Polytone mini teeny brute problem

[prulup]

Hi there, I have a strange problem with this amp (Polytone mini brute, it has 378 power amp schematics). It burned down: R17 100ohm resistor, drivers, LM391, one 0.15 5W resistor, power transistors. I replaced the resistors, the LM391 and the drivers. I checked the voltages with bulb limiter inserted and without the power transistors. I got:

MJE 182 (NPN)
E 0
B 0,6
C 29

MJE 172 (PNP)
E 0
B -0,6
C -29

I know the MJEs have a different pinout than PU57s.
The bias potentiometer works, I can set the -0,6V on the MJE 172.
On the pin 7 of the LM391 is cca 1,2V.

and now the strange thing:
When I put the power transistors the bulb starts to light and I have about 90mA on the power resistors (0.15 ohm). The bias potentiometer has no influence on it. The power supply voltage gets down from +-29 to about +-8V.

When I take the LM391 out and connect the pwr transistors on, I get these voltages:

MJE 182 (NPN)
E 28,9
B 19,8
C 29,4

MJE 172 (PNP)
E 28,8
B 28,2
C -29,2


Any suggestions please...? Thank you

[prulup]

basicaly this schematics, pwr transistors are MJ4502 and MJ802 (so was the originals in there)

[nickb]

First remove the power transistors. Next measure the voltage across R18 and R19. Also, observe if the bias pot has any effect on them at all. Check the values of R15, R14 and R13 in the bias network. Check the soldering to pin 6 and 7 too, just in case.

[prulup]

Well, I removed the PWR transistors. And after power on the R18 and R19 starts to burn. I had to remove the chip and now it is ok. I am not sure if the chip is OK yet:-/ So I must now replace the R18 and R19 and check again without the chip. Soldering on the chip is OK, R13, R14 and R15 are OK, bias pot works. Does anyone have the voltages on the chip socket with and without the chip?

[nickb]

Well, I removed the PWR transistors. And after power on the R18 and R19 starts to burn. I had to remove the chip and now it is ok. I am not sure if the chip is OK yet:-/ So I must now replace the R18 and R19 and check again without the chip. Soldering on the chip is OK, R13, R14 and R15 are OK, bias pot works. Does anyone have the voltages on the chip socket with and without the chip?

Did you do this? What was the result?

Check the values of R15, R14 and R13 in the bias network. Check the soldering to pin 6 and 7 too, just in case.

Also, have you tested the 92PU07 and 92PU06?

[prulup]

Yes, I checked the bias circuit, it works. I replaced the burned resistors, and I with the bias pot all the way up there is no burning now. The 92PU07 and 92PU06 were replaced by MJE172 and 182 as I wrote earlier. But the replacement chip is gone as I see -28V on most of the pins. I have to order another

[52 Bill]

have you been using the light bulb limiter for your testings?

[nickb]

Yes, I checked the bias circuit, it works. I replaced the burned resistors, and I with the bias pot all the way up there is no burning now. The 92PU07 and 92PU06 were replaced by MJE172 and 182 as I wrote earlier. But the replacement chip is gone as I see -28V on most of the pins. I have to order another

I have annotated the schematic with typical voltages:





Since the LM391 was bad, it's hard to be certain that the bias network was OK so it's best to proceed in baby steps.

1) Replace the LM391.
2) Set the bias pot to the lowest resistance.
3) Temporarily put 1K in place of R19 and R18, Do not fit the power transistors. Do not connect a speaker
4) Power up using limiter
5) Check voltage drop across R20 & R21 - should be zero and nothing should be getting hot.
6) Replace R18 and R19 with 100 ohms and recheck voltages.
7) Replace the power transistors and check voltages
8) if OK the adjust the bias to get no more than 10mV across R20 & R21, aim for 5mV ( wild guess - you really at least a scope to set it properly)
9) Recheck for 0V on output, if OK try it with a speaker.
10) Remove limiter - adjust the bias back to 5mV if necessary. Check for 0V on output. If OK connect speaker and test.

[prulup]

have you been using the light bulb limiter for your testings?

yes, I have

[prulup]

Nick, this is excellent, thank you very much, I will let you know when I got new LM391s!

[prulup]

Well... I tried to replace the LM, and I can still see strange behaviour, do you have the voltages on the LM? I am suspecting that they are fake...:-(

[prulup]

great, the PWR trans. are burned, so now to get a new pair.

[nickb]

Well... I tried to replace the LM, and I can still see strange behaviour, do you have the voltages on the LM? I am suspecting that they are fake...:-(

I'm not sure what you are asking for I put all the voltages and steps a couple of posts back.

On the transistors, did you follow steps 1-7 before putting them back in?

[prulup]

I'm not sure what you are asking for I put all the voltages and steps a couple of posts back.

On the transistors, did you follow steps 1-7 before putting them back in?

For voltages on the LM. Yes, I followed the steps, both the LM and drivers seem to work properly. I don't know why the PWR trans burned, the bias was set on the minimum.

[nickb]

Okay doke. Pull the power transistors and report the voltages on the bases of the two driver transistors.