SX-850: what to recap?

Big Ben

Active Member
So, as mentioned in http://audiokarma.org/forums/index....peaker-silent-until-volume-cranked-up.847285/, I recently acquired a serviced, but not recapped SX-850. Besides the relay issue above, the device sounds good. I checked the DC offset at the speaker terminals and got something around 10mv in both channels, which would indicate to me that the fellow who sold it to me indeed did some adjustements...

That being said, since I’m about to make a mouser order for a new relay for the issue above, this would be a good occasion to also order parts for a partial, or full recap/diode/transistor replacement. So the question is what is worth recapping on a working unit? From lurking around here, it looks like people are usually advised to do the power supply and protection boards at least. Is that right?

As a note, while the unit works well, I do get quite a bit of noise when listening over (low impedance) headphones. Some of the noise is constant, some increases with volume. Lacking a unit to compare, I can’t tell if this noise is a feature of the 70s design, or a sign of leaky/failing components...

Thanks!
 
If you are going to do that much you should do the whole damn thing, the 850 is a nice easy unit to work on. There are current BOM's for the 850 & 950 all over Audiokarma
 
The above. If you're gonna do it, do it right. This unit needs a lot of transistors replaced, as well as the zeners and trimmers you mentioned. I up the wattage on quite a few resistors in the PS too.
Careful with the BOM's, I've had to add or remove a few parts every time I use one, most times I just make my own from the schematic and looking at the unit. Safer.
For the relay I know there's the cool standoff trick, but the case is deep enough on the 850 that you can use the flanged version and simply mount it in the bottom instead of it coming through the midsection.
 
So I started preparing a BOM to order from mouser. I used http://audiokarma.org/forums/index.php?threads/sx-850-restoration-record.831103/ as a reference. For the power amp board AWH-059, Q5 and Q6, that thread suggests 512-KSA1381ESTU as replacement for 2SA818/2SA898. Unfortunately, 512-KSA1381ESTU is currently backordered. I guess waiting 6 weeks would not be the end of the world at this point, but would anybody have a suggestion for a suitable alternative?
 
take a peek at KSA1142 and see if adequate....

I have not looked at the BOM, but do you need a compliment as well?
 
So, here we are: I replaced all electrolytic capacitors and all BJT transistors (except for the main outputs) on all boards except the tuner, replaced the speaker relay, dc offset and bias trimmer and adjusted the dc offset and bias. I had way more fun doing it than I thought I would :)

The flaky output on 1 side went away with the relay replacement as expected, and I'm generally very happy with how the unit sounds. However, the audible background noise over headphones is not gone. When listening on speakers, the noise is only audible when putting my ears right next to the tweeters, but it is fairly noticeable over headphones when no music is playing. The noise doesn't change with volume, and mostly goes away when unplugging the jumpers between the preamp and amp at the back of the receiver. The tone switch is off. I guess this may point to the noise originating in the flat amplifier? Is an audible background noise expected on receivers of this era? If not, any idea on how to investigate the source?

Thanks!
 
Try a pair of headphones with a higher impedance 1st, before blaming the 850. My headphones of choice since Jr. High (1969) have been Koss PRO-4A's (260ohms) or more recently PRO-4AA's (260Ohms). I also carry Koss Porta-Pro's(60 ohm) for listening on my "not so smart" phone. (I can't stand buds). I DO get more noise from the porta pro's than I do with the PRO-4AA's (2yrs old) and the Pro-4A's (50yrs old) on any full sized stereo I listen to, be it tube or Solid State.
 
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