Category Archives: Pioneer SX-650

Pioneer SX-650 receiver

I had decided I needed a “proper” hi-fi system, so began looking on the local Craigslist for anything in a decent price range and of a style I liked.

Enter a modestly-priced ($65) Pioneer SX-650, found in uptown New Orleans from a guy in the hi-fi club who bought it as “working” but soon realized it was indeed not, and sold it as “parts”.

It was in good cosmetic condition, despite being grimy and some of the photo-finish on the corners of the wood coming away. I got it home and gave it a good old clean up.


Once acceptable to place in the living room, the top came off, and a rudimentary inspection given. Everything inside was very original looking, nothing hacked up or nasty looking. Brought it up gently on my Variac, connected to a couple old computer speakers.

First power up

At about 60 Volts the beast came alive! Pops and crackles out of the speakers, then I managed to dial in a local radio station. The sound was good, considering. Watching the current, I brought the voltage up slowly. At about 95 Volts the current began to run away, so I killed the power. Not soon enough, the main filter capacitors both boiled and vented. Needless to say, that’s an unpleasant smell and I was not the most popular person in the house.

Main filter capacitors

The caps are fairly large, considering the device. There’s the originals, side by side to a regular “D” cell battery for comparison. At that point I decided that if those ones had gone bad, the rest of the electrolytic capacitors on the board probably weren’t that far behind. Shotgun time! Got onto Mouser, and from a useful parts-list from AudioKarma purchased a big box of capacitors.

New caps

Started the long task of matching up all the capacitors in their bags and writing on each bag the location on the board that it was due to replace.

Organizing capacitors

All set up on the table, there was much of this, removing old, adding in new:

Kitchen table workshop
In bits again

Soon enough I had a box full of old capacitors.

Old caps

Comparing the sound of the device at each step along the way was surprising. I’d replaced the capacitors as per suggestion that some of them by the original design were running very close to their maximum tolerance.

Power amp board

Higher voltage rated ones with better impedance and crosstalk characteristics were fitted. Higher temperature ones were used where I could get them.
Soon, I had a set that was able to be switched on and left on.

FM on

With a rudimentary antenna, 100.3 KLRZ-FM coming in at 5 on the scale. Replacing the capacitors in the FM stage helped a lot in removing the FM stereo “hiss”. In that image, the scale illumination bulbs had all burned. Only the power and stereo lights were operational. I decided to try and see if I could use a fluorescent tube from an old computer screen to light the scale:

Fluorescent tube

It was too blue for my liking, as interesting as it was. That came out, in favor of some flashlight bulbs of the correct voltage and current rating from Radio Shack.
I had then started to try and track down the source of some pops and crackles. There’s a speaker/FET protection circuit built in. It monitors the resistance across the outputs and also times a period after initial turn-on to prevent damage to loudspeakers from the THUMP you get as everything charges up. The audio output goes through a relay, which I removed from the circuit.

Protection relay 1

Isporopyl alcohol to clean up the gold-plated contacts. They were quite dirty:

Protection relay 2

That helped a little bit but there were still crackles when I moved the speaker selection switch, so off came the front and out came the switch, a multi-wafer affair with some very dirty contacts.

Selector switch
Selector switch dirty

That got a clean up, same as the mode switch, which was the same but more complex:

Mode switch 1
Dirty mode switch

Careful use of wet-n-dry paper and DeOxIt D100 spray, along with cramped fingers:

Clean mode switch
All cleaned up and low resistance. Reassembled it in the correct order, then soldered it back in.

Mode switch fixed

Pops and crackles begone! I had some parts on back-order for the record player preamplifier boards (below):
The silicon was also going noisy with age, so in the interim, I built a stereo preamplifier board out of spare (and non matching parts) to a design I found online:
Preamp on breadboard

That allowed me to at least hook up and listen (with much background hum) to my records until the parts arrived:

Records playing

Everything set and tested:

Stereo scope

All set up and well within limits. It should deserve the sticker it received from a previous home, one of the government offices in Los Angeles:

Safety tag

Nice to have a little bit of history with it. Got it all tidied up next to my record player:


Looking and sounding good!