Thanks, I appreciate it.
How about the S (sense) circuits? Looks like I should connect R214 and R225 somewhere to get the whole circuit working...
You can get Sanken Thermally compensated Darlington Power Transistors from Digikey
see for example Blocked who have stocks exceeding 700 for both of your output versions. Prices look reasonable probably less than mail charges that would be incurred.
You would want to be sure there have been no consequential transistor failures elsewhere. I found a forum where failure of the output Darlington stage in an Arcam amplifier took out other transistors - albeit the short circuit protection had been disabled.
My rule is to check and don't assume. In an early post it was noted the voltage readings for various transistors were inconsistent with what one might expect.
see for example Blocked who have stocks exceeding 700 for both of your output versions. Prices look reasonable probably less than mail charges that would be incurred.
You would want to be sure there have been no consequential transistor failures elsewhere. I found a forum where failure of the output Darlington stage in an Arcam amplifier took out other transistors - albeit the short circuit protection had been disabled.
My rule is to check and don't assume. In an early post it was noted the voltage readings for various transistors were inconsistent with what one might expect.
Thanks. Indeed, prices look good. What power rating would you recommend for the external .22R resistor?
No doubt about it! That's why I came here to ask for help on how to do a proper troubleshooting, which would otherwise be beyond my repair skills.
I have followed your suggestions to use base to emitter resistors. Thanks again for suggesting it.
The voltage across the output terminals is now 145mV. I have checked voltages on all transistors, and they are mostly the same, compared to the working channel, with only a few deviating by no more than 200mV.
Would this be a pass mark?
Ok,so main amp looks fine.
Power transistors, one definitely out, the other HIGHLY suspect, replace both.
Remember first to plug amp into a bulb limiter and no load whatsoever,then measure voltages.
If *absolutely* not available, you may replace them by similar specs power darlingtons and add your own emitter resistors (somewhat easy) and biasing which will be somewhat more complex but *specially* very well thermally coupled to heatsink, maybe using a TO220 Darlington Vbe multiplier as heat sensor, because original circuit relies on superbly well coupled *internal* diodes, who can beat that?
But you can approach it.
Power transistors, one definitely out, the other HIGHLY suspect, replace both.
Remember first to plug amp into a bulb limiter and no load whatsoever,then measure voltages.
If *absolutely* not available, you may replace them by similar specs power darlingtons and add your own emitter resistors (somewhat easy) and biasing which will be somewhat more complex but *specially* very well thermally coupled to heatsink, maybe using a TO220 Darlington Vbe multiplier as heat sensor, because original circuit relies on superbly well coupled *internal* diodes, who can beat that?
But you can approach it.
BDW93/A/B/C
Hammer Drivers,
Audio Amplifiers Applications
• Power Darlington TR
• Complement to BDW94, BDW94A, BDW94B and BDW94C respectively
TIP142\147
Add external 5W emitter resistors.
Add external chains of SI diodes. Install on radiators near transistors. Epoxy resin. Connect with wires with PSB.
Hammer Drivers,
Audio Amplifiers Applications
• Power Darlington TR
• Complement to BDW94, BDW94A, BDW94B and BDW94C respectively
TIP142\147
Add external 5W emitter resistors.
Add external chains of SI diodes. Install on radiators near transistors. Epoxy resin. Connect with wires with PSB.
Thanks, but shipping to Romania would be prohibitive. Anyway, looks like I can source them relatively cheaply from the local market
Lead-out is indeed different. To use an STD instead of a SAP, the emitter needs to be soldered where the sense lead used to be, and then the external .22R soldered across to where the emitter used to be... Looks like I am going to stick with the SAPs and go to a higher power rating to increase reliability.
Will surely replace both
How would you choose an appropriate power rating for the light bulb?Can't beat that, indeed
I'll try to stick to SAPs.Thanks for the suggestion, OldDIY. I think all these modifications would be a bit too much for me to do...
Greetings!
The replacement transistors arrived yesterday and had them in circuit today. I took all the precautions suggested here (dim bulb, no load on the output) and got the amp working.
However, I am worried about the value of DC across the speaker terminals on the left channel, the one which was defective before. It measures about 145mV at zero volume, dropping to about 30mV with the volume half way up. DC across the right channel is about 3mV, dropping to about 1mV, under same conditions...
It the DC I am seeing acceptable, or does the amp need further troubleshooting?
The replacement transistors arrived yesterday and had them in circuit today. I took all the precautions suggested here (dim bulb, no load on the output) and got the amp working.
However, I am worried about the value of DC across the speaker terminals on the left channel, the one which was defective before. It measures about 145mV at zero volume, dropping to about 30mV with the volume half way up. DC across the right channel is about 3mV, dropping to about 1mV, under same conditions...
It the DC I am seeing acceptable, or does the amp need further troubleshooting?
There could be some dc leakage across a series capacitor either side of the volume control. Also check the decoupling to earth leg of your feedback divider - capacitor, continuity to earth, and solder joints. If the capacitor in that network is leaky it would take some time to come up to working level - perhaps close but still leaking - after all the output stage did fail.
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Hi mjona! Thanks a lot for replying.
I have done the checks, as follows:
1. Checked the network of resistors and capacitors between CN201 and the base of Q204. All resistor values check out, and there is continuity to ground. I have replaced C201. However, there is still the same DC voltage (145mV) across the speaker terminals, left channel. There is no DC voltage on CN201, so I don't think I should extend the check to the preamp. Base voltage on Q204 is 262mV, about 140mV higher that the base voltage on the corresponding transistor on the right channel...
2. I assumed the feedback divider is the network composed of R209, C204, R210, R204, C203. Please correct me if I am wrong. The resistors check out, there is continuity to ground and I have replaced C203 (which was bulged and had only about half the capacitance). However, there is still the same DC voltage at the output of the left channel.
Have I missed something?
I have attached the left channel schematic, for quick reference.
I have done the checks, as follows:
1. Checked the network of resistors and capacitors between CN201 and the base of Q204. All resistor values check out, and there is continuity to ground. I have replaced C201. However, there is still the same DC voltage (145mV) across the speaker terminals, left channel. There is no DC voltage on CN201, so I don't think I should extend the check to the preamp. Base voltage on Q204 is 262mV, about 140mV higher that the base voltage on the corresponding transistor on the right channel...
2. I assumed the feedback divider is the network composed of R209, C204, R210, R204, C203. Please correct me if I am wrong. The resistors check out, there is continuity to ground and I have replaced C203 (which was bulged and had only about half the capacitance). However, there is still the same DC voltage at the output of the left channel.
Have I missed something?
I have attached the left channel schematic, for quick reference.
The base to emitter voltages for your LTP transistors Q204 and Q203 should be close enough to equal and be a close match to those corresponding levels for Q212 and Q222.
It is possible you will have otherwise due to some leakage of current through C202.
If you measure the resistance of R202 which is in parallel you can see if the resistance of the combination is the value of R202. Check this against the corresponding parallel pairing of R247 and C227 in the working channel.
It is possible you will have otherwise due to some leakage of current through C202.
If you measure the resistance of R202 which is in parallel you can see if the resistance of the combination is the value of R202. Check this against the corresponding parallel pairing of R247 and C227 in the working channel.
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