what to do when a problem goes away for days on end

[Randall]

This concerns an EDEN bass head that came back for a muting issue. It ran for about 15 minutes back in my shop and then cut out. I powered it down a few times, let it set, and repeated with the same results. Once it goes quiet, it stays quiet. So I take it out of the cab, set it on the bench and hooked up a DVM to monitor the gates on a couple of FETS when it goes quiet, except now it will not go quiet. For the past 4 days I have had an Ipod playing thru it, and it just won't quit. On for hours at a time, or shutting it off and on randomly, doesn't matter. This is a music store brokered deal, (they make some extra $$ on my bill to them), and everyone wants to know when it will be fixed. Meanwhile, it's not a small amp, and is clogging up my small shop. I don't know what to do. Surely it didn't magically heal itself, and is pretty likely to fail on the customer if I return it. But, I can't really fix what I can't really see.

What to do?

[drewl]

Get the hammer.

Beat it until it quits.

[nsubulysses]

does it need physical vibration to fail? Have you used a chopstick to push on the PCB and slighly flex it? Could be cracked solder connection or broken trace and tapping around on the board would potentially display the problem.

As to what to do with the amp now. Most of the time I don't charge for things I dont' fix. this is not a good idea. You should tell them that you cannot get it to fail but have spent many hours with it and you need to be paid something, and you cannot guarantee that it will not happen again.

Edit: I usually don't charge for things I can't fix because most work is done for friends. In this case you are working for a shop. You do their work because they can't or don't want to. And they make money off of you, and you make them look good. So they must pay.

[nsubulysses]

Yes drewl is right break it and then tell them it cannot be repaired. Then you don't have to explain this whole story. Use a rubber mallet so there are not dents in the chassis when you return it.

[Randall]

I've done all the usual probing, flexing, banging, heat gun, etc. If I can't get it to fail in a reasonable amount of time, I'm thinking of disabling the mute by pulling the two FETS that ground the signal. It is only muted in two ways, if there is no input jack plugged in, and who really cares about that? Or with the optional footswitch, which he doesn't have.

[The Dude]

Just spitballin' here. Is it maybe heat related and, since you took it out of the cabinet, it's getting more air? I've had times I had to cover a removed chassis with an amp lid or something to trap the heat in. Also, as suggested, try chopsticking around. It could be an intermittent connection.

Edit: <simulpost> I see you've already tried those things.

[nosaj]

Just spitballin' here. Is it maybe heat related and, since you took it out of the cabinet, it's getting more air? I've had times I had to cover a removed chassis with an amp lid or something to trap the heat in. Also, as suggested, try chopsticking around. It could be an intermittent connection.

Edit: <simulpost> I see you've already tried those things.

What I'm seeing here is that it has run fine for several days outside of cabinet. Put it back does the problem come right back? Stick a temp probe in there to monitor temps or check the inside of the cabinet for anything that might contact PCB.

that's all i got.

nosaj

[drewl]

I know it's frustrating as hell, and as Enzo said, probably needs to heat up to fail.

Kinda' hard in this cold weather!

Take a heat gun or hair dryer to it.

Had a Hot Rod Deluxe that came back twice that I just could not get to fail.

Owner sent me a video of it cutting out when it was on a concrete floor cranked the F- up.

Finally V1 failed enough for me to duplicate the problem if I banged the hell out of the tube while playing at full volume.

[Chuck H]

When I was pup (before I started working on amplifier circuits) I had a JCM900 2100 head that I was using with my band at the time. It started to make intermittent crackling noises when played through. I took it to CAE Sound in Santa Clara, Ca. They billed me for a repair and gave it back. The problem started again almost right away. I took it back to them three times with no joy. Finally I got sick of the problem at a practice and told my band mates "I gotta do this." I pulled the chassis, set up the amp in play mode and started poking at all the components with a drumstick. I found a capacitor that would crackle when I pushed on it. The practice space was an outbuilding at the drummers house. "Steve!" I said, "Have you got a soldering iron?" Steve went to the main house and returned with a clunky old pencil iron. I pulled the board up to get at the solder pads and remelted them with the iron. No more problems with that amp. Of course I would have done better to add some flux, but I didn't know how to do this stuff then. But the repair worked anyway.

CAE was a respected shop. I'm sure what they did was a money grab though. They found something they could call a repair and charged me. Whatever their test criteria was it never found the problem. I remember them saying that they burned it for 24 hours on two occasions without incident. Well f#@&!ng fooey on you guys! Are you playing rock and roll through it at live drummer levels? Did you poke at the caps and resistors with a stick like I did?

They never saw me or anyone I knew again.

Burn in's are good for heating an amp when it's in the cabinet. Running a generic program through it into a dummy load while it sits out of it's cabinet on a bench is a waste of time for intermittent faults.

[g1]

Did you prove it was a mute fault, or is this just a best guess?
Even if that was the previous issue, could it be a different kind of cutting out this time?
If you are positive, I'm not terribly opposed to your post #5 solution. I've had manufacturers suggest the same in problematic cases.

[The Dude]

FWIW: I have had switching jacks be very intermittent, also. So, yes...... be sure it is a mute problem, first.

[glebert]

There was a post here recently from someone working on an SWR (California Blonde IIRC) that would blow a fuse in the cabinet but work fine with the chassis on the bench. It turned out to be a broken power transistor leg that would flex just a little differently when put into the cabinet. These are the amps my son calls "possessed."

I've actually been putting off working on one of my amps because I know that as soon as I take it out of the chassis the problem will go away.

[Randall]

I have not 'proven' it is a mute circuit problem, but it acted in the same way as it did when it first came to me, very, very faint signal at the preamp out, and worked with signal aplied to power in jack.

[The Dude]

That's also a symptom you often see when you lose either the +15v or -15v supplies and/or could be a number of other things.

[Randall]

I ran it for two solid hours today with music playing thru it, and a cardboard box covering the top to mimic it being in the box. It got well warm enough, but not a glitch while probing, flexing, tapping. Frustrating.