Peavey XR684 turn on/off thump.

[Check.. 1,2]

Hi all ... first post here .... I've had a good look over the forum and seems like I'm in the right place ...

I have an old XR684 domestic version with the jack outputs and bridge mode that wasn't working on the right channel ... that has now been fixed by replacing U202.

The remaining problem is very dramatic thump when turning the unit on and off.

I am a sound engineer and did basic electronics when I was training back in the 90's so can get around a schematic and have a scope, signal generator, good multimeter etc. that I usually use for gain structure calibration etc. in audio systems.

I'm not good at circuit fault diagnosis and some component testing methods ... I have the schematic here (400sc) and looking over it I can see components that I think maybe involved such as transistors Q105, Q205 but on other amps I have here there are delayed relays that are a lot more obvious so I wonder if I'm barking up the wrong tree.

XR684 power amp.pdf

If anybody can help point me in the right direction it will be well appreciated ...

[Check.. 1,2]

Hmmm tried to upload the diagram ... but seems to have turned into "clear.gif" ...

I'll try again ... XR684 power amp.pdf

[DrGonz78]

The schematic needs to be put in a zip file or something altered from the original. Problem is the database got screwed years ago leaving some downloads defunct. Courtesy of member G1 here is that downloaded file.
http://music-electronics-forum.com/a...-schematic.zip

[Check.. 1,2]

Thanks ... I'll maybe get some help now ...

[Jazz P Bass]

Looking at the schematic, I can see a few circuits where the signal is 'muted'.

I would start there.

[Check.. 1,2]

a few circuits where the signal is 'muted'.

Thanks for the reply jazz ... "a few" ... could you be a bit more specific please and I'll have a close look at those areas and try to get my head round it ...

[doctor]

This effect appears when capacitances of filter capacitors after high-voltage rectifier are different. In this circuit, they are C2 and C3 (each 10,000uF 80V).
The best way is to replace both. Good luck.

[Check.. 1,2]

Ok ... bit of a delay there to find time to have another go at this, here's where I am with it:

Filter caps are good and the 60v rails are bleeding properly and not holding charge.

The "X" JFET clamping/muting circuit is working, I replaced the JFET's (Q105,Q205) anyway.

Grounding Pin 1 (U100) doesn't make any difference but If I short Pin 7 (U100) to ground it stops the thump ... so it looks like the transient pop is coming in after the FET's?

I tried lifting the drain leg of Q105 (links to pin 1 of U101) and jumpering to Pin 7 of U100 but that didn't make any difference to the thump/crack transient pops.

Any suggestions on what to try next?

[Check.. 1,2]

Here is schematic: http://music-electronics-forum.com/a...-schematic.zip

[Jerry]

Check Q1000,1200,1201. They're on the preamp schematic, or, if you have the preamp board out, they're around their respective controls(main and monitor master volume controls). I talked to Gene at Peavey one time and he told me of a dealer in Texas, and the service guy used to pull all of the muting transistors on new mixers so that he wouldn't have to pull or replace them at a later date.

ehh, they're before the ones you noted.

[Check.. 1,2]

Check Q1000,1200,1201. They're on the preamp schematic, or, if you have the preamp board out, they're around their respective controls(main and monitor master volume controls). I talked to Gene at Peavey one time and he told me of a dealer in Texas, and the service guy used to pull all of the muting transistors on new mixers so that he wouldn't have to pull or replace them at a later date.

ehh, they're before the ones you noted.

Hi Jerry .... thanks for your reply ... yes I was already considering them.

But because the power amp JFETS (Q105, Q205) short to ground it should mute anything that happens in the preamp anyway so I think they are not the problem.

Do you think I could still get thump/pop from the preamp JFETS even though the power amp muting circuits seem to be working?