Pioneer SX-950 PS Rebuild or Not?

[MBuras]

I know, that's a loaded question.

I've restored a few SX-950's over the years. A friend recently picked one up and just wants the bare minimum to get it into a reliable state, not trying for a full restoration.

Was planning to rebuild the power board (usually very toasty in my experience) along with some pre-emptive replacement of known noisy transistors (SA725/726/SC1313/etc) in this unit. But now that I have the bottom cover off I see a power supply board cleaner than any I've ever seen. It's immaculate. No signs of overheating. Most of the e-caps even have their wrapper properly secured on the end and not slipping off from shrinkage/heat. Truly must be a very low hour unit.

So back to the question. Because it's such a low hour unit and it's currently functioning within acceptable parameters, do I still go ahead and rebuild the power supply or leave it for the next person who will likely want to do a full restoration? Maybe it's a risk assessment only I can answer but still wouldn't mind a few opinions. And I know you guys are full of those!

Thanks,
Mike

 

[nicholas2381]

I'll be interested to see the consensus.

 

[ivandezande]

Well it's gonna have heat issues eventually, so fix now so the board will continue to be that nice.

 

[markthefixer]

Thermal grease will still dry out, and the Sky Blue Sanyo "Capsistors" will still fail - you will want to check on those..

Some of those failures will shut down protection sensing, so you're NOT really protected.

There may be 2sa726's and 2sc1451's in the power amps too. I wouldn't let them stay in a unit my name was connected with.

 

[MBuras]

Thanks for the advice

 

[rcs16]

It is one of those where do you start and where do you end. Every old component changed increases reliability. Certain ones make a bigger difference than others.

 

[tsd71]

Rebuild the power supply first including transistors and both zeners you will be happy you did.

 

[goat67]

That sure is a clean unit. I would for sure replace the known bad transistors and rebuild the Power supply and Protection.... Thermal grease. Add Fly back diode IIRC this is not present in stock form.
I just got a good capacitance meter and I tested the Grey Elna caps out of a Kenwood KA-7300 and they all were above or right at there capacitance values compared to new ones that are always below spec but within 20% ESR was also very low. So what does this mean I do not know some of these old caps seem to hold up better than others. Now the Rubicon and the Sanyo caps were way out of spec and high esr

 

[MBuras]

Thanks for all the wisdom. Very helpful in making my decision.

BTW...in case anyone ever doubts freshening the thermal grease, what was left on Q8/Q9 in the power supply section fell off as dust when I removed them from the heatsink.

 

[jboerger3]

Gents, As this is a fairly fresh thread, and I am toe-deep in restoring my SX950, I have a question regarding C14 in the power supply board. The PS is the first board that I am working on. I purchased parts kits from Peace.Love.and.Music via EBay to save some time and effort. From what I can tell (I am a newbee) they are quality parts and, mostly, correct for my needs. However, the oem cap, on C14, is a 47uF/25v. That spec matches the oem schematic that came with the unit. The service manual shows a 220mF/10v and the P.L.a.M instructions say that some C14 caps are 220uF/10v and some are 100uF/10v. He supplied a 220uF/16v. I read part of a justanswer thread where the questioner's c14 was also a 47uF/25v. As I purchased the receiver new, in 1977, I am guessing that it was a late production model and the 47uF/25v was a revision. So, I'm thinking that I should keep the oem cap, start a parts cart with a 47uF/35v higher temp cap. This will allow me to move on with the project - so I can test along the way. I may have some other changes as I work through the other boards etc. Unless someone can tell me that it would be ok to use the 220uF/16v....?

 

[jboerger3]

I just found mattsd's comment regarding my question: "C14 is used to slow the ramp up of the voltage for the function and speaker indicator lamps to avoid thermal shock. The 220uF is fine, all it will do is slow the ramp up a little more, 0.9 seconds opposed to the 0.183 seconds for the 47uF." It was in thread https://audiokarma.org/forums/index.php?threads/recapping-sx-950.364598/page-2 Thanks mattsd! Lots of great info. I'll try to find existing info before asking questions.

 

[Watthour]

Given the position of that e-cap, 220µF should be fine, and 16V is safe. From what I have seen, that online vendor has evidently done some homework and normally provides good quality components from reputable suppliers. Incidentally, that's not me, nor am I affiliated, but I've looked at some of those ebay listings while searching for things in general, and found that some of those "kits" are sketchy at best, but those listed from that seller seem to incorporate current information (probably getting data/BOMs from AK) and are not insanely priced or hyped.

I really get a kick out of some of those "upgrade service" listings, where everything magically becomes new and wonderful for a fixed fee.

 

[jboerger3]

Thank you Watthour, for your confirmation. I had heard some horror stories about many of the ebay kits. P.L.a.M has only positive feedback, is from Milwaukee, and is, likely, a Packer fan. How could I go wrong? All the e-caps are Nichicon's rated at higher temperatures. Lots of Wima film caps for the other boards. And getting instructions to reverse the order of certain transistors made it worth the price of admission. I did, however, separately order multi-turn trim pots for the power amp, upgraded a few caps to a higher audio-grade, for the phono board, and ordered replacement caps for the 22,000mF filters.

 

[Watthour]

Thank you Watthour, for your confirmation. I had heard some horror stories about many of the ebay kits. P.L.a.M has only positive feedback, is from Milwaukee, and is, likely, a Packer fan. How could I go wrong? All the e-caps are Nichicon's rated at higher temperatures. Lots of Wima film caps for the other boards. And getting instructions to reverse the order of certain transistors made it worth the price of admission. I did, however, separately order multi-turn trim pots for the power amp, upgraded a few caps to a higher audio-grade, for the phono board, and ordered replacement caps for the 22,000mF filters.

I'm not sure whether everyone around there is a Packer fan. I'm not so much a fan, just an owner.

It's probably good to add in the new trimpots if there are any suspicions about the originals. It probably wouldn't hurt to swap out some of the caps in the EQ section, and if the main filters have any questionable characteristics changing those is good insurance.