Pioneer SX-850 Cleanup Thread

rave0035

rave0035

Disassemble first, ask questions later.
I snagged an SX-850 yesterday, along with a pair of CS-99a's for a great price. The amp is a gift for a good friend opening a tattoo shop in a few weeks - this is going to be the daily driver at the shop, displayed prominently in the lobby :)

Just documenting the before/after for fellow forum-goers. All functions work as expected, and all the lamps still light... Just needs a little TLC.

Tips on how to bring the veneered case back to life? I've used Restore-a-finish in the past with feast or famine results.

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Mike
 
If it's going to be a daily driver (or even if it isn't) it should really be recapped, at least power supply, power amp, and protection boards. Definitely replace the bias and offset trimpots, they get flaky in their old age and if the bias pot opens up it'll rail the outputs and let out the magic smoke.

And that looks like a 950 judging from the power amp board (4 emitter resistors per channel instead of 2, and wiring for 4 outputs per side.) I just worked on one not long ago. They're marvelous sets. She deserves a proper doing-up. :yes:

TP
 
It's an 850, they have the same amp board. One of the easiest Pioneers to work on, Good Luck!
 
Doh! I had the board from my 838 still in my head. ;) Surprising to see the 850 having four trannies a side like the 950...they put a lot of $ into that thing for a middleweight receiver. Even at full tilt that output stage would be loafing along, methinks...
 
I've got a LOT of vintage gear, my SX 850 is one of my favorites. It has a beautiful sound to it, and looks every bit the beast my 950's do!
 
The thing has four outputs per channel for 65 watts per cannel.

Of course that new stuf is better. Teice the power, half the weight, and can almost drive your speakers at 80 Hz.

And mine has subsonic problems.

Yeah, I wil just throw it in the dumpster and buy a new unit off Amazon without ever hearing it.,

(that was a calibration check for your sarcasm monitor. It should have read soewhere between 4.2 and 5.6. If not, check the calibraiton)
 
I'll recap 'er this winter. For the time being, she just gets a good bath and pot cleaning.

The belly of the beast:
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Grill gets a paintjob:
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Mike
 
OK, I think I've got some dirty relay issues. Left channel is distorted, and I sometimes can't get the audio to "pop" on until I raise it to a certain level.

I've seen a couple of threads on this. What say ye all... replace or try to remove and clean the contacts? How well do these relays take to being cracked open?

Mike
 
Nice job on the cleanup in Aisle 3...

The loss of signal could be the relay, other connections (speaker switches, etc.) or the controls. To determine whether it is the relay you might install mini-grabber or small alligator clip jumpers across the contact lugs, turn off all speaker switches, power up the unit, wait five seconds, and turn on the speakers. If the distortion remains it probably isn't the relay.

If the relay is still suspect, try opening the relay by releasing the plastic cover from the base. It may require cutting some glued joints. There is really nothing to lose since the relay is not hermetic.

Once opened the contacts can be inspected, aligned, and burnished (NOT sanded or filed). If it is determined the relay is beyond repair, replacement is not impossible.
 
Thanks, Watthour! So this is one of my first forays into the SS world. You're suggesting I use mini-grabbers to jumper the relay's lugs, essentially bypassing the relay switch altogether?

Mike
 
Can I ask what you painted the grill with? I have a 1050 whose grill is scratched and am looking to re paint it.
 
Duplicolor Engine Enamel - Ford Semi Gloss Black You can pick it up at any auto parts store. It lays on really nicely and is extremely durable. I would recommend prep work with a scotch bright pad, some 400 grit sandpaper, and 2 light coats vs. one heavy coat.

I personally love the sheen, but it does have a little gloss to it, so stick to satin if you're going for that.

I used Howard's Restore-a-Finish on the walnut veneer, with fantastic results, and metal polish on all the faceplate stuff up front after a light bath in warm, soapy water. Turned out minty.

Now if I can just get the distortion figure out... I'll jumper the relay tonight.

Mike
 
Also, to make absolutely sure I'm doing this right, do I have this right?

Relay.jpg
 
If you read my thread http://www.audiokarma.org/forums/showthread.php?t=655152, 2 months ago I've fixed a SX850 with protection issues:

The problems: differential pair transistors on the power amp, shifting the DC offset, and, once fixed them, transistors on the protection board were bad too. I've replaced them and the unit is stable now.

Check out the power supply board, check for broken solder joints. That "upside down" design seems to generate too much heat, damaging the soldering. I don't mean this will cause your distortion, it's just a point to check in this units.

I'd say: check DC offset, bias current, and DC at the relay (yellow and orange wires against ground)
 
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I strongly recommend that you check the amp adjustments.

I strongly recommend that you replace the amp trimmers before trying to adjust them.

As mentioned, they are known to go bad and when they do, a LOT of parts loose the magic smoke. Like almost all the transistors on that channel.
 
Update: jumpering the relay did NOT reduce the distortion. :( so much for an easy fix.

I'll get the readings that elnaldo suggested and report back.

blhagstrom: how ginormous of a job is it to swap out the trimmers?

Mike
 
The trimmers are right there on the front of the amp board.
Two white, two blue on your first photo of the insides.

Its been a while so I have forgotten how difficult it is to lift the amp block out and "open" it to get to the solder traces.
 
( I didn't replace those pots! !!! just the differential pair transistors!!!) I hope they will last
 
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