Hi Guys,

Fired up the SX-850 after an extensive cleaning with Deoxit. Upon close inspection all components appear physically good, no burned or darkened PC boards. Wirewraps looked fine. No blown fuses. All fuses of the correct rating.

However, after power up I heard no relay clicking in and no sound. All lights on the receiver are working, and the tuner seems to react normally to antenna signals.

Any pointers on what to look for? Perhaps resolder a connection or two? Or can the relay be taken apart and cleaned?

Thanks in advance,

crooner

Is there a headphone jack you can monitor?

Yes, as you can see in the picture, I have a set of headphones connected to the receiver.

There is no sound either. Does the headphone output operate independently of the relay?

Regards,

crooner

Not a Techie, so please take this for what it may be worth.

Could be Relay or Outputs shorted?

Don't know much bout De-Oxit, but was everything completely dry prior to fire-up?

Well, Deoxit is non-conductive, AFAIK. Caig Labs even mentions spraying it with the equipment energized. So no biggie here.

I did spray some DeOxit on the relay. It penetrated the contacts.

Unit looks really good. As I mentioned before, no blown fuses. One would think a fuse would blow if the transistors short out.

I suspect it's a cold solder joint or bad wirewrap connection somewhere. This is the weak point of Pioneer's 70s equipment. Wirewrap connections everywhere!

Regards,

crooner

I'll ask 2 hopefully answered questions, just for the halibut.....

1) Tape monitor switches off?

2) Pre/main rear outputs jumpered?

Sometimes we miss the obvious........,

But, WTH, better to fess up to an oversight than to sling solder.

Well, I would guess if the tape monitor or pre-main jacks would be the culprit, I could still hear the relay clicking.

But, further examination has determined the cause:

A tiny half watt resistor close to the large heatsink assembly on the main regulated power supply board, is toast. It is not exactly black or burned. However, the coating is flaking.

I will try to take a good pic with my humble camera.

If this resistor is indeed toast, then the regulating transistor right next to it is probably burned out as well. It seems to me the receiver had service in this area before. The TO220 encased devices (Sanyo brand) appear to be replacements. Strange that these would go, given the generous heatsinking.

I've got to many projects right now, but eventually I will get the board out and do a wholesale replacement of parts. It shouldn't be too expensive.

Best regards,

crooner

Here's a shot of the main regulator board.

The resistor that is discolored/flaking is indicated with an arrow.

The heatsink shown right next to it, contains the two TO220 devices that are suspect.

Powering up showed this heatsink to be cold to the touch, confirming the transistors are shot.

BTW, the other two TO220 transistors (also shown in the pic), have individual heatsinking. These were warm to the touch when unit was powered up.

It wouldn't be a bad idea to take your
ohm meter and check all of the
transistors on that board. Also check
the rest of the resistors in that area
to make sure none are out of tolerance.
If the smaller electrolytic caps have never
been replaced this would be a good time
to do it, they wouldn't cost you much to
switch them out. A leaky cap can ruin a
perfecty good repair.

Let us know how it goes.

4 out of 5 times it's a regulator on a Pioneer, but it's generally a pretty easy fix. All output voltages on this need to be checked. If you don't know what that board is supposed to output, get a manual.

I'll take the board out and replace the two transistors and other associated passive components on the board.

But, what's the best way to deal with wirewrap connections?
Are these wrapped and then soldered or just machine wrapped?

I would like to take the whole board out, since it would be much easier to service this way.

Regards,

crooner

I don't recommend removing the board. Once wirewrap is 'unwrapped', it has to be trimmed and re-wrapped. There usually isn't enough wire to do this, so the only option is to replace the wire. Wayyyy too much trouble.

One way or another you should be able to find a way to flip the board over to get to where you need to.

Thanks for the suggestion, Echowars

I will work with the board on the set.
There should be enough space.

Thinking about replacing the regulator transistors on the heatsink and the surrounding passive components on the board, including the charred resistor. Hopefully it should work ok. I don't feel the need to replace all the parts on it, specially the large caps (glued to the board) near the rectifiers.

This is actually my first experience with a Pioneer repair. I have been lucky with my earlier Pioneer pieces, since they only needed cleaning.

I have repaired a McIntosh MA5100 integrated, and C24 preamp in the past. And I completely rebuilt a Dynaco Stereo 80 amp back in my earlier days. None of those units used wirewrap.

Wirewrap was apparently introduced by Scott in the late 60s, and adopted by the Japanese the following decade. Perhaps they bought the tooling when Scott folded in 1972. Wirewrapping, Scott explained in the early ads, was used in military and aerospace applications and was a way to prevent solder joint failures.

I would have preferred plug in connectors such as those used by McIntosh and others. I believe Pioneer uses plug in connectors in the C-21 Series 20 preamp. Wirewrap on that unit is minimal.

Best regards,

crooner