Peavey 260 C (1978 model) buzzing

[Andcarl1]

Hi All,

got a Peavey Centurian III 260 C bass amp head that buzzes really badly when turned on with a speaker connected. I checked the output and found +38V DC on the output to the speaker. I checked pretty much the entire power amp section and found all the DC bias voltages out of whack. I tried isolating the source of the output DC by removing various components (transistors) but they always seem to check out fine out of the circuit. Finally I put everything back in circuit again. It was then I noticed that when I turn the compressor limit switch - off the DC disappears on the output and all the dc transistor bias voltage return to normal around the circuit. I then hooked up a speaker and found it still buzzed though nothing like the violent one way "suck down" I experienced with the DC. I measured the output and found 18.0 VAC which must be my buzz - it sounds like 60 hz but not sure. Questions are.

1. why did the limiter switch off return the DC voltages to normal and could the compressor circuit be the source of my problem?

2. Any idea on how to isolate and repair the source of the ac voltage?

3. Could 18 V ac be a power supply ripple voltage?

any help or suggestions gratefully received - I have some electronics back ground but I'm no expert.

Thanks Andy - California

[Enzo]

First, disconnect the speaker load, and don;t connect anything to the output until we have no DC or AC there at idle.

So it doesn;t blow fuses, that's a plus.

Look at the schematic, the power stage runs off +42VDC and -42VDC. Check them. Are BOTH at about 40-45VDC? It doesn;t matter if they are at 42, 40, 45, or whatever, but we do want the positive and negative to be about the same voltage. Also, measyre each again but set your meter to AC volts. That will measure the ripple on each. If one of your filter caps is failing, the DC will read low and the AC on it will be substantial. I expect less than a volt of ripple.

There is also the +/-15VDC suplies. Same thing there, they should both be real close to 15v and about zero ripple.

Those 5000uf 55v caps have not been available for years and years. Replace them with common 4700uf 63v caps if they are bad.

Look at the schematic, upper left quadrant. See the compressor LED? The compressor switch connects together the top two pins of that four pin connector. The LED is the bottom two pins. Now step back and look at the circuit. The output of the amp is sampled into U1a, then U1b, on through Q1 and into U2, which controls U1d. All of that is DC coupled. So DC offset anywhere along can screw things up. Now opening the switch would point the finger at U1a.

That TL074 had its outputs on the corner pins. Leave the switch off and see if pin 1 has DC on it. Several volts, not worried about a few millivolts. But check the +/-15v rails first, because an upset 15v power system can leave these sourts of troubles.

[Andcarl1]

ok speaker load disconnected -(I only plugged it in for a very short while - it's an old one)

Input AC = 110.8 VAC steady.

Readings taken at Cathode CR14 , Anode CR15, with compressor switch off and on

Switch off - CR14 +39.1VDC , CR15 -26.04 VDC,
CR14 varying AC, CR15 - 11.5 VAC solid

O/P reading - +57 mV DC, 17.9Vac solid.

Switch On - CR14 +39.3VDc , CR15 -37.9 VDC.
CR14 varying AC, CR15 varying AC

O/P +38.0 VDC, varying AC

Both 15 V regulators solid +15.02 VDC and -15.10 VDC both have varying ac on them at all times.

Pin 1 of U1 has -2.75VDC on it with switch off.


Could I have 2 problems here? AC ripple and bad compressor circuit chip?

Thanks for the help - Andy

[Enzo]

Well right away your V- is 14v low, plus it has 12V of AC on it. As I said above, if a filter cap is failing, then the DC will be low and there will be substantial AC. C27 is the filter for V-. I'd replace both of those main filters. This thing is over 30 years old after all. The 4700uf 63v or 80v caps with "snap-in" leads all seem to have 10mm lead spacing. This is not the same as the old parts. Looking at the traces on the board, it would be easy to drill new holes for it for C27. For C25, ther is a trace that runs between teh legs, and that might be a tight fit for new holes. If nothing else you can glue the cap to the board updide down, and run a couple wires down to the board. or get creative and mount caps off the board adn run wires back to it.

Fix the power supply first. Nothing works reliably with bad power supply. Other problems may or may not be present and may or may not disappear when power is restored.

Those 15v rails look OK, when you say varying AC, all I am worrying about is like several volts. I am not concerned when we see a couple tenths of a volt.

[Andcarl1]

thanks Enzo,

I agree the power supply caps do appear to be suspect - the Negative rail one is the original large metal can with support feet type - the positive one has already been replaced - though the solder job is rather poor - I'll probably just replace both and then see how things work.

Something that does puzzle me - with the switch off the negative supply is at -26V which is 14V low I agree - any idea why it returns to around -37 v when the switch is on?

I'll hit you back after I change the filter caps and let you know how things go - thanks again for the help

Andy

[Enzo]

Who knows, if I have to guess, I'd say the offset that happens when the switch is on is unloading the cap.

Fix it, and then we see what still needs work.

[Andcarl1]

Caps arrived from Newark - installed them easily enough with a little gentle bending of the feet to get them to fit the existing holes. Result success! Both supply voltages now +40V / -40V and no ripple. No output ripple or DC. Checked all the transistor bias voltages - all normal Then I tried the compressor limiter switch - switching that to on had no visible effective effect for about 10 secs and then R29 started to boil. I turned it off pretty quick. Decided to replace R29 and I also replaced C15. Turned it all back on minus the switch. Tried a speaker on the output and the amp is working fine. Then I tried the switch again watching R29 like a hawk = this time everything was fine. I can hear compressor circuit working and R29 is cold to the touch. Not sure what that was about maybe there's a problem with the unloaded amp or maybe all that ripple ac damaged R29 or C15? I think I'm going to leave it there and have them try the amp as is. I did order another Op Amp but I could not find a TL074CM only a TL074CN ($1.00!!) - they appear to be the same - any idea if they are?

Thanks again for all the help. Andy

[Enzo]

If you find any sort of TL074 in the 14-pin DIP like the one you have, it will work.

The compressor should only compress when peaks clip. And it shopuld not cause the amp to oscillate - which is what burnt up R29.

Way over on the left, see C9? Marked "tantalum." Also note the polarity. Tantalum caps are VERY sesitive to proper polarity.

That caps smooths out the compression feedback signal, so the poor little amp doesn;t try to track every little waveform. I'd replace that cap, even if you don;t have a tantalum, use a regular e-cap.