Recommended procedure before returning repaired amp

[pvbandit]

Hello,

I would describe myself as an amp repair hobbyist with some good repair skills. I am constantly challenging myself and learning / reading. I want to try and improve on making sure the amps I work on 100% will not return with other issues. It is maybe the weakest point of mine and the rate at which it happens makes me reticent to step into offering it publicly, and has me considering leaving it to others, as much as I love working on vintage tube amps. For example, I recently worked on a friends 1966 twin. It was blowing fuses due to a bad rectifier tube. I replaced it, checked critical voltages, sent a 1k sine wave through the amp for a good 30 minutes, chopsticked around the amp to look for bad connections. It passed that test with no issue. A week later, I get a call that the pilot light is lighting up but not the tubes. Which wouldn't be possible without there being a bad connection on one of the tube sockets. Maybe it is something I overlooked but I don't think so. It's an issue that did not come up at all while I had the amp.

Anyway, this kind of thing has been a source of stress for me, just not knowing whether I have gone through the amp thoroughly enough. I would like to trust my own work and build trust with people who bring their amps to me. Does anyone else deal with this? Any recommendations for a "checklist" of things to do before returning an amp and guaranteeing no issues come up later? There are no right answers on this topic since every repair is different, obviously. It is likely that I have some serious holes in my knowledge, but I feel though I am educating myself through trusted materials like "art of electronics", merlin blencowe's book and taking online electronics courses after getting into amp repair being completely self taught. I am getting to the point where I feel as though I am not cut out for it, any help or advice is appreciated. Sorry if this is not an appropriate question for this forum but I'm sure other wanna be techs deal with these issues as well. Thank you in advance. I'm sure there are many experienced techs here that have a very low rate of returns that can share what they know.

[Timatmit]

I've had similar experiences with return repairs, when you're dealing with equipment that is over 50 years old it's to be expected. All you can do is check what you can when you have the amp on the bench but that's no guarantee that you won't get another possibly unrelated failure a week later.

[mikeydee77]

What do the amps come back with... could you have prevented it? are they common faults? chopsticks is good and some parts we know we can expect failures so need to cover them thoroughly like fx jacks, tall parts like caps, input jacks etc etc

I would lile to suggest longer test and multiple cycles. I like to simulate 3 x 60m gig or rehearsal warm up and cool down cycles. Heat expansion and contraction can reveal a lot. I advise this on old amps especially when they haven't been used for a while. I also like to take a 24hr break on the last test just so my ears come to it fresh.

Returns on amps happen so I try to let the client about it and prepare them to almost expect possible next faults kind of like a garage and my old car. i also tell them not to hesitate to return it if anything seems bad. Usually the return fault is something newly discovered rather than part of the original repair.

This might work better for me in a city where most clients are travelling a few miles only. My return rate is about 3%

[Mick Bailey]

Repairing a fault and checking the amp over isn't the same as restoration. You fix a specific fault and do a quick check to see if there's anything else wrong that may need to be done that may be obvious, but you're doing a repair, not insuring or offering a warranty on the whole amp against breakdown. If I have a 50 year old car and take it in a garage to have a wheel bearing replaced, should the garage take time to check over the whole of the car under my driving conditions to find and fix every possible fault that it may have? If my radiator hose leaks a week later, should the garage have picked that up in advance when they changed the wheel bearing?

With many regularly seen models of amps you get to know that there will be a bunch of faults that need checking even if the amp currently shows no symptoms. Take the 90's Fender Deville series (and related amps). You can bet those grey Illinois caps are on their way out, the tube sockets are cracked, the zeners and dropper resistors have cooked the PCB and maybe the tracks are lifting. It may just come in for a re-tube, but you know it will be more than that. Chances are, if you just re-tube it will come back sooner or later with noise or channel switching issues. The same with Marshall DSL/TSL amps 1998-2003. You can bet that there's going to be bias creep due to a faulty main PCB, the bias PCB will have bad joints on the pot pads and there may be loose connections on the PT and OT terminals.

My return rate on equipment was very low - maybe two or three of items a year needed a second look and almost always this was due to an unrelated fault that could not have been predicted. Brand new amps have component failures and warranty returns/repairs. Old amps are full of old parts, many beyond the anticipated or predicted service life that the original manufacturer(s) intended. You do the best you can within the reasonable economics of the service you are offering. Don't feel bad about a return or re-work. If you can learn from it and maybe improve a procedure, test or diagnosis, then that's a bonus.

[Tom Phillips]

Mick's reply is spot on!
I'll emphasize that it's important to make it clear to the customer what service is included when you are taking the amp in. For example, write on the work order the agreement with the customer such as "Just fix the reported problem."

I, of course, give attention to finding other obvious issues. These are fixed as I go if they are easy and offered as paid additions if they are time and parts significant. You cannot expect to do a deep dive looking for additional issues on every old amp. If you do, you are sure to find things. Even then, you can't guarantee that something will fail (Even a new part) after the amp goes for a ride in the customers car. If the customer requests a full restoration then that's a whole different category of work and warranty.

The best you can do is:
1) Manage the customer's expectations up front
2) Perform good workmanship
3) Expect some returns and graciously do some free warranty work when appropriate. Almost all customers will appreciate that to the extent that it enhances your reputation.

If a customer is unreasonable, unappreciative or disrespectful then you have the option to put them on your black list.

An interesting experience:
I picked up an amp for service at a local music store. The store's customer requested that the power cord be replaced and upgraded to a three wire grounded cord. I did the requested work, set the humdinger pot which was way off and sound checked the amp.
The store's customer called me after he had used the amp. He was very upset saying that the amp now had a high noise level and it had ruined his recording session. He said that, after the power cord was replaced, the amp should have no problems of any kind. I said that I would take another look at the amp.

When I got the amp back, it turned out that the volume and tone controls on the unused channel had all set to maximum level. The amp was an old Ampeg that did not have shorting switches on the input jacks. The amp was fine with the volume turned down on the unused channel or with a guitar plugged into that channel. I'm sure that there are threads full of similar type stories on MEF.

[Chuck H]

All this

And I'll add...

Sometimes an amp that's repaired is newly able to stress parts or components that that were only holding on because they weren't being pushed into fully operational performance. It's hoped that any such issues will be caught with a 30minute bench run, but not always. That's just the brakes. Old, well used amps are full of old, well used parts. My own test includes a 30 min bench run at expected service levels. Like playing a music program at full clean power. Then for vintage style tube guitar amps I take it one step further with a cranked into heavy clipping test. I'll just plug into an attenuator and riff/noodle for about twenty minutes. Something I don't even need to charge for because I'd do it anyway This is sufficient in my opinion to stress other components after a repair, but still no guarantee against another component failing. Example:

You put new power tubes in an amp (assuming a proper bias check/adjustment) and (assuming) it's operating at full performance. Whereas it may not have been with the old tubes. Newer made tubes (and probably older ones too FAIK) can drift in performance for tens of hours. That's an unreasonable bench test time. So suppose that the the operating conditions for the tubes, while still safe by test standards, are now stressing an old screen grid resistor and it blows. You couldn't have prevented it and I would take the posture that it's a NEW problem. Albeit brought on by the repair. But the repair WAS tested. I wouldn't hesitate to charge for the second repair. Though I would probably do it for the 'nice price' provided there was no animosity and the customer wasn't an A-hole.

[g1]

As far as the OP goes, I think that was just bad luck. Second fault seems unrelated to rectifier or fuse blowing.
But pvbandit mentioned return repairs being a 'weak point', so I wonder if most of the returns are related to original fault or not.
You will never be able to guarantee 100% against failure, but if you're constantly 'unlucky' you may not be thorough enough.

[The Dude]

Agree with the above. I'll just add: This is sort of a mixed bag. You will have some customers ticked off that you didn't catch something unrelated to the requested repair and others that will be ticked off that you fixed something they didn't ask to be fixed ("I didn't ask you to solder the tube sockets. I just wanted a new power cord"). As silly as it sounds, it does happen. The best way to TRY avoiding both scenarios is to communicate with the customer and have a good understanding up front. Then, continue to communicate with the customer if something unexpected is discovered during the repair process. For instance and only as an example, if a customer came into my shop and wanted his power cord replaced, I'd ask him if that's all he wanted, or if he wanted me to inspect the amp for other potential problems. This provides at least a partial understanding of the scope of work and the customer can have some expectation of costs.

[Chuck H]

... if a customer came into my shop and wanted his power cord replaced, I'd ask him if that's all he wanted, or if he wanted me to inspect the amp for other potential problems. This provides at least a partial understanding of the scope of work and the customer can have some expectation of costs.

And time. Also an issue sometimes.