Sansui Eight help - hum on one channel
- Thread starter KeithD
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- sansui eight
Those were never out. They have been in the whole time. I may have gotten confused, I didn't see where you said to pull them.
bob777
Active Member
Ok so everything is installed except the outputs and the voltage at (o) is approx 0volts.
Do you have 2 multimeters? Need to monitor (o) for 0 volts and monitor and set the bias current by removing f001 or f003 whichever side we are working on and measuring current set vr803 for 8 to 10 ma. Please read page 2 of this thread for clarification before continuing if not familiar with procedure.
Do you have 2 multimeters? Need to monitor (o) for 0 volts and monitor and set the bias current by removing f001 or f003 whichever side we are working on and measuring current set vr803 for 8 to 10 ma. Please read page 2 of this thread for clarification before continuing if not familiar with procedure.
The OT are also in and were in for all of the testing I did. I think when you joined the conversation you said you would remove all OT and driver transistors and check PS voltages, but the conversation quickly moved on to other things, so I never removed any of those transistors. They are all new I checked they were not shorted to ground, so I was not expecting any problems with them, and PS voltages seemed ok for where things were at.
Yes, will follow Jim's adjustment procedure (from QRX restore) in this thread:
https://audiokarma.org/forums/index.php?threads/eight-driver-board-adjustments.707373/#post-9493418
In the past I was not able to get idle current down to 8 to 10 ma. I think 13 ma was the lowest I could get. Will try to set idle current and then use Jim's procedure for the other settings. I do have three DMMs with enough minigrabbers for the test points. I will be measuring DC offset at a speaker output. Jim's procedure is to measure that at the headphone jack, but I don't have the kind of plug he uses to do that. Will do all adjustments on DBT and report back before going on. Still need to replace trimmers on the other driver board before doing the other (R) channel. Only been working on the L channel.
bob777
Active Member
Just to recap, we found nothing wrong, all parts were good.
The vr801 and vr802 and vr804 were far enough out of adjustment that they were able to adjust the balance or output voltage to +40 volts. Incredible !!
I was under the assumption the driver and output transistors were removed. Sure glad we did not lose any of that magic smoke!
The vr801 and vr802 and vr804 were far enough out of adjustment that they were able to adjust the balance or output voltage to +40 volts. Incredible !!
I was under the assumption the driver and output transistors were removed. Sure glad we did not lose any of that magic smoke!
Yes, that is correct. Overall, I did replace all 4 OT and the 4 driver transistors. Replaced e-caps on driver board and PS. That's it. I too am shocked that setting the driver board VRs all to the starting positions per SM resulted in such high DC offset. On the other hand, who knows? I have found that once in a while removing and putting transistors back in solves a problem for some reason - possibly a cracked solder joint?
We'll see if I can get all of the adjustments in spec. Keep my fingers crossed!
We'll see if I can get all of the adjustments in spec. Keep my fingers crossed!
Sometimes in these instances especially for plastic cased transistors (TO-92 in particular in my experience), there can be an internal bad connection in the device. The heat from de-soldering and re-soldering the device has fixed it, but only temporarily in the vast majority of cases.
Thanks John. We'll see what happens with the other driver board where I only replaced TR801 and 802. If I can get the R channel to spec just by adjusting the VRs then I guess that was the issue, but if not and I need to go through the same procedure of removing TR005-8, then there would seem to be a transistor issue on these boards.Sometimes in these instances especially for plastic cased transistors (TO-92 in particular in my experience), there can be an internal bad connection in the device. The heat from de-soldering and re-soldering the device has fixed it, but only temporarily in the vast majority of cases.
The transistors on the drivers do match the SM. Other than TR801 and 2, I did not turn up recommended replacements for the other transistors if it comes to that. Any recommendations?
I can only go by the schematic and Service Manual, so I don't know what you have in your boards - can you give me a list?
TR803, 804: 2SC627 = KSC2310
TR805: 2SC735 = BC3374 (=BC337=Mouser) or BC639 or BC637
TR806: 2SA562 = KSA733 or BC337 (watch pinouts on KSA733)
TR807: 2SC708A = KSC2690
TR808: 2SA537A = KSA916
I got these by simply searching AK for the type numbers in your list, and seeing what others have used to replace the devices. It took a bit of trawling to weed out the NLA devices, all of the equivalent devices in the list should be freely available.
TR805: 2SC735 = BC3374 (=BC337=Mouser) or BC639 or BC637
TR806: 2SA562 = KSA733 or BC337 (watch pinouts on KSA733)
TR807: 2SC708A = KSC2690
TR808: 2SA537A = KSA916
I got these by simply searching AK for the type numbers in your list, and seeing what others have used to replace the devices. It took a bit of trawling to weed out the NLA devices, all of the equivalent devices in the list should be freely available.
OK, used Jim's procedure. Set main amp current. Was able to get it to about 8 mA. It became very touchy above that. One thing to note, the Bourns replacement for VR803 appears to work the opposite of the original. SM says minimum is clockwise, but the Bourns minimum is counter clockwise. Just highlighting that for future reference. This may be one reason I was getting such odd values, as I thought I had set VR803 to minimum.
Then adjusted VR801, 802, 804. Was able to get 0.5 V across R804 pretty easily. DC offset adjusted to -44mV and VR804 to about 1mV. Could not do much better than that. If I adjusted VR804 up, I could get lower DC offset, so this was a balance.
I think next will be the R channel. After that it should be ready to be off DBT and on full line voltage?
(Edit, DC offset corrected to be -44mV, not -0.44mV I had originally in there)
Then adjusted VR801, 802, 804. Was able to get 0.5 V across R804 pretty easily. DC offset adjusted to -44mV and VR804 to about 1mV. Could not do much better than that. If I adjusted VR804 up, I could get lower DC offset, so this was a balance.
I think next will be the R channel. After that it should be ready to be off DBT and on full line voltage?
(Edit, DC offset corrected to be -44mV, not -0.44mV I had originally in there)
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Thanks John, I did peruse the Sansui transistor replacement thread that's been around for a while, but did not see the ones I have. Thanks so much for doing the leg work on this!TR803, 804: 2SC627 = KSC2310
TR805: 2SC735 = BC3374 (=BC337=Mouser) or BC639 or BC637
TR806: 2SA562 = KSA733 or BC337 (watch pinouts on KSA733)
TR807: 2SC708A = KSC2690
TR808: 2SA537A = KSA916
I got these by simply searching AK for the type numbers in your list, and seeing what others have used to replace the devices. It took a bit of trawling to weed out the NLA devices, all of the equivalent devices in the list should be freely available.