Friday, September 25, 2015

Sansui G-8000 Repair High-Light

Sansui G-8000 

I wanted to highlight a unit that came in for repair on this blog. Here we've got a Sansui G-8000 Receiver. These are pretty damn nice, stepped volume potentiometers, independent shielding, robust redundant protection networks, an excessive amount of 27K/Bank microfarads of filtering and a big ol' torodial Pformer.  One thing primarily unique on this model are the side input.output banks, the speaker and RCA panels are located on the sides of the chassis. Ive noted this on a few SX Pioneers as well or Sansui, many customers note they prefer the ease of access.

The reason were high-lighting this failure is its very common problem with a very rare result. Initially the unit came in high no protection engagement on either bank of outputs. 

Initial readings tests indicated an excessively high DC offset (1>Vdc+), initial offset at (>5mVmV-), we removed the rear heatsink mount and pulled the TO-3 output devices to ensure their okay. The output devices (to-3) here like many high series G-XXXX units mount the heatsink and outputs to the rear chassis, unlike the 7XXX the emitter networks are fastened to the TO-3 mounts on the 8000, much smarter design.

After ruling out output devices, I found Offset would not adjust below 250mV+, so we need to examine the F-2806 driver assemblies. As noted too, this is a sansui, I recommend replacing any fuse-able resistors now with MOX types. All final through first stage Bi-Polar transistors (BJTs) checked out find along with no diode reverse or forward issues. We did replace all electrolytics below 330Uf here due to heat damage from the large wire-wounds with audio grade Elna Silmic and WIMA Film capacitors. The final test was the main course dc potentiometer, which at this point in their life tend to gunk up badly and lose their tracing. Sure enough both 100 ohm potentiometers were completely shot with no resistance tracking at all! So new caps and pots.

I do want to point out a proper location for Offset on the G-8000, I recommend for a more accurate reading to test at points R61/62 (2.2ohm) prior to each speakers actual relay device with note to star ground (slightly isolated chassis here!) I was finally able to adjust with 0-15mV left and right respectively.

Yet we still did not have protection disengagement, we double and triple checked out protection tracing after confirming all protection and mains rail voltages. Traced back to the F2802 switching assembly as a common point of contact, speaker A/B switching, no matter what I did the only way to kick on the relay was to short the incoming speaker inputs at Pin 23/22 of F2802, yet this removed the entire B bank from operation. After vigours cleaning I decided to pull the switching devices and check continuity. All lo-behold each speaker switch A/B had constant continuity, I could also see some shaft damage from the pull. Comparing like switches from the Tone circuit I was able to verify that Yes, this was not factory intentional and each switch had a broken shaft. Now these are hybrid pushpull/toggle switches, almost impossible to replace of course.

Well, after examining the switching for DOlby and MPX which are fairly irrelevant nowadays and consulting with the customer I found a work-around. Fixing the broken switches into the off position I swapped all Dolby.MPX and speaker switching. Now Ive got both engagement with proper Offset and bias..ta'da Thanks again joe for going back and fourth not he switching.