peavey musician mark III 400 GH 1978... loud constant buzz

[wheatley]

hey i've got a peavey mark III musician head that recently stopped working. after months of satisfying use it started to make a loud, low pitched hum when turned on and smells hot. i've got it opened up to see what i can see but before i can take many measurements the amp starts to smell hot. here's what i noticed...
1) the heat sink gets really hot after about 1 minute at the bottom left side near the power transistors below the large power supply caps
2) the transformer seems to make a clicking noise like a kitchen timer or a fast clock
3) there is 6.8 volts dc at the speaker out... which i think i WRONG
4) there is 15 v dc and - 15 v dc at the connector to the preamp... which i think is correct

any advice on where to go from here would be great... i have come to really like the sound of this amp

[olddawg]

You probably have shorted output transistors (and usually more) caused by a catastrophic failure or a shorted load condition. Don't keep turning the amp on. You need to start troubleshooting.

[DrGonz78]

Do not connect a speaker until DC on output is less then .500vDC... Check for shorted output transistors and other components as OldDawg was getting at in his post. So there is a short in the circuit path of the power amp section of the amp. >>> Perhaps that clicking is the PT arching (click click click...)??? Relays click and so on... Clicks of this nature would make me think of arching of power and you do not want your PT to take all this stress. So follow the advice of not turning on the amp until you find/replace shorted components. I read another link that you had up from the summer time (about this same amp) and it seems you are very familiar using Lamp Limiters. So that is good thing and it will be very necessary after finding the shorts to power it on safely. Is this a Peavey 260c power board?

Forgot to ask: Did any fuses blow? I guess not...? That is strange here too if the fuse did not blow. I am guessing but those old filter caps might need to be replaced as to really keep this amp working properly in the future. First thing first fix all parts that are shorted.

[wheatley]

thanks for the responses... the board with the power caps says "400 gh drive bd" and the one with the power transistors says "400 g/b output". None of the fuses blew. Do i need to unsolder the power transistors to test them? are the filter caps the 35v 1000 microF caps or the same as the power caps which are 55v 5500 microF. how would i go about testing the capacitors?

[Enzo]

The 400GH is the same as the 400BH, except for the 400BH has the compressor. 400GH circuit is the same but for that. However the component numbers ar all different, so Q1 on the 400GH is Q8 on the BH. SO contact customer service at Peavey and get the schematic.

The two 15v supplies you found at the preamp connector serve only the preamp. The power amp also needs +/-15v for its IC, but those are zener derived right on the board.

Pins 4 and 11 of the IC are its power pins, are both supplies present?

And are the main power rails OK? About 50v each.

Since ther is DC on the output, DO NOT connect a speaker until it is gone. So then, without a speaker, does it still get hot?

The schematic includes an output matching transformer. Does yours have that? If so, disconnect it, connect speakes directly to the board. If the amp tries to run DC through that transformer, it will struggle and overheat for sure. PLus the transformer could be loading down the voltage.

When you say the transformer ticks, did you mean the largte power transformer, or the smaller matching transformer?

Anything is possible, but I am not getting shorted transistors on my radar. That would put 50v on the output, or seriously blow fuses.

[wheatley]

I took these measurements with a light bulb limiter wired in series, first with the small transformer UNplugged...

i got -40v at the mains listed at -52v dc
37v at the mains listed 52v
39v at a point on the schematic listed as 26v
and on pins 4 and 11of the ic +14.6v and -14.6v
the light bulb got dim I the first seconds after turning on

I then took the same measurements with the small transformer plugged in

I got -5 v at mains -52v
4 v and the =52v mains
-3.5v at the point listed as 26v
and 1.5v and -2.7 volts at pin 4 and 11 of the ic
the light bulb never dimmed with the small transformer plugged in

also for both scenarios the heat sink did not get hot and the ticking noise didnt happen either so I cant say where exactly the noise came from

[Jazz P Bass]

I believe your output transistors (at least one of them) may be shorted.
By disconnecting the 'small' transformer, you removed any path to ground that the transistors may have.
When you reconnected it, there was the path, and the voltage took it.
Thus your whole power supply collapsed.

[wheatley]

how would i go about plugging speakers directly into the board without the matching output transformer? would i have to remove the transistors from the board to test them.

[Enzo]

The matching transformer is an option, only used on certain models. That same amplifier board is used on many models. The transformer plugs onto the power amp board on a couple pins. Your speaker would connect to those pins directly.

BUT

Until you know for sure there is no DC voltage on th speaker connections, DO NOT connect a speaker there. (Or transformer or other load)

You can look for shorted transistors right where they sit. Nothing about the circuit can make a shorted one look as if it were OK.

[bsco]

I actually own one of these heads. I bought it new in 1978.... I had problems later with mine....It was stored for a couple of years and when I took it out and plugged it in in blew the fuse...Opened it up and I had 4 transistors shorted...So I replaced all eight of them, soldered up a bunch of bad connections, and I believed I replaced the filter caps....it was a few years ago so I can't say for sure....While I had it apart, I also went through the preamp board and soldered a bunch of bad connections there as well....been using it now for the past 10 years..... great head....no problems since....I run mine direct as it has a connection in the back for direct to spkr or through an 8 ohm put as well....which would be transformer out.....you don't even need to remove the transistors to test them...you can check them right in the circuit just like Enzo stated...I also had to replace the 10W resistors as well...can't remember the values of the top of my head...Hope this helps....

[wheatley]

so yea all 8 of power transistors are shorted and im gona replace the filter caps because theres only +/- 39 volts at the mains. where is a good place to buy the mj15003 transistors, i keep seeing things about counterfeits and prices ranging from 3 to 8 dollars per transistor online. also there is like a white grease between the case of the transistors and the board, do i need to reapply more of the stuff, and what is it?

[Enzo]

It is possible, but unusual for ALL the outputs to be shorted. remember they are wired in parallel, so one shorted one makes the whole row of them appear shorted. Start removing them and testing them when loose from the board.

I ONLY buy parts from reputable suppliers like Mouser. Some guys buy their parts from ebay, and that scares me. If they do it successfully, more power to them, but not for me.


White silicone heat grease is necessary for proper thermal transfer from transistor to heat sink. You can find it at parts suppliers too.

Peavey will also sell you transistors direct, and probably even have tubes of heat grease.

Are you still using the light bulb limiter? If so, that is probably why your voltages are low. if your 52v rails were down to 39v because of bad caps, you;d have 10-20vAC of ripple on them or more. The bulb is a great tool for preventing damage to an amp from applying full power. But it has the side effect of not putting full power into the amp, so of course all your voltages will be low.

You might need new filter caps, but please don't go changing yours out until this amp is repaired.


If you do swap out the caps, you will not find any of those 5000uf/55v types. Those are non-standard and have not been made in years. The replacement would be a common 4700uf/63v type. Also, the connetions on the cap will be different, so you would have to fiddle around to mount them. I usually wind up drilling a couple holes for the leads.


And just a technicality. "Mains" refers to the AC power coming out of your wall outlet, the common 120vAC in the USA. We do not refer to the 52vDC rails in the amp as "mains". It may be that the 52v rails are the main DC power supplies in the amp, but the term "mains" is already in use for your AC power coming in. "Rail" is a common term, as in the 52v rails and the 16v rails, or in this case high voltage rails and low voltage rails. We also often use terms like V+ and V-.

[wheatley]

thanks, lots of good info there... i am taking the measurements with a light bulb limiter so i'll hold off on the caps. so i took all eight mj15003 transistors out and tested them with the diode function of my DMM and they all checked out fine. when i put them back in the circuit though they read a voltage drop of 0 volts going both ways from emitter to base and base to emitter. the resistors all check out fine but i found that diode CR6 reads open both ways so i'll replace it and see what happens.

also on the schematic there is something labeled T.B. in the middle of the 8 power transistors but in my amp it is bolted on top of Q14. should i leave it on top of Q14 or move to where the schematic shows it

[Jazz P Bass]

If you look at the schematic you will see a 1 ohm resistor between the base & the emitter of Q4 & Q14.(R28 & R62).
So that is what you should read. One ohm.
If you are truly reading zero ohms, then there is still a base/ emitter short.
As to the TB bolted to Q14, leave it there. (it is the thermal breaker & it needs to sense the output transistor temperature)

[Enzo]

should i leave it on top of Q14 or move to where the schematic shows it

Just fix whatever is wrong, don't go changing anything.

they all checked out fine. when i put them back in the circuit though they read a voltage drop of 0 volts going both ways from emitter to base and base to emitter.

If you look at the schematic you will see a 1 ohm resistor between the base & the emitter of Q4 & Q14.(R28 & R62). So that is what you should read. One ohm.

The lesson here is that the circuit is what is important. Your parts do not operate as individuals alone in a vacuum. They are always connected to other things - the circuit. IF your transistors SEEM shorted, we then take them out and measure them alone. We also look at the circuit as Jazz points out to see what resistances we OUGHT to see in a normal circuit. In this case the 1 ohm is normal, and by taking the transistors out, you found that they were not hiding any defects behind that low 1 ohm resistance.