Sansui Eight F-1278 Driver Rebuild

[petesui]

I'm struggling mightily to order the correct trimmers for my restoration projects. I need to replace the trimmers on the F-1278 driver cards. The cards plug in vertically and the trimmers are at the top of the boards. They go in "sideways," if you will and the adjustment screws face up towards the ceiling. The pinout is also more or less a straight line with the middle pin being slightly offset as opposed to being oriented in a more defined triangle. I have several values of Bourns 3386P, 3296W, and 3296Y and they all appear to be the wrong orientation. What series do I need? I'm struggling to figure out the right parts on Mouser and DigiKey because their pictures are of a generic style. I need 1k, 3k, and 100k ohms for the Eight. Everything I have has the screw adjustment on top, thus facing to the side if installed. This is confounding the hell out of me. Any guidance and tips are much appreciated. For context, I have the left channel adjusted fine - the trimmers survived the procedures. VR801 on the right channel, however, is shorted/open and I can't dial in the adjustments. Thus, I'm just going to go ahead and rebuild the drivers. I have everything else I need except the damn trimmers! Thanks!

 

[KeithD]

I used these on my Eight:

 

[petesui]

I used these on my Eight:

Yes! You are the man! Thank you so much @KeithD! Exactly what I was looking for. I have a stupid stock of useless trimmers. Thank you.

 

[KeithD]

Yes! You are the man! Thank you so much @KeithD! Exactly what I was looking for. I have a stupid stock of useless trimmers. Thank you.

No problem. Just FYI, that picture was taken before I replaced all the transistors on the driver boards. I was noticing a bit of static; replacing all the transistors cleared that up.

Best wishes on your restoration work.

 

[petesui]

Best wishes on your restoration work.

Thank you. Really appreciate your helpful response to my nearly unhinged rant of a post! I found all the 3362X and they're en route. Patience is not my strong suit which makes this a tricky hobby.

This Eight is really very clean and original. I thought I'd listen to it for awhile as is and not do anything to it other than control cleaning and SM adjustments. Alas, it was not to be when I couldn't get the differential amp and bias adjustments dialed in. Trimmers on the right were out of range/spec and I couldn't get them close enough to let it ride. Thus, I've decided to rebuild the drivers entirely and replace all the transistors, etc. I'll get the right side dialed in first and then match the left side. I was planning to restore my AU-317 next but this beautiful Eight came out of nowhere and intervened. I suppose that's a good problem to have.

I tried cleaning the original trimmers first, too. I don't know if that was a good or bad idea. In any event, it didn't work. Perhaps it hastened their demise.

 

[Hyperion]

This Eight is really very clean and original.

I thought that - it really is very nice and clean.

 

[petesui]

I thought that - it really is very nice and clean.

Thanks! A few nicks and light scratches on the wood case, but the face, rear panel, and the rest of the exterior are in exceptional condition - truly not a scratch on it. Internals are in great condition, too. It cleaned up well. I have replacement tags for the push button switches and a new light kit. Only the FM Stereo light is out. The seller's brother brought it back from Vietnam and it stayed in the family which is usually a good sign.

 

[AlexHempel]

Six was the first receiver I ever worked on (still have it). The build quality of the single digit series is really something. Always wanted a Seven or Eight -- yours looks great!

 

[petesui]

Six was the first receiver I ever worked on (still have it). The build quality of the single digit series is really something. Always wanted a Seven or Eight -- yours looks great!

Thanks. I wasn't looking for it and I was perfectly happy with my 7000, but when this Eight popped up nearby I really had no choice in the matter!

 

[petesui]

Waiting for the trimmers so I've gone as far as I can for the time being. Replaced all transistors and the two caps on the right channel F-1278. It's a little bit of a pain to work on. Every single component lead is bent flat to the board and soldered down with its neighboring component lead on the shared trace. Makes de-soldering and bending the target lead straight up for extraction very delicate and tedious. I managed to go 27/28 without lifting a pad. Of course, I lifted the last one because I got quick and careless. At least I know what to expect in the left side now.

 

[petesui]

These trimmers are tiny buggers. The leads are very short and the lead spacing very narrow for the original through hole pattern. Really not much to work with. I had to revise one leg on VR801 because my joint was cold and it wasn't connecting with the TR803 collector. All good now. Still waiting on the 1k ohm VR803. I think I'll use some lead clippings to make extensions for that one and the left channel trimmers when I get to it. Anyway, some progress.

 

[petesui]

Both F-1278 drivers for the Eight rebuilt. Full transistor, cap, and trimmer swap. Successful DBT result upon reinstallation. As I mentioned above, the legs on these trimmers are impossibly short for the spread of the through holes. Another centimeter or two on the trimmer legs would be a huge help, Bourns! I painstakingly soldered extensions on to the outside trimmer legs to give me something to work with and to create some purchase for the solder joint. Seems to have worked. I'm holding off on adjustments until I can refresh the thermal paste on the pre-drivers and outputs. Fresh TO-66 mica insulators, from our friend peace.love.and.music (@EastPoint!), for the pre-drivers are en route.

 

[petesui]

Addressed the outputs today. Right side was rather liberally pasted. Somebody had been in there before and put on a healthy layer. I cleaned it all out and redid it. Left side was original - dry mounted. No thermal paste, just a mica insulator. I treated the the left to a fresh mica and its first pasting. The gain on the outputs was a mixed bag. On the right channel (previously serviced and pasted), the rear output was 116 hFE and the front was 82 hFE. On the left side (dry mounted and seemingly unmolested), the front output was 84 hFE, similar to the right front, and the rear was 66 hFE. So, that makes one curious about that 116 and the liberal pasting on the right side. Also, under each output on the heat sink, between the socket holes, there were little rectangular scraped/abraded spots. Perhaps the mark of some manual factory smoothing before installation?

The pre-drivers mounted to the chassis are dry mounted as well. I'll do those later this week. The brass bronze-colored heat sink and the little round Sansui covers over the outputs on the Eight are very cool. Great little detail. You really can't even see the Sansui covers under the exterior grill.

 

[KeithD]

The brass bronze-colored heat sink and the little round Sansui covers over the outputs on the Eight are very cool. Great little detail. You really can't even see the Sansui covers under the exterior grill.

Agree, Sansui went to so much effort to craft those heat sinks, it boggles my mind that they just covered them up with a black sheet metal grill. Some leave the grilles off to show the beauty of those heat sinks!

 

[petesui]

Some leave the grilles off to show the beauty of those heat sinks!

This is not a bad idea at all. I put them back on for the time being, but may consider doing that. Now that they're clean and shiny, they do show through the grills a little better. At least under the light of my bench they do.

 

[Plimpington2]

Waiting for the trimmers so I've gone as far as I can for the time being. Replaced all transistors and the two caps on the right channel F-1278. It's a little bit of a pain to work on. Every single component lead is bent flat to the board and soldered down with its neighboring component lead on the shared trace. Makes de-soldering and bending the target lead straight up for extraction very delicate and tedious. I managed to go 27/28 without lifting a pad. Of course, I lifted the last one because I got quick and careless. At least I know what to expect in the left side now.
View attachment 3710444View attachment 3710445

Ugh. That's what I have to look forward to! I hate when the leads are bent over like that.

 

[Plimpington2]

Both F-1278 drivers for the Eight rebuilt. Full transistor, cap, and trimmer swap. Successful DBT result upon reinstallation. As I mentioned above, the legs on these trimmers are impossibly short for the spread of the through holes. Another centimeter or two on the trimmer legs would be a huge help, Bourns! I painstakingly soldered extensions on to the outside trimmer legs to give me something to work with and to create some purchase for the solder joint. Seems to have worked. I'm holding off on adjustments until I can refresh the thermal paste on the pre-drivers and outputs. Fresh TO-66 mica insulators, from our friend peace.love.and.music (@EastPoint!), for the pre-drivers are en route.
View attachment 3715658 View attachment 3715659

I am on record here (in another thread somewhere) complaining about what is currently available in terms of trimmers. These Bournes are functional, yes. But they seem heap and suspect. The leads are too thin, too short and the whole package just inspires no confidence whatsoever. All these vintage amplifiers were spec-ed with big, meaty and stout trimmers that, admittedly, have failed after 40 years. But still - I wish we had better options today.

 

[petesui]

Ugh. That's what I have to look forward to! I hate when the leads are bent over like that.

I've since learned they are called "clinched" leads, and, yes, they kind of suck. The Hakko gun gets the bulk of the solder, but it doesn't get the layer underneath holding it to the board. Liberal flux, solder wick, and your iron are your best friend here. That technique is much more effective and it went much better on the left channel when I figured that out.

 

[Plimpington2]

I've since learned they are called "clinched" leads, and, yes, they kind of suck. The Hakko gun gets the bulk of the solder, but it doesn't get the layer underneath holding it to the board. Liberal flux, solder wick, and your iron are your best friend here. That technique is much more effective and it went much better on the left channel when I figured that out.

Ah yes, the Hakko. I've only had mine for about 6 weeks, but it has changed my life. I don't know how I managed without it. But you are right, first hit with the gun, then solder wick.

 

[petesui]

I am on record here (in another thread somewhere) complaining about what is currently available in terms of trimmers. These Bournes are functional, yes. But they seem heap and suspect. The leads are too thin, too short and the whole package just inspires no confidence whatsoever. All these vintage amplifiers were spec-ed with big, meaty and stout trimmers that, admittedly, have failed after 40 years. But still - I wish we had better options today.

I was chastised by one of the purists in a Sansui FB group for using the Bourns. Complete with crying eyes emoji. He did, however, make me aware of these trimmers which match the original style and pinout spacing. I was not aware of them or I might have considered them. I don't know if they're any good, but they would certainly be easier to install. Curious if anyone has used them and can opine on their quality. I have an 8500 in the queue with similar trimmer orientation and I'd be inclined to use these.