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

[wheatley]

sooo i let this project sit for the better part of a year and got back in it a week or so ago. i've been reading other threads about the same amp and here is some new info that seems important.

ic data
pin 4 +14.8v pin 5 and 6 +7.93v pins 8-10 +.81v 11, -14.6v 13, +12.4v 14, +4.5v the rest were zero

after taking all the power transistors out and testing them im confident that they are fine. Im pretty sure the problem is on the drive board of the power amp as ive tested all the components.

[wheatley]

Vr1 voltages are +21.3v ; +.67v ; +15.6v

vr2 voltages -15.6v -21.3v -.69

Q6 which is a 5331 on my schematic +37.3 +39.3 -38

Q7 (5332) +36.8v -39.6v +37.5v

Q2 another 5331 -39.6v -39.6v +37.5v

Im pretty sure the order of the previous three measurements is E C B

[Jazz P Bass]

In order to help you, you must help us.

Unless you can positively identify the pins of the transistors, any measurements that you post are meaningless.
Look up the datasheet for the MJE243 (5331) & you will see the proper pinout order.
Redirect Notice
Note that this shows the transistor from the rear.

At any rate, it appears that you do indeed have dc volts on your output.

Q2 voltages are way off.
Look at the schematic & you will see that the ECB should be:
E: +0.58Vdc
C: Power supply +Vdc (lower if using a limiter)
B: +1.17Vdc

You need to find why there is negative voltages on Q2.

As to equipment: does your meter have a 'diode check' function?

Also, are we on the same schematic?
http://music-electronics-forum.com/a...0-pv-400bg.pdf

[wheatley]

heres a link to the schematic i'm using. its got the whole Musician mark iii 1978 head schematic

https://pod51034.outlook.com/owa/ser...kyJs-H3P5mD7e8.

[Jazz P Bass]

Bad link.

[wheatley]

this one should work ... https://workspaces.acrobat.com/?d=xt...1ZH1ZfLsbok5Ow

it looks like the labels are different between the schematics.

on mine schem Q2 (5331) E -39.6 C +37.5 B -39.6

Q7 (5332) E +36.8 C -39.6 B + 37.5

Q6 (5331) E +37.3 C +39.3 B +38

there is also +36.8 v at the white wire on the speaker jack.

[Jazz P Bass]

That link is also a dud.

+36.8 volts on the speaker out is not good.

Use your meter to 'diode check' Q2.(power off)
It sounds as if it is shorted.

Can you compare your schematic (the power amp) to this one?
http://music-electronics-forum.com/a...0-pv-400bg.pdf
They should be the same.

[wheatley]

I'm going off of page 5 and 6 of the schematics i posted.

i believe the translation is Q6=Q2 Q7=Q12 Q2=Q9 mine schem. = your schem.

anyways i pulled these three transistors and they all tested fine with the diode function on my dmm.

[g1]

Here is the schematic from post #21

[wheatley]

yea thats the one, thanks for that. i was having difficulty uploading that thing

[bsco]

Hi there...I am not as experienced as these other techs but maybe if those output transistors have mica insulators, one of them could be bad and causing your dc voltage at the output....I am not positive they they are used in this unit but if they are check those...they tend to get brittle after awhile and there could be a very small piece which has broken away and allowing the transistor case to short to the chassis..It has been so long since I had mine opened up I can't recall if the unit uses these insulators but if it does then check them...there has got to be a reason for this dc to appear at the output and if your output transistors are good, and your filter caps are good then this might be something to check into....hope this helps you out....
Cheers,
Bernie

[Jazz P Bass]

Thanks for the schematic GOne.

It appears that Q6 is shorted.(at least)

[Tone Meister]

Was this amp ever repaired?

[wheatley]

Was this amp ever repaired?

i had to let it sit for a few days.. it was drivin me crazy cause i had pulled like every other transistor on the drive board probably twice. i'm new to this and kept questioning my readings. anyways i found a diode that was open CR19 that i replaced, but that didnt take the 39 volts off the white wire of the speaker jack. i then pulled one of the TL074CN ic from the pre amp but was confused because the ic in the power amp was a 4741CP which i think was a replacement because the schematic specifies a TL074CN on the power board. the ic from the pre amp took care of the DC voltage on the speaker jack so im currently hopeful that it will be up and running soon.... can i run the preamp without on the that ic to test the amp while i wait for the part. i feel like ive tested almost every component that i can on this thing, mostly out of ignorance. is there anything else i should look out for since the ic was bad?

[g1]

Are you saying you removed an IC from the preamp and the DC at the output is now gone?
Which IC was it on the schematic?