Kustom PA 300 DC on output and lots of it!!!

[thintheherd]

Hello. I am currently working on rebuilding a vintage Kustom 300 PA. When I got it, it had a blown power fuse. Turned out one of the 2N3055 type transistors was blown.I replaced it and (I have tested all transistors in the power section) I brought it up slow on the variac and with my current limiter I could see there was a short still somewhere in the circuit. Well I disconnected the power to the power amp and was getting normal readings from the power transformer and filter caps, so that is good. After checking every transistor on the power board through and through by using my diode setting on the meter, they all were reading fine. There are portions on the power board that appear to have been repaired with scorch marks and the whole bit but those components were replaced by a tech at some point and they read fine on the meter as well. I checked for dc on the output and I am getting immediate dc reading that are climbing to 15 volts dc at half ac power from variac.

I went ahead and replaced two (there are 3) of the NPN 40409 drivers on the power board but I am still getting DC on the output, there are 3 of the 40410 too.DC readings same as before. Any ideas where to start? Any help would be appreciated. Thanks!

[Techknowman]

I haven't worked on that particular model but I have found after checking the transistors that there would still be defective components. usually when a transistor goes, it sometimes takes resisters with it, Especially in close coupled amps.
Some resisters are made to fuse to prevent catching fire and they can look good but be completely open.
also check the diodes in the circuit as some of these are used to bias the output transistors and could allow it shift to one power rail or the other especially if resisters are bad in the string as well.

[thintheherd]

Thanks, Ill check it out and report back.

[Techknowman]

If you have a schematic it might help.

[thintheherd]

I do have a schematic but it glued to the inside of the chassis. By the way i just checked EVERY I mean EVERY resistor and diode on the power board and they are all totally fine. Hmmmmm.

[thintheherd]

ok, this board is the same exact as mine except this is the 200 and mine is the 300. mine has one more amplifier stage (2 more outputs, 2 more drivers, same circuit otherwise.)

http://music-electronics-forum.com/a...8-dscf2808.jpg

[Mark Black]

Don't know this to be true of that particular amp, but I have run into a few (usually older, like that Kustom) amps that wouldn't stabilize until they reached something approaching normal line voltage and really didn't like being brought up on a variac.

It could also be possible one of the output transistors you did not replace becomes leaky at operating voltage. These days I tend to replace them all (at least the ones in parallel) when one goes bad. Actually there is a recent thread with a nice post at #8 by jrfrond extolling the virtues of replacing seemingly good (but subjected to stress) transistors here: http://music-electronics-forum.com/t21760/

[Enzo]

What is the board number for the board in your amp? Your example was a PC5065, what does yours say?

Your meter does a very basic check, but what it does not do it put real world voltages on the parts and draw real world currents through them. Just because a transistor acts like a diode at 2 volts from a meter does not mean it doesn;t leak like a sieve at 40v. (sieve? seive?)

It isn't always bad parts, an open trace somewhere can do the same to you.

And at this things age, caps are always suspects, I see looks like four small e-caps. Those probably need changing even if they are not yet at fault.

Well now just a minute, I HOPE we are working with no speaker load until we get rid of the DC. The thing is putting out DC, but without a load is it drawing excess current? Or does it just put DC out? And if it is not blowing fuses, have you gone all the way up to mains voltage? A lot of amps are unstable at half mains voltage and could easily act funny then.

[thintheherd]

Hey guys, thanks for the replies. The board number is PC5029. I looked in the forum archives and the schematic was there but has since been removed. There is no load on this amp when testing DC on the output. I havent tried swapping the e caps out yet but I certainly will now. I will dig into this more and Enzo if you have this schematic could you post it? Thanks!

[olddawg]

ok, this board is the same exact as mine except this is the 200 and mine is the 300. mine has one more amplifier stage (2 more outputs, 2 more drivers, same circuit otherwise.)

http://music-electronics-forum.com/a...8-dscf2808.jpg

If you have a working output amp and an identical non-working output amp, can you not compare one to the other component by component?

[thintheherd]

hey olddawg, the "board" i was referring to up above, the 200, was a link to a schematic of the 200. i was trying to say that the schematic i linked to is the exact same power amp setup as my messed up 300. if i had another power amp board i would most certainly compare. only referring to the link below my post. thanks.