The dropper resistors are 5W parts but whilst warm did not seem to have got to the temp I would expect for 3W dissipation. The current changes up when I touch the resistor so I guess its less when not being measured.
With the base and emitter resistors added the voltage measured -14V and seemed more stable, there seemed to be more dissipation in this configuration so I would guess when not being measured without the resistors it was even closer to the rail.
Andy
With the base and emitter resistors added the voltage measured -14V and seemed more stable, there seemed to be more dissipation in this configuration so I would guess when not being measured without the resistors it was even closer to the rail.
Andy
Tr206 is new and has been checked before each test. R216 is also OK and was measured before last test. I am thinking Tr207 is the problem, it measures OK with a diode test but might be subtlely damaged.
See above I suspect the 9v is a measurement effect and tempoary whilst being measured.
Thanks for your help,
Andy
See above I suspect the 9v is a measurement effect and tempoary whilst being measured.
Thanks for your help,
Andy
Valid point! Sorry i did not see that earlier. I will take it out and check it again. Its hard to measure as the 120R swamps the forward junction diode voltage so I was probably just measuring that.
It was replaced and it did not fix it though, so there must be another issue as well. I can't remember if it was replaced before I replaced the feedback capacitor (to ground) which was dead short.
Many thanks,
Andrew
It was replaced and it did not fix it though, so there must be another issue as well. I can't remember if it was replaced before I replaced the feedback capacitor (to ground) which was dead short.
Many thanks,
Andrew
If that base is at 9V and the emitter at 30 something then R216 would be smoking, measure the voltage at the emitter as another check.
OK I checked TR206 out of circuit and to confound all expectations found it was O/C base collector not base emitter as you would expect. I am beginning to mistrust my own measurements.
I have replaced it. (currently the 1K base resistor and 68R emitter resistor as still in place)
I then checked the other transistors that the forward diode function is swamped by a resistor across the base emitter. TR208, 204 and 205 by detaching the base either at the transistor (TR208) or by removing the base resistor TR204,5. They all show functioning diodes.
I have not powered it up again yet as I am trying to think what else could have blown TR206. It appears I have only replaced this once (as I bought three and now have one left).
Which means it was still in when the feedback cap was S/C (C203) but I have measured the base collector junction of it probably 5 or 6 times and compared it to the other channel, I don't understand how it could have been faulty all this time and I not have seen it. - Hang on just checked diagram again - I might not have been very careful about the polarity of the measurement and been measuring the base emitter of TR207, I guess this is my inexperience in fixing things, I need to be much more careful taking measurements!
Should I try powering up? Or is there anything else to check first.
R216 is OK and still measures 120R - I am going to check that TR207 is Ok with TR206 removed as I cant see how Base emitter of TR206 could not be blow if current could flow through that path.
Regards,
Andy
I have replaced it. (currently the 1K base resistor and 68R emitter resistor as still in place)
I then checked the other transistors that the forward diode function is swamped by a resistor across the base emitter. TR208, 204 and 205 by detaching the base either at the transistor (TR208) or by removing the base resistor TR204,5. They all show functioning diodes.
I have not powered it up again yet as I am trying to think what else could have blown TR206. It appears I have only replaced this once (as I bought three and now have one left).
Which means it was still in when the feedback cap was S/C (C203) but I have measured the base collector junction of it probably 5 or 6 times and compared it to the other channel, I don't understand how it could have been faulty all this time and I not have seen it. - Hang on just checked diagram again - I might not have been very careful about the polarity of the measurement and been measuring the base emitter of TR207, I guess this is my inexperience in fixing things, I need to be much more careful taking measurements!
Should I try powering up? Or is there anything else to check first.
R216 is OK and still measures 120R - I am going to check that TR207 is Ok with TR206 removed as I cant see how Base emitter of TR206 could not be blow if current could flow through that path.
Regards,
Andy
I powered it up. The base of Tr207 was at 13v so I turned it off again.
I checkedby disconnecting base and doing diode check tr206 it seems ok.
I measured the rail voltage after the resistor which was at -22v the rail is at 34v so the 150R protection resistors are dropping 12v. I make this 80mA currectly the bias resistance is set to 0R so it seems there is still a problem with something.
I wonder if this current is increasing and the 13v measurement indicates the rail is at 13.7v as I cant see how Tr206 is not failing otherwise.
Looks like something is still wrong but I cant work out what, I dont want to leave it on for long. The transfromer also buzzes quite loudly like its under alot of load but 80mA doesnt seem like it should do this.
Any suggestions?
Regards,
Andy
I checkedby disconnecting base and doing diode check tr206 it seems ok.
I measured the rail voltage after the resistor which was at -22v the rail is at 34v so the 150R protection resistors are dropping 12v. I make this 80mA currectly the bias resistance is set to 0R so it seems there is still a problem with something.
I wonder if this current is increasing and the 13v measurement indicates the rail is at 13.7v as I cant see how Tr206 is not failing otherwise.
Looks like something is still wrong but I cant work out what, I dont want to leave it on for long. The transfromer also buzzes quite loudly like its under alot of load but 80mA doesnt seem like it should do this.
Any suggestions?
Regards,
Andy
I would replace TR206 207 and R216 with a 6.2k resistor (~30V / 5mA) between IC202 base and negative rail and measure the voltage.
Thanks, Herve, I will try that when I next get some time, I think I have had enough for now. Back to work tommorrow so it might have to wait till next weekend now.
Regards,
Andy
Regards,
Andy
I would suspect TR210 or the surrounding circuitry, have you tested it?
Note that everytime you blow an output device there is a chance that it
takes out other devices.
That would be TR210 in the newer schematic in post #30
Note that everytime you blow an output device there is a chance that it
takes out other devices.
That would be TR210 in the newer schematic in post #30
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Hi Pete, TR210 is not present in the design of amp I have it equates to TR208. I did a diode check on this and it was OK but perhaps it has other issues. Thanks for the idea I will remove it and check again.
Regards,
Andy
Regards,
Andy
If you take it out, leave it out and that eliminates the drive to the outputs.
The 1K resistor would then go to the base of IC201, and first I'd have you
leave the 1K disconnected, turn it on and take voltage measurements. on
each pin of the output stage transistors that we are debugging.
TR203 is still out, correct?
Have you checked for solder bridges?
The 1K resistor would then go to the base of IC201, and first I'd have you
leave the 1K disconnected, turn it on and take voltage measurements. on
each pin of the output stage transistors that we are debugging.
TR203 is still out, correct?
Have you checked for solder bridges?
Last edited:
Tr203 is in at the moment I will remove both then try with no connection and the 1k to ic 201. Pobably be a few days before I get back on the bench though.
Regards,
Andy
Regards,
Andy
Hi, Sorry this has taken so long!
No solder bridges as far as I can tell.
TR203 and TR208 removed.
0R bias resistance
1K from 0v to collector of TR203
See GREEN text and grey modification for CCT diagram voltages.
Note: 1K from base of TR207 has been removed (I used it as the 1K collector resistance)
The voltages were so odd I rechecked them, I especially don't understand voltage drop over -Ve 150R and no voltage drop on +Ve 150R!
I have left it in this state (Turned off obviously) so if you want any other measurements or any repeated, let me know.
Many thanks for any ideas:
Regards Andrew.
No solder bridges as far as I can tell.
TR203 and TR208 removed.
0R bias resistance
1K from 0v to collector of TR203
See GREEN text and grey modification for CCT diagram voltages.
Note: 1K from base of TR207 has been removed (I used it as the 1K collector resistance)
The voltages were so odd I rechecked them, I especially don't understand voltage drop over -Ve 150R and no voltage drop on +Ve 150R!
I have left it in this state (Turned off obviously) so if you want any other measurements or any repeated, let me know.
Many thanks for any ideas:
Regards Andrew.
Attachments
I did a bit more experimentation today more progress!
I tested Tr208 it has diode function but its hfe was 8 compaired to a new part at 170. So
I replaced it. I also replaced Tr207 its hfe was 100 vrs 170 (I suspect this isnt low enough to cause misfunction but it might be the source of instability).
I then checked Tr206 which measures ok for diode function and it measured 0. So I checked a new part, that measured 0 as well. So I checked the pin order and I had it right.
I then checked the other channels part number to discover it was a bc183b not a BC183LB like the part I had replaced. I should have checked not assumed it was the origional part, obviously someone had been in before me and replaced it with the wrong part! Sure enough the pin out is different. It is not practical to cross wire as it is the thermal compensation part and needs to sit against the heatsink, so I will have to order a replacement. At least this explains the very high bias current and why the forward voltage accros what I thought was a base emmitter was several volts. There is basically nothing is turning Tr207 down so it is running open with a 10k input resistor.
I think I might finally be getting close.
Andy
I tested Tr208 it has diode function but its hfe was 8 compaired to a new part at 170. So
I replaced it. I also replaced Tr207 its hfe was 100 vrs 170 (I suspect this isnt low enough to cause misfunction but it might be the source of instability).
I then checked Tr206 which measures ok for diode function and it measured 0. So I checked a new part, that measured 0 as well. So I checked the pin order and I had it right.
I then checked the other channels part number to discover it was a bc183b not a BC183LB like the part I had replaced. I should have checked not assumed it was the origional part, obviously someone had been in before me and replaced it with the wrong part! Sure enough the pin out is different. It is not practical to cross wire as it is the thermal compensation part and needs to sit against the heatsink, so I will have to order a replacement. At least this explains the very high bias current and why the forward voltage accros what I thought was a base emmitter was several volts. There is basically nothing is turning Tr207 down so it is running open with a 10k input resistor.
I think I might finally be getting close.
Andy
It is finnaly fixed!
I cross wired the BC183LB as it seems the B parts are very hard to find now. I had to put insulation on the legs so they could cross over.
Thank you ever so much for your help I dont think I ever would have got there without it.
Regards,
Andy
I cross wired the BC183LB as it seems the B parts are very hard to find now. I had to put insulation on the legs so they could cross over.
Thank you ever so much for your help I dont think I ever would have got there without it.
Regards,
Andy
Excellent, I think the key to this was eliminating things to close in on the real problem.
Nice work!
Nice work!
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