We need to go back to some basics because something is amiss here.
If the circuit really is as shown in the diagram and the base of the two outputs are connected together as here:
Then it is not possible for the outputs to conduct and pass current in themselves. In other words they run at zero bias all the time no matter what voltage you have on the base.
So you must check that without disturbing anything. With the amp ON there should be ZERO volts between the two base leads. With the amp OFF there should be zero ohms between the two base leads.
Be careful measuring with it on. We must prove this point before going further.
If the circuit really is as shown in the diagram and the base of the two outputs are connected together as here:
Then it is not possible for the outputs to conduct and pass current in themselves. In other words they run at zero bias all the time no matter what voltage you have on the base.
So you must check that without disturbing anything. With the amp ON there should be ZERO volts between the two base leads. With the amp OFF there should be zero ohms between the two base leads.
Be careful measuring with it on. We must prove this point before going further.
OK I will double check tomorrow but I did initially double check that the bases are not connected, and there is no continuity when powered off, but I didn't do with it on so I'll do tomorrow morning, now its time for TV and some food 👍
Same here
If the base's are joined no current can flow.
If they are not joined then whatever is in the middle is effectively a bias generator which generates a volt drop and which in turn allows the Darlingtons to conduct. So if not joined we need to know exactly what is there.
You also should be able to link the bases's together as a quick test. It should draw no current.
Same here actually.
If the base's are joined no current can flow.
If they are not joined then whatever is in the middle is effectively a bias generator which generates a volt drop and which in turn allows the Darlingtons to conduct. So if not joined we need to know exactly what is there.
You also should be able to link the bases's together as a quick test. It should draw no current.
so i did as suggested
no continuity between the bases power off
linked the 2 bases out as suggested and something burned out in the middle of the amp, i heard it fizzle, so now i have to find out what that is before i go any further..
no continuity between the bases power off
linked the 2 bases out as suggested and something burned out in the middle of the amp, i heard it fizzle, so now i have to find out what that is before i go any further..
so heres a weird cenario ive not come across before
take out the rail fuse from the faulty circuit and the other channel is ok, no lamp on and i am getting the correct voltages at the output transistors with no DC v at the speaker terminal on that channel
turn off the amp, put the fuse back in and i loose all the voltage from the good channel and the lamp is on
turn off the amp, remove the fuse again from the faulty channel, turn on, then turn the amp off again
now put the fuse back in with the amp off and i get a flash and it blows this fuse
is this just a surge left within the amp?
take out the rail fuse from the faulty circuit and the other channel is ok, no lamp on and i am getting the correct voltages at the output transistors with no DC v at the speaker terminal on that channel
turn off the amp, put the fuse back in and i loose all the voltage from the good channel and the lamp is on
turn off the amp, remove the fuse again from the faulty channel, turn on, then turn the amp off again
now put the fuse back in with the amp off and i get a flash and it blows this fuse
is this just a surge left within the amp?
So the first thing you have to do is physically look why there is no continuity. The circuit shows there is. (I think I have seen versions with a diode or two between the base's so you need to check and draw what you have)
That obviously should not happen. Even if there is a different version with a bias generator between the base's it should still be safe to link them.
We need to see a correct circuit diagram for what is there between the base's.
That sounds normal. Bad channel isolated, good channel powers up OK
CHECK THE CONTINUITY of the base's on the good channel. Do they show the same as the bad channel i.e. not connected and so different to the diagram.
Expected. The bad channel pulls the supplies down.
It sounds like there is a dead short in the bad channel and the rail fuse is dumping a fully charged PSU reservoir cap into this short.
I think you have this version. Two diodes as a bias generator but these should be safe to link out.
Also where did you get the replacement outputs from. Are you sure they are OK and are genuine. Never discount faulty or problems with new parts.
replacement parts are genuine ones and i have tested them again, they are fine
ive just traced it out and this looks about right
It should be 100% OK with those diodes shorted (base's joined). Think the theory through. The output which is taken from the emitters does not go to anything that can draw significant current. The emitter voltage follows the base less the B-E drop. The base can safely be any voltage between 0 volts and supply. No current can flow if those base's are linked because the two transistors can not both turn on together.
Do a basic check with the amp off to see if you can read a short across the supply rail on that amp. So that is measuring from the collector of the upper NPN Darlington to ground. Do the measurement with the rail fuse removed so you do not get confusing results from the PSU.
Do a basic check with the amp off to see if you can read a short across the supply rail on that amp. So that is measuring from the collector of the upper NPN Darlington to ground. Do the measurement with the rail fuse removed so you do not get confusing results from the PSU.
I would replace Q5, Q6 and Q7 along with R43 and suspect the 3300 uf output cap as being leaky. Those darlingtons can be very unreliable from my past experiences and when they go, they take the driver with them, especially when there's no series R between them. Its amazing what you can do with 5 transistors...
so there might be something in what you are saying here.
the parts i brought were from a reputable supplier that i use all the time.
i have recapped the original faulty side and swapped over the transistors from the good channel, now this is ok
the 3300 cap was replaced at the first run
Q5,6 and 7 were also replaced
when i checked the ones i removed(these were new) they are both shot
so i have put the last 2 new ones in that i have and the lamp is on now, with the original good channel
what is odd(unless this is correct with a darlington), they both test as if they were both NPN not one of each
the parts i brought were from a reputable supplier that i use all the time.
i have recapped the original faulty side and swapped over the transistors from the good channel, now this is ok
the 3300 cap was replaced at the first run
Q5,6 and 7 were also replaced
when i checked the ones i removed(these were new) they are both shot
so i have put the last 2 new ones in that i have and the lamp is on now, with the original good channel
what is odd(unless this is correct with a darlington), they both test as if they were both NPN not one of each
That's not correct. Meter on 'Diode' and for the NPN Darlington and with the red lead on C and black on E it should read open. Reverse the leads and it will read. Red lead on base and black on emitter will read as will Red on base and black on collector.
The PNP should be opposite polarity wise.
so im going to leave this for now and source new parts
both were dead shorts again when i removed them
are there alternatives to the BTC64/65C
both were dead shorts again when i removed them
are there alternatives to the BTC64/65C
There probably are but I would have to look.
BDW94C for the PNP and ???? is it the BDW93C? Seems so... there are lots of possibles. Have a look at RS.
https://uk.rs-online.com/web/p/darlington-pairs/1024103
https://uk.rs-online.com/web/p/darlington-pairs/1024092
BDW94C for the PNP and ???? is it the BDW93C? Seems so... there are lots of possibles. Have a look at RS.
https://uk.rs-online.com/web/p/darlington-pairs/1024103
https://uk.rs-online.com/web/p/darlington-pairs/1024092
Just to say there might be more suitable ones than those (there are TIP type's as well. It's a case of doing a bit of research.
Off out to play 🙂
Off out to play 🙂
so the difference is then there is no diode between b/c and would read as a dead short?, well thats how the diagram reads to me
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