Help with a acoustic 150 Transistor overheating

[Slam]

  Hello, a friend of mine brought me an acoustic amp that had a blown fuse. I replaced the fuse with the amp open and noticed the resistor (.5ohm 5watts) across one of the transistors was glowing bright red. I shut her down and replaced the 2n3055 with a mj15022 (higher rating) and replaced the .5ohm with a .33 ohm. when i fired it up the two transistors on the side i replaced got alot hotter than the other two but nothing fried or glowed in the short time i had it on. There was no output from the amp at all (Had speaker and guitar hooked up). I have repaired several tube amps but am at a loss with SS. Might be easier with a schematic but i couldn't find one online without having to buy it.
  I really would appreciate some input, this is a favor for a buddy. 

[teemuk]

http://acc.homeunix.net/images/manuals/150/150manual.pdf

...Also, you need to replace those emitter resistors with an exact value. They are for current sharing, equaling the current flowing through the output transistors. If you replace the emitter resistor with a lower resistance the transistor will "hog" most of the current and run too hot.

If you replace output transistors it might be a good idea to replace all of them (instead of just a single one that's gone bad). That is especially if you fit in a substitute device.

[Slam]

  Thanks teemuk, Right after i started this thread i ordered the 4- 2n3055's and 4- .5ohm 5 watt resisters, i kind of knew i wasn't going to get away with that. I still think i should have got some kind of output with these other values but it could just be my ignorance showing.
  Thanks for the manual, this is obviously going to be a big help. I will update the thread for future enthusiasts to reference.

[teemuk]

No, you are right. There should be ouput despite the mismatch. Something else is causing that issue. I just didn't really have time to write a longer answer so I merely pointed out that matching thing and posted a link to service manual. There are deeper issues in your amp but there are also quite a many scenarios that could cause them so it's kind of futile to start guessing (e.g. the interstage transformer could be busted, the driver amp, driving the output via interstage coupling could have failed etc.). The service manual/schematic should get you started, though, and help in visualising what to measure when troubleshooting.

Just don't power that amp up without some kind of current limiting unless you're sure it works allright. That could ruin all the previously done repairs in an eyeblink.

[gbono]

I would start by checking if the preamp boards have output. Are you sure there is a signal going into the power amp board? The driver transistor on the PA board is also usually damaged when I get these amps after they have output issues. Also check the large output capacitor on the same chassis that the power transistors are mounted on. Last there is a zobel network on the output speaker jack make sure these components aren't damaged as well.

[R.G.]

It took me a few years, but I do understand how those amps work now.

Compare the power amp schematic to that of the Thomas Vox Super Beatle. Notice any similarities? They could almost be the same amp, with only minor differences. The Acoustic is a cleaned-up version, but is very much like the Beatle.

The red-hot emitter resistor indicates that the thing is trying to drop the entire power supply voltage across one emitter resistor, with poor results.

The secret to fixing it is probably divide and conquer. First off, remove any loading significant loading from the amp. This amp will not go crazy if unloaded, like many tube amps will. I would remove the output capacitor and also open up the driver transformer primary. It will help a lot if you have a light-bulb limiter like I'm always preaching about. With the output connection open and no signal to the output stage, the output stage is going to try to simply sit at a DC level. The voltage dividers set the bias for the output devices: 400 ohms and 6.8 ohms just barely trickle the bases on. If the 6.8 ohm resistor goes open, the output devices are turned full on and death ensues as all of the base current from the divider goes into the bases. If the 500uF cap shorts, that side is permanently underbiased and bad crossover distortion results. Check the values of each part - there really aren't that many - and also the continuity of the wiring/pcb. Bring the power up slowly with the primary of the driver transistor open and watch the junction that would feed C311. This point should rise with the power supply, following approximately 1/2 of the total power supply voltage. If it doesn't, something is wrong with the 400/6.8 dividers, the driver secondaries, the output transistors or the wiring.

Once you get the output stage producing about 37Vdc where C111 would connect, you can start debugging the input/driver side. Reconnect the primary and disconnect both secondaries, being sure to note how they're wired so you can get them back in the right places. The driver can put out a few watts from one of the secondaries, so you can put 8 ohms across one of the disconnected secondaries and get the input side working.

If it were mine, I would arbitrarily replace every electrolytic cap in the amp. I have sometimes been called a miracle worker for getting Thomas Vox amps running, when all I did was to replace them all. They're old enough that if a cap doesn't fail today, it might tomorrow.