Pioneer SX-850: Under the hood

[Pacific Stereo]

If there's one thing we have plenty of around here, it's vintage gear that needs some TLC. Here's an SX-850 that just got done in the Pacific Stereo lab. The 850 is a special piece to me, as it was my very first "real" receiver. I remember buying mine from the Pacific Stereo store in Anaheim (a store I later worked in) and spent many, many hours in front of that beauty. I remember all of my friends were insanely envious of the 850 and indeed, one of mine ended up buying that unit from me and I do believe it is still in service today. So it really was very cool to have one come across the bench some 35 years later!

This one was an eBay buy that the new owner wanted restored. It arrived in reasonable shape, though a number of the switch knobs were bent due to the way it had been packed.

The 850 is a 65 WPC beauty with great FM and excellent sonics. I always liked the way mine sounded, and this one was no different. So let's dive under the hood!



Like many Pioneers, this one has a solid middle chassis; that means working on a board requires that it be lifted and rotated in order to get to the solder side. In an ordinary 1977 world when one is going to work on just one board and maybe change a few parts, this isn't a big problem. But it makes a rebuild a lot more work, since we are going to be changing a bunch of parts.

Like most products, the first thing I like to do is get the power supply refurbished, since that section is the most critical and can cause the most trouble. Here's what it looks like before we start:



Like many of their designs, this supply runs hot, though not nearly as much as the one in the 1280, for example. But it's still needing a refresh. This one gets new caps and new semiconductors.



Also, this one needed new filter capacitors. New parts are smaller than the originals, so there's a little work to do to make them fit. The good news is that they are also better than the old ones in terms of impedance and ripple-current capacity.



Next is the power stage. This unit uses two paralleled devices per signal half, making for eight devices in total, which is unusual for a 65WPC amplifier.



New semis, good electrolytics, film caps and new bias and offset pots make for a happy output stage.



So what's next? Well, we should probably work on the phono and line stages. The phono section looks like this:



This is a nice design; simple, quiet and very sweet-sounding. It gets better with even quieter semis and some nice film caps.



So you want to adjust bass and treble? Of course you do. If I had a dime for every tantalum Pioneer used in their products...



Here, in addition to changing out the transistors for quieter parts, we use a combination of low-leakage and film capacitors in place of those little blue distortion-factories.



The flat amp looks like this:



Happily, there are fewer little blue monsters on this board. Same treatment; quieter semis and Wima, Wima, Wima on the label, label, label.



Shall we look at the tuner?



This is a far-simpler design that that of the 1050 or 1250, and yet it performs very, very well. Sensitive and musical, too. TLC (and a complete alignment) makes it even better. The bad news for the technician is that because the only way to get to the solder side of the board is to lift and separate (sorry, I just could not resist!), we have to destring the tuner mechanism, which is never much fun.



But wait, there's more! The protection circuits in the 850/950 are notorious for going a little squiffy, either shutting off when they are not supposed to, or worse, NOT shutting off when it's time to save speakers and output devices!



This is generally caused by leaky devices, so this board gets new replacements and better caps (originals can leak). The relay drive transistor also tends to get tired, so we change that to a better part, too. Oddly, Pioneer didn't utilize something very standard in relay-drive circuits, and that's a diode to save the drive transistor from the back-EMF spike the relay makes when the field collapses. This adds a smidge of delay for the relay to shut off, which may be why Pioneer chose to omit it. Myself, I'd much rather live with the possibility of a slight turn-off thump than a fried driver transistor.



Well, what's left? I guess a look at the bottom might be in order.



And the top, now that we're all done. This one's relay contacts were too far gone to refurbish, so the guts of the relay were changed while keeping the outer case.



And of course, we have to look at her!





She is just a beauty, and sounds terrific. And for me, the walk down memory lane was awesome!


All photos, copyright ©2013 Pacific Stereo. All rights reserved, no use without written permission. I better not see these on eBay!

[shadowlord]

awesome work as always.

If i was in the US, i'd send you my A-77X for an overhaul in a heartbeat.

[DaveInVA]

Nice work!!!


Dave

[Tasuke]

fantastic job!! perhaps you 70's enthusiasts have a point there;
these vintage amp designs of the era do appear singularly easy to service.

that said, how much experience have you with newer SS receiver designs
such as my 1990 VSX-D1S? i got my current example as allegedly almost NOS,
and it has a single, minor issue;

occasionally the motor-drive remote controllable MASTER VOL. seems to glitch out when adjusting VOL by remote, resulting in the VOL. dial taking off on it's own in whatever direction i last pushed. this is usually remedied
by hitting another button. also, this only ever happens
when i attempt VOL adjustment within moments after power up,
leading me to believe that there is perhaps a faulty discrete component
such as a cap in there or something, i dunno.

any suggestions as an experienced tech?

[Pacific Stereo]

"Easy to service" is not a term I would use to describe these units. This 850 took several days to complete. Insofar as your D1S, there's no real way to know without getting into it and troubleshooting it (unless this is a known problem with a published solution). The signal from the remote eye is serial data right to the microprocessor. Since the remote eye can't make meaningful data by itself, that leave the micro, power supply (noisy, spiking the micro) or possible a leaky drive IC for the volume control motor. Those are the areas I would want to look at first.

[Tasuke]

Originally Posted by Pacific Stereo

"Easy to service" is not a term I would use to describe these units. This 850 took several days to complete. Insofar as your D1S, there's no real way to know without getting into it and troubleshooting it...

thanks. sorry, i should have said -relatively- easy to service.

i've seen a lot of receiver designs, and many, if not most,
don't appear to do the serviceman any favors,
and indeed there were some late 80's PIO designs
(VSX-29TX, -39TX and others) that pretty much required one
to dismantle the entire unit to get at the main mother PCB.

at least some of the better late 80's/early 90's PIO designs
provided a removable bottom panel for ease of access
to the main PCB. if i'm not mistaken, those 70's units
are pretty much the same, even the entry level models?

[20tajk7]

Hi, just a little question about the driver transistors, I have a 838 who has burnt output transistors (the board looks quite similar to the 850 but there is only one pair of output transistors by channel), the previous owner has tried to fix it by using a pair of cheap 2N3055 !
So now the 3055's and 2SB528 / 2SD358 are burnt
I was able to find 2SA747 / 2SC1116 in France but no 2SB528 / 2SD358
Does 2SB528 / 2SD358 can be replaced by BD139 / BD140 ?
(according to a French data book they are equivalents but I have some doubts).

By the way great job on this 850

[Pacific Stereo]

I would have to go and look to see if those can be subbed with those parts. But littlediode.com shows both parts in stock (albeit in small quantities). I have those parts as well, but I would not sell them for any less than LD sells them for, so it would make sense to get them from them.

[Loddestuen]

Hi Pacific Stereo,

Great work - that really looks nice!

I'm refurbishing a similar model SX-838 right now. It had problems with the caps in power supply board and it had defective driver transistors that were popping and scratching.. It has a partly defective transistor for the protection relay.

So i have ordered the parts and changed most of the parts when i saw this thred.

So, i have already ordered electrolytic caps for everything - most af them where available in "Audio Grade" quality but now i see that you prefer wimas for some of the replacements. Why? And which ones should be wimas?

I like the old school analog hifi sound that these units can produce, i dont want the sound to be to modern (Flat, bright and digital) sounding. What should i use for the control and EQ and most important the phone stage? Wimas like you did?

I would appreciate input on this subject as im trying to learn and understand all the tricks in the book as repairing old hifi is my absolute favorite hobby - especially during the dark and wet winter time here in Denmark.

Best regards from DK

[Tasuke]

3rd of this month (3/3/16) i was up at OHSU at PORTLAND OR. again for a medical appt. and i was able to swing by my favorite audio haunt up there again on the way out,
"AUDIO SERVICE SPECIALTIES LTD." the small, privately owned service shop where i got my beloved 1990 TOTL PIONEER VSX-D1S receiver back in 2003.

he had some VERY NOTABLE gear for sale, such as a pair of KLIPSCHHORNs (those bastards are HUGE)
and an almighty SX-1980 he had up on end on his work bench with it's top off, another first time, impressive sight to see in the flesh!!

he also had one of these SX-850s on the sales floor for a mere $350, it was hooked up to small speakers and running off FM reception.
i fiddled around with it some, gave it a heft, it really was NICE, especially for the price.

i kind of wish i could have left the shop with it now, it was truly beautiful to behold in person. it would have been a lovely computer sound amp,
i wonder if it could have matched or exceeded the 1985 TOTL YAMAHA R-9 receiver i employ for computer sound? it may not be nearly as lovely in physical appearance,
but it sure does get the job done more than adequately all the same...

[Pacific Stereo]

Both are excellent but different-sounding products. The 850 doesn't have as much power, but it also lacks CMOS switching and thus I think is a little more transparent. I like them both.

[Tasuke]

yeah, it would have been a handsome piece. it would have stood out like a sore thumb against all thee black in my computer setup,
but it sure would have looked and felt good doing it, a solid construction with what appeared to be and actual wood veneer overall, and actual glass for the display face.

hit all that wood veneer with a good OLD ENGLISH polishing job, and that receiver would have SHONE, probably sounding equally fantastic all the while...