Pioneer SX-950 Restoration
This Pioneer SX-950 arrived recently as part of a trade. Can you tell we love these Pioneer SX-_50 units?
The unit was a bit dirty, but overall in good condition. It functioned well except for the phono stage distortion, scratchy controls, and some of the lighting being out. The case was a bit dried out as well. A perfect candidate for a restoration!
Upon opening the unit, dust just poured out of it.
After lightly brushing out the big, loose stuff, we took compressed air and carefully sprayed out all the fine dust that blanketed most of the top side of the unit. As annoying as that can be, it is nice seeing all that dust because that means the unit was never opened… and THAT means no one has ever needed to ATTEMPT to repair something.
This is a nice, untouched, original unit!
The original filter caps were replaced with direct replacements… 22,000uf 63v Nichicon caps. Same dimensions as well. They fit perfectly.
However, you can see that the capacitor terminals were different, pin vs screws. Slight modification will be needed, but nothing too crazy or difficult. All the wires were attached to ring terminals that will be clamped down by the caps’ screws. The original wiring can be brittle, so the wires were cut and re-stripped at the end to expose fresh wiring for a better connection. The original bleeder resistors were still at spec and just transferred over to the new caps. These prevent the capacitors from staying charged while the unit is off, which prevents thumps when the unit is powered on.
These came from Mouser with clamps that had the same chassis mount dimensions as the originals, which is nice. Except for the termination difference (which I prefer), these caps were about as drop-in replacement as you can get.
Next was the power supply board. A LOT of parts get swapped on this board.
All of the electrolytics get changed to Nichicon PW series, computer-grade high-temp/high reliability capacitors for filtering.
The zener diodes get replaced with Fairchild modern equivalents. All 4 of the heatsink mounted regulators also get changed and new thermal compound paste is applied. The smaller transistors all get replaced with modern equivalents as well. This board, similar to the SX-1010, has the heat producing power supply board mounted underneath the unit… meaning heat builds up as the unit runs, and has less opportunity to escape. Over time, these parts get a lot of abuse. To prevent further complication, we swap out all of parts and beef up some with heavier duty components for better reliability.
When the unit is complete, we triple check voltages to make sure the unit is running as it should… which it is!
The phono EQ board is next. Before moving on, we need to sort out the distortion coming from the phono stage.
In this case it was simple. The first transistor in the phono signal path is the 2sa725. Notoriously bad, and we swap them out with new, MATCHED, KSA992 in every unit that has them. They get noisy with age. A safe bet this was the culprit, and it turned out to be correct. The other 2sa1313 transistors also fail often with time, and those get swapped with new KSC1845.
The old gray electrolytics are filtering caps, and get Nichicon PW caps. The light blue and orange caps are low noise, so WIMA caps go in for the 1μf input caps, and the 3.3μf caps are replaced with Nichicon Fine Gold caps.
Next was the protection board. This board also gets many components swapped out.
One main thing that everyone on the internet recommends is adding a flyback diode to the relay circuit. This prevents any spikes of voltage that occur from the relay operating from doing any potential damage. An easy upgrade!
It is very important to make sure to get the polarity of the 1N4004 diode correct! Correct orientation is: cathode (banded end) goes to pin 10 and the anode to pin 9.
As for the rest of the protection circuit, Nichicon PW series go in for all of the old gray filtering electrolytic caps. The two small blue, 0.22μf Sanyo caps get replaced with WIMA caps. The transistors also all get changed to new, high quality, modern components. The original 2SC1384 that drives the coil get changed to a Fairchild KSC2690.
Another common trend we have seen cause problems on some of these Pioneer SX receiver Protection boards are diode failure, causing the unit to kick in and out of protection. We change them all, D1-D7, to new 1N4148 diodes. However, D8 we beef up to a 1N4004 diode.
More of the same for the tone amp board. Nichicon PW for the two filtering caps, and the low noise/tantalums get WIMAs or Fine Gold/KL low noise caps.
The 2sc1312 transistors are changed out for KSC1845.
The flat amp gets the same treatment as the tone amp board.
LEAVE the 2SK30 FETs alone!
You should swap out the 2SA725 with KSA992. We also change the 2SC1885 with KSC2383.
The amp boards are easier than they look. The 330μf caps get changed to Nichicon PWs. The 33μf caps below the trim pots also gets a PW series cap. The 2.2μf, light blue, Sanyo input caps get changed to low noise Nichicon Fine Gold caps, and the 1μf Sanyos get changed to WIMA film caps.
The big thing to change on the amp boards is the 2SA726 input pair transistors, Q1-Q4. These often fail and become noisy, sometimes creating pops through your speakers. Very bad! The originals are matched, but the new ones don’t have to be (this unit has DC offset adjustment). As long as they’re close enough to balance the DC offset, no worries. Units without DC offset adjustment, matching input pairs in VERY important.
The unit was very clean except for some dust, but that dust can stick to the old thermal paste. This can prevent it from dissipating heat as efficiently as it could. The outputs were all pulled, one by one, and cleaned. The old paste was removed, replaced, and new Mica washers were added.
It is VERY IMPORTANT to make sure you put the outputs back EXACTLY as you found them. You definitely don’t want to short any of the output transistors.
The dial lamps and indicators were all changed to custom made, soft-white LEDs. Prevents heat, more reliable, and maintains its soft, warm glow.
The case was pretty dried out, but in great shape otherwise. We applied 3-4 coats of Howard’s Restore-A-Finish, then a few coats of Feed-N’-Wax. The grain really pops, it looks fresh, and smells like oranges and honey!
The above left is the case when the unit arrived, the other two images are after a little elbow grease was added.
Feed-N’-Wax is a good thing to have one hand. Every month or so adding some will help keep the wood from drying out and looking color.
That’s about it. The controls were all flushed with Caig DeOxit, and re-lubed where needed with a light amount of Caig Fader Lube. The inputs were all tested. Offset and bias quickly checked to make sure the unit was safe and now the unit was ready for final touches. The tuner was dead on, so it was left alone for now!
The unit was run for about 15 minutes and DC offset and bias adjusted once the unit got up to a good operating temperature. It has run in for a few days after the work was finished to burn it in and test it. It sounds just as good as we’d expect! These SX-750/850/950/1050/1250 units are amazing. We prefer them to the later X80 series units.
So this unit had the following service done:
- Main filter caps replaced
- Power supply recapped w/ new transistors
- Protection board recapped with new transistors and diodes
- Amplifier recapped with new input transistors, alignment done
- Phono section recapped with new transistors
- Tone board recapped with new transistors
- Preamp board recapped with new transistors
- Switch assembly board recapped
- All controls cleaned and relubed
- Tuner checked and works great
- LED conversion on all lights in the unit
- Wood case restored and waxed
- Faceplate, glass, and knobs cleaned
- Burned in and play tested
This item will be for in our store shortly!