First paid repair

[Chuck H]

Since both channels are summed by I1a, at PCB PC5066, I1a can't be guilty.

Thanks. I did find the schems right away when I took the job. I know preamp board for ch1 (the "selectone" preamp #pc5067) isn't at fault. So why does the problem show up on ch1 with the channels jumpered and all the controls on ch2 at zero or otherwise, but no problem on ch1 when the channels aren't jumped (ch1 still summs at I1a)?!? Rhetorical. I don't expect an answer. It's strange. I'll probably go back in there tomorrow.

[J M Fahey]

Not that rethorical.
On the "Normal" channel (PC5066) (please label them so, rather than channel 1 or 2) you have leaky/noisy CR1, CR2 or Q1.
Said noise is amplified by its own channel (that's why it responds to its own controls) or, when jumpered, gets injected into the "Selectone" channel and responds to its controls.
IC's are innocent.
To test this theory (so far it's only that) short the node R1 C1 C2 to ground on PC5066 or, in other words, short across C1.
Good luck.

[Chuck H]

Thanks. Looking at the schem it's hard to imagine how the problem could be early enough in the preamp for jumpered inputs to "inject" the problem when the problem seems to manifest two or three amps down the line at the volume controls PAST the chips. I'm sure you see what I mean. I'm not experienced with SS designs though. I suspected a leaky cap as soon as I heard the problem and I'll certainly try your suggestions. Thank you for the help.

[mort]

That's cool Chuck, that you're making a serious go at teching. I'm just trying to get started learning all I can, primarily focused around old Fender tube designs. I'm looking into getting some books on the subject here soon. This site has been by far the most helpful so far(even though half of what I read here goes right over my head but I keep reading anyway).

So far I've only worked on my own stuff, and a Fender 30 for a neighbor of mine. He found it at a Farmer's Market for $60. It was trying to run on both channels at the same time and sounded awful. After some digging around I found some loose connections on a ribbon cable terminal that were confusing the comparitor chip that controls the channel switching. That's the extent of my experience with anything chip related, and it took quite a bit of digging on the internet to half ass figure out what was going on with that oddball channel switcher.

I might have an old Fender Champ to work on here in the next few days. A friend of a friend has one that is having some sort of trouble and I offered to look at it for free. Heck, I'd work on any old Fender for free just to have something to work on.

Anyway, good on ya Chuck. I hope it all works out well for you and that you learn all you need to. You're light years ahead of me! lol

[Chuck H]

Well... At 43 the challenge hasn't lost it's shine. That's always something. But with responsibilities comes the bottom line. I don't expect this venture to be profitable before my feet are nice and wet. Being as I'm SS impaired, diagnostics and troubleshooting will be slower at first. I'll get it though. Experience tells me that. There's not many tube amp problems I couldn't fix in short order. But almost nothing in repair is likely to be a tube circuit. Most modern amps are full of silicooties that seem to fail much more regularly. It's sort of poetic that my first gig is a SS amp almost as old as I am!!!

[g1]

On 5066, even though the volume is down after I1B, signal through I1B could internally affect I1A if the chip is faulty. So I wouldn't rule out I1.
Or perhaps something in the bad ch. is putting noise on the low voltage supply, but only when signal present?

[Chuck H]

OK... This one went sour in a hurry. I found another cold solder joint and suspected another so I just went to task re flowing the preamp boards. Didn't take that long. I'm finding the tiny bias voltages off everywhere. Where the schem says something like .1V I might read .23V sort of thing. Not sure how significant a 110% difference here is to a transistor since I've never worked with them. Supplied voltages are fine. All the polar caps are those icky blob tants. Also lots of CC resistors that of course may have drifted. Hard to troubleshoot PC5066 since PC5067 must be removed to get to it and the amp doesn't work without both boards in place. Power transistors all seem to be working but the amp only makes about 25 clean watts. Probably an operating condition problem but I don't have the know how to test that theory.

I guess first move will be to replace all the polar caps and any film or ceramic DC blocking caps. Not hard to do. There's not that many and the boards come out easy enough. After that I'll check voltages again. Background noise tell me there's probably a bad amp in there too. I suspect I1(a/b) on PC5066. I don't really have the right iron for this sort of thing but I should be able to replace it with what I have (and not do more harm than good).

vintagekustom.com use to have a link to any available crossover numbers for Kustom transistors, op amps and those funky dual gate fets. But the website went belly up a few days ago. Why the hell didn't I copy that info when I had the chance?!? Now I'll have to do it myself... The Google way. Should be very rewarding. Like watching paint dry.

[J M Fahey]

Dear chuck, don't work beyond what was asked, unless it's your personal amp and your passion is restoring it to "better than new" for your own pleasure, no cost or time constraints.
Just repair the f****** thing and call it a day.
Cold solder? Fine, specially since it raised its ugly head and *begged* to be resoldered.
Tiny bias voltage differences? *May* be important or not at all. The main question is; does the stage (reasonably) work as intended or it distorts/farts/oscillates/hums/buzzes/whatever?
Don't fix what's not broken.
Same for caps and resistors.
Troubleshoot the boards when and if they fail. If they work properly, leave them as is.
Inject 1 volt into the power amp(PC5065?) input jack, pin 2, into R2 (2200 ohms) and you will get full power, around 20 volts on the speaker out.
SS amps do not wear out and lose power like Tubes, they either work or die.
If the amplifier does not work without a PCB, try to make some extension cables so you can pull it to one side but remain functional.
Get a fine point 30/40 W soldering iron.
Even those cheapies sold in dollar stores work, only their plain tips do not last.
But for a couple weekends they will do.
Better than using your 160W opr 250W tin roof soldering monster
Good luck and keep ahead.

[Chuck H]

+++
I'm only considering replacing all the polar caps because it would likely be easier than taking the board out and reinstalling it repeatedly each time I find a cap leaking. I already know there are incorrect voltages here and there and even in a couple of places where they shouldn't be. It might be a dozen or fifteen caps total. But I hear what your saying loud and clear. And I'll likely make some extension cables too.

Thank you.

[Chuck H]

Hah... Ok, serious green horn moment here. I had the volts/division set for 5V but didn't realize the tuner at the end of that coltrol was misadjusted AND I didn't do my probe division. The amp is actually putting out about 23 volts into any load I put on it. I read that this amps "actual" output is about 125W into a four ohm load so that seems correct. And since I no longer question the output power this means that the PC5067 board is fine too. The customer complained that the reverb was a little weak. The last guy to work on the amp replaced the reverb circuit control pots for a non failure related problem. He said the pots were destroyed during board removal due to corrosion. Seems odd to me since all the other pots are stock and fine. I also don't think that tech can be trusted because he used 2.5k pots where the schem calls for 10k value. Easy fix on that one. So now it's the PC5066 board and the leaky voltage at the "to module" jack and this dog should hunt.

[Enzo]

Off-voltages. Like anything else, context matters, which I believe is what Fahey alluded to. FOr example, if it is the voltage across an output emitter ballast, then doubling the voltage means doubles current through it. But in some preamp stage, you might have 20v of headroom and the signal level typically 2 or 3 volts. then while it may be optimal to center the steady voltage in the middle, it really won't matter a lot if the whole 2-3v signal window is pushed a volt or two up or down.

[Chuck H]

Thanks for the guidance guys. I haven't been back in the thing since the day I last posted. But after reading your posts I just reflowed and cleaned some things. That actually helped a bunch and cleared a couple of small issues that were confusing the troubleshooting process. I'm 90% that I need two semiconductors. One is the 14 pin IC that I suspected from the start. It's marked sl14228. Which I read can be directly replaced with an RC4739, XR4739, NTE725 and a few others. All of which are obsolete with no current crossover. And the other is marked RCA 80848. Some looking around seems to indicate that this may be a CA3080. Pinout is the same and (not that I'm that sure what I'm looking at) the specs seem right. I had to go to Ebay to find the parts!!! Twelve dollars for the RC4739 and six for the CA3080!!! Didn't want to do the "dead bug" thing. Probably two to three weeks for payment transfer and shipping. Everyone will have moved on with life by then. I think I've got it now.

Thank you

[52 Bill]

Twelve dollars for the RC4739 and six for the CA3080!!! Didn't want to do the "dead bug" thing. Probably two to three weeks for payment transfer and shipping. Everyone will have moved on with life by then. I think I've got it now.

Thank you

It's tough to find replacements for the 4739 chips, but you can redo the board to use any standard 8 pin dual opamp. There may be compensation parts on the board, as the original parts were spec'd for the uncompensated uA739 chip. These parts can be removed if you use the XR chips.

Jameco still carries the dip 3080, I think for $3.

[soundguruman]

"I'll try a couple more repairs and if they go as well or even close I'm definitely going for the business licence."
Besides the business license, the is usually a license from the "bureau" of appliance repair, electronic repair certification, or contractor license, etc...
The business tax license is usually with the city, the professional license, with the state or Provence...

[Chuck H]

Yupper. I need to look into that. But here in Washington state they don't regulate near as much as elsewhere. I think the reason is that the poulation per square mile is a bit smaller and they just want to collect the excise tax. I was almost offended a how easy it was to get my current business up and running. Anyone with ten percent of my qualifications could do it and be my competition in the field. But of course I'll look into it.