Just replacing outputs doesn't always work.
You need to find the reason why they blew in the first place.
Could be just driven too hard.
Or sometimes outputs going can take driver with them.
Or a fault in the driver/Vbe multiplier circuit.
Leave output transistors out and check bias voltage and driver stage is correct.
You need to find the reason why they blew in the first place.
Could be just driven too hard.
Or sometimes outputs going can take driver with them.
Or a fault in the driver/Vbe multiplier circuit.
Leave output transistors out and check bias voltage and driver stage is correct.
Hi Guys,
Thanks for your suggestions. All transistors check out OK (in circuit) no obvious shorts (except for the power transistors which I have removed),when compared to readings of the good channel . Do i have to get them off the PC board to check or is there something i'm missing.
Thanks again,
Thanks for your suggestions. All transistors check out OK (in circuit) no obvious shorts (except for the power transistors which I have removed),when compared to readings of the good channel . Do i have to get them off the PC board to check or is there something i'm missing.
Thanks again,
Even experienced repairers need to use a current limited power supply when testing an unknown amplifier or similar. The reason is to ensure that not enough current can flow to cause more failures if there has been a blunder in refitting parts or the original reason for failure hasn't been identified and cleared.
The usual affordable method is to power the amp. via a dim bulb tester - AKA lightbulb limiter which you can easily find lots of suggestions for making, on the web. I would think a 60W incandescent bulb would be OK for testing purposes before hitting it with full mains voltage and current. If you don't do this and have no idea whether the original problem was cleared, you're just risking another failure etc.
You can't really check the transistors in-circuit unless the amp. is powered so that you can compare essential operating voltages of the semis. This is why you need a current limited supply such that you can still safely take the appropriate measurements, such as Vbe.
The usual affordable method is to power the amp. via a dim bulb tester - AKA lightbulb limiter which you can easily find lots of suggestions for making, on the web. I would think a 60W incandescent bulb would be OK for testing purposes before hitting it with full mains voltage and current. If you don't do this and have no idea whether the original problem was cleared, you're just risking another failure etc.
You can't really check the transistors in-circuit unless the amp. is powered so that you can compare essential operating voltages of the semis. This is why you need a current limited supply such that you can still safely take the appropriate measurements, such as Vbe.
Excellent advise.
Although you can test most components in circuit bearing in mid other components in parallel.
Resistors tend to burn out open circuit.
Old electrolytics lose capacitance and can go short.
Check diodes and transistors for correct junction resistances and also for dead shorts.
On difficult faults I sometimes remove output transistors and power up amp.
You can then look for obvious wrong voltages.
Beware of amp blowing speakers so when you repair amp and connect to burned speakers the amp blows again.
I got caught out with one amp that didn't work right.
When I removed all transistors I had one with a Hfe of 1 !
Not in your case but I have been caught out with previously repaired amps where someone has given up on it.
Turned out they replaced NPN transistor with a PNP ! junctions checked right but wrong voltages on power up.
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Sure, had thought of powering up without output transistors but assumed absence of output transistors could cause some disruption in the voltages due to the lack of feedback .Plan to go ahead after i do a check on all resistors and diodes.Thanks Nigel et al
Another thought , about a year ago, on switching on, the amp audio output would completely cut off with a relay click sound, stay off for 10-15 secs then come on again . this would happen regularly just when switched on. After about 5 mins of cut off/ cut in the amp would 'stabilize' and play for hours without any problem. Could this be the fore-bearer of the bad news to come ?? Does this give any clues to the possible component that could be defective??
The amplifier protection circuit, based on QN04 (TA7317P) is a very common problem in old amplifiers. The circuit often becomes erratic when its small electrolytic timing and filter capacitors begin to fail after about 20 years, depending on climate, hours of operation, proximity to heat sources in and around the amplifier.
This only explains the erratic starting behaviour, probably not the cause of the power amp. failure though. If the protection no longer works properly as a result, since it senses both amplifiers, it could have left either prone to overload and failure.
Now, after a couple of failures, there will probably be more than one problem to solve but first you need to fit a dim bulb tester unless you like to keep replacing expensive parts and eventually damaging the PCB. Do that before anything else.
This only explains the erratic starting behaviour, probably not the cause of the power amp. failure though. If the protection no longer works properly as a result, since it senses both amplifiers, it could have left either prone to overload and failure.
Now, after a couple of failures, there will probably be more than one problem to solve but first you need to fit a dim bulb tester unless you like to keep replacing expensive parts and eventually damaging the PCB. Do that before anything else.
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