One thing just caught my attention... On the first photo on the left side, just below the big Illinois cap - those diodes are the main rectifiers, or Zeners?
I guess they are the rectifiers, because they are bypassed with snubber ceramics. If that is the case you should replace them with high current TO 220 case rectifiers (like BYW 29-200), because the PCB is pretty badly discolored underneath them due to the excessive heat they are producing....
I guess they are the rectifiers, because they are bypassed with snubber ceramics. If that is the case you should replace them with high current TO 220 case rectifiers (like BYW 29-200), because the PCB is pretty badly discolored underneath them due to the excessive heat they are producing....
Thanks for the advice and for the tranny sub list Jeromach. Just to confirm:
The rectifiers in my unit have been completely replaced for presumably high-speed diodes. They are larger than the originals and I don't think I have any issue with them as, as advised, I am getting good power up to a certain point in the circuit,
No issues with relays - yet,
I have also replaced the little 5-pin input device with matched 992's, good results there,
I'm halfway thru cleaning all the connectors now. After that will replace all the zeners and will report back. I'm concerned as the offset is fluctuating only on one channel, while the other side holds its values dead on.
The rectifiers in my unit have been completely replaced for presumably high-speed diodes. They are larger than the originals and I don't think I have any issue with them as, as advised, I am getting good power up to a certain point in the circuit,
No issues with relays - yet,
I have also replaced the little 5-pin input device with matched 992's, good results there,
I'm halfway thru cleaning all the connectors now. After that will replace all the zeners and will report back. I'm concerned as the offset is fluctuating only on one channel, while the other side holds its values dead on.
Then be ahead and solve the last possible failure/obstruction in good sound before it occurs and you have to take it apart again
Yes, they are rectifiers.
I followed up ilimzn's advise and replaced them with similar (heavy!) diodes, only risen more from the board to facilitate cooling of them and the whole of that side of the amplifier. It works, heating up is much more even on both sides.
Why not replace the caps that suffered most from ripple, aging and use ? Arguably also one of the changes one hears directly.
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OK - there is still an issue here.
I replaced all the zeners on the dodgy board. I checked and rechecked all the molex connectors, cleaned them all out, sanded the pins and reseated them in the PCB and cleaned and tightened all the sockets.
I have bias current again on both sides, all good. However on the side that was was previously dodgy I still have issues - I set the DC offset to 0V, then it climbs and drops - from about 0 up to 40mV and then up to 200mV at times, then sometimes peaking at 400mV, before dropping again.
It seems to cycle once the amp has reached OT. The other amplifier side holds fine.
I will need to shotgun all of the transistors based on Jeromach's parts list in the powersupply and amplifier section unless someone could give me an idea of where to start, just to narrow down what I replace. Any ideas? thanks!
I replaced all the zeners on the dodgy board. I checked and rechecked all the molex connectors, cleaned them all out, sanded the pins and reseated them in the PCB and cleaned and tightened all the sockets.
I have bias current again on both sides, all good. However on the side that was was previously dodgy I still have issues - I set the DC offset to 0V, then it climbs and drops - from about 0 up to 40mV and then up to 200mV at times, then sometimes peaking at 400mV, before dropping again.
It seems to cycle once the amp has reached OT. The other amplifier side holds fine.
I will need to shotgun all of the transistors based on Jeromach's parts list in the powersupply and amplifier section unless someone could give me an idea of where to start, just to narrow down what I replace. Any ideas? thanks!
Well, I'd start with the differential transistors Q1, 2SA798.
In a topic on Audiokarma I explained how to make them;
Technics SU-8077 Rebuild | Page 3 | Audiokarma Home Audio Stereo Discussion Forums
The paired transtistors need to be matched.
In a topic on Audiokarma I explained how to make them;
Technics SU-8077 Rebuild | Page 3 | Audiokarma Home Audio Stereo Discussion Forums
The paired transtistors need to be matched.
Funnily enough I have replaced them - the problem was there before i did this however - replaced both with KSA992's with closesly matched hfe.....
They are hfe matched however not thermally connected. What are your views on doing this in this application? I wouldnt have thought it was necessary for an input pair - they dont appear to get warm. I was also advised on another site by an expert that in his view I wouldnt need to.
Supporting that view is the fact that I have done both amplifier boards and only one has a problem.....
I am about a third of the way thru shotgunning the trannys anyway on the problem board - following your list except using KSA260A and KSA1220A to replace the 2SC1903 and 2SA898....
Why would Pioneer use the double transistors if it wasn't necessary or at least recommended? The gain of these transistors also reacts to outside temperature, which is not something to ignore in an M-22.
It's not so difficult to thermally connect them, just take care the casings are connected with some thermal paste in between and then heat shrink them.
But since the problem already was there it indeed might be something else. The next suspects for me would be Q3 and Q4 and I'd say these should be matched too.
Now I do no think you will replace these with KSA260A but with KSC2690A. Which I think is overkill btw compared to the KSC35303, but if you have the 2690's in stock they most likely are just as good.
It's not so difficult to thermally connect them, just take care the casings are connected with some thermal paste in between and then heat shrink them.
But since the problem already was there it indeed might be something else. The next suspects for me would be Q3 and Q4 and I'd say these should be matched too.
Now I do no think you will replace these with KSA260A but with KSC2690A. Which I think is overkill btw compared to the KSC35303, but if you have the 2690's in stock they most likely are just as good.
Please check the link to the audiokarma topic I posted here before, there you will find directions;
Technics SU-8077 Rebuild | Page 3 | Audiokarma Home Audio Stereo Discussion Forums
Reply #54 and #55
Technics SU-8077 Rebuild | Page 3 | Audiokarma Home Audio Stereo Discussion Forums
Reply #54 and #55
Right - I think Im making progress here - The issue is now evident in both channels - so now I think I've come full circle and agree that the input differential pair KSa992 that I used to replace the 2SA798 I replaced need to be thermally coupled. In the meantime I have replaced all the original 2SC1903 and 2SA898's with hFE matched KSA1220A and KSC2690A.
I have been working this amp for so long now I am going cross eyed and am paranoid - the last time I plugged it in the bulb glowed bright after a couple of seconds - its a 100W bulb. I think this is however due to the current drain of the amp of the amp (rather than a short) causing this to happen. The amp doesnt get enough current thru the DBT to be able to trip the speaker relay. However if I take it off the DBT then I risk blowing the amp up if there is a short - I go thru a large stepdown of 240VAC to 120VAC for this amp.
I also noticed that each set of the two 2sc1903s on each board (AWK057 and AWK058) are facing different ways. I wish I took a photo before I took these out to replace them.
Jeromach are you please able to confirm that there are not any silk screening errors on this PCB to your knowledge, with respect to the 2SC1903 transistors - being Q3 and Q4 on both boards......? I have installed the new trannies according to the silk screening
I have been working this amp for so long now I am going cross eyed and am paranoid - the last time I plugged it in the bulb glowed bright after a couple of seconds - its a 100W bulb. I think this is however due to the current drain of the amp of the amp (rather than a short) causing this to happen. The amp doesnt get enough current thru the DBT to be able to trip the speaker relay. However if I take it off the DBT then I risk blowing the amp up if there is a short - I go thru a large stepdown of 240VAC to 120VAC for this amp.
I also noticed that each set of the two 2sc1903s on each board (AWK057 and AWK058) are facing different ways. I wish I took a photo before I took these out to replace them.
Jeromach are you please able to confirm that there are not any silk screening errors on this PCB to your knowledge, with respect to the 2SC1903 transistors - being Q3 and Q4 on both boards......? I have installed the new trannies according to the silk screening
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Hi Korrah - no this is an M22 for the Euro market which needs 120VAC. I understand that this was the mains voltage for a while before it changed.
I do have some 100VAC gear, but it was all sourced direct from JP - including a Sansui AU-999 - and yes I do run that on 100VAC!
Thanks for your help with this Jeromach. Before I heatshrinked the input pair of 992's I put a meter on the speaker terminals and then pressed the faces of the 992's together and you can see the DC offset change before your eyes.
All the transistors have been changed and the input pairs of 992's heatshrinked together with a small squirt of heatsink compound put in the mix. Now both sides are fluctuating to a very minor extent as the amp reaches OT, up and down by about 10mV but mostly holding stable at around 3mV per channel. I think this is probably the most stable this amp has run for a while!
All the transistors have been changed and the input pairs of 992's heatshrinked together with a small squirt of heatsink compound put in the mix. Now both sides are fluctuating to a very minor extent as the amp reaches OT, up and down by about 10mV but mostly holding stable at around 3mV per channel. I think this is probably the most stable this amp has run for a while!
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