Put your meter on 'Diode' range.
All 2SA and 2SB devices are usually PNP's and all 2SC and 2SD are usually NPN polarity devices.
For the NPN devices and with the red lead on the base you should read around '600' which is the forward volt drop in millivolts across the junction when probing the collector and emitter.
With the red lead on the collector and the black on the emitter there should be an open circuit (as if the leads were not connected).
The PNP is the same but the red/black meter positions are reversed.
Using normal ohms ranges on a DVM is open to interpretation of the results as it depends on the range selected and the meters internal test voltage but generally the open circuit condition between C and E should be valid.
All 2SA and 2SB devices are usually PNP's and all 2SC and 2SD are usually NPN polarity devices.
For the NPN devices and with the red lead on the base you should read around '600' which is the forward volt drop in millivolts across the junction when probing the collector and emitter.
With the red lead on the collector and the black on the emitter there should be an open circuit (as if the leads were not connected).
The PNP is the same but the red/black meter positions are reversed.
Using normal ohms ranges on a DVM is open to interpretation of the results as it depends on the range selected and the meters internal test voltage but generally the open circuit condition between C and E should be valid.
Yeah, that's how I've been testing. Just done it again and this is what I got.
A1264
black on base, red to collector, followed by red to emitter
Got these readings 553, 558
Reversed = OL, OL
C-E
got OL, then reversed and a reading of 1500 or so that kept climbing so effectively an OL.
So that means a good transistor
c3181
Red on base, black to collector, then black to emitter
reading = 557, 559
Reversed = OL, OL
C-E
Black on collector, red on emitter
Result = OL
Reversed = OL
Good transistor
1508
Red on base, black to collector, then black to emitter
reading = 684, 1222
Reversed = OL, OL
C-E
Black on collector, red on emitter
Result = 1882
Reversed = 1540
1508 looks dodgy although I got similar results on same transistor on good channel.
A1264
black on base, red to collector, followed by red to emitter
Got these readings 553, 558
Reversed = OL, OL
C-E
got OL, then reversed and a reading of 1500 or so that kept climbing so effectively an OL.
So that means a good transistor
c3181
Red on base, black to collector, then black to emitter
reading = 557, 559
Reversed = OL, OL
C-E
Black on collector, red on emitter
Result = OL
Reversed = OL
Good transistor
1508
Red on base, black to collector, then black to emitter
reading = 684, 1222
Reversed = OL, OL
C-E
Black on collector, red on emitter
Result = 1882
Reversed = 1540
1508 looks dodgy although I got similar results on same transistor on good channel.
The 2SD1508 is a 'Darlington' type. What you are reading is the volt drop of the two B-E junctions in series. So all good.
Darlington transistor - Wikipedia
Darlington transistor - Wikipedia
Update
So I soldered in the new resistors, checked the output transistors one last time (out of circuit) and all is well.
Applied some fresh compound and cleaned up the pcb using isopropyl alcohol.
Switched on unit with DBT is place and all seems fine.
The bias was fairly high at 20mV so I adjusted to around 14mV and then tested with speakers - audio is fine and importantly no more distortion in the right channel.
Then I just set bias to zero and turned off unit and removed speakers.
Re-adjusted bias on mains to around 13.8mV and am now gonna do some monitoring.
Hopefully, that's everything back to normal.
Thanks to everyone for the help, much appreciated.
So I soldered in the new resistors, checked the output transistors one last time (out of circuit) and all is well.
Applied some fresh compound and cleaned up the pcb using isopropyl alcohol.
Switched on unit with DBT is place and all seems fine.
The bias was fairly high at 20mV so I adjusted to around 14mV and then tested with speakers - audio is fine and importantly no more distortion in the right channel.
Then I just set bias to zero and turned off unit and removed speakers.
Re-adjusted bias on mains to around 13.8mV and am now gonna do some monitoring.
Hopefully, that's everything back to normal.
Thanks to everyone for the help, much appreciated.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.