7900Z Sansui

Wow! That would do it.. I should have paid closer attention to the SM... to make it even worse I initially ordered 68K !!! Thank you for pointing that out
 
Many thanks for pointing that out Rob, as putting in the correct value of resistor did the trick, and was able to dial it right in at 2mv. This thing sounds very nice!
 
I ran it for about an hour, both sides set to 2mv, sounds wonderful, very pleasing to my ear. Equal output on both sides. However, I looked at it with the thermal camera and the right side (the side that was damaged initially) was running hotter than the other side. Bias seems to be correct so I'm not sure what else could cause this. Note that this is the side with the Ebay outputs... I wonder if it would be wise to swap them to test?

In photo (bottom of unit) right side is at the top and you can see one output q13 2SA1169 is hotter.
 

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I would not trust any 2SA1169 on eBay unless it was pulled from a working unit. Even then, it’s a crap shoot.

Did you post a WTB in barter town. Someone may have some, or others in that package that would work.

Rob
 
Those replacements worked fine up until they were pushed a little harder... when I connected to my daily drivers (L112) and gave them some volume I had a fireworks then smoke show! Still not sure if it was down to being knock offs or other problems on that channel, but a predictable result, a few resistors fried. I am now going to convert all of them to a TO3P package based on what I have read. Digikey has the 2SA1943 and there is a seller that has adapter plates

Interesting thing is how similar they looked at first, but I noticed that when I cleaned off thermal grease with 99% alcohol, it took the printing away on the fakes but the originals did not fade at all
 
The originals are pretty tough. I used to run a 7900Z in the garage with two pair of 8 ohm speakers, so a 4 ohm load, at very loud levels for long periods of time. I never had an issue.

Swapping to TO-3P is probably a good idea.

Rob
 
There was just recently a post on here in which it was stated that there was a semiconductor manufacturer apparently getting ready to spin up production of MT-200 devices again. The timing, and ultimately the availability of those, had not been formally announced, however.

My take is that in the long run, conversion to TO-3P devices is probably the most sensible solution, as we can have the greatest confidence in their continued availability.

As to the hardiness of the original factory output devices, I've always been very judicious in how heavily I loaded receivers of this series. That said, I had a 7900Z that I sold to a buddy of mine quite a number of years ago now, and he has had two pair of 8 ohm rated speakers connected to it ever since then. I know for a fact that he runs it hard, and that unit has never once had an issue (so far}. I'd say that speaks pretty well for the original Sanken outputs.
 
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I very rarely have the opportunity to run them hard but it sounded great with both L112 and L100 so I was running through some known "test tracks" and liking what I was hearing.

I did temporarily tack in the new ones, replaced the cooked resistors, and it did not come out of protect so it did some damage further up the line.

I have learned a ton on this one so far so a good experience and a cool receiver.
 
I did install some new 1943/5200 straight from DigiKey, was fairly easy to remove heatsink drill and tap the 3mm holes. One side is working just fine as before.

The bad side (right side) is challenging me greatly...I'm getting full negative rail voltage -57V (on dim bulb) where it should not be. Pulled and tested transistors, whatever I could find. Learning a lot about reading schematics but clearly have a long way to go. I'm completely stumped...

I'm obviously still learning here but by comparing the sides I was trying to start at the input and compare voltages on good side to bad. Seems that trouble starts at Q6/Q8 where the negative voltage is. Transistors test OK.
 

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Here are the voltages that I am seeing... all transistors are new on Right side... tried everything I could think of to look for bad traces, etc.

I think my next step will need to be removing this small driver board from the large board to inspect and test... it appears that desoldering is the only way?


Right (Bad Channel)
E, C, B
Q5 58, -1.8, 57
Q7 58, 58, 57

Q6 -1.8, -58, 0
Q8 -56, 57, -58


Left (Good Channel)
E, C, B
Q5 58, 1.8, 58
Q7 58, .4, 55

Q6 -1.4, -58, 0
Q8 -58, -.05, 0
 
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Yes, that "sub board" is soldered on. That was one of the cost saving measures seen on this line, instead of having "pluggable" connectors.
 
Correction!
I made an error after rechecking the voltages. Still plan on removing the board though.

Right (Bad Channel)
E, C, B
Q5 58, -1.8, 57
Q7 58, 58, 57

Q6 -1.8, -58, 0
Q8 -56, 57, -58


Left (Good Channel)
E, C, B
Q5 58, 1.8, 58
Q7 58, .4, 55

Q6 -1.4, -58, 0
Q8 -58, -.05, -58
 
Going to set this one aside for now... can't seem to find the fault but will revisit when I have the experience to tackle it!
 
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