Xr500 compression kicks in when any signal is put thru

[6t7goat]

I have an old xr-500 series 260c. When ever i put audio thru any channel the compressor light imeadiatly lights up and the signal is very distorted and sounds real bad. I have cleaned all the pots,jacks connectors etc. I have done a visual inspection as well as used a chopstick to check for any loose connections or bad solder joints and found no issues. Any guidance would be appreciated. I am a guitar guy but try to do basic repair on my amps when possible. Thanks in advance.

[Enzo]

The light is on the front, but the circuit is in back on the 260C board.

Are both polarity main power supplies present, roughly 42v each? Scope the output, what does the distortion look like? Missing half waveform? CLipping severely? Other?

Your main IC on the 260C board is a TL074. There are several of those on the preamp board (mixer, front panel). Swap the TL074 in your power amp for one of the ones up front. Any change?

[6t7goat]

No scope available. I can swap the IC and check pwr supply voltage. Thanks for the info. Will let you know what i find.

[Enzo]

I used to have an XR500.

WHile at it, make sure the two 15v supplies are OK as well.


Use the patch jacks on front. Plug a signal into the GRAPHIC IN jack to bypass the preamp, sound OK? Run a cord from MAIN output jack to some other amp and speaker, does the preamp sound OK? We want to narrow it down to power amp or preamp.

[J M Fahey]

Please somebody post the schematic.

Youīll have to troubleshoot, which means making some tests, evaluating results and making decisions accordingly.

Random/Blind parts replacement will not help you and probably increase/cause damage.

In principle you will have to take some voltage readings, starting, as Enzo suggested, with +/-V supply rails, +/-15V ones, presence of DC on speaker output, and transistor voltage between pins, but for any concrete suggestion we must both refer to same schematic.

Later Iīll suggest how to use a software oscilloscope and signal generator , plus making the necessary cables and signal attenuators so PC/Notebook can be used for both functions.

[6t7goat]

Problem appears to be in power amp section. I bypassed the pwr amp and ran from main out on front of unit to a different amp and signal was good sounded perfect. No compression happening. I ran signal directly to the graphic in to bypass the preamp section and the amp imeadiatly started compressing and breaking up. Think we are narrowing it down. I did not have time to open it back up.

[Enzo]

260C power amp

[J M Fahey]

Oh, it comes out (for us) as a single white dot "clear.gif"

Maybe you can zip it, give it another name, whatever

[Jazz P Bass]

PV 260C 78.zip

[J M Fahey]

Thanks JPB

So we have:



and, just in case:

[J M Fahey]

1) in principle, *excellent* DDT Compression used by Peavey works by comparing power amp output and signalinput.
So if output can not track input (damage/overload/short circuited load) and we feed signal in, it will trigger (itīs its duty) and as a side job light the Limiting Led.

2) check that all main voltages are present: +/-42V rails.+/-15V ... also check these reach T074 power pins 4 and 11, that you have 0V at speaker out rail, at TL074 pins 1 and 14, all this withpout speaker connected, without signal in.

IF TL074 has different voltages than shown in schematic, you can replace it with a fresh one as suggested and recheck.

Although I personally suspect some power or driver transistor is damaged, and amplifier can not drive a proper load.

But letīs go step by step.