I powered the amp up again today via the lamp limiter, unfortunaly the 1.25a fuse just blows after a couple of seconds. I assume I must have managed to short the speaker outout with the cable while I was setting up for the testing with the frequency sweep.
Not sure where to go from here. Should I disconnect transformer output wires from the board and test the output? I always thought it was a bad idea to power up a transformer without a load on it? If anyone could advise on that that would be most appreciated.
The 2 fuses for each channel are ok, tested them for continuity. Its the fuse before the input side of the transformer which is blowing...
What should I be testing? I've put this much time into it might as well carry on. Its all part of the learning curve!
Cheers
Not sure where to go from here. Should I disconnect transformer output wires from the board and test the output? I always thought it was a bad idea to power up a transformer without a load on it? If anyone could advise on that that would be most appreciated.
The 2 fuses for each channel are ok, tested them for continuity. Its the fuse before the input side of the transformer which is blowing...
What should I be testing? I've put this much time into it might as well carry on. Its all part of the learning curve!
Cheers
The mains transformer will be fine, they are bomb proof.
Audio output transformers like are used in valve amps are the ones that generally shouldn't be run with no load but mains transformers are fine.
It sounds like you might have zapped an output transistor. First simple check is just to measure them in circuit and see if any read low ohms or short, typically between C and E.
Just one thing though, a bulb tester shouldn't allow the fuse to blow even if there were dead short across the mains within the amp. Make sure the fuse is a slow blow type and that the bulb is an ordinary household 100 watt or 60 watt filament type.
Audio output transformers like are used in valve amps are the ones that generally shouldn't be run with no load but mains transformers are fine.
It sounds like you might have zapped an output transistor. First simple check is just to measure them in circuit and see if any read low ohms or short, typically between C and E.
Just one thing though, a bulb tester shouldn't allow the fuse to blow even if there were dead short across the mains within the amp. Make sure the fuse is a slow blow type and that the bulb is an ordinary household 100 watt or 60 watt filament type.
Hi, Mooly. The bulb is a 100w filament type, 1.25A fuse has a T stamped on the metal before the value so think that's a slow blow type?
I have the lap limiter wires: mains>switch>lamp>socket outlet
Just doing some testing now on the OT's. If I take TR8 (BD139) as an example, With the meter set to 2MΩ I'm getting ".109" between E and C (+ probe on C) and .202 (+ probe on E).
On TR8 (BD140) I'm getting -.188 (+ probe on E) and gets to 1.000 and keeps going higher with the probes the other way around.
I have the lap limiter wires: mains>switch>lamp>socket outlet
Just doing some testing now on the OT's. If I take TR8 (BD139) as an example, With the meter set to 2MΩ I'm getting ".109" between E and C (+ probe on C) and .202 (+ probe on E).
On TR8 (BD140) I'm getting -.188 (+ probe on E) and gets to 1.000 and keeps going higher with the probes the other way around.
BD139/140's sound more like the driver transistors than outputs. TR11,12,13 and 14 are the outputs.
Put your meter on the diode range and just test the outputs in circuit. Do any read really low? The collector is probably the middle pin on these oddball devices. Read from collector to emitter initially. If that is short circuit then they are duff.
Put your meter on the diode range and just test the outputs in circuit. Do any read really low? The collector is probably the middle pin on these oddball devices. Read from collector to emitter initially. If that is short circuit then they are duff.
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They all show around 600 between base and collector with the probes one way and infinity/no read with them switched around except for TR12, which shows 585 one way. and 759 the other way. I think we have our culprit?
Wonder why the fuse still blows with the lamp limiter before the mains input though?
Wonder why the fuse still blows with the lamp limiter before the mains input though?
Aan excellent time to get help from an experienced person. You tried and that is good but please have a repair man have a look at it.
If the fuse blows when a lamp current limiter is used one would say that there might be a short in the mains cable. Then the lamp would not glow. Did it glow at all? You know things are wrong when it glows at full brightness!?
If the fuse blows when a lamp current limiter is used one would say that there might be a short in the mains cable. Then the lamp would not glow. Did it glow at all? You know things are wrong when it glows at full brightness!?
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Both situations need investigation, the bulb tester and the different reading on TR12 although that is not really definitive proof at this stage that it is faulty in itself.
The bulb should really be a normal old fashioned filament type with the spidery filaments and (not a halogen type) which have much higher inrush currents but even so...
So we need to prove the bulb tester is OK. A 1.25A slow blow fuse will withstand around 20 amps (yes 20) for 10 milliseconds falling to around 7 amps after 100 milliseconds. These are fuse manufacturer data sheet values.
Only you yourself know what is in front of you (thinking safety aspects now) but I think the first step is to check the bulb tester on its own and make sure it works OK. I can't imagine a 100 watt filament bulb blowing a 1.25AT fuse.
Make sure the fuse really is a 1.25 and not a T125MA which would be just 0.125A.
Starting Characteristics
The bulb should really be a normal old fashioned filament type with the spidery filaments and (not a halogen type) which have much higher inrush currents but even so...
So we need to prove the bulb tester is OK. A 1.25A slow blow fuse will withstand around 20 amps (yes 20) for 10 milliseconds falling to around 7 amps after 100 milliseconds. These are fuse manufacturer data sheet values.
Only you yourself know what is in front of you (thinking safety aspects now) but I think the first step is to check the bulb tester on its own and make sure it works OK. I can't imagine a 100 watt filament bulb blowing a 1.25AT fuse.
Make sure the fuse really is a 1.25 and not a T125MA which would be just 0.125A.
Starting Characteristics
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Repair men started off somewhere, Jean-Paul, as did most of the already competent DIY'ers on this forum. Which is what this forum is for, isn't it? for *DIY* audio. I like doing this, I am learning. Its only an amplifier at the end of the day.
It must be exhausting to be so constantly pessimistic
That sounds more like it
If the bulb is lit brightly then that points to a problem within the amp, something drawing lots of current... which comes back to the output transistors.
So logical steps.
1/ Make sure no speakers are connected.
2/ Measure the rails and see what is actually there. They are shown as - and + 37 volts.
3/ Recheck that transistor that read differently. Compare with the other channel and make sure you compare by keeping meter lead polarity the same between tests.
4/ Check the two 0.47 ohm resistors still read OK.
If the transistor is still looking suspect then its time to remove it and test it out of circuit. TR12 is NPN and so should not read any resistance (or show a reading on the diode range) between C and E when C is connected to the meter red lead.
B to E should read 0.600 ish on the diode range with the red lead on B.
B to C should read similar.
Reverse the leads and you should get an 'open circuit' result.
The PNP devices are tested the same way but the readings vs lead polarity are reversed.
If the bulb is lit brightly then that points to a problem within the amp, something drawing lots of current... which comes back to the output transistors.
So logical steps.
1/ Make sure no speakers are connected.
2/ Measure the rails and see what is actually there. They are shown as - and + 37 volts.
3/ Recheck that transistor that read differently. Compare with the other channel and make sure you compare by keeping meter lead polarity the same between tests.
4/ Check the two 0.47 ohm resistors still read OK.
If the transistor is still looking suspect then its time to remove it and test it out of circuit. TR12 is NPN and so should not read any resistance (or show a reading on the diode range) between C and E when C is connected to the meter red lead.
B to E should read 0.600 ish on the diode range with the red lead on B.
B to C should read similar.
Reverse the leads and you should get an 'open circuit' result.
The PNP devices are tested the same way but the readings vs lead polarity are reversed.
Repair men started off somewhere, Jean-Paul, as did most of the already competent DIY'ers on this forum. Which is what this forum is for, isn't it? for *DIY* audio. I like doing this, I am learning. Its only an amplifier at the end of the day.
It must be exhausting to be so constantly pessimistic
You see this wrong. You lack knowledge that may bring your safety in jeopardy. You fail to see this as you state it is only an amplifier but that is not the point. Safety is. Also experience is gained by learning the basic stuff first not by needing guidance with every step on a forum. Your lamp current limiter did not work OK but you don’t tell why which IMO is essential if you want to get help.
It should be a two way communication.
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I had a the wrong type of bulb in it, Jean-Paul that's why! Was trying not to say. Are you happy now?
Beta people need feedback to understand a situation and to be able to give correct information which is only in the interest of both parties. The bulb current limiter not working ok could be a risk to you or the amp so any detail is essential for correct understanding and better and more efficiënt guidance. “Trying not to say” when one makes an error is plain silly and unnecessary. Every person makes mistakes.
Has nothing to do with being happy or not. You are the one that should be happy that people care about your amplifier and your safety.
Has nothing to do with being happy or not. You are the one that should be happy that people care about your amplifier and your safety.
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Measured the transformer outputs in circuit with the amp powered on with the lamp limiter.
I'm getting 9.7V?
3/ Recheck that transistor that read differently. Compare with the other channel and make sure you compare by keeping meter lead polarity the same between tests.
Rechecked them all in circuit exactly as described, have not yet tried TR12 off the board...
If the transistor is still looking suspect then its time to remove it and test it out of circuit. TR12 is NPN and so should not read any resistance (or show a reading on the diode range) between C and E when C is connected to the meter red lead.
TR12 shows 1515
TR11 shows 1605
B to E should read 0.600 ish on the diode range with the red lead on B.
TR12 = 595
TR11 = 571
B to C should read similar.
TR12 = 590
TR11 = 559
Reverse the leads and you should get an 'open circuit' result.
TR12 = 766
TR11 = OPEN CIRCUIT
TR14 and TR13 give results as expected from what you described.
4/ Check the two 0.47 ohm resistors still read OK.
Getting 0.7 on all 4 of them.... then I realised this meter doesn't auto null the resistance of the leads (which is 0.3) so actual result is .4 which is ok.
Thanks, Mooly.
I'm getting 9.7V?
3/ Recheck that transistor that read differently. Compare with the other channel and make sure you compare by keeping meter lead polarity the same between tests.
Rechecked them all in circuit exactly as described, have not yet tried TR12 off the board...
If the transistor is still looking suspect then its time to remove it and test it out of circuit. TR12 is NPN and so should not read any resistance (or show a reading on the diode range) between C and E when C is connected to the meter red lead.
TR12 shows 1515
TR11 shows 1605
B to E should read 0.600 ish on the diode range with the red lead on B.
TR12 = 595
TR11 = 571
B to C should read similar.
TR12 = 590
TR11 = 559
Reverse the leads and you should get an 'open circuit' result.
TR12 = 766
TR11 = OPEN CIRCUIT
TR14 and TR13 give results as expected from what you described.
4/ Check the two 0.47 ohm resistors still read OK.
Getting 0.7 on all 4 of them.... then I realised this meter doesn't auto null the resistance of the leads (which is 0.3) so actual result is .4 which is ok
.Thanks, Mooly.
It sounds like they need testing out of circuit although tbh I would have expected one or more to have shown a more definite short.
The low voltage you measure indicates a low impedance path somewhere but not a dead short.
What would I do if I had this in front of me...
1/ Check the outputs out of circuit. These are still favourite at this point.
2/ Jean-Paul I think mentioned checking the bridge rectifier. Just quickly do a diode check across each diode of the bridge looking for shorts. You'll get some weird readings because of the large caps... just look for shorts.
3/ If an output transistor measures faulty we know what to do, replace them both and I would also replace the driver transistors as well.
4/ If they appear not to measure faulty then it all gets more interesting and we need to then confirm or otherwise that the rails actually come up to normal with the output transistors removed from the circuit, however before switching on and doing that we need to do a couple of things first to make it safe to do that.......
So first thing is to check the transistors out of circuit.
The low voltage you measure indicates a low impedance path somewhere but not a dead short.
What would I do if I had this in front of me...
1/ Check the outputs out of circuit. These are still favourite at this point.
2/ Jean-Paul I think mentioned checking the bridge rectifier. Just quickly do a diode check across each diode of the bridge looking for shorts. You'll get some weird readings because of the large caps... just look for shorts.
3/ If an output transistor measures faulty we know what to do, replace them both and I would also replace the driver transistors as well.
4/ If they appear not to measure faulty then it all gets more interesting and we need to then confirm or otherwise that the rails actually come up to normal with the output transistors removed from the circuit, however before switching on and doing that we need to do a couple of things first to make it safe to do that.......
So first thing is to check the transistors out of circuit.
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