground noise (not ground loop, but just background noise...some hiss and light hum)...but nothing i would have considered at the time out of the ordinary.

    About the previous work that was done. I noticed at the time I originally repaired this item, some power supply filter caps and power resistors were replaced. Also someone had install a biasing pot....which based on my evaluation did not seem to be very effective. I believe this either had a fixed or self adjusting bias....cant recall, but if it is self adjusting, then i would not expect the pot to really make any difference....and now i am thinking about it, i will probably remove it and put it back to 'stock' configuration.

    Now, let me explain what is specifically going on with it. My Customer brought this amp back to me complaining about the 'clean' channel (1) having a lot of hum while the effect channel (2) is much quieter. I wont say completely quiet, but significantly less than channel 1. This hum is reproducible without anything plugged into it (no guitar, cables, pedals, etc.) It happens with or without the reverb tank connected to the main board. This is (what i am told) a rev 1 system with the hum balance pot....which by the way is adjusted to minimize the hum...but comes no where near making it go away. The channel 1 volume does affect the overall volume of the hum....as you turn it up, it increases, but seems to max out when you get it mid way up. If you turn it all the way down, the hum is low, but does not go away and is still much louder than channel 2 if it was turned all the way down. If i pull the input tube (V1), the amp is dead quiet....no hum at all...but this is kind of expected. I have replaced V1 with a different 12AX7, no change. All tubes in the system were tested and check out. I even rotated them out with other known good tubes; again, no change. Channel 1's bass, mid, treble do not really affect the hum at all....in fact, they only seem to attenuate it...iow....turning the bass all the way down, make the hum less 'bassy' if you take my meaning.

    So, with all this said, my gut is telling me ground loop. Everything seems perfectly tight in regards to wiring. Chassis screws are solid and i seem no evidence of a bad solder joint in and around the channel 1/input section (which is where i am focusing on.) I pull a couple of caps and found C39 to be out of range and the installed part does not match the schematic parts spec (which may or may not be an issue.) It is supposed to be a 22uF, 50V per the parts list, but a 35V is installed. I also measure a little over 40uF instead of 22uF...so this one is going to get replaced. I am in process of continuing to evaluate pretty much all the components in this area of the board, but while i am doing that, i wanted to get this information out here for you guys to consider and provide input. Any feedback you can provide would be most appreciated.

    Thanks in advance...

    Nabeel Sayegh

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  2. #2
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    Nobody interested in touching this one?

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    I think if you were to post a schematic so we are all on the same page / know what you are talking about, you would get a better response.

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    Click image for larger version. 

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    Seriously thought i had attached these....apparently not.

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    Lifetime Member Enzo's Avatar
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    You sure you don't mean BV-6212?

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    batting 1000! A little bit of dyslexia i guess. Yes, the BV 6212.

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    So a bit of an update.

    I replaced the electrolytic near V1 and V2 and also replaced the ones around the biasing circuit. I also removed the adjustment pot that someone had added in and put it back to stock. Lastly, i removed the Hum balance pot and wired in a pair of 220K's (as was discussed in a different thread.) In that same thread, there was discussion about a bad jfet in the brightness circuit. I checked it out and also found I had a bad jfet. All of this is now sorted and put it all back together. STILL getting hum....but a different behavior now. First...the hum is around the 120hz mark. second, it almost appears to be two different hums going on (if that makes any sense.) Once that seems to start up as soon as the amp warms and another shortly afterward....they almost blend together. Now here is the odd part. If i removed V1 now, there is no difference in behavior. Hum is loud in channel 1 and drops much lower (but is still there) for channel two. I put V1 back in and pull V2....I get a lower volume hum, but changing channels does NOT affect it. If i pull V3....i get no hum at all...amp is dead quiet. Of course, i started to shuffle around the tubes and even replaced V1-V3 with other known makes of good tubes....no change in behavior (other than some slight variations that i would attribute to tube strength and individual tube characteristics.)

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    To clarify my first statement above...I replaced ALL the electroylytics near V1/V2.

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    ...another real quick update. I have spent that last couple of hours going through and pulling/testing resistors, transistors and even some of the really small caps in the input section and i am still coming up short. Going to put this animal back together one more time and trace all the power supply feeds to make sure no DC supply has any AC elements to it. I really could use some help here is anyone has any suggestions here. I am even considering putting back in the hum balance pot as it did not seem to improve the situation at all.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    It's a DC filament, the hum pot does nothing.

    The resistors should be 220 ohms not 220k.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    The problem with comparing the two channels is it's apples and oranges. I suggest you put it on clean and measure the ACV hum across the speaker and let's see what we are talking about.

    How do the vol & gain pots affect the hum now?

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    Last edited by nickb; 03-01-2018 at 05:28 PM.
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    Ok, so first, i doubled checked the two resistors i put in place of the hum balance pot, and they are 220 Ohm, not 220K so we are good there. One thing to mention; i removed R112 (100Ohm resistor) that tied the original pot wiper to ground and wired the two 220 Ohms directly to ground effectively bypassing R112. I don't think this is a problem, but just throwing that out there. So, the AC/DC measurements at the speaker output terminal (with no speakers attached) read 0.14VAC, 0VDC when Channel 1 is engaged. Switching to channel 2 and it drops to 0.001VAC, 0VDC.

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    Quote Originally Posted by nickb View Post
    It's a DC filament, the hum pot does nothing.

    The resistors should be 220 ohms not 220k.
    Based on what i am seeing on the schematic and what i am measuring across both sides of the 220 Ohm resistors, this is an AC filament, not DC.

    I went through all the front panel pots with channel one engaged and found that as you increase channel one volume another hum (more like a traditional grounding noise) starts to increase in volume, peaks around the mid point, but then go away when turned all the way up. nothing else really affects the output other than some addition volume when cranking up the master gain and/or reverb. With channel two engaged, you have to crank up the bass/treble/mid along with the input and master gain and then you will get a good amount of hum...but that is controlled by the dials...with all dials at zero, channel two is pretty quiet.)

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    don't forget the joker g1's Avatar
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    From what I see, V1 is DC filament, the others are AC.

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    unfortunately, the schematic is not 100% accurate in this case. I am measuring the voltage across pins 4/5 and 9 and it is 6.7VAC....there is no DC present. In addition, the section that shows the FIL 1/2 to DC FIL 1/2 does not exit on this rev 1 board. There are other differences such as it not showing the hum balance pot that i originally had.

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    Could and opto-coupler be a potential cause of this? I just noticed that one of them is acting like a microphone. I tap on it and i can hear the tapping coming from the speakers. Is this normal behavior. The one in particular is OC1 I think.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by thespeakerport View Post
    Based on what i am seeing on the schematic and what i am measuring across both sides of the 220 Ohm resistors, this is an AC filament, not DC.

    I went through all the front panel pots with channel one engaged and found that as you increase channel one volume another hum (more like a traditional grounding noise) starts to increase in volume, peaks around the mid point, but then go away when turned all the way up. nothing else really affects the output other than some addition volume when cranking up the master gain and/or reverb. With channel two engaged, you have to crank up the bass/treble/mid along with the input and master gain and then you will get a good amount of hum...but that is controlled by the dials...with all dials at zero, channel two is pretty quiet.)
    If we are looking at the same place it's DC:

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    If you are not seeing DC then the rectifiers and caps would have to be shot in just the right way. That seems very, very unlikely. Measure the ACV on one of the DC filaments using a 0.1uf cap in series with your meter leads and see what you get.

    That's a clear difference in hum. Repeat the hum test but measure at V1 pin 7. The signal will be small and a scope might be a better too. Repeat at the junction of C11 and R15.

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    "Thermionic Apocalypse" -JT nickb's Avatar
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    Quote Originally Posted by thespeakerport View Post
    Could and opto-coupler be a potential cause of this? I just noticed that one of them is acting like a microphone. I tap on it and i can hear the tapping coming from the speakers. Is this normal behavior. The one in particular is OC1 I think.
    I was working toward that. I've asked you to make some more measurements to narrow it down. My test for opto hum is simply to bridge the the one that is low resistance. The hum could be from the opto supply - in that case add a new cap across C46 to test, but we're jumping the gun.

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    Thanks Nick....

    I will certainly do the tests you suggested above. But regarding the DC filament: the schematic shows that AC to DC rectification circuit for supplying DC filament voltage to V1...but that simply does not exist on this amp. For instance, there are is no D21-24 nor is there a 4700uF capacity anywhere on the actual main board. This must have been added in on a later revision for some reason; I can assure you, V1 is being fed with the AC filament supply only.

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    So these optocouplers were intriguing me and so got distracted looking them up to learn more about them. The three physical OC's on the board are VTL5C3's identified as OC1-3. I pulled each one and tested them out of circuit. The LED side tests good. The "cell" side which i think behaves as a variable photoresistor measures as an open circuit on all three. Based on what i found out about them, this is expected. Now, i wanted to try and test them so this is what i did. I applied 1.6VDC @ 20mA to the LED side and put the cell side on my ohm meter. Here is what i am getting on each:

    OC1 - immediately reads 5K but then continually climbs up in value....@2-3 ohms per sec.
    OC2 - immediately reads 10K but then continually FALLS down in value...@ 4-5 ohms per sec.
    OC3 - immediately reads 750 but then continually climbs up in value...@1-2 ohms per sec.

    Looking at the schematic...(and to my surprise)...OC1-3 are identified as being VTL5C6, not 5C3's. Looking at the parts list, there is a notation (2) that reads it was redesigned per E950123 (which i assume to be some revision/TSB bulletin of sort.) SO...first and foremost, what do you guys think about what i am getting as far as results for these Opto-couplers? Clearly, the values are vastly different from one part to another. I know these are not high precision components, but it is normal for them to float this significantly in value? More importantly, could this in anyway be causing hum?

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    Quote Originally Posted by nickb View Post
    I was working toward that. I've asked you to make some more measurements to narrow it down. My test for opto hum is simply to bridge the the one that is low resistance. The hum could be from the opto supply - in that case add a new cap across C46 to test, but we're jumping the gun.
    I went ahead and replaced both C46 and C47 with new ones i had in stock....no change in behavior.

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    Been working on this more today and mapped out all the voltages (AC/DC) at every pin on all tube sockets. A couple of things i noticed. With channel 2 engaged, the amp is pretty much dead quite with nothing plugged into it. I can go around all pins on every tube and take both an AC/DC reading from them. With channel 1 engaged.....things get weird. First, those Opto-couplers are suspect to me. OC2 and OC3; if I hover over either of them, the hum exponentially increases. OC1 is part of the reverb circuit and it does not seem to have this issue....or at least, i did not try any further testing to see if it has the issue or not. The other odd thing is that when I try to take either an AC or DC reading from pin 6 or 7.....V1 makes a very unnatural buzzing sound....and the output to the speakers sounds like the end of days. I am going to document all of my finding including the schematics test point results...which also seem off to me. With post them in a few....

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    Attached is a spreadsheet with the voltage mappings. Also includes some interesting finding about the opto-couplers. I am concerned with OC3 specifically...it looks like it is not getting turned on when it should and that is leaving the other side as an open circuit (for the most part). I do not think 0.4V is enough to turn it on. I traced this back to what could potentially be an issue with Q14. I tested R58, R59 and R60 and they are all good. I tried to do an in-circuit test of Q14 and compare it to the reading i get from Q13 and i think Q14 may be jacked. Going to have to pull it and test it out of circuit to be sure. Will see if i have some replacements.

    Any thoughts/feedback would be appreciated.
    Attached Files Attached Files

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    For those interested....I pulled Q14 and it tested good, however, I replaced it anyhow and channel 1 hum is virtually gone. Still bit louder than channel 2, but night and day difference. I did order up some replacement optocouplers and plan to replace OC1-3 and still plan on swapping them out. I see some noise coming from the cell side of OC3 on my scope and I think it is warranted to replace them and see if things improve further. Once they come in and I get them installed, I will post a final result.

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