Crate Blue Voodoo 2612 hum

[thespeakerport]

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.

[nickb]

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:




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.

[nickb]

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.

[thespeakerport]

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.

[thespeakerport]

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?

[thespeakerport]

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.

[thespeakerport]

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....

[thespeakerport]

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.

[thespeakerport]

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.