Help with old Peavey amp

Could someone help me understand this old amp? It's intermittent - input but no output. Power supply is good, all semi's & caps are OK. It uses a couple of op amps plus an OTA (operational transconductance amp), I think for the compression circuit or maybe as a current source. Also - my other dumb question - why does the center tap from the transformer go to the speaker? This should be so simple but I can't seem to even follow the signal path after it gets out of the preamp.

I have it working now but it has a horrendous squeal. I think one of the op amps or maybe the OTA (unobtainable) is bad. I'm not too worried about actually fixing it, just trying to understand the circuit. Any help would be appreciated.
 

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The current amp U3a is used to cram the signal up to rail on U2a to prevent excessive clipping on the speakers. Peavey still sells them, but you have to phone in, they don't sell over the Internet. I found some 3080's (original RCA version, harrison 13080 is a copy) at jameco.com in CA, thanks to someone who tipped me off here. But my PV1.3k works okay without the 3080.
I had an oscillation caused by a bad solder joint, that I had made. This amp had a curse on the B channel that made it whang the output to +85V and trip and melt the DC voltage on speaker detector. The cause of that was a bad solder joint on a 4558 at the factory.
I wouldn't start "updating" the op amps, the slow slew rate of the 4558 is suited for a guitar amp, and this circuit has not been tested for ultrasonic oscillation with faster parts. My PV1.3k had a long parallel wire run betweeen the input and output of a 4558, that could have caused a radio oscillation with an amp with a faster slew rate and more output current.
Semi okay is a variable concept. I found output transistors that would pass the double diode test, would also blow up with 85 volts on them. I had to test used transistors for leakage with b-e shorted and 17VDC on the collector (or minus on pnp) through a 47k resistor and a DVM ammeter scale. Some of the OT's would leak more than others, which were not leaky at all at the 2 v the meter diode scale uses. I also had blown resistors and ceramic capacitors due to the original OT failure.
 
Thanks for the replies. Are you saying you can just remove the OTA? I hadn't thought about slew rate. I usually just stick in some Burr Brown chip. Good point about the old outputs. I know better. I need to take them out and check them out of circuit. I've only checked them with the diode test. (How did you know?) Did you replace the drivers?
 
Uh, yeah, 18 of 20 output transistors were bad. I used the other two that passed the leakage test to replace MJ15020-21 driver transistors, which were cheaper than MJ15024-25 back in the day but aren't now. I put a capacitor on a car battery charger to make a steady 17 vdc, then through 47k and the ammeter, to make a Vceo tester that wouldn't stop my heart (that voltages over 25 can do).
C4, C6, and C17 are accidents waiting to happen after 15 years. E-caps dry out, and if C17 is open the following op amp has an open input (where it can pickup RF). U1a is your input buffer, u1b does the tone controls and echoes the output. Any IC with a jack on it can be blown up on a dark stage by having the wrong thing (an amp output) plugged into it. U2a give the 3080 an opportunity to cram the signal down to nothing if there is too much high frequencies in the input signal. U4 drives the output transistors and is vulnerable to being blown up by OT's. One of my CR15's in the heat sink was blown up, available only from Peavey or as a 1n part from Avnet "call for a price". That part is hard to measure with a DVM. Q1 Q2 are soa protectors, try to protect the OT's from overdrive. With my +-85 v rails I had everything blown up back to the op amp, including some 50v ceramic capacitors (green, look like resistors). I used MPS8099 and MPSA56 for the 6530-6533 transistors. MPS8599 and MPSA06 are the more expensive complements. I don't know if your OT's are TO3, Peavey used MJ15024 and MJ15025 for years. Since there is only one OT, you can get away with MJL21193-4-5-6 depending on price. IF they are TO247 plastic, you can use MJL4302&MJL4281 or MJW21195-96. The drivers, the best are MJE15028-29-30-31 with the lower voltage ones cheaper. You could probably get away with TIP41c-42c drivers. Don't forget the heat sink grease.
 
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Semi okay is a variable concept. I found output transistors that would pass the double diode test, would also blow up with 85 volts on them. I had to test used transistors for leakage with b-e shorted and 17VDC on the collector (or minus on pnp) through a 47k resistor and a DVM ammeter scale. Some of the OT's would leak more than others, which were not leaky at all at the 2 v the meter diode scale uses.

As a last resort I take out all the semis and check them on a diode/transistor tester. I once had a transistor with an Hfe of 1 and of course it was the last transistor I came to of about 12.
Once all the semis are out I check the passives.

I also got caught out with an amp that worked but sounded terrible where someone else had replaced some transistors and put in an NPN instead of PNP ! It checked out with an ohm meter ok but screwed up the voltages in the circuit.
 
I can explain the speaker/center tap thing, but let's get it working first. if you are dying to know, look up flying rail amaplifiers or probably grounded emitter amplifiers.


Isolate the problem.

You say it functions but squeals? OK, does it squeal with nothing plugged into the input jacks? Do ANY of the controls have ANY affect on the noise? That is, can you turn the noise up and down with a gain knob or change the tone of the noise with tone controls?

Plug a cord into the preamp out jack - all the way in. Run that cord to some other amp. Is the squeal present in that preamp signal so it comes out the other amp?

Does plugging all the way into that jack silence your speaker in this amp?


What we want to know is if the trouble is in the preamp or power amp, and if preamp, where along the signal path is the noise signal.

Do you have a scope? If not, how about a signal tracer? If neither, MAKE a signal tracer. All a tracer is is an amplifier with a probe as its input. We also add a series cap in the probe to block any DC from getting to the tracer amp. Remember we will need a ground conection on our probe to connect to the chassis of the unit under test.

Google signal tracer, and use the simple examples. The ones that include diodes are for listening to RF and beyond our needs.

A signal tracer or scope lets you go stage by stage through the amp to see where the problem is and is not.


You can remove the OTA at U3 during tests. When those have issues, the result is usually no output or very low level output, because the compression is stuck on. I never saw one oscillating, but anything is possible.


May I also say don't start upgrading anything. FIX the amp first, THEN play with fancier parts if you wish. Slew rate, schmew rate, the amp has worked the last 30 years without higher slew rates.

If the thing amplifies anything, even noise, then I doubt your outputs are blown. Yes, the outputs are TO3. The SJ6392 is an MJ15015, and SJ6505 is an MJ15016. If you don't have those, MJ15003 and MJ15004 would work fine. I doubt you would be happy cramming TO220 TIP41 into the place of the little TO126 5331 and 7036 (5332) drivers.

Peavey sells all the parts in the thing at reasonable rates. WHy screw around?

See R29 across your speaker? Make sure it has not gone open.

And see the three pairs of small diodes on points in the signal path. CR1-CR6. Make sure none of those has shorted, puting DC on the signal path.
 
Peavey sells all the parts in the thing at reasonable rates. WHy screw around?
Something about screaming my debit card number over the noise of the diesel truck parked next to the payphone at the gas station just strikes me as being wrong. Not to mention trying to communicate part numbers in that noise environment.
Or I could buy a cell phone for $45 a month plus a direct tap into my bank account and a contract for life. That's not going to happen. I retired when my knees started hurting so badly by not getting myself into those types of financial contracts. The fine print on my AT&T landline is getting so long and vicious I'm thinking of reverting to US mail to communicate. Thank you State of Indiana Communications Agency, for being total wimps.
 
Well, I am not sure what story you are trying to hint at, but most of us can find a telephone other than at the gas station. Friends or relatives? Work phone? And we don't have any reported phone use problems from the original poster, hence my sugestion o fgetting the real parts from Peavey rather than searching the internet for (hopefully) equivalent parts.

If you have a land loine, why would you have to go to the gas station and use a cell phone?

Just as an option, I have a cell phone that I buy $10 of minutes whenever I need them. No contract, no billing. And in fact, I can even buy my T-mobile minutes at the gas station if I want.


None of which gets the original poster closer to a working amp.
 
I'm hearing a squeal when I put a 1kHz sine wave in the input and turn the volume up. The output is present on the scope but is very distorted. So it's amplifying just with no linearity. I'll check those caps mentioned and I'll try the preamp isolation. I should have some time tomorrow. Thanks again for all the help and the explanation of the circuit. I know it's simple but I'm not familiar with that current amp chip. It was really mystifying me. I'll look up grounded emitter.
 

unclejed613

Member
2006-12-28 12:19 am
i noticed they did something very odd here... they're running the +/-15V rails off of the "flying" rails. if any of those caps dry out, you will get audio from the output rails leaking onto the rails for the op amps, which would cause at the least instability, at worst a lot of oscillation.
 

djk

R.I.P
2001-02-04 4:23 am
USA
"i noticed they did something very odd here... they're running the +/-15V rails off of the "flying" rails. if any of those caps dry out, you will get audio from the output rails leaking onto the rails for the op amps, which would cause at the least instability, at worst a lot of oscillation. "

+1
 
I've been out of town and haven't been able to work on this thing. Anyway, I checked the output semis out of circuit as Indianajo suggested. They don't have any leakage but the beta of the pnp is 30 and of the npn is 3! So I think I'll replace them along with the drivers if I can find decent substitutes. Also the caps test OK on ESR, etc. but two of the PS filters are bulging a little, I'll just recap everything. I'll report back as soon as the parts come in. Again, thanks for all the help.
 
Finally got the parts. I recapped it, replaced the drivers and outputs (one of the outputs had very low gain, but not shorted). Everything worked except the signal was distorted at just after the preamp input to the power amp (power amp side of R17). I finally realized that the OTA was injecting the noise. I pulled it out and the signal is clean. Thanks again for all the help.
 
Congratulations. If the DDT circuit (with an OTA IC in it) is working correctly it should only mess up the sound if you are putting square waves in the amp (which will melt black widow speakers, Harley Peavey said, that is why he had DDT invented). So something is wrong with that. I don't know how to test DDT, but pulling out the Operational Transconductance Amplifier is a good way to shut it off permanently.
 
Yeah, I think it was one of your earlier posts that convinced me that it is OK to just pull the OTA and leave it out. I promise, my bass guitar won't put any square waves into it. I happened across a sub for the OTA on Mouser's site (for $7.50!) and it was tagged "not recommended for future product development". Kind of cool though. Until this little repair I didn't know that OTA on a chip existed. Thanks again.