Peavey XR684

[jondoe]

Peavey XR684 schematic.zip

Hi all, I have one of these on my bench at the moment, here's a fun story for you.

It wasn't powering up fully, no lights on the front preamp, but the fan was spinning and the fuses were good. I found it had a dead +15v regulator, I read elsewhere on the forum that this isn't uncommon. I swapped it out and everything lit up like christmas, excellent I thought. The power output was there now, but low. I figured I could plug in directly to the power amp input and bypass the preamp to test, so I plugged my phones 3.5mm output with a tone generator app running and sure enough I had a strong output on the power amp. I swapped the jack over to the Right PA input, the phone turned off, something fizzed and the preamp rebooted.

Now my android phone has a blown headphone output, it's royally screwed, the Right PA IN was actually putting out +15vdc On a closer look CR1313 is dead short, which probably explains why the voltage regulator went bang in the first place.

I learnt a valuable lesson today on not using my nice android phone as a tone generator on old broken amps.

[DrGonz78]

I learnt a valuable lesson today on not using my nice android phone as a tone generator on old broken amps.

Yup get a nice old tone generator and use the phone to take pictures of the amp and look up parts.

[Helmholtz]

the Right PA IN was actually putting out +15vdc

So the right channel PA input coupling ecap is shorted?

[jondoe]

Peavey_XR684F_XR696F_PREAMP.pdf

I've attached the preamp schematics, I didn't realise the zip I uploaded didn't have them, apologies.

So the right channel PA input coupling ecap is shorted?

CR1313 is an LL4148 diode (PAIN2 on page 17 preamp pdf), I removed it this morning and it was dead short, which cap do you think it is?

I think the 15+ Voltage regulator is getting a little too toasty, maybe I've more issues to find.

[Helmholtz]

which cap do you think it is?

PA input coupling caps are C107 and C207, 2.2µ/50V NP. I guess C207 is right channel. If they are ok, you can't have DCV at the inputs.

CR1313 is an LL4148 diode (PAIN2 on page 17 preamp pdf

Don't see CR1313 on the power amp schematic. Which page is No. 17? The pdf only has 10 pages without numbering.

[jondoe]

Sorry about the missing PDF back there

There should be 28 pages to the preamp pdf attachment, the actual label on the sheet is "SHEET 13 OF 17" in the bottom right corner, I'll post a screen grab below for reference

[Enzo]

PV protects their inputs and some outputs with a pair of diodes, reverse biased from the port to the +15 and -15 rails. If one of those diodes shorts, you get 15v on the jack.

There should not be 15v on the inside end of the coupling cap anyway.

[jondoe]

PV protects their inputs and some outputs with a pair of diodes, reverse biased from the port to the +15 and -15 rails. If one of those diodes shorts, you get 15v on the jack.

There should not be 15v on the inside end of the coupling cap anyway.

Once bitten twice shy, hopefully I won't get caught by that again. I'm waiting on delivery of some diodes, I'll replace them all while I'm at it and see how she fares.

[jondoe]

So diode(s) are replaced, I don't know what rating the originals were but I didn't go for the largest 2A versions. I was hoping that might be the end of it but I still have problems.

The power amp inputs now pass signal cleanly on both channels and the peavey puts out a clean signal. Using the line input causes creates a slight distorted signal, the output wave form becomes a little fuzzy looking. t

I've used me cheap scope to trace a clean signal in the TAPE INPUT section, through U902A/B, through to LEFT/RIGHT BUS (page 7) onto the output of U1202B and U1204A, the sine wave still looks clean and stable.

I've just discovered the FXIN+ is carrying a huge amount of hissing noise to the effects board, so down that rabbit hole I go!

How hot do the voltage regulators run on these, does anyone know? The mains bridge rectifier, negative and positive voltage regulators all share the same thin metal plate heat sink and they are VERY hot.

[jondoe]

After some poking around I realised the hissing was down to one of the channels not being turned down, it was turned all the way up

I've discovered the output of FDBKOUT1/FDBKOUT2 do not look healthy, they are both clipping the + signal and flatting out the top of the sine wave. The signals are fine all the way to FDBK1IN+ and FDK2IN+, so maybe the DSP board is foobar?

[Enzo]

I have the power amp drawing you posted, and a 28 page file for the mixer part. I cannot find where FDBKOUT1/FDBKOUT2 come from. Can you point that out for me?

If two signal like that exhibit the same symptom, my first thought is do they share a dual op amp? Is it faulty? Is it missing one power rail?

[jondoe]

I have the power amp drawing you posted, and a 28 page file for the mixer part. I cannot find where FDBKOUT1/FDBKOUT2 come from. Can you point that out for me?

If two signal like that exhibit the same symptom, my first thought is do they share a dual op amp? Is it faulty? Is it missing one power rail?

Apologies, I should have mention where it was, there is a lot to trawl through! FDBKOUT is on the preamp schematic, its "sheet 11 of 17" . There is a 40 pin ribbon cable that runs to the preamp board to the DSP board (J1100), there is no schematic of the DSP board itself. My only other observation is the +15v rail is a little low at 14.7v, the -15v rail is bang on -15, but the signal looks perfect coming across the preamp board.



I took a picture of the wave forms shown on the FDBKOUT lines. I can trace these back to a 2068 dual opamp on the DSP board, which itself is receiving a similar clipped signal on pins 3 and 5, I can't quite work out where it's getting those signals from right now.

The DSP board has some feedback reduction system, I went over the manual and tried using the bypass setting, but that didn't seem to make much difference, although it did seem to lower the gain unless it's turned off completely (holding down button till it blinks twice)

[jondoe]

I've spent some time hunched over the DSP board trying to trace the source of those nasty looking wave forms. The signal to U102 (right hand side) pins 3 and 5 seem, to be localised to that area of the board, I can't find it anywhere else on the whole board other than right next to it. There are two AK4528 DAC's on the board, the only one receiving the test signal is below U102, on pins 2,3,4,5, its also putting out a clean signal on pins 25,26,27,28. All the caps across the middle and the diodes have a clean waveform, where one is passing.

I double checked the power amp is working via the Power Amp Inputs and its loud and clear, but any other input has low strength and distorted output. I might be barking up the wrong tree with the DSP, any thoughts are welcomed

[jondoe]

I thought I'd add the logical layout of the from the manual. There is no mention of the feedback system in that diagram, but I'm assuming the Line Input (channel 9) doesn't terminate on the left/right bus until it's been through the feedback control? Or everything goes through the feedback control then terminates on the bus? FDBKOUT1/FDBKOUT2 (sheet 11 of 17) from the 40 pin connector don't appear anywhere other than the header in the pre amp schematics, unless I'm missing it.

[Jazz P Bass]

This may be the correct DSP schematic: Pvy XR696F DIGITAL BD.pdf