SX-636 - Restoration Help

[68krazy]

Hi everybody!

I am in the middle of restoring a Pioneer SX-636 receiver. When I got it, there were multiple blown fuses in the power supply, a dead channel, and very faint sound out of the working channel. Thanks to reading through tons of old threads here at Audiokarma, I was able to get her up and running 100% again!

The fuses and dead channel were due to a shorted output transistor. Once I replaced with modern Onsemi, she came right back to life. Then I went around re-building one circuit board at a time using new Onsemi for all transistors, Panasonic NHG for general caps, and Nichicon KL for low-leakage. She truly is sounding wonderful now.

SX-636

• 68krazy
• Mar 17, 2023
• pioneer sx-636

My Pioneer SX-636, mid-restoration. March 2023.

I am going for a full-blown, as-meticulous-as-possible restoration, and I have some components I'm having trouble figuring out replacements for. If anybody knows, would you mind pointing me in the right direction? Here's what I need help finding replacements for:

Power Amplifier board:
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Does anybody know of tested, or at least potential, modern replacements for the following components?
-2SA777 transistors (2)
-2SC1509 transistors (2)
-W06A diode (2)
-1S2473 diode (2)
-2 potentiometers/variable resistors (VR1 and VR2, I believe 100 ohm?)

Tuner board:
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I am afraid of messing with this thing but would like to be as thorough as possible. If anybody knows what they are doing with tuners - please chime in!
-2SC945 transistor (2)
-2SA763P or 2SA725 (2)
-2SC461 transistor (1)
-1S2076 diode (2)
-1S2473 diode (1)
-Additionally, does anybody have guidance/wisdom for aligning the tuner pointing dial with the station? The lower half of mine is off by about ~.4. Top half of the tuning range seems fine.

Any guidance/info/wisdom would be much appreciated. Thanks everybody!

 

[markthefixer]

First off, tuners can ruin your day.
These are ok to swap out:
2sc945 >> ksc1845
2sa763 or 2sa725 (725 is a shoot on sight transistor!!) >> ksa992

Don't even TOUCH (or even the stuff around them)
2sc461, 2sc535 & 2sk55 transistors !!!!!!!!!!!! It WILL ruin your day.

1s2076, 1s2473 >> 1n4148 but I would leave them alone.

power amp
W060a diode - do not touch.

1s2473 >> 1n4148 this is a "shouldn't" touch

2sa777 & 2sc1509 are complementary (opposites) drivers. ksa1013 & ksc2383 ** don't mix a & c part numbers**
2sa777 >> ksa1013
2sc1509 >> ksc2383

100 ohm pots - you want single turn, this sets idle current, and if goes open circuit, it could be the reason you had a blown output.
The other possibility is the ksc1451 - a REAL troublemaker - get rid of it!! >> ksc3503

Someone should have a better fitting part number 100 ohm idle current pot than I was using.

 

[68krazy]

@markthefixer, thank you so much for chiming in. It was mostly your posts that I was referring to when I said that I had been reading through old threads. Thank you for all the knowledge you've been sharing over the years - there is no way I'd be putting the finishing touches on this receiver without you and everyone else on here.

Alright, let's proceed!

[...]

Don't even TOUCH (or even the stuff around them)
2sc461, 2sc535 & 2sk55 transistors !!!!!!!!!!!! It WILL ruin your day.

1s2076, 1s2473 >> 1n4148 but I would leave them alone.

power amp
W060a diode - do not touch.

1s2473 >> 1n4148 this is a "shouldn't" touch

noted.

I got the KSC1451 swapped out along with the new output transistors. That's an interesting observation on your part - maybe that's what put her out of commission in the first place? It's hard to say because I rebuilt the entire power supply and a lot of the power amp board all in one go.

Another question: I want to get all the tantalum capacitors in the unit out of there as well. I work on old computers a lot and have come to distrust tantalums. Can you (or anyone) recommend a good replacement for tants? I've been looking at film capacitors but they don't seem like they are polarized. I'd like to replace with something as close to the original polarized tantalums as possible.

And if anybody has a lead on a replacement idle current pot that fits well - I'm all ears.

 

[markthefixer]

ideally NO capacitor should be polarized.
A non-polarized cap of the correct capacitance and an adequate voltage is a better replacement for any polarized cap.

swap film caps for tants? perfect! superb! Yes!! Probably even sound a bit better....

Capacitance has to do with surface area of the capacitor, two parallel plates.
Electrolytics get more surface area to store electrons on by etching the wound foil and turning it into a microscopic sponge.
To access that surface area brings in the concept of liquids, chemistry and unfortunately, polarity.

The only detraction about film caps is their physical size and therefore, cost.

 

[larryderouin]

WIMA film caps are very popular as they are smallish for their capacitace and voltage. They aren't any more expensive than comparable other brands. In some cases quite a few cents cheaper. once you get over 4-5uf in size at 50-100v they grow like any other cap.

 

[68krazy]

Gentlemen, great news. I have completed the following using your guidance:

-Replaced the 2SC945 and 2SA725 in the tuner. There were indeed two 2SA725s in there - glad we got them out!
-Replaced 2SA777 and 2SC1509 on the power amp board
-Replaced 100ohm pots with modern Bourns equivalents
-Replaced all tantalum capacitors with Kemet film (I know, not WIMA, but it's what Digikey had)

End result: everything sounds very nice, tuner continues to pull in lots of stations (and is sounding VERY warm and smooth, might I add). I also re-transistored the tone control board and it made more of a difference than I thought it would -- the receiver sounds smoother, slightly warmer, and a bit bassier than before.



We have two remaining issues before I can call it good:

-First, any guidance on getting the pointer dial to align with the actual station the tuner is tuned to? The top half of the tuning range is fine, but the bottom half is off by ~.2 to .4. Is it safe to try to fine-tune this, or is this a "tuners will ruin your day" item?

-Second, and more importantly, we are having an adjustment issue on the power amp board. I am not sure what VR1 and VR2 adjust, is it idle current or bias? Either way, the service manual tells me it should be 20mv, but I can't get it to go any lower than 25mv on either channel. Is this a serious issue? Did re-transistoring the power amp board throw that adjustment range off? If so, anything I can do?

 

[68krazy]

Edit - removed this post, I was describing what may be a speaker issue. Will need to track down a different set of speakers to do A/B testing then will come back here if issue persists.

 

[markthefixer]

VR1 & VR2 adjust IDLE CURRENT.
there is NO dc offset adjust.

On semi transistors has been migrating the Fairchild transistors to their chemistry, which has a lower overall Vbe by a few millivolts.
This upsets the calculations done back then by the Pioneer Engineers. Idle currents are more easily reached, and there is slightly less voltage drop per stage.

You could remove D3, substituting a piece of wire, and then seeing of the 100 ohm trimpot has enough adjustment range.

Frankly I would leave it at 25, and not doing some risky fooling around. If that goes open circuit - it's the smoke and flames thing....

 

[68krazy]

Hey all - checking in again after a few months. I didn't take this restoration any further for a while because I hooked the SX-636 up in the living room with some KLH Model 6 and was enjoying it too much! I left the idle current at 25 on both channels and haven't had any issues.

I've got her back on the bench though because there is a lingering problem. The channel that originally had the blown output transistor (right channel) is quieter than the other channel (left channel). The highs seem muffled and the bass is not as strong. I've ruled out speakers and sources - this is consistent across multiple different sets of speakers and happens with all inputs, including the tuner.

How would I go about troubleshooting this last issue? Are there any components I can easily check that might have gotten damaged when the output transistor went?