Newbie seeking transistor Advice

[Appaloosa]

With the Fluke on diode test, you will see .600 (about) on a good junction.
Collector to Emitter should read nothing.(out of circuit)
By the way, this is the "bias" circuit for the output transistors.
When everything is correct the bases of the outputs should be "barely" on)

When I test like this does the transistor need to be out of the circuit? Or can I get a reading with it in circuit and amp off?

[Jazz P Bass]

Basically, yes, test the transistor junctions in circuit.
But....
It all depends ON the circuit.
If a low value resistor is in the circuit it could throw off the reading.
When in doubt, pull the component.
Please keep in mind that transistor Q11 is a NPN & Q10 is a PNP.
When diode checking a NPN the Red Lead goes on the base.
Black then will read to the collector & then the Emitter.
PNP testing the Black Lead goes to the base.
Red to C & E.
Checking the transistor junctions is a good first step.
Keep in mind this is a static test.
Afterwords, voltage readings of all three terminals (Base, Collector, Emitter) will define how the transistor is functioning.
A dynamic test.

[Appaloosa]

So I got a chance to get back to this amp. I've had it powered up with the chasis out off the cabinet for about an hour and it hasn't failed yet!?

But Q10 and Q11 are still very hot to touch.

With the amp running and the PC board in the amp I tested the legs of the transistors in question.

my test results Q10: B=58vdc, C=13.vdc, E=58vdc. I think I have a problem because from my understanding the Base of Q10 is also Test Point 11 which I got 58vdc and shouldn't I get 48VDC? this isn't in range is it?

On Q11: B=-57.5vdc, C=-1.06vdc, and E=-58vdc. this also seems wrong to me because the base of Q11 should read -48vdc according to Test point 12.

With the amp running I also tested the voltage on the base of Q9 and I got -.449. according to the schematic I should get -.473vdc as the base of Q9 because this is Test Point 13. Is this difference in range or should the values be very similar to the schematic?

where should I go from here? does this show that Q10 and Q11 may have some short in them? Do I have enough info to blame Q10 and Q11?

thanks again!

[Jazz P Bass]

Is that 13 Vdc on the collector of Q10 a misprint?
Thevoltages on Q10 & 11 are not correct.
Remove them fron the PCB.
See the above post about testing transistors with your Fluke meter.
Dollars to donuts, they are both bad.
When you purchase replacements, get Q9 also.

[Appaloosa]

yes, thats a misprint thanks. it should be 1.3vdc. I think I will just go ahead and replace those parts and see what happens. thanks!

[Alex R]

Well nobody said it yet, so I will - did you check the effects loop jacks?

Some parts do run hot, and it isn't necessarily a sign that you have leaky transistors or whatever. To cut to the chase, you need to get the amp into its fail mode and then find out what is making it go quiet. Those transistors would not be the first thing I checked.

[Appaloosa]

What would you check first? The reason I checked or suspected Q10 and Q11 was simply because of heat indecation and because I don't know anybetter, just seemed to be a problem area or a place to start. If I can get it to fail I will check the effects loop. What will this tell me? That the problem is in the preamp if it doesn't work and output section if it does? Thanks

[bkahuna]

If I can get it to fail I will check the effects loop. What will this tell me? That the problem is in the preamp if it doesn't work and output section if it does? Thanks

I think what Alex is saying is that the FX loop jacks (the return in particular) are known to cause this type of problem. There is a switch inside the return jack that is normally closed when nothing is plugged in. The preamp signal runs through this switch, and if it opens up or gets dirty, it will cause problems similar to the one you are experiencing. There are several ways to use the FX loop to help you isolate the problem. If you plug a cord from send to return when the unit is failing and the sound comes back, you most likely have a bad or dirty return jack. Deoxit will usually fix this.

Other uses for the FX loop: 1)When failing, try inserting a signal into the return jack. If it makes sound, you know the power amp and speaker are working. 2) Run the signal from the send jack into another amp/speaker or a mixer/amp/speaker. This will tell you if the preamp is working or not.

Good luck!