Diagnosing problem with an old TKO 80

jonnylbluejeans · #1 08-21-2012, 07:18 AM

I got an old Peavey TKO 80 (3knob EQ style) from an older lady off Craigslist. I didn't have my bass to test it out but it was pretty clean(and cheap), so I got it.

The input jacks are solid, w/ no crackle when adjusting any of the knobs. However when using either the high gain or low gain input, it sounds like I'm running through a fuzz/distortion pedal - no matter how low the pre-gain is set. Bass, middle, and treble all seem to function normally. The post gain seems to do what it should.

I don't do much with electronics, but nothing on the circuit board looks fried. I removed the speaker, and it looks in good shape.

Where would you guys lay money on the cause? (or is there just not enough info to make a good guess)

anderbass · #2 08-21-2012, 08:34 AM

I just ran into a situation with a Peavey bass speaker that had a very slight voice coil dragging problem. The speaker was still loud and clear but the slight coil-drag made it sound like the clean tone had been blended with a distortion pedal...

You might try connecting a different cab to your combos power section as a quick/easy test...

jonnylbluejeans · #3 08-21-2012, 09:18 AM

I don't know why I didn't think of that. I connected the pre-amp out to the head on my guitar cab. Same exact sound. So, not the speaker then.

Is there anything else I could try before taking it to the local shop for repair?

will33 · #4 08-21-2012, 10:49 AM

A good cleaning never hurts on something that old.

Try running some other unit into the power amp in. If it still does it, might need fresh caps in the power supply or something. Would at least help to further isolate the problem.

jonnylbluejeans · #5 08-21-2012, 11:10 AM

First off, I really appreciate the quick responses. Will33, I might be dense, where should I hook the other unit in?

will33 · #6 08-21-2012, 11:19 AM

Well,

. I assumed most units that had a preamp out would also have a poweramp in. Apparently that one doesn't. Unless there's any other jacks on the back.

jonnylbluejeans · #7 08-21-2012, 12:27 PM

Oh, OK. I was hoping that you didn't mean trying to jump something on the circuit board. LOL

Nope, sorry. No input on the back.

MAMMOTHvolume · #8 08-21-2012, 03:22 PM

Quote:

Originally Posted by jonnylbluejeans

I got an old Peavey TKO 80 (3knob EQ style) from an older lady off Craigslist. I didn't have my bass to test it out but it was pretty clean(and cheap), so I got it.

The input jacks are solid, w/ no crackle when adjusting any of the knobs. However when using either the high gain or low gain input, it sounds like I'm running through a fuzz/distortion pedal - no matter how low the pre-gain is set. Bass, middle, and treble all seem to function normally. The post gain seems to do what it should.

I don't do much with electronics, but nothing on the circuit board looks fried. I removed the speaker, and it looks in good shape.

Where would you guys lay money on the cause? (or is there just not enough info to make a good guess)

does the pregain alter the gain on the input at all? i'd say take it to the tech but check the soldering on the pot

nashvillebill · #9 08-21-2012, 04:50 PM

The preamp out is your means to test this amp--IF you have another amp that functions OK.

Go out of the TKO's preamp out, into the input of the other amp. Keep the other amp's gain moderately low. Do you still hear distortion? If so, it's the preamp section of the TKO. If not,it's the power amp or speaker of the TKO. And even if you don't have another amp, do you have atape deck or something that you can record into? That could work too (just don't overload the input of the tape deck, you'll hear distortion from overloading the tape deck's input).

I believe this version has opamps, if so they are either 5532 or 4558's. Peavey for years put their opamps in sockets. Very easy to swap an opamp, you can even get the common 5532 opamp at Radio Shack I believe (it will sub for a 4558). If you call Peavey with the serial number, they will send you a schematic free.

Or, if you live near Nashville, I can look at it for you. (A decent tech, with decent test equipment, should be able to nail down your problem in less than an hour...oops, I hope I wasn't trying to brag on myself, it might take me an hour and a half).

jonnylbluejeans · #10 08-22-2012, 11:59 AM

NashvilleBill, thanks.

I do have a working guitar amp. I replaced the the op-amp with a 5532 from Radio Shack. The pre-amp out to my guitar cabinet is clean.

(Last night it sounded exactly like the TKO though the other amp. So either I'm an idiot, which I may be, and accidentally had OD1 on - or I had two issues and not just one. Either way. Op-Amp is replaced, and the pre-amp out is good to go.)

Since I have "distortion" or "fuzz" with ANY pre-amp gain on the TKO itself. I still have a problem.

The pre-amp gain gives me fuzz or MORE fuzz at any setting other than 0. Post amp gain give me *some* change in volume, but not a lot. Definitely not what I'd expect.