Xr600c mixer amp, advice.

[Oldmactech]

Gifted a dead XR600 C mixer amp. Makes a noise like an elephant through a speaker even disconnected from the preamp side.

Big picture probably not worth saving as one or more of the XLRs and 1/4” inputs are broken. But…I’ve got it apart and replaced the two IC 1000uf caps that were leaky hoping that would help. No.

I’m Not all that knowledgeable so from the symptom, what would you expect the problem would be?

Thenks

[The Dude]

I would suspect shorted outputs or something in the amp section. Firstly, don't hook up a speaker until you verify that there is no DC on the output. Step one would be to measure the speaker output and see if your noise is AC or DC voltage. If it's DC, it would likely indicate an amp problem. If AC more likely a power supply issue.

[Tassieviking]

If there is no DC voltage on the speaker output, (a small amount is ok, mV), then isolate the problem to the mixer or power amp circuit.
To isolate the power amp, stick a plug in the power amp input ant that will separate the power amp from the pre amp (mixer) part.
The socket is on the front right bottom of the front panel (PWRAMP), best would be to insert a shorted plug, like a guitar lead with the other end shorted with some wire.
If it is still noisy the power amp is your problem, if quiet then the problem is in the mixer somewhere.
I would never throw out an 800 watt amp, if you cant fix it yourself, then some one else would be happy to have it and repair it.
Peavey-XR-600C-Schematics.pdf

[Oldmactech]

thanks.

The Dude please clarify. Outputs refer to transistors 6357 on the schematic?
I have the mixer section apart from the Amplifier section, can I measure Measure Voltage at the speaker Jack on the power section?
Help me understand the terms.. Difference between the amp section and power supply?

[Oldmactech]

Voltage at the output jacks

.6 DC
19.76 AC

[Tassieviking]

0.6V DC seems a bit high to me, but I am not really qualified to comment.
Hopefully someone else will chime in who knows more.
The output transistors are Q4, Q5, Q6, Q14, Q15, Q16 on the schematic I posted above, measuring at the speaker jack is good.
Did you try plugging in a shorted plug into the PWRAMP socket on the front of the mixer ? That will make sure no signal is injected into the power amp.
The power amp schematic is on page 3 of the manual, and the PCB layout is shown on page 4.
The signal comes into the power amp through C9, 2u2 50V non polarity electrolytic capacitor.
If you short out R35 (22k resistor) there should be no signal going into the power amp, if you still have noise on the output the problem is in the power amp.
It looks to me that Q17 is set up to short the signal as well, most likely a mute function from a mute button somewhere ?
U1 is a basic 4 channel OP amp chip (TL074), easily obtainable.
U2 is a OTR OP amp chip that is no longer made (CA3094), there are lots of counterfeits from China so beware if you need one of those.
I wish I could help more but I don't think I am good enough to go into a power amp circuit, I might just lead you astray.
Good luck, I hope someone else helps you out as well.
Peavey_Cross_Reference.pdf

[Helmholtz]

If you have around 20Vac at the output you likely have a power supply issue. Check the big filter caps (5000µ, C35/36).

[Oldmactech]

I have replaced the 1000uf caps, one was oozing a bit and the ILlinois brand caps are notorious.
Can the 5000uf caps be tested in circuit? And if they are failed, can I find suitable replacements?

[Helmholtz]

Can the 5000uf caps be tested in circuit?

Depends on your C-meter. Consult the manual.
Always most reliable to measure out of circuit.
You can always wire caps in parallel to add up capacitance.

Can your DMM measure AC on DC?
If so measure the power rails for ACV.

A scope would be great here.

[Oldmactech]

Thanks.
Dont believe I can measure AC on DC.
No scope either….

[Helmholtz]

Dont believe I can measure AC on DC.

Test your meter by measuring a battery in ACV mode. Should read zero.

[Tassieviking]

Kind of a late question, but...
What voltages have you got on the supply rails ?
You should have +52V and -52V at the power amplifier, measure across capacitors C26 and C30, measure DC as well as AC please.
You should have +16V and -16V at the power amplifier near Q2 heat-sink, measure across C41 and C42.
You should have +15V and -15V on the power amplifier, measure across C43 and C44 please.
If these are all ok then we go from there.
When you measure AC on a DC voltage you are looking at how large the ripple voltage is, the bigger and better the capacitors are the less ripple voltage you have.
This is not always true as it also depends on other variables like the load etc, but it is a good start.
I am also wondering what sort of noise your amp is making, is it like a mains hum, 50 or 60 hertz / 100 or 120 hertz ??
The E string (top) on a guitar is 82Hz and the A string is 110Hz for reference.
A 7 string guitar has 62Hz on the B string, in case you have one of those.

I think there are some cheap D class amps on Ebay that can run on +- 52V, from memory.
If you cant fix it then it might be cheaper to throw one of those in, you will have to make up a power supply board though to provide the voltages needed.
I live on a few acres and I use a Behringer 600W Euromixer to play music on outside, an old computer full of MP3's provide the tracks.
I have been over 100meters away with earmuffs on and chainsaw going flat out and still I could hear it really well, (so could the neighbors 2km away).

[Tassieviking]

The capacitors should be available, any will do as long as the voltage rating is above 55V and the capacitance is 4700uF or higher.
The critical thing with the capacitors is the distance between the leads and the diameter of the capacitor, it looks like the capacitors are clamped in with a circular mounting bracket on the drawing ?
You will also want to know if they have leads or snap in pins that solder them in place.
You might have to use a higher capacitance to get the same physical size (diameter) and lead spacing, but it does not matter at all, try to get low ESR caps if you can.
If you measure the lead spacing and diameter I can look some up for you, it might depend on what country you are in though.
at a rough guess I would say about 25mm diameter and 15mm lead spacing ?

XLR might be this type,
https://www.fullcompass.com/prod/045...-for-xr-series

This mixer is worth fixing, if for no other reason then as a really good learning tool for you, the best way to learn is with experience.

[Oldmactech]

Thanks for the testing tips.


Across c41 and c42 0 ACV 31.6 DCv
C43 and C44 5.9 ACv 10.9 DCv
Cr16 and c30 13.1 ACv 71.5 DCv

keep in mind my testing is a little suspect as I’m pretty cautious about electrocuting myself. I do have a light bulb limiter though.

the sound it makes is somewhere between a mating elephant and a car slamming on the brakes at high speed. And the preamp section is completely disconnected, still makes the noise if a speaker was connected.

[Oldmactech]

Thanks, I just opened the pdf and that answered my testing question.