First paid repair

[Chuck H]

Actually, not as a pro since I have no business licence in place yet for this service. But I've decided to "scab" a few jobs to see how it goes before jumping in.

This repair was on a Kustom 250. First, I don't know how many here have seen the inside of one of these amps but the build quality rivals ANY PCB tube amp of any era. Just fantastic and easy to work on.

The amp came to me not working. I looked up the schem and identified the "to module" jack as a way into the power amp. I injected a signal there and the power amp worked fine. So I plugged into the preamp and got only a very very faint sound. But, I could hear the reverb operating in the faint sound. That being the case I surmized that the preamp was also working fine since whatever amp powers the tank must be working. So the problem had to be between the effects and the power amp. This amount of trouble shooting took about five minutes.

So I opened up the amp, injected a signal and, following the schematic, traced the signal. I was expecting a bad op amp from the effects to the power amp. But it was even easier than that. Just a cold solder joint on the cable plug from the effecs board to the power amp board. No parts required. I had to remove the effects board to access the bad connection. Sucked out the old solder and reflowed the joint and the amp works perfectly.

It took longer to take the amp apart and put it back together than it did to diagnose and repair it. I'm charging the customer for an hour just because that seems to make sense as a minimum charge.

I'll try a couple more repairs and if they go as well or even close I'm definitely going for the business licence.

Thanks to all here that encouraged me.

[J M Fahey]

Congratulations.
Way to go.

[Jazz P Bass]

It is always a good feeling when everything goes right.
Way to go.
(Now, about the ones that won't 'go away'....grrr.....)

[JoeM]

Keep it going Chuck!

[Enzo]

Yes, even if it takes you five minutes, always charge at least an hour bench minimum.


Just think, your band gets paid the same amount whether the crowd requests Louie Louie or MacArthur Park. Your doctor office visit costs the same whether you have a cold or cancer. Ease of repair is not the issue.

[g1]

Yup. And it will make up for some of the 10 hour ones you don't feel right charging 10hrs. for.

(Now, about the ones that won't 'go away'....grrr.....)

Or the ones that come back...
Good stuff Chuck, keep it up!

[Steve Conner]

Congrats!

Worse than the repair jobs that won't go away, are the ones where you get it all fixed, then blow it up again by plugging an internal cable or card into the wrong socket during reassembly.

I just did that last week, and to add insult to injury, I did it to a piece of equipment that I designed while I was being paid to upgrade it. A classic Friday afternoon f*** up.

[pdf64]

Well done, keep at it!
From a ISO 9001 perspective, you've done the remedial action (fixed the breakdown) but consider corrective actions (any other similiar joints with the likelihood for a similar failure?) and preventive action (a bit more tricky, maybe secure with cable tie / glue etc items that may vibrate and fail?).
And a checkover / remedial action for any common failure modes.
And double check that everything is re-assembled securely (cable headers latched into place etc).
Then test it at high volume.
You don't want to be in the position of having the customer bring it back in the near future because you neglected to attend to one of the above tasks.
The downside of fixing something up so well that it doesn't fail again for years is that the customer forgets who did the job!
Pete.

[Chuck H]

Then test it at high volume.

Yes. Well... That did occur to me after full reassembly. I had checked both channels, BUT, I only checked channel one at high volume. Turns out there is a problem with channel two at high volume. So I need to go back in now. I had already called the customer when I discovered it too. A little embarassing but I do that pretty well most of the time. I still consider the initial repair a success. I just didn't do my follow up testing well enough. These lessons can be learned much harder than this I'm sure. So it's alright.

(Now, about the ones that won't 'go away'....grrr.....)

I'd best acquire a taste for it.

Thing is, it's a really obscure problem. Both channels go through volume controls and are summed into one op amp (which is one half of a dual op amp chip). But when channel two is turned up it clips, crackles and fizzes. This is directly proportional to the volume control setting which is right behind the junction of the two channels. Yet channel one works fine. So it seems to me that channel two is fine since all amps are up stream of the volume control and the summing op amp is fine since channel one is operating correctly. Therefor nothing is wrong. Yet there is a problem. Kind of fun actually. The volume control seems fine. Maybe it's some obscure op amp failure since these are dual op amp chips? No electrolytic caps in the preamps but I havent tested the films for leakage. Still. If the amp was thrown off by an incorrect voltage it should affect both channels at the failing circuit. There is a little extra noise in general also. Hiss with little crackles like you might hear with a moist carbon comp resistors.

If I don't find anything obvious like incorrect voltages, grossley drifted resistors or leaky caps I'll trace the problem with my scope and see exactly where it is and what it looks like. If I still don't have a clear path I may just try replacing both of the 14 pin dual op amp chips and be done.

[Jazz P Bass]

Congrats!

Worse than the repair jobs that won't go away, are the ones where you get it all fixed, then blow it up again by plugging an internal cable or card into the wrong socket during reassembly.

I just did that last week, and to add insult to injury, I did it to a piece of equipment that I designed while I was being paid to upgrade it. A classic Friday afternoon f*** up.

I did that 'one pin off' on a Crown Macro Tech.
Not a recommended coarse of action

[Steve Conner]

Did you ever get the burn marks off the ceiling?

Chuck, I think op-amps can go bad in such a way that they spit noise current back out of their inputs. If the input was hooked to a volume control wiper, it would be quiet when the volume control was turned to zero, because the noise is shorted to ground. So the noise is affected by the volume control, even though it's coming from the stage immediately after it, in a blatant violation of the usual troubleshooting signal flow rules. (which to be fair, the electrons have never heard of)

You may have seen the effect in tube amps where a noisy tube makes your guitar volume control crackle, this is no different.

Of course discrete transistors and FETs can go bad and start to hiss, pop and fizzle too.

[Chuck H]

Of course discrete transistors and FETs can go bad and start to hiss, pop and fizzle too.

Yup. I'll be scoping this thing to trace any noise before ordering parts. There is one very old design FET I probably won't be able to source. I was actually surprised to find the dual op amp chips. This thing is over fourty years old!

I would like to replace the power supply filters too but the customer doesn't want the added expense. If you've never seen one of these amps, the filters are HUGE. They look like two coffee cups with screw terminals. I sourced them and they'll cost between $60 and $100 US. There are no hum issues and the caps look to have been replaced at some time since the originals were Mallory and these are not.

[J M Fahey]

the filters are HUGE. They look like two coffee cups with screw terminals. I sourced them and they'll cost between $60 and $100 US.

That is if you want to replace them with leaky dry old exact NOS ones.
Moden ones, same capacity and voltage will be less than 1/2 size and 1/10 the cost.
Not to mention the reliability factor.

PD:what board numbers are involved here?
or ... post the Power amp and PSU

[Chuck H]

I am looking at new replacements. And not much smaller than the originals. These don't appear to be your average filter caps. Much more NASA looking. I could get less robust caps that would do the job, be much smaller and cost less. But they wouldn't be in keeping with the nature of the existing parts. Which are very obviously of extreme grade and quality. Since there are no symptoms of any kind that relate to power supply filters, and I can see that they've been replaced at some time, I think I'll solve the problems the amp has for now. Saving the customers money at his request.

The amp is the K250-2 . Since you asked for the board numbers I'll guess that you have access to some Kustom schems. The board numbers are:

power amp - pc5065
preamp ch2 - pc5066
preamp ch1 - pc5067
effects - pc5068

PSU is all chassis mounted and wired PTP.

I just found a really odd symptom. With the amp operating I found that when plugged into channel two the problem is volume and tone control analogous. That is, higher settings result in more of the problem. Each channel has two inputs. Channel one has no problems on it's own or with a cable plugged into channel two. But if I jumper the inputs (a cable to join the two channels at the unused inputs) I get a slightly different manifestation of the same problem on both channels. Even with all the controls on channel 2 set to zero channel 1 displays the same volume sensitive problem. But the volume controls are at the ends of the preamps?!? Just to be thorough I checked for DC at the input jacks anyway, but found none. Wierdness. I won't really know anything until I open it up again and chase the gremlins with a scope.

[J M Fahey]

No special links with the Gods, but all your schematics can be found at http://music-electronics-forum.com/t18421/ .
Please identify which is channel 1, which is 2
Since both channels are summed by I1a, at PCB PC5066, I1a can't be guilty.
Good luck.