Peavey XR-400 (4 channel PA Mixer/Amplifier)

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5String
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Joined: Mon Apr 17, 2017 1:09 pm

Peavey XR-400 (4 channel PA Mixer/Amplifier)

Post by 5String » Mon Apr 17, 2017 1:59 pm

Peavey XR-400 (4 channel PA Mixer/Amplifier)

Hello.

I am having very big trouble with a PA mixer/amp that hums, has very weak output.

I’m using a known good Peavey PV22 Mic with a known good mic cable

The output is connected with known good 12ga/2cond audio cables (they’re power amp cables for amp to speakers, not just some guitar cables), with known good ¼ inch jacks (TS not TRS), to a pair of known good 8 ohm Full Range JBL TR125 speakers

http://www.jblproservice.com/pdf/tr-Series/tr125.pdf

They are in parallel for 4 ohms total (the output jacks on the rear of the amp is parallel anyway)

The amp is connected/powered with a grounded 3-prong typical house wall outlet (original amp power cable which still has the original ground prong)

XR-400 Amp power switch powers on (in either direction) with an loud audible pop, but not too loud, (and pops when powered off too)

Power LED turns on,

There is very noticeable hum, which increases when I increase the gain (but the hum is not too loud, just very noticeable),

All 4 channels (both highs and lows inputs < A and B input jacks >) function
(Gain barely increases audibly only when I get up to generally 8, 9, or 10, and then is still relatively weak- my yelling voice is actually louder), an increase in gain on any channel while the master gain is at about 8, 9, or max results in only some audio output, just certainly not 100 watts worth- not even close.

Master gain operates (gain barely increases audibly only when I get up to generally 8, 9, or 10, and then is still relatively weak- my yelling voice is louder)
Effects operate normally… (I can hear the reverb increase while operating the Effects knob, on each channel, and Reverb knob)

All 5 band graphic equalizer sliders operate normally and slightly help to improve the output power, but only just barely

While speaking into the mic, I can hear weak audio (gain barely increases audibly, -from 1 to 8 is generally useless, then only when I get up to about 8, 9, or 10 is any kind of real amplified audio, and then is still relatively weak- my yelling voice without the mic is louder)


I have not used any of the other connectors (aux in, eff out, mon out, main out, graphic in) as I have no power amps, rack effects, powered speakers mains, or powered monitor wedges

I opened the back of the amp and looked at the power section (200B power module – I think it’s a 200B but I’m not sure) circuit board to see that a ceramic resistor (R27, 5watt, 100ohm) has browned the circuit board beneath it some time ago, but no other noticeable damage anywhere else on the board, or on the preamp/controls board either, and I don’t smell any smoke or burning.

Is there anyone who can tell me what might have caused this 5watt resistor to draw enough current to get so hot and brown the circuit board beneath it?

I have looked at the underside (the soldered side) of the circuit board and there is no sign of shorting, arcing, etc

Probably, this resistor condition is the result of some other component malfunction.

On the back side of the board, there was nearly no solder attaching the board to the pins all of the 4 power transistors -as though some troubleshooting/repair by the previous owner stopped in mid stream, so I soldered the pins to the board and this did not correct any of the problems.(the pics in the link at the end of this post are prior to resoldering)

I have downloaded the related schematic diagrams and circuit board layout diagram but there is an apparent variation between my board and that of the board layout diagram as some components do not match locations but most do (probably 95% are accurately located)

music-electronics-forum.com/attachments ... 200h-b.pdf

music-electronics-forum.com/attachments ... module.pdf

The markings on the back of my 200B power module board :

(98830000) (KBB) (200H) (041883)

I know how to use a basic Fluke 115 digital multimeter, a good Velleman soldering iron, a solder sucker, solder wick, etc. I don’t have an oscilloscope. I have a very basic knowledge of some electronics, but it has been a very long time.

You might say that this is a basic circuit -ok for you, -but not for me. hahaha- sure I could take this to a tech, but what would I learn and where would the fun be in that?

Ok wordy, yes I know, but I wanted to be as accurately descriptive as I can, and explain it in as good detail as I can for you. (it might help someone else with another bad XR-400)

I wish I knew how to post the pics directly on this forum post.
I hope that you can see the 15 pics here if i did this correctly(please let me know):

https://postimg.org/gallery/ocg6idje/

Any detailed replies are appreciated. Please let me know how to properly troubleshoot this circuit and repair this problem (be patient-spell it out). Thank you in advance.

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