Acoustic 370 resistor toaster.

[ManRay]

Q304 sets the bias for the output stage. A problem with it can make the drivers and output devices burn up.
[ATTACH=CONFIG]33173[/ATTACH]

So it stands to reason that I'm going to need all new output transistors. How can I measure the current ones to test them?

[patlaw]

So it stands to reason that I'm going to need all new output transistors. How can I measure the current ones to test them?

No. You should diagnose the problem and replace the defective parts. If all transistors are bad, they need to be replaced. If they are not all bad, you may not need to replace them all. You can do a rough test with an ohm meter. If there is a short between the base-emitter-collector pins, the transistor is bad. If there is a 0.65 diode voltage drop from base to emitter on an NPN (your meter has to have a diode test function), the transistor is probably okay.

[Enzo]

The collector is the case, but it is still just a terminal of the transistor. Use your ohm meter, is there low resistance between that case and either pin underneath? If so, it is shorted.

[ManRay]

The collector is the case, but it is still just a terminal of the transistor. Use your ohm meter, is there low resistance between that case and either pin underneath? If so, it is shorted.

Do I need to remove the transistor from the circuit to conduct this test? There is resistance between the case and both pins of each while in the board.

::Edit::
I've pulled Q306 and Q311 and neither has resistance from either pin to the case. Should I keep0 pulling transistors, is there another test I should run on these while they are out or should I look to something else?

Thanks

[52 Bill]

Do I need to remove the transistor from the circuit to conduct this test? There is resistance between the case and both pins of each while in the board.

::Edit::
I've pulled Q306 and Q311 and neither has resistance from either pin to the case. Should I keep0 pulling transistors, is there another test I should run on these while they are out or should I look to something else?

Thanks

Each of the power transistors should be mounted with an insulator and heat sink compound. If you remove them, be careful not to damage the insulator and replace the compound when remounting.

If you just remove the two mounting screws and leave the transistor in place the contact from the circuit to the case/collector will be broken and it can be tested without removing it. If your meter has a diode test function use it, if not set it to a low ohms range.

Place the red lead on the base lead of the transistor and then touch the black lead to the emitter lead and note the meter reading. Next touch the black lead to the case/collector and note the meter reading. Reverse the two meter leads and repeat the same tests. Finally touch the red meter lead to the emitter lead and the black meter lead to the case/collector and note the reading. Again reverse the two meter leads and repeat the test.

In none of the tests should you see a zero or short reading. In some of the tests you should see a low reading with the meter leads connected in one direction and a high or open reading when the meter leads are reversed.

Test all of the transistors in the power amp in this fashion. Test all of the diodes in the power amp as well.

[ManRay]

The results of the test are as follows:

Meter set to diode function.

Q306, Q311, Q313, Q314
Positive lead to Base, negative to Emitter=520
Reverse the leads= No reading

Positive lead to Base, negative lead to Collector= 520
Reverse leads =No reading

Q312, Q307, Q308
Positive lead to Base, negative lead to Collector=7
Reverse leads= 3

Positive lead to Base, negative to Emitter= No reading
Reverse leads= no reading

Q309
Positiver lead to Base, Negative to Emitter= No reading
Reverse leads =No reading

Positive lead to Base, Negative to Collector=408
Reverse leads= 947

What does this test tell me about the state of the transistors? And do I need to remove the diodes from the board if I am to test them as well or can I test them in place?

Thanks

[The Dude]

It would appear that Q307, 8, 9, & 12 are all bad. Also check the ballast resistors. They are the .1 ohm 10W. I notice that you didn't check collector to emitter on the transistors. You need to do that also and make sure there are no shorts. There is enough evidence of a large failure that you may want to just replace all of the outputs.

[ManRay]

It would appear that Q307, 8, 9, & 12 are all bad. Also check the ballast resistors. They are the .1 ohm 10W. I notice that you didn't check collector to emitter on the transistors. You need to do that also and make sure there are no shorts. There is enough evidence of a large failure that you may want to just replace all of the outputs.

R322 and R323 are measuring .7 Ohms. Is this too far out of spec? Should they be replaced as well?

[The Dude]

Are they cement wire wound resistors? If so, they rarely change value. Because they are wire wound, they usually work or go all the way open. I suspect maybe your meter is not zeroed. Touch your probes together and note what you read. If you don't have a zero function on your meter, you'll need to subtract the probe to probe reading from the resistor reading.

[ManRay]

You are right. The leads have .6 ohms resistance. I don't have a zero function, but that would put the ballast resistors back in spec. Thanks for the tip.

Is there anything else I should be measuring? What may have caused my power transistors to go?

[Enzo]

ManRay - I am not picking on you, but please do not say "no reading" when you mean an open indication or for that matter zero ohms. No reading implies the meter went blank. OL or OS meaning over the scale or other indication of infinite resistance or open circuit is still a reading, just as zero is still a reading when it shows up.

I agree with Dude, collector to emitter is very important. In the Q306 et al, we see the B-C and B-E readings that are identical. That may be OK, or it may mean the C and E are shorted together. You need to check.

Look at Q312,307,308 in the schematic, see there is a 1 ohm resistor connected from the base to the emitter in each case. That will certainly affect the B-E readings, because that resistor is parallel to the transistor. Now you report "no reading" on that B-E, which I assume means open circuit. A 1 ohm resistor would not read that way unless it was open itself.

[The Dude]

The resistors burnt because shorted transistors caused excessive current through them. You need to find ALL shorted components. Check the entire circuit (all transistors and diodes) thoroughly. Unhook the speaker and, using the light bulb limiter or variac with current monitoring, bring up the amp and verify it is working correctly. Make sure there is no DC on the speaker leads. Only after this, hook up the speaker for testing.

Edit: Power transistors can short for lots of reasons. Some are:

1) A short at the output (i.e. shorted speaker cable or short in the speaker cabinet itself.
2) A load impedance that is too low (somebody hooked up too many speakers).
3) The amp was run too loud for too long causing it to overheat or possibly even a thermal runaway condition.
4) No reason at all. Sometimes parts just go south.
5) Etc.

[The Dude]

Look at Q312,307,308 in the schematic, see there is a 1 ohm resistor connected from the base to the emitter in each case. That will certainly affect the B-E readings, because that resistor is parallel to the transistor. Now you report "no reading" on that B-E, which I assume means open circuit. A 1 ohm resistor would not read that way unless it was open itself.

Agreed. I assumed, maybe wrongly, that the transistor measurements were made out of circuit with the transistors removed.

Edit: The 1 ohm resistors should be checked, regardless.

[ManRay]

Agreed. I assumed, maybe wrongly, that the transistor measurements were made out of circuit with the transistors removed.

The transistors were measured out of socket. when I said no reading I meant that the reading was the same "1 . " That my meter shows when the leads are open. I was in diode function (which I am not familiar with) so no reading may well mean open curcuit.

[ManRay]

The E-C reading on Q306 in diode function is the same "no reading" or "open?" or "unchanged state?" that I was describing before. What would you guys prefer I call this reading?

Also, do I need to remove the diodes to test them?