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]

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]

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

[Streamer]

Hi Jazz
I'm not sure if this is relevant in checking the Vdc, but I'm trying.
From the back of the amp VR1 has three prongs pointing towards the front with my tester on VDC set at 500 the left prong reads 14, middle one doesn't read, and the righthand one 27.
the middle one reads .3 with my tester set at 200

VR2 with the prongs pointing back the left prong doesn't read middle prong -27 and the righthand prong -19
the left prong reads -42 with the tester set at 200

Is that a help please, thankyou

[J M Fahey]

Somewhat confusing alignment but Q1, Q7 and Q10 , so far, look shorted.
I guess you are measuring them onboard; carefully (good solder pump, maybe some solder wich for final cleaning) pull them and remeasure outside.
Now that I check the schematic (I *hope* you follow its numbering) you are measuring small, short protection transistors.
Yes, they look bad, but what about *power* and *driver* transistors?

[Streamer]

Hi Juan, thankyou for your thoughts, I am measuring them on board - or trying to - I'm new to this - as you may have guessed.

I'll see what I can test on the others as you suggest.
Diode mode between the two soldered joints I can see
On the curcuit board diagram Jazz sent, Q13 is on bottom left of the output board
screw fixings are bottom left and top right, soldered joints are diagonally opposite the screws, soldered jopints bottom right and top left

I've called bottom right 1 and top left 2
..............1............2..................1......... .........2
...............+....>.......- .......//.......- ........>........+
Q6................ 000..............................000
Q5................ 000...............................000
Q4.................000...............................000
Q3.................000...............................000

Q16...............000..................................0 00
Q15...............000..................................0 00
Q14...............000..................................0 00
Q13...............140..................................I----... (meter doesn't actually move)

On all the above if I test between the screwfixings either way or the screws and 1 and 2 either way round I get 000

Q13 between 1+ and top right screw- 140 1- to top right screw+ I----(meter doesn't actually move)

I've just noticed that TB is fixed to the top of Q4 and is not mounted between Q3 and Q16

Hope this helps, and thankyou for your time

[Jazz P Bass]

It would help if you knew what you were testing for.
You need to consult each & every transistors datasheets to know what pins are what.
Then you need to apply that info to the schematic.
In the case of the output transistors, there is a one ohm resistor from the base pin to the emitter pin, so any reading that you get is meaningless, unless you (A) pull the transistor or (B) lift one leg of the resistor.
Q3 & Q13 are the odd ducks, where there is a 47 ohm resistor from base to emitter.

[Streamer]

Hi Jazz,

Thankyou,

I had the feeling someone was going to say something along those lines, I shall have to send for a solder pump and get some more data
so it will a few days before I have more questions - but I'll be back - I bet you can't wait

Thanks again for all your help

[Jazz P Bass]

Here is the Peavey Cross Reference.
It may help when looking up the 'real' transistor numbers.
I have also included the output transistor pinout (TO3 package)
The 'Collector' is the metal case.

[Streamer]

Brilliant Jazz, thanks for those

[Jazz P Bass]

Another pinout.
The MJE243 (TO225 package, NPN) & it's PNP mate MJE253
Case to the top
Leads to the bottom
Left to right
Emitter, Collector, Base.

[Streamer]

Another pinout.
The MJE243 (TO225 package, NPN) & it's PNP mate MJE253
Case to the top
Leads to the bottom
Left to right
Emitter, Collector, Base.

Thanks again Jazz, your a star,
I am waiting for a reply from Peavey parts for some parts pricings,
I'm also waiting for a solder sucker to arrive, as that and my soldering
iron and solder seems to be the thing to remove parts from the board.

Are the control pots interchangeable, say the perhaps the monitor channel level and the reverb main, or say the channel bass
for the channel treble, sorry if that's a daft question.

Thanks again.

[Enzo]

Look on the schematic, they either have the same value or they do not.

[Streamer]

Look on the schematic, they either have the same value or they do not.

Hi Enzo,

Thanks for your reply, I was just being lazy, I haven't got a schematic for that part of the amp - I'll get on it and download one
thanks again

[Streamer]

Hi again,

I have to say Peavey spares USA have been really helpful,

[Streamer]

I'm Back,

I have replaced Q2 the transisiter I blew up, and have had Q13 Q14 Q15 and Q16 out of the board, and tested them against a new one. Q14 seemed to be shorted so I renewed that one and put the others back in.

I have the pre amp power amp and transformer all out of the case and connected together on the table.
When I switch it on - the fuse doesn't blow, so that a start, there's a heathly hum but -46VDC at the speaker socket, so I haven't plugged a speaker into it or loaded it.

there are no VDC at the pre amp out socket but if I plug that into another power amp I get a hum but no sound.

At R66 i have 9 and 49 VDC, at R68 -9 and -49 VDC, at C43 9VDC and at C44 -12 VDC.

With the board in front of me fuses to the left at VR1 i have (left to right) 14 00 and 26 VDC
at VR2 00 -25 - 13 VDC


the collectors on Q3 Q4 Q5 and Q6 have -49VDC
and Q13 Q14 Q15 and Q16 have 49VDC

R31, 30,28,29, 63, 62,60 and 61 all have -46VDC

I hope this info helps, any help always very much appreciated, thankyou

[Streamer]

Good news the pre amp is working - I had the channel gain up but the master down, turned the master up and the pre amp works fine through another power amp.

What can I say, how dumb can you be,

Okay just the power amp to sort now - uumm isn't that where I started, never mind I'm learning all the time.

[Streamer]

Good news the pre amp is working - I had the channel gain up but the master down, turned the master up and the pre amp works fine through another power amp.

What can I say, how dumb can you be,

Okay just the power amp to sort now - uumm isn't that where I started, never mind I'm learning all the time.

[Jazz P Bass]

At R66 i have 9 and 49 VDC, at R68 -9 and -49 VDC, at C43 9VDC and at
C44 -12VDC

Something is amiss with these voltages.
The indicated +9 & -9 should be +15 & -15Vdc.
Either something is seriously loading them down or both zener diodes are bad. (CR20 & 21)

[Enzo]

You say you have 49 and 9 volts on both those resistors, yet you have 9 and 12 on the two caps. Since the caps and resistors are wired together, the voltages at the right ends of the resistors ought to match the voltages across the two caps.

In any case, since both low voltage supplies are low, it MIGHT be two bad zeners, but I'd be more suspicious of a bad IC on the board. Does either IC get warm just sitting there? Remove the ICs from their sockets, now does the voltage at those two supplies rise back to 15v?

By the way, your preamp works? Fine, disconnect the cable to it from the corner of the 400BH board and set it aside. That will allow more room for you to work.

Sort out these low voltage issues, and you may find the rest of the amp falling into place.

[Streamer]

Hi Jazz,

thanks for your help , I've removed C20 & C21 and I think they are okay.
with my tester on resistance 20K neg lead on the dark ringed end I get 12.30 swop the neg and pos leads around and the meter just stays on 1.
so something is happening one way and not the other - does that make them good?

Thanks as well for the attached file , I'll leave C 20 and 21 out for the moment until I hear that you think they are okay.

cheers for now.

[Streamer]

Thanks Enzo,

when I have the okay to put C20 and C21 back in the board, I'll disconnect the pre amp, power her back up and check out the ICs.

Thanks again.

[52 Bill]

with my tester on resistance 20K neg lead on the dark ringed end I get 12.30 swop the neg and pos leads around and the meter just stays on 1.
so something is happening one way and not the other - does that make them good?

You are charging the cap with the battery in your meter. When you reverse the leads your meter is seeing the small voltage that is being stored by the cap, that is why you are getting the 1 reading. If you keep the leads attached the voltage will slowly drain off and then you should see a rising number probably back up to 12.30.

While this test does not actually test the capacitor, it does prove that they are not dead shorts and are probably ok. I'd put them back in, watching the polarity, and go on to the ic test that Enzo suggested.