Kustom Profile one 100w PA head help!

[Bruce / Mission Amps]

I have a friends Kustom PA mixer/head in for a repair and can find no serious info online.
The issue is screeching oscillation on one channel only.
I saw some other problems from owners on fixya.com regarding this screeching thing but actually zero fix info.
Anyone have anything on this one?

[J M Fahey]

C'mon Bruce, you already have all the skills needed to repair this and then some.
Anyway, and as an exercise:
1) Does it respond to any of the controls?
2) Do you have a scope?
3) If not, build an RF test probe, to measure the presence and severity of oscillation:

4) Either you have a severe grounding problem (cracked track or solder, broken decoupling capacitor pin, etc.) or you'll find the oscillating stage.
Remember you must not have oscillation on ground and power rails
Good luck.

[Chuck H]

Of course Bruce can fix it. But since it appears to be a common problem how much better to learn about it and fix it in half the time Especially since this is for a friend (could be a freebe ). Bruce is an important man and his time is much better spent bringing affordable boutique amps to market IMHO

Sorry I don't know about this one Bruce.

[J M Fahey]

This page suggests some possible problems and solutions:
Kustom Profile One 100w PA -loud bang! - FixYa
although these seem to appear on all channels, not just one.
Of course, marginal PSU filtering might just barely make one channel oscillate and not the others, if the PCB layout was somewhat poor.
"Fully automatic" PCB layout programs look useful, but must be human checked .
Maybe this particular PA system had its fair share of problems, because it was discontinued, although they seem to have dumped remaining stock in Brazil and Argentina, they are everywhere around here.
No , have not had one of them yet.
If the channels use DIP 8 pin op amps, try bypassing pin 4 and 8 to ground with a .1 or .047 ceramic, or just pin 4 to 8. It may stop oscillation around that particular IC.
I suspect it may be an SMT board, ugh!!
Good luck.

[Bruce / Mission Amps]

Yes it will be a freebie.
There is no bang on start up and the squealing is only on channel 4... none others.
That should make it a lot easier to narrow down.

Actually, I have seen and heard the PA in use.... my friend brought it to a little gig that I played in with him a New Year's Eve.... and one channel just squealed like banshee if a mic, with an XLR, was plugged in... that is the extent of my experience with the PA head/mixer. ha ha.
To be honest, I had never seen one before, let alone have opened one up... but after looking around on the NET,
Fixya.com was the only place I saw any conversation about this problem and or any other issues.
But, being the lazy butt I am, I was hoping maybe someone here with Kustom's warranty or repair shop status, already knew about this problem and had some dirty little secret factory band-aid, since it appeared that it was not an uncommon complaint...
He's coming by my house later today again before my gig and I'll open it this time for a peek inside .... to be continued.

[Enzo]

I don't know this unit Bruce, but is it recent vintage or from the old days? If old, what is the board number for the preamp channel?

My first reaction is an open gain control in the bad channel.

[Bruce / Mission Amps]

Well, pulled it all apart and found nothing really wrong... bummer.
I did look at the main PCBs with my 4x loop, finding nothing really odd. No unusual ripple on the +- regulator rails
But I went through it and reflowed all the solder joints around the XLR jacks, 1/4" jacks and all the trim pots associated with that one bad channel.
As mentioned in the articles on fixya, I R&R'd the two 100 ohm 1 w MtlOx resistors and installed two 130ohm 2 watters and put it all back together to see if anything changed. I went back and replaced the two 47uF filter caps on +- rail with 100uF 50v caps.
Ugh... I hate this.... it now doesn't squeal at all... Damn I hate it when I fix something and have no idea what fixed it.
FRUSTRATION
My friend thinks I'm smart... just clueless parts swapping with no firm ideas.

[J M Fahey]

C'mon Bruce, you *repaired* it. How's that?
¿Clueless parts swapping?
Don't think so, everyday we read here about and try to help musicians turned into technicians , who do exactly that ... and more often than not they do *not* solve the trouble, specially on first try.
Chance and randomness play a part, too, but those better prepared , unsurprinsingly fare better with great regularity.
Good Luck and feel warm about it, you deserve it.
PS: did you take any pics? Never opened one of those (yet).
PS2: I vote on

reflowed all the solder joints around the XLR jacks, 1/4" jacks and all the trim pots associated with that one bad channel.