Peavet 400BH power amp problem

[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.