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Hi folks. I just purchased a Peavey Deuce VT Series Amp to fix up and I just have one last issue to resolve, and being that these IC-driven circuits are not my forte, I'm having some difficulty and appreciate any guidance. Everything is working properly after complete recapping but the effects channel has the same issue as before, it distorts at all volumes, and behaves like a fuzz pedal with improper bias. The input signal from a guitar is almost gated until you play hard and then it has a fuzzy distortion but achieves nearly full volume with phasor and reverb working. The normal channel works perfectly and sounds great. I scoped out the effects channel and the clipping is being introduced somewhere AFTER the master volume and before the power amp but with the several ICs involved in that part of the circuit, it's hard to determine the precise culprit. I'm assuming that improperly biased transistor sound probably indicates an improper biased or bad IC? I triple-checked all of the solder connections and everything looks correct. I'm hoping someone with more experience with these types of circuits and/or this particular amp might help me pinpoint the problem. Thanks so much. Schematics link attached.
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If it is good up to the effects channel post-gain, and the normal channel is good, I think the only ICs that are involved are U1 (A and B) and U17. Unfortunately U17 is obsolete, IIRC if that is bad the usual course of action is steal U16 and hardwire the reverb to be always on. Or something. -
Here is a pdf of the schematic and board layout. I believe it is the same.
Peavey_Mace_Deuce_VT_Schematic.pdfIt's not just an amp, it's an adventure!Comment
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U17 is a TL604, and yes they are obsolete, but right now all we want to do is determine the problem. SO there is another TL604 in the reverb return circuit. Swap it out for the U17. Did the problem resolve. If the IC is bad, we have ways around it.Education is what you're left with after you have forgotten what you have learned.Comment
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Indeed it is U17 that is the culprit gentlemen! I swapped out the reverb IC U16 for U17 and the distortion is gone, effects channel sounds great! Reverb still works with swapped IC but it is very weak now. I'll pull the bad IC out and I figured out where I place the jumper to keep the reverb engaged! Thanks so much for your help! I just purchased 10pcs of "new" TL604 chips from Ebay Seller in China with a good rep. Finger's crossed.Originally posted by glebert View PostLast edited by jasonguitar; 06-01-2022, 12:34 AM.Comment
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Digikey has them, albeit sourced from another seller.
https://www.digikey.com/en/products/...kBsAGALCAugXyA"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22Comment
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Holy cripes! Sorry, I didn't even look at the price.
"I took a photo of my ohm meter... It didn't help." Enzo 8/20/22Comment
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Maybe they are just buying the fake ones out of china and then testing them to see if they actually work and then reselling those that do.Originally posted by The Dude View PostHoly cripes! Sorry, I didn't even look at the price.Comment
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I bought these: https://www.ebay.com/itm/274838850733Originally posted by glebert View PostComment
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I know that some of the earlier Peavey hybrid amps didn't use the TL604, they just shorted out the signals with the footswitch. I have a couple of Roadmaster amps that do this and surprisingly it doesn't seem to add noise to the signal. If the ebay parts don't work and you really want to switch off the reverb that might be an option.Comment
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I'm pretty sure those will be fake. Don't want to flog the issue, but just a few tips for others that may come across this.Originally posted by jasonguitar View Post
1) Obsolete parts that you find sold as 'new' for cheap (10 for $4) rather than being sold as NOS for higher prices are almost always fake. No one is allowed to make them now under the original part #, so there is no reason anyone would have new ones. However, you will find obsolete parts available that are used from legit US sellers.
2) for ebay sellers, go straight to their negative feedback, look for the longest time period available.Originally posted by Enzo
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I'm really curious what kind of fake these could be. With something like power transistors I can totally see that someone would put an undersized die into a big package, and it still works as a xxx transistor or whatever but can't handle the voltage or current the real part would. On something ilke this, though, if it is actually a functional die is there a chance that it will work fine? Also, I don't even know how the intellectual property rights work for parts like these. If I look at the datasheet for an LM741 from different vendors there is no details of who owns that spec. Can anyone make a 741? I have no idea.Originally posted by g1 View PostComment
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In a power transistor they don't bother to make new under spec parts, they just take already made transistors and label them as something more valuable.
ICs won't be lower voltage or lower current versions, they are just more likely not to work, These are not just op amps, In the case of op amps, they could easily take something that really was an op amp and mark it as something more modern.
Data sheets are not for someone making parts, it is for someone using parts. As to who owns the design of a chip, that isn't someth9ng I'd expect to see on a data sheet.Education is what you're left with after you have forgotten what you have learned.Comment
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Didn't Peavey make a little PCB that could be wired in to take the place of the TL604 ?WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
REMEMBER: Everybody knows that smokin' ain't allowed in school !Comment
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