Peavet 400BH power amp problem

[Streamer]

Hi, my peavey XR600B mixer amp has very little sound coming out of it and I'm pretty sure it's the power amp that is at fault.
If I plug a lead from the "pre amp out" of the mixer amp into the "power amp in" of another amp everything works fine. If a plug the "pre amp out" from another amp into the "power amp in" of the mixer I get zilch.

I've had the power amp out and can't see anything with a burnt oiut look about it. There seems to be getting some power to power amp as it is nicely warm. I've cleaned the speaker sockets and changed leads etc. The fuses all look okay.

I'm not a tech and I live in rural France at the moment and the chances of getting it fixed by a tech are very unlikely - so I'm really keen to have a go at fixing it myself.

Any help would be much appreciated, thankyou, Rogan

[Jazz P Bass]

Peavey 400BH Schematic

Here is the schematic.
First rule: check the power supplies
Second Rule: look for shorted conditions where none should be.

[Enzo]

Is there any background sound when the amp is powered on? A small amount of hum or maybe some hiss. The kind of sounds any amp makes when it is turned on but not amplifying anything? That would tell us at least the power amp is powered and might want to amplify if we let it.

The power amp will work by itself, and that is how I usually service them. I let the rear panel lie down on the table as I face the rear of the amp. In the corner other end from the fuses is a four wire cable in a five hole connector. That cable runs up to the front panel. Unplug that from the power amp board. The wires are +15v, -15v, ground, and signal from the preamp. The pin away from the corner is the input to the power amp. With a speaker connected and the amp powered up, if I touch that input pin with my finger, it should make a hum out the speaker. Can't make a simpler test that that.

You are not blowing fuses, so I tend to doubt something is blown up.

Do indeed l;ook for missing voltages as Jazz suggests. Note that there are two sets of dual 15v supplies. The two with voltage regulators make the 15v sup-plies for the front panel. The power amp has a couple ICs of its own, and they use the zener regulated suppies instead. So finding 15v on the connector we pulled off doesnl;t tell us if the 15v for the powr amp is OK.

There are a couple ICs on this board.ALWAYS NOTE WHICH WAY THEY GO IN THE SOCKETS. You can remove the smaller 8-leg one during tests. ANy help doing that?

[Streamer]

Thankyou for sending the schematic Jazz, I've now downloaded a "how to read a schematic" file as well.
I'll get back to you later when I have done my homework, thanks again,

[Streamer]

Hi Enzo thankyou for that,,

That's a great way to work on it so simple, why didn't I think of that.

the plug from the front I have unplugged, the furthest pin doesn't make any noise when I touch it.

from the corner the first pin green wire, pin -13v to -19v dc on first touch then it goes to zero
then the gap gap
then bare wire with transparent sheath pin 0 v
then black wire pin 0 v
then red wire pin 15v dc consant
then white wire pin 0 v

removing the 8 pin IC didn't make any difference

the red wires with the red/yellow wire in between have 40 v ac on them
and the orange ones 21v

I'll test more when I have read my "how to read schematic" file

thanks again, hope my reply makes some sense

[Streamer]

Good news and bad news

Well I was doing okay, I was making sense of the schematic and getting some usefull readings, and am pretty sure
that big IC seemed okay but had a feeling something was amis to the right of that section.
Then i got interested and was looking at readings here and there and was probing the near one those 3 pronged black things bolted down to the metal plates with 8 flat ends bent up, where the bad luck (carelesness - useless amature moment struck) i slipped with the teste and there wwas a flash and the fuse blew.

I've replaced the fuse, but it won't power up without blowing the fuse, I can plug the orange wired connector in but not the red.
It will power up with the orange in and not the red and it still hums.

I know I've bungled, but if you have the time, it's just made me more determined.

Cheers

Hi Enzo thankyou for that,,

That's a great way to work on it so simple, why didn't I think of that.

the plug from the front I have unplugged, the furthest pin doesn't make any noise when I touch it.

from the corner the first pin green wire, pin -13v to -19v dc on first touch then it goes to zero
then the gap gap
then bare wire with transparent sheath pin 0 v
then black wire pin 0 v
then red wire pin 15v dc consant
then white wire pin 0 v

removing the 8 pin IC didn't make any difference

the red wires with the red/yellow wire in between have 40 v ac on them
and the orange ones 21v

I'll test more when I have read my "how to read schematic" file

thanks again, hope my reply makes some sense

[Enzo]

You probably destroyed the part you shorted. But what you need to do now is NOT assume any parts that were good before are still good now. That now bad part could have easily caused parts further downstream to fail. And yes, they can fail a LOT faster than a fuse can blow or your fingers can move.

Don;t assume anything, treat the amp as if it just now came through your door.

Do not connect a speaker or load to the amp until it no longer blows fuses AND it does not make DC voltage on the speaker jack.

[Streamer]

Thankyou Enzo,

I have found that if I disconnect the three wire plug with blue red and white going to the output board the fuse doesn't blow.
on the short pin I have 0 V on the middle pin 115V Ac and on the far pin 0 V

with my meter on resistance 20K and probing in the plug none of them seemed shorted to ground,
blue to red or white it reads 1 and between white and red zero.

The part I shorted out 522 5331 has 115v on the middle pin nothing on the other two
on resistance between left and middle 7. between right and midlle 4. and left and right .1 but the readings do seem to climb around a bit.

Hopefully these findings may have some relevance, thanks again nfor your time and patience

[Jazz P Bass]

The 5331 transistor is shorted.
Replacement number is MJE243.
Besides shorting out the amp, you still do not know the original problem.
A static (ie: no power on) check of the transistors may indicate the original fault.
The power supply check that you made was only Vac.
The next step would be to check the relevant power rails in Vdc mode.
As Enzo stated, until you have the amplifier in a stable state, do NOT connect a speaker load.

[Streamer]

Hi Jazz,

Thankyou for the info re the 5331 transistor and it's replacement, I'll get one ordered. I'll have a look at the transitors and then see what I can make of the power rails. As soon as I have any info I will get gack with the results, thanks again.

[Streamer]

Hi again,

I've tried to test the transistors with my tester on diode mode, as I understand current will pass from B to C and from B to E but only in one direction and not between E and C.
+ - meter - + + - meter - + meter + - meter - + meter
so my findings transitor 6531 C - E 000 , C - E 000 B C 000 , B - C 000 , B - E 000 , B - E 000

6534 1354 173 1264 176 001 001

MPS 8097 014 014 015 015 000 000

I can't see EBC on 2N 5400 AA713 so from the
3
back of the amp its flat side is on the right and I've numbered then 2
1

+ - - + + - - + + - - +
1 - 3 1 - 3 2 - 3 2 - 3 1 - 2 1 - 2
2N 5400 AA713 162 165 366 224 235 180
flat side on the left 2N 5400 AA713 286 305 613 217 215 1000

2 transistors side by side
I can't see the numbers
flat side on the left
both test the same 184 246 004 004 190 248

[Streamer]

Oh man, I spent ages typing that out as an easy to read table for you, but when I hit post it's lined it all up with the margin,
Now it will make even less sense.

I'll see if i can re aline it

[Streamer]

Oh man, I spent ages typing that out as an easy to read table for you, but when I hit post it's lined it all up with the margin,
Now it will make even less sense.

I'll see if i can re aline it
Oh man, I spent ages typing that out as an easy to read table for you, but when I hit post it's lined it all up with the margin,
Now it will make even less sense.

I'll see if i can re aline it

Hi again,

I've tried to test the transistors with my tester on diode mode, as I understand current will pass from B to C and from B to E but only in one direction and not between E and C.
..........……...... +....-...meter …...- ...+.meter…-..+ .. meter..+..-....meter
transitor 6531..C - E ...000…....C.-...E 000…..B..C....000…...B -C….000…..B..-..E ...000....B..-..E...000
……………. 6534………….,,,1354……,…….,..173.............1264........... ..176.. ..............001...............001

...... MPS 8097............014..............014...............015.. ...........015.... ............000...............000

I can't see EBC on 2N 5400 AA713 so from the
------------------------------------------------------------------------------------------- 3
back of the amp its flat side is on the right and I've numbered then........2
----------------------------------------------------------------------------------------------1

............................................. +....-..........-....+..........+ - .............-....+.............+....-........- +
..............................................1..-.3.........1..-..3..........2 – 3............. 2 - 3 ........... 1 - 2 ...... 1 - 2
---------------2N...5400...AA713........162...............165.......... ...366..................224............. 235 ..........180
flat side on the left
---------------2N...5400..,AA713..........286..............305......... ....613..................217.............215..........10 00

2 transistors side by side
I can't see the numbers
flat side on the left
both test the same---------------- ----184.................246............004-...----------004---------190 ...........248


Hope this works when I submit it

[Jazz P Bass]

Each transistor on the schematic has a 'Q' label.
It would help if you used that identifier.

[Streamer]

Hi Jazz,
That's a good idea, I've found the Q numbers, okay starting at the top of my list (6531 -Q1) (6534 -Q7) (MPS 8097- Q10) (2N 5400 AA713 -Q8) (2N 5400 AA713- Q11) and the last 2 are-Q16 and Q17,

Thanks again for help and your patience