polytone brute bass amp back again....

[pontiacpete]

I repaired this amp about 8 months ago. It came to me with R11 looking burnt and sounding very very distorted. Changed out R11 and still farting sound of distortion. Found Q1 - Q1A was shot so I replaced it with the recommended replacement, two 2N5210 and it finally works and lasts for the past 8 months. Now it comes back and Q3 is clearly blown and the collector wire 'C' from Q5(perhaps refer to the layout)had come detached from the board. Reconnect it and replaced Q3 and left the diode because it seems to have survived. I brought out the power slowly to about 10v and there's huge hum and I haven't gone further.
This board has taken a beating , keeps me from pulling the transistors out , might have to cut a new board.
Anyway, ,, ,, any suggestions?

pete

[Enzo]

If you are bringing it up on a variac, get rid of the speaker, and watch mains current draw instead. A lot of solid state amps are unstable at low mains voltage and will slam over to one rail until the voltage clears the minimum it needs.

If the excess current doesn;t appear on the meter, then check for DC on the speaker wires. If there is no DC there, then connect he speaker up and flip the power switch on.

[pontiacpete]

thanks Enzo

I tried bringing it up measuring the current across the output, at 50v on the variac there was 21mA and B+ had 5vdc. mmm doesn't seem good to go further.

pete

[rf7]

21ma is nothing, keep going up to 120V. What current is it drawing at the correct input voltage? If you're drawing several amps at 50Vac then you've got a problem. A lot of amps don't fully come on until they're close to 120Vac due to relay switching. If you're not drawing a lot of current, then you're not going to be blowing something up.

[pontiacpete]

THanks,

At full power it worked , plugged in the speaker and played a bit and R11 smoked, like it had the first time I worked on this amp. I'm thinking that Q1 and Q1A are out.

[Enzo]

"Current at the output?"

I was suggesting measuring the current the amp draws from the wall outlet. Nothing to do with the output current. Only thing on the output to concern is DC voltage there. Also scope it for RF.

What kind of current was it drawing from the wall when it was trying to work?

Set the bias control to the coldest setting - the setting that draws the least mains current from the wall - should be the zero ohms setting for R14. make sure R13 is not open, and make sure R14 works.

Make sure that R19 is not open. That is important.

Q3,Q5, and even Q2 are a lot more likely to burn up R11 than Q1.

[pontiacpete]

oops mental meltdown...

R13 , R19 and R14 appear to be good, not open and the R14 is working.
I took out Q5, that tests fine. Q3 I am presuming is still good because I replaced it. Q2 I don't know about. Is there a way(hope this isn't too rediculous a question) Q2 can be tested without removing it? The traces are really bad in this area.

[Enzo]

To a point, yes. use the diode test on your meter.

[pontiacpete]

Tested Q2 with the diode test in circuit and it appears fine i.e. fluke churps when diodes of transistor are good, so it seemed good. Q4 seems unusual when tested the same way so I tested out of circuit with ohm meter, using the other method I know of testing transistors with probes hooked to emitter and collector and touching the base to the emitter with a 10K resistor, meter moves from open to 15 or 20K. good? perhaps
Powered up with R14 turned to lowest point with no speaker, seems fine so played through it for a while. Sounded a bit distorted then R11 burns again.