Kustom 36' Coupe not working

[shadygreentree]

Hi,
I have one Kustom 36' Coupe. It is not working, power lighs comes on but no sound . One thing I noticed was that the channel lights were not coming up. I opened up the amp and found the two fuses on 28V CT side were blown. I replaced the fuses and the amp worked for a while and then one of the capacitors (2200uf 35V) blew away. It was like a bullet going off!
I replaced all three capacitors on the 15VDC circuit and amp worked fine for another day. Today again the amp stopped working channel lights are gone too. I checked fuses but they are intact and there is 15V DC available.
I am at total loss now what to do. Can anyone please help? I do not have schematic and I understand Kustom has stopped giving schematic. Can anyone please help with schematic?

[shadygreentree]

Help Please!

[Audiotexan]

Hey shady!

Might try asking for help with the schems in the Schematic Requests area. I've had that suggested to me before, with the reason being that some of the kind folks that help out with this watch that thread almost exclusively.

Also, it sounds like things are getting progressively worse as things go. :/

Make sure that you don't sub-out fuses with higher ratings 'to test' as these will definitely allow things that 'might not have blown' to blow. Always replace fuses with the same rating/type as the original.

Also, just a reminder, make sure to properly discharge the caps before you go to work on things!

I opened up the amp and found the two fuses on 28V CT side were blown.

Can you tell us if the PT (primary transformer) has multiple secondaries? Or was the 28v line just a sub-section of the main PSU (which is what I would expect).

Given that the amp has what appears to be 6L6's in it, I'd assume the main secondary to be much higher than 28v with a center tap.
An additional secondary for heaters would also not be out of the norm.
So perhaps they could be taking 28v down to 15v (as you mention below) for additional rails, and then down to 12v for the 12ax7 heaters. But that'd be a *very* unique way to do things IMO. But again, I've not worked on/seen this amp before now. So the 28v (as referred to above) is kind of throwing me. Just taking stabs in the dark.

First thing to do would be to make a light bulb limiter if you don't have one already. (see: http://music-electronics-forum.com/t37189/#post354114)
These will limit the current during testing (and keep from blowing more components).

I replaced the fuses and the amp worked for a while and then one of the capacitors (2200uf 35V) blew away. It was like a bullet going off!

As well as smelly ;P lol

I think you were on the right track in checking the PSU section for preliminary issues. However, you should also check to make sure that you don't have a bad/shorted tube in the mix.

Have you also pulled the tubes, and then powered it up?
I'd pull outputs first, try it out. Then with the outputs out, pull preamps, repeat.

If it continues to blow without tubes, then you know it's probably a fault on the pcb at this point. If all seems well without the tubes in it, then you know it's probably a fault with a tube somewhere. Preamps can fail, but it's rare they short, though it's still possible, given that you mention the 28v/15v lines. It's far more common however for output tubes to short. Especially if the amp's been driven hard (loud volumes for extended periods and frequently in this condition.) The harder you drive the tubes, the more often they should be checked/changed.

I replaced all three capacitors on the 15VDC circuit and amp worked fine for another day. Today again the amp stopped working channel lights are gone too. I checked fuses but they are intact and there is 15V DC available.

-Might also check any HV (high voltage) mains caps.
-Have you checked diodes in the PSU? (if you have one that's just leaking, and not simply shorted, that could be operational until it gets to it's 'working limit' and then cut loose and cause things as you've described.)
-Could also be the same for a cap that you've yet to check.
-Are there any burned/discolored components/pcb areas?

I am at total loss now what to do. Can anyone please help? I do not have schematic and I understand Kustom has stopped giving schematic. Can anyone please help with schematic?

Keep your fingers crossed that someone has them, but try the above for starters. Hopefully you'll get some additional input here!
Stay safe!

[shadygreentree]

Hello Audio Texan
Thanks for taking time to reply to my query. I will follow your suggestion and post the schematic request in correct forum.
You have given me some very good pointers, let me work on it and see if I get get the amp up and running again.
I do have the " Bulb Limiter" as suggested by you and I find it a very valuable tool / test jig when fixing things.
Thanks once again, I will keep you posted on how it is going.
Thanks and Regards

Sandeep

[g1]

Schematic: http://music-electronics-forum.com/a...m-72-coupe.pdf
(Thanks vintagekiki !)

[shadygreentree]

Thanks everyone for help (special thanks to Vintagekiki). The fault was in thru the hole connection, power was not getting from one side of the PCB to other. Fixed it and now amp is working very well.

[km6xz]

That plate-through does not explain the fuses blowing or the capacitor exploding. Not much can explode a cap like that except AC or reverse polarity across it. Have you checked the bridge rectifier of the 28 volt supply? And ignore all the references to tubes and high voltage, that is an uninvolved distraction since it is not a high voltage problem.

A general rule of good diagnostics is gathered data about conditions or symptom has to match the proposed solution. If the proposed diagnosis does not match the symptoms, it is wrong.

An open does not fit the symptoms so it can be assumed that an intermittent problem is at hand and the open might very well be an additional symptom in that high current, as required to blow that cap and repeated fuses could also be expected to open a weak point in the current path.

[shadygreentree]

Hi Km6xz
Thanks for your reply. I think I know why the cap exploded. It was bit under rater and old cap. it was 25V rating on 18VCT AC supply. With age I guess it just gave up and that was the reason that blew fuse. When I replaced the fuse, the cap blew.
This problem started after I replaced capacitors. I guess my soldering somehow destroyed plated thru hole connection.