WHICH RESISTOR BURNED UP?
Also, I see three BE voltage drops between the bases of Q4 and Q6, so either a diode is missing or one of the diodes is not a 1N4148. That is a puzzle indeed, as most amplifiers include diode drops to match the B-E drops in the output section. But I also see only 2 diodes in the photo, so it does seem a puzzle indeed. If R19 burned, it was because of no base current drawn by Q7 otherwise there should only be one B-E drop of the output transistor across R19 and R20.
So both driver transistors were turned on solidly. That points at Q3 being either shorted or driven into high conduction for some reason.
Direct coupled amplifiers are SUCH a pain to analyze when things go wrong!!!

 

I don't know what diodes are used in the circuit. I substituted 1N4148 for now.
AC simulation gives OK results.

Shorting across Q5 emitter and collector with 30Ω resistor gives similar effects as reported. This gives me suspicion that Q5 is bad.

 

I don't know what diodes are used in the circuit. I substituted 1N4148 for now.
AC simulation gives OK results.

Shorting across Q5 emitter and collector with 30Ω resistor gives similar effects as reported. This gives me suspicion that Q5 is bad.

In a direct coupled circuit like this, any transistor failure leads to a string of damages.

 

I did say R19 in post #74.

So your are just learning. Here are some trouble shooting tips.


You did not state the sign, +19V or -19V.
Where was the RED meter probe connected?
Where was the BLACK meter probe connected?

Details such as these can make or break the diagnostics.

From what I surmise, the original fault was a bad transisor Q5, BFR79.
Whatever replacement you use, make sure that you check the transistor lead identification and that the transistor is installed correctly. The PCB hole orientation might not match the transistor.

.
yeh. did not note polarity on speaker test. noted .
will take care on tranny hook up .
thanks for the tips.

 

two new transistors fitted . waiting for mail delivery to replace blown resistor .

 

Remove the two large transistors.
Remove the burnt resistor R19.
Measure the resistance across collector-emitter in both directions, Q5, BFR79.
If you measure resistance lower than 100Ω, do the next step.

Remove Q5, BFR79 paying close attention to the lead orientation.
Again, measure the resistance across collector-emitter in both directions, Q5, BFR79.

If you still measure low resistance, go ahead and replace R19, 100Ω and Q5, BFR79.

Do not install the two large transistors in the metal can as yet.

 

Remove the two large transistors.
Remove the burnt resistor R19.
Measure the resistance across collector-emitter in both directions, Q5, BFR79.
If you measure resistance lower than 100Ω, do the next step.

Remove Q5, BFR79 paying close attention to the lead orientation.
Again, measure the resistance across collector-emitter in both directions, Q5, BFR79.

If you still measure low resistance, go ahead and replace R19, 100Ω and Q5, BFR79.

Do not install the two large transistors in the metal can as yet.

so , Im probably going to need a new Q5 and should order one ? or can it be re connected ?

I fitted the two big resistors already . so before taking them out to do those tests you asked for , I tested them inline and got 0.972 swapped polarity and got 0.660. the 2nd one measures 0.104 both ways . does that make any sense ?

 

so , Im probably going to need a new Q5 and should order one ? or can it be re connected ?

You have not told me as yet what resistances you measure across collector and emitter.

 

You have not told me as yet what resistances you measure across collector and emitter.

I measure three ways . 0.5 , 0.4 and 0.2 . that sound right ?

 

I measure three ways . 0.5 , 0.4 and 0.2 . that sound right ?

So the transistor is bad and needs to be replaced and installed.
I would suggest installing new R19, 100Ω, new Q5 , Q6, Q7 and then do a power ON test.

You may want to find substitution for Q5, BFR79.
Make careful note of the transistor pinout since your substitution is very likely to be different.

 

So the transistor is bad and needs to be replaced and installed.
I would suggest installing new R19, 100Ω, new Q5 , Q6, Q7 and then do a power ON test.

You may want to find substitution for Q5, BFR79.
Make careful note of the transistor pinout since your substitution is very likely to be different.

ok. thankyou . more waiting then . cest la

 

https://dsmcz.com/prestashop/en/pnp-transistors/243-bfr79-pnp-transistor-90v-2a-08w.html

BFR79 is available here..

 

https://dsmcz.com/prestashop/en/pnp-transistors/243-bfr79-pnp-transistor-90v-2a-08w.html

BFR79 is available here..

oh . brilliant Dave . thanks mate

just out of interest, what would an alternative be ? I have a few spares boards lying around with plenty of these on them ...what are my chances are finding something ?

 

Again, still pay attention to the transistor pinout because the legs of the BRF79 E-C-B were twisted to match the C-B-E layout of the PCB.

 

Some alternatives here, make sure you get the pinout in the correct way when fitting it.

https://www.web-bcs.com/transistor/tc/bf/BFR79_sim.php

 

Some alternatives here, make sure you get the pinout in the correct way when fitting it.

https://www.web-bcs.com/transistor/tc/bf/BFR79_sim.php

nice one dave . Ive got.... C9014's for Africa . could open a shop . I will makes sure it goes in as it comes out . got that . thanks

 

OK. my part arrived at a last , if anyone is still around . Have fitted resistor and new transistor and hooked up the speaker again . but I'm not going to turn anything on before checking with you guys

 

Remove the loudspeaker.
You need to measure the DC voltage at the output connection that would normally be going to the loudspeaker.

 

Remove the loudspeaker.
You need to measure the DC voltage at the output connection that would normally be going to the loudspeaker.

Hey there Mr Chips . thanks for getting on. is that ok powering up with no speaker attached though ?

 

Hey there Mr Chips . thanks for getting on. is that ok powering up with no speaker attached though ?

Yes. It is ok.
You are measuring DC conditions with no source input.
I know that you are not supposed to run tube (valve) amps with no speaker attached. This is not the case here.