Yamaha G30-112

A case study in an intermittent power amp issue involving finding and replacing a faulty transistor.

Symptom:

Output cuts out intermittently. Seems to be worse at louder volumes.

First, a note on anything "intermittent." Intermittent issues are tricky. Always tell the customer so. If you think you've fixed an intermittent issue, don't proclaim sure success to the customer. Let the customer know you think you've fixed the issue, but warn the customer the issue could be hiding. Your less likely to alienate the customer and send them running to the competition if you're frank and honest about intermittent issues.

Verify the symptom:

I began with a straightforward test of playing guitar through this amp. Per the customer's notes, I turned the amp up. At first, no issue. While I played, I banged on the amp with my fist. This may reveal a bad connection, which is often the problem with intermittent issues. This amp did not react to banging on it, so I continued to play. Eventually, the sound began to cut out, momentarily sounding like a tiny fuzz pedal, then returning to full volume.

Once the symptom is verified, we can move on to amp disassembly.

Methods: Divide and conquer, and gather clues

This is a combo amp. First things first is to separate amp and speaker. Bad speakers sometimes do funny things, so its best to eliminate that possibility straight away. This speaker seemed fine when hooked up to my tester power amp, so I put the cabinet and speaker aside while I focused on the amp.

After leaving the amp off while I tested the speaker, the symptom took some time to reappear. Once I got the symptom going again, a test signal could actually keep the problem going in a fairly consistent pattern, almost like an oscillation. The amp volume was way up. I used an attenuator to listen, or I just used the dummy load and watched the signal come and go on the scope. Lowering the volume caused the problem to go away.

How do we divide and conquer this issue? I tried the controls. None of the controls seemed to affect the issue. Yes, lowering the input signal seemed to help, but while it was cutting I could not stop it by slamming down the volume or anything like that, so the problem was likely in the power amp. If a control was able to remove the issue, I would look at things before the control in the signal path. I also continued to poke and prod with a Sharpie marker just to double check all connections were good.

The power supply to the power amp is about +60V, and the output sits at about +30V. Just to see, I monitored these voltages and noted that they jumped quite a bit when the signal cut out. That to me was an additional clue to look at the power amp.

The fact that the issue took some time to develop, and then could be fairly consistent made me suspect heat being involved. The output devices are bipolar junction transistors. BJTs have problems with heat and a positive feedback effect from heating up. Sure seems like a tricky thing to troubleshoot, right?

Method: Freeze spray and heat gun

One way to snuff out a bad transistor is to alternatively blast it with hot and cold and see if you can control the symptom that way.

I waited until I got my symptom going very regularly, then I started shooting squirts of freeze spray at each transistor in the power amp.

When I sprayed Tr15, the NPN output device, the cutting out stopped. I then applied heat by holding my soldering iron close to the device. The cutting out started up again. A blast of freeze restored normal operation. Eureka!

Cure:

Replace the faulty transistor. I chose a TIP29C to replace the transistor.

Replacing an output transistor is never as easy as replacing a preamp or driver transistor. They are always clamped down to a heatsink, and there is thermal grease to deal with. Remove all the old thermal grease well. This heatsink even had some extra gunk/dirt build up that may have even been adding to the issue. This stuff was so caked on, I actually lightly sanded it off with fine sandpaper. Once the heatsink is smooth and shiny again, apply fresh thermal grease. Don't have this? You need it. Buy a tube. Don't install new transistors without this stuff!

I also prefer to replace the insulation between the transistor and the heatsink, but I was out the TO-220 size, so I let the old ones ride.

After installing the new transistor, I biased the circuit according to the service manual which asks that the emitter resistors have 3mV +/-1mV across them when the amp is turned on with no signal applied.

A long burn in test followed. The amp was fixed.