Cuda's SX-850 Refurb Thread

[cuda440]

I picked up an SX-850 last summer off CL, a bit of a mess inside, but a pretty good price I thought. Ran it for an hour this morning to make sure it still works as it did when I purchased it. Scratchy balance control, adapter switch needs cleaning, and a bulb is out, but that's it. Starting on the refurb now.

First up is the power supply (AWR-101), but I have a question.

When I put together the parts list for this one some time ago, the common replacement for the 2SB507 (Q2) was a KSA940TU, which I have in stock, but I now see the MJE15033G is recommended as the 940 is going obsolete.

Any reason not to use the KSA940?

I know I like to see picts of refurbs so I'm going to post a bunch of this one shortly.

 

[cuda440]

Here she is as I got her last summer. You may notice the grill is missing from the case - I sent it with the grill from the SX-1050 to the powder coater since there was a minimum order that I had to fill. Should get them back soon.

Anyway here she is. The front.

 

[cuda440]

The wood case is not in bad shape. A few scratches but no water spots or gouges. I'll refinish it as the very last step.

 

[cuda440]

And now around back. Plenty of rust so I'm thinking she spent many years in a garage. Here in CA we don't get humidity (unless you're on the coast) so maybe it was on the floor of a garage near a vent. The jumpers are rather ugly.

 

[cuda440]

Now to the inside. Spider webs can be seen if you look closely. This is a first for me. Rust spots scattered about on the chassis, especially around and including the transformer. I'll strip and repaint the top of that. The clamps holding the filter caps and other various painted black pieces need cleaning and painting, if not stripping as well. Fortunately the rust barely made it onto the back panel, a couple of spots right at the top edge.

This morning I put CLR on some of the rust spots, which takes off a good part of it, but it also removes the cadmium plating. Other that doing a rotisserie restoration and sending the chassis out for replating (not worth the effort or cost), what is the best practice for treating the problem?

 

[quaddriver]

<insert popcorn eating thingie here>

my uncle gave me a mint 850 he got from the local gw for $20 that works and lights...I was gonna do a bottom up restore so I will be watching....be verbose. list your subs! take zillions ofpics!

PS: KSA940TU is still around. and its compliment which I have tossed into smaller amps. I expect the MJE device is meatier and hence will survive the environment better but I defer to the experts.

 

[cuda440]

Here's the power amp. I didn't think anyone had been in here before me, but a close inspection this morning when I measured/set the DC offset and bias revealed that the amp board isn't sitting in the two slots that hold the bottom of the board in place. No evidence she took a severe blow that would jar them loose.

The power transistors look kind of grungy, at least the top ones exposed to the elements. Not planning to replace them, just apply winter rules (clean and place) with new grease of course.

 

[cuda440]

A friend gave me a Mitsu RA-20 and a Pioneer PL-500 (to get the 500 I had to take the 20), everything else I've purchased. I've done well I think, though never anything as good as your uncle's score.

 

[cuda440]

Here is the inside of the bottom cover. Notice the strange pattern of corrosion, especially in the last pict. Looks like a metal eating termite has been there. Anybody able to explain what's happening here?

 

[cuda440]

And the rest of the underside. Pretty clean, as most are.

 

[cuda440]

Lastly, check out the underside of the filter caps. One is clean and the other is not. I've only refurbed 5 units (SA-9500ii, SX-737 (two), SX-727, and SX-1050) in my short career and I have yet to encounter a cap of any size that had obviously gone bad and was leaking. Might I have one now? The residue is dry and the color of chocolate powder.

 

[cuda440]

Oh and can anyone tell me what kind of stripper to use on the black painted parts like the bottom cover, the filter cap brackets, and the top of the transformer?

 

[tjg79]

It appears to have a previous owner's ssn engraved on the back. That's a likely indication that the previous owner was in the military service. It was recommend practice for the troops to engrave their property with their ssn. So, that receiver has likely moved a lot (possibly overseas) during it's life. The strange pattern of corrosion is due to fine scratches in the coated surface with subsequent storage in a damp area, likely near the door or a vent on the garage cement floor as you suggested. I've got a tool box that has a very similar corrosion pattern that was stored in my garage.

 

[THD]

Here she is as I got her last summer. You may notice the grill is missing from the case - I sent it with the grill from the SX-1050 to the powder coater since there was a minimum order that I had to fill. Should get them back soon.

Anyway here she is. The front.

I'd rather see the Cuda

 

[cuda440]

So, looks like someone has been here (the power supply) before me. The photo below shows Q8, Q9, and Q5. The two mounted on the heat sinks are not 2SD313 as the service manual calls out, but T D234 something.

The lone transistor below (Q5) is a 2SC1845, not the 2SC1313 original.

This is a good time for me to ask about Q5 anyway. The BOM I found for the SX-850 has the Q5 replacement as a KSC2690, yet the other 2SC1313 originals on this board have the KSC2383 as the replacement. The 2690 is a beefier part, so a better choice for the job it has to do?

 

[cuda440]

More curiousness. I have found an original cap, C14, that has the value 47uf 25v and not the 220uf 10v as in the SM. Doesn't look like it was part of the previous work done.

Another AK'r found the same thing back in October on his SX-850 (documented here:http://audiokarma.org/forums/index....x-850-protection-circuit-troubleshoot.840387/)

My board is labeled AWR-101C. Anyone else have a different rev/part?

 

[cuda440]

I completed the power supply this morning and the unit powered up properly on the DBT, and again without it. I measured the voltages coming off and they were all within spec except for pins 2/3. The SM specifies 5.4v, but I found 7.3v. This part of the circuit is exactly where C14 is, so could the 47uf (I replaced with the same value as advised in that other thread, instead of a 220uf per the SM) here account for this difference?

 

[tsd71]

Use the KSC2690 for Q5

 

[tsd71]

More curiousness. I have found an original cap, C14, that has the value 47uf 25v and not the 220uf 10v as in the SM. Doesn't look like it was part of the previous work done.

Another AK'r found the same thing back in October on his SX-850 (documented here:http://audiokarma.org/forums/index....x-850-protection-circuit-troubleshoot.840387/)

My board is labeled AWR-101C. Anyone else have a different rev/part?

I've done many 850-950 and the 47uf is used predominately, just replace with same value that is there

 

[Watthour]

My analysis of the circuit (take that for what it's worth) is that C14 is pretty much running on clean DC at that point, and it is fairly well clamped by ZD11 (14V zener) so the differences are going to be driven by the voltage divider created by R21/22, and the cap is just riding along smoothing out any switching variations for the two trannies. Given the forward voltage drops of Q9 and Q10 may be slightly different in replaced components, that might account for some variation in what shows up at Pins 2 & 3 on the supply. All you can do is verify the 6.6V at the base of Q10 and adjust R21/22 to get it there. If those resistors are 20%, that could also drive some variation.

Keeping it all in perspective, it shouldn't make a great deal of difference, since the result is that you might end up with slightly brighter bulbs in the function/speaker indicators.