My dad recently dropped off his guitar amp, an Ampeg B-80N from the late 70’s or early 80’s, for repair saying that it wasn’t working. It was behaving oddly, then just stopped working one day. Figuring it should be easy to fix, I gladly said I would take a look. First thing was obviously pulling it apart and seeing if anything obvious was wrong. The trick to taking these apart is to remove the speaker grill, then the 9 screws that hold the speaker on, drop it down and unplug the spade connectors on the driver. Then unscrew the amp on the back, and pull it straight off, and then pull the speaker wire out of the hole that it threads through down to the sound box. Make sure to note which lug plugs into which spade on the speaker, btw. I just snapped a quick picture with my phone.
Anyway, pulling the amp, there was nothing obvious wrong other than being a rather old design of a, I believe, class AB amp. It uses a diode pair (STV4H) to bias the output transistors, and unfortunately, that diode pair is at this point, unobtanium unless you want to spend $60 on ebay.
Okay, anywho, after the initial look, I contacted Ampeg (or rather, loudtechinc) to get the schematics, and manual for the amp. Sadly, they don’t post schematics on their site like Fender, but if you contact them, you can ask, and they’ll send you back an indemnity clause that you agree to not electrocute yourself, then they’ll send you the info. Took about 2 business days to get the schematics1.
Checking those, there didn’t seem to be a lot that could go wrong. Certainly not that would cause the amp to not power up at all, so I took the unit to the bench, and started with simply checking the transformer output. Nothing. Hmm. How about the input. Nothing. Well crap. Fuses are good. And there’s no way the amp was pulling down the AC input to ground (circuit breaker would have tripped). So, I checked for power cord continuity. Bingo. Neutral and ground were good, hot was open. Noticed also that the hot spade wiggled just a bit. With the meter hooked up, I wiggled the spade and eventually got continuity. Plugged the amp in, and bingo, it worked. Crap. I hate simple solutions when I’m looking to actually fix something. I WANT to have to power up the scope. =/
Anyway, I grabbed a old computer power cable from the bin, cut off the IEC C13 end, and soldered it in place (which was a bit of a process since they soldered the original cable on after wrapping it around the fuse holder, ground lug, etc). After that was done, I put it back together (again, making note to plug the speaker connections in on the correct sides), and powered it up. Worked great, other than the pots being a bit scratchy (not surprising given their 30+ year old age).
- btw, I find these old schematics funny given they use the old symbol for polarized capacitors [↩]