Peavey TKO 65 Bass Amp Noise

[tguen63]

Hello all,

I am a first time poster here. I am a hobbyist who has built and repaired some tube guitar amps, but I confess to knowing very little about solid state.

I have a Peavey TKO 65 bass amp that is making some random background static type noise, but works fine otherwise. It is not like hiss, but rather a fizzle bacon frying type sound that comes and goes randomly. It is generally present most of the time, but occasionally goes away for a minute or so then returns. The settings of the volume or tone pots do not increase the volume of the noise. To me the sound is just like what I have experienced many times with cheap/noisy pre-amp tubes. I have isolated the problem to the power amp section by utilizing the pre-amp out and power-amp in jacks.

I read some other posts that led me to believe the problem might be the output transistors or filter caps, so I replaced all of those with no change. I have a schematic in hand and am using a "listening amp" (don't have any oscilloscope) to go through parts of the circuit to see if I can figure out where the noise is introduced, but not having much luck.

Does anyone have a suggestion as to what type of component might introduce this kind of noise? I have not experienced resistors or capacitors making this kind of noise, which leads me to believe it is being generated in one of the many transistors or ICs. I suppose if those aren't too expensive I might just replace them all.

Any help would be appreciated.

Tom

[Twist]

I'm not sure how old our amp is but check the plug in molex connectors. Look inside the connector at the female pin. Sometimes they get spread out and loose connectivity. Also check the pins that are soldered to the board for cracks. The noise you're describing could be a failing component.

[Gtr_tech]

Check the 15v supplies. Sounds like an op amp on the way out to me, but start at the supplies. Should be +15 and -15 via 7815 and 7915 regs.

[tguen63]

Twist - It appears to be about 1985. No molex connectors. I visually checked the solder connections and everything looked really good. I even tried chopsticking every component and could not affect the noise.

Gtr tech - I don't know what the 7815/7915 regs are and don't see those parts anywhere on the schematic or layout. Please let me know if I am missing something. I did check the voltages at the filter caps for both 15V and 43V supplies and got +15.5, -15.1, +43.2, -43.1.

[Enzo]

No Molex? Not even where the power transformer wires enter the board? And the speaker connector?

This amp uses zeners instead of 7815s. Your power supply is fine.

Frying bacon is usually a noise semiconductor, in this case I suspect one of your op amps.

Isolate the problem.

Turn ALL the controls to zero, stil noise? If yes, then plug a guitar into the power amp in jack and turn ITS volume to zero. Still noise? If yes, the power amp is noisy, if not, then the preamp.

Preamp noise. If the post control doesn;t kill the noise, then U2 is suspect, try a new 4558. The ICs are in sockets, right? If the Post does kill the noise, it is earlier in the circuit. ANY control that has ANY effect on the noise is after the source of noise. Or is the source of noise itself. I am betting on a semiconductor though.

Miving along, if post kills noise but tone controls have no efect, then U2 other half is suspect. If the tone controls affect teh tone of the noise, then U1 is suspect, try a new 4558 there.

SInce the amp basically works, I doubt there are any DC problems in it.

Output transistors are rarely noisy, and bad filter caps usually make hum.

Power amp noise? Pull U4 from its socket. Kill the noise? If not, you have a noisy part in the powr amp proper, probably one of the diffy pair. If Y4 gone takes the noise with it, then try a new U4.

Right behind the presence control is a 8 leg IC labelled 87478. It is the DDT compressor. Pull it, the amp will run without it. Noise gone? And if that is no help, right behind the 87478 is Q2, a small transistor, it is a mute, remove it. ANy help?

[tguen63]

Sorry, I thought Molex was the ribbon cable stuff like in my Classic 50. Yes, there is Molex on the transformer and speaker connections. I had already cleaned those and they are really tight.

I checked again and it is definitely the power amp. I pulled U4 and 87478 (U3) but no change. I will remove Q2 and report back. Can I pull U5?

[Enzo]

Look at the schematic, I already put mine away for now. I think U5 does things like turn on the LED,and also drive the input to a transistor that controls the 87478, so yes you could pull it. I mean try it, if the amp goes silent, put it back. But seems to me it is not in the signal path and won;t therefore contribute noise.

HOWEVER, one thing I preach is this: NEVER think up reasons NOT to check something. SO I won;t violate that rule here. Pulling U5 will take all of 10 seconds as a test.

If the noise is there with U4 missing, then I doubt Q2 is involved.

At this point I'd have my scope on it, but lacking a scope, you will have to swap parts. With noise and no U4, I'd be thinking the diffy pair - and replace them as a pair. We really do0n;t want two odd ones there, even if the same type.

[tguen63]

Wow Enzo you are the man! I replaced the two 2N3904 transistors (I assume that is the "diffy" pair), and bingo! Noise gone and it works great. Even found the 2N3904's at Radio Shack.

Thank-you so much!

Tom

[Enzo]

I am sorry. That pair of transistors at the input stage of the power amp us called the "differential" pair. The two form a differential amplifier stage. Diffy is just shorthand for that. Bad habit, using slang when trying to instruct.

Great, glad it works.


Classic example of systematic troubleshooting.