Output transistors keep blowing on SX-2500

[PoDuck]

Yeah, I know. That's what my problem is. On radios I repair, I often get a low signal from the output even if the final transistor is blown, because it is measuring the driver signal passing through a blown final. I originally thought that might be my problem here, but I don't think it is, since I replaced them with no results. For some reason audio amplification seems to work a bit differently than RF amplification. I haven't figured out why yet.

 

[PoDuck]

I see pin 12, but not pin 1 on the board, but I get 80.1 volts from pin 12. If it is the pin right next to that one, away from the output transistors, then that is also 80.1 volts.

 

[PoDuck]

I think that is pin 1 that I tested, because it connects to R51.

 

[merlynski]

I think that is pin 1 that I tested, because it connects to R51.

Yes, that means the thermal switch is closed, which it should be unless it overheats.

 

[petehall347]

cant see the resistor id but was looking at 2.2k 1/2w watts . R10 R11 ?

 

[merlynski]

Check that R46 an R48 are 15kΩ and that their junction connects to the emitter of Q2. I'm thinking maybe there is not enough negative feedback??

 

[PoDuck]

Both are 15K, and there is continuity between each of them and Q2.

As for the mystery about the C1/C2 R7/R8 junctions, on the boards, they are 10K resistors. This is what the service manual I have shows in the board view though:

 

[merlynski]

Both are 15K, and there is continuity between each of them and Q2.

As for the mystery about the C1/C2 R7/R8 junctions, on the boards, they are 10K resistors. This is what the service manual I have shows in the board view though:

View attachment 1348098

I see what you mean, they show as 330Ω on the board picture. Isn't that the area you had bad traces?

 

[petehall347]

my scem says 10k

 

[PoDuck]

cant see the resistor id but was looking at 2.2k 1/2w watts . R10 R11 ?

That's not what I see here. Both R10 and R11 are 150 ohms. Do you mean R17 and R18? If so, that's what I have.

 

[PoDuck]

I see what you mean, they show as 330Ω on the board picture. Isn't that the area you had bad traces?

I had pulled traces where pins 2 and 3 are, and a solder bridge from pin 11 to the trace on the positive side of C2.

 

[petehall347]

That's not what I see here. Both R10 and R11 are 150 ohms. Do you mean R17 and R18? If so, that's what I have.

yes thats them now i look again . are voltages same both channels ? measure right across them first

 

[merlynski]

cant see the resistor id but was looking at 2.2k 1/2w watts . R10 R11 ?

On my crappy schematic R17 and R18 (2.2kΩ) could easily be taken for R12 and R11 (150Ω). R9 and R10 are 3.3kΩ

 

[petehall347]

crappy schematics and crappy eyes is my problem .

 

[PoDuck]

yes thats them now i look again . are voltages same both channels ? measure right across them first

The actual board is a bit different than the one in the SM. R51 is on the wrong side, but it would do the same thing in either place. R17 I get 9.2V across it, and R18 I get 8.8V.

 

[merlynski]

I had pulled traces where pins 2 and 3 are, and a solder bridge from pin 11 to the trace on the positive side of C2.

Hmmm - any chance there is more damage in that area, wrong parts, etc?

 

[PoDuck]

Well, considering that I replaced a good chunk of those traces, and I'm confident there are no breaks, the traces shouldn't be bad. As for wrong parts, I'll look, but I don't think so.

 

[merlynski]

The actual board is a bit different than the one in the SM. R51 is on the wrong side, but it would do the same thing in either place. R17 I get 9.2V across it, and R18 I get 8.8V.

R17 and R18 feed the output bias (idle current) control circuit.

 

[PoDuck]

Is it possible that there might be a bad capacitor somewhere acting like a resistor instead of a dead short? That seems to be my other option here I guess, unless you guys have something else.

 

[PoDuck]

Anyway, the wife is wanting me to come home, so I guess I'll get started pulling caps and testing them tomorrow, because that's where my mind is at right now. Unless, of course, you can give me another place to look.