Troubleshooting guide

Hi. Haven't worked on that one, but I'll bet if you post this message to sci.electronics.repair, there's a better chance someone has.

Good luck Chris

Thanks Chris. I'll do that.

-phaeton

You most certainly *should* be messing around with that circuit!

Even if the pots don't seem bad, they might benefit from cleaning. Also drrrrrrrrrrrrrrrrrrrrrryyyyyyyyyyyyyyyyyyyyy it *really* well. (You probably did both already but it bears repeating.)

For a shotgun approach, you can try heating and/or cooling things, rapping (gently) on parts, flexing (gently) the board. You've got some respectable voltage in there, so caution is mandatory.

Next, if you have no test equipment other than the DMM, you can use a cheap audio amp and speaker as a signal tracer. You'll need to couple the signal into the amp through a capacitor, and may need to attenuate the signal so you don't overdrive the cheap amp. Since the thing plays correctly sometimes, you can fiddle with the tracer so you know how it should be set up at the various input/output points. When the sound is good at the input to a stage and bad at the output, you've found the bad area. Your idea of monitoring the voltage couldn't hurt, either. When you find the bad electrolytic cap that is causing the problem, let us know. (That's my way of expressing what has to be a predjudice - I always suspect electrolytics.)

Ed

Hello again. Glad we could help.

Most likely a corroded or dry solder joint, component leg. Another likely cause is a bad/dirty pot. It could easily be a combination of those things. Given the history of the amp, you should give all the pots a squirt of contact cleaner just on general principles. If there are any trimpots on the board, mark their settings with a permanent marker

Next step:

Put a continuous tone into the amp, & gently poke around the board with a non-conductive (wood or plastic) stick. Start with the physically large & heavy components like the electros, power resistors & heatsinked transistors, & work your way down in size. Don't forget to check the pots & connectors! If you hit one that affects the sound, label or mark it so that you can find it again.

After that, you do a close physical inspection with a magnifier under the best light source you have. (The back porch on a bright day is perfect.), again, marking/labelling any corroded/burnt spots, or 'grainy' looking solder joints.

You should generally replace any parts that are burnt, pitted or bulging. Grainy, dull, or cooked-looking solder joints need to have the solder removed & replaced with new solder. For a typical amp, a $5 RatShack solder sucker is fine for this.

All the above diagnostic techniques are SOP for any piece of electronic equipment, & will solve most problems without requiring any electronics expertise. I've trained many junior tech's with zero electronics background to do all the above very successfully. All they need is good eyesight, good hand/eye coordination, & a methodical attitude.

How indeed? I started learning electronics in the exact same way when I was about eight, & I'd recommend the same approach to anyone. ;^)

Not at all. Your symptoms are all typical of a device that's been through all that.

My pleasure. Good to see that you're sticking with it.

IME, the most reliable way of determining the experience level of a tech is to see how long he takes to blame the electros. You can spot a /really/ experienced tech because he keeps the spare parts drawers with the electros right on the workbench, & automatically assumes that any faulty device will have /some/ sort of cable/connector problem.