I'm working on a G771 recently purchased by my son as non-working. The right channel doesn't function but rather than a simple switch issue it turns out that the right side driver amp is pretty hacked up. The cosmetic condition's pretty good and well worth saving. I've identified some suitable driver transistors and am looking at 2SC2581/2SA1106 output transistors which are presumably NOS on Consolidated Electronics website. Anyone got experience with these folks?
Sansui G771 aka G7700 on the bench
- Thread starter DougBrewster
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DB-7
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I have purchased outputs from them as well.
They look and work legit to me.
Time for an update on this one. I bought outputs from Ceitron as suggested above and they were good. However I've since been back to him for more as the result of having two blow-ups in the meanwhile. In fact I've cleaned out his stock of 2SC2581/2SA1106 and am now using 2SC4468/2SA1695. First blow-up occurred after having had some trouble with bias (sitting at 10-20mV and unadjustable) which turned out to be a bad FET which presumably had occurred in a prior blow-up before it landed on my desk. Anyway after getting it working it blew the new outputs bias trimmer and R52 driver emitter resistor. All of this happened the first time the volume level was turned up above normal listening level - peaking at up to 25W say. Some reading here revealed how sensitive these units are to load impedance leading me to measure my bench test speakers which though labelled as 8ohm are actually a shade under 4ohm.
So, rebuilt it again and this time used a pair of Sansui speakers I have but again, they measured well under 8ohms so they are seriesed up with the 4ohm bench tests such that the total impedance is in the 9-10ohm area. Got it running without much trouble and must say that the bias and offset were super stable. Blew up again (this time with a light show from the outputs) after several hours of operation and when I was function testing all of the switches. This was when I decided it must be going into oscillation and have replaced as a precaution the Miller caps on TR8, 10, 16 and 18 with 47pF up from 22. Also replaced C24 and C26 in the bias chain with like for like just in case.
Testing today and scoping the output revealed no tendency for oscillation at 1KHz to 20KHz both sine and square wave. On square wave the power was limited to 12.5W but on sine wave it went to 162W without clipping, just ran out of volume knob travel.
@ivandezande got it right when he said elsewhere in this forum that "these amps are fussy"
So, rebuilt it again and this time used a pair of Sansui speakers I have but again, they measured well under 8ohms so they are seriesed up with the 4ohm bench tests such that the total impedance is in the 9-10ohm area. Got it running without much trouble and must say that the bias and offset were super stable. Blew up again (this time with a light show from the outputs) after several hours of operation and when I was function testing all of the switches. This was when I decided it must be going into oscillation and have replaced as a precaution the Miller caps on TR8, 10, 16 and 18 with 47pF up from 22. Also replaced C24 and C26 in the bias chain with like for like just in case.
Testing today and scoping the output revealed no tendency for oscillation at 1KHz to 20KHz both sine and square wave. On square wave the power was limited to 12.5W but on sine wave it went to 162W without clipping, just ran out of volume knob travel.
@ivandezande got it right when he said elsewhere in this forum that "these amps are fussy"
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About an hour after my post last night this thing blew up again. It was playing Einaudis' Pathos from his Summer Portraits album; a demanding track. Bizarrely it was playing the same track last time it blew up. Guess there's a lesson in there. On the bright side I've found a strong root cause candidate which is a bad soldered joint at L02 in the Zobel network. I was able to move the coil around a little and watch the ohm meter jump to open and back momentarily. Happily I've still got some spare output transistors....