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Pioneer SX-850 DC Offset Adjustment

OK, thanks. I shall proceed to recap the power supply.

I wonder if this is related...Fairly frequently this happens: maybe 30 seconds after turn-on, the tuner section, both FM and AM, will stop working & will not produce sound and the tuning meters quit working. If I shut the receiver off & turn it back on again, tuner functions will work again. Would new caps in the power supply board likely resolve this too?
 
one never knows until it is tried. i would watch the 13v supply Q8 may be failing. it could be hurting the 5.4 and 51.5 / 36.5 / 28v supply too.
 
Getting a BOM together to buy parts is not as easy as expected due to two parts being back-ordered. Are the following transistor substitutions acceptable?

1) For AWR-101 Power Supply Board, Q1, Q8 & Q9: substitute 512-KSC2073H2TU for the back-ordered 863-MJE15032G. This substitution was suggested in another SX-850 thread, yet these transistors seem to have different characteristics; and

2) For EQ board AWF-011, Q1 & Q2, and Protect Board AWM-062 Q5: substitute KSA992FTA for the back-ordered KSA992FBU; These transistors seem identical except for the substitute part having kinked legs (which seems easy to overcome).
 
Yes, substitute KSA992FTA for the back-ordered KSA992FBU

Beside the kinked legs, the packaging is different:
TA = Ammo Tape
BU = Bulk
 
Q1, Q8 & Q9: substitute 512-KSC2073H2TU for the back-ordered 863-MJE15032G.

863-MJE15032G is the equivalent of carpet bombing the function - i.e. hard to break.
KSC2073 will do quite nicely, and be robust, but it cannot be AS badly abused as the 15032....
 
Board rebuilding is underway. Transistors Q1 & Q2 on power supply board are mounted to a heat sink, see photo. What's the correct way to remove the transistors from the heat sink? It looks like a screw and old, dry thermal paste hold things together and that once the transistors are de-soldered, removing the screw and breaking the bond made by the paste should be relatively easy. Does this sound right? And was a mica insulator likely to have been used here? tran.jpg
 
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sounds right and YES, a mica insulator!!

Looking at the oval shape of the white area, there ARE mica insulators in use.

Q1 collector (& metal tab) is at +62v dc, Q2 collector (& metal tab) is at -62v dc

never should the twain meet!!!! :yikes:

The screws, when you look close, are threaded into brass receptacles mounted in plastic insulators, thus they are isolated, without needing shoulder insulating washers to isolate the screw.

upload_2022-7-27_19-27-37.png
 
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All but one of the electrolytic caps on the power supply board were replaced. Preliminary testing & review of the work done suggests that the receiver's two remaining problems were resolved (intermittent shutdown of tuner section and thump at shutoff). Hooray!

The C14 cap was not replaced due to a found discrepancy: All written documentation, from both Pioneer as well as from other AK users shows it to be 220uF/10V. It is actually 47uF/25V and is original. Any ideas as to what this cap's function is? Could it be that in this case, such discrepant values don't matter?
 
After looking around some more, I found that Mark already answered this discrepant cap issue in another thread.
 
The work is done, for now, anyway. All issues are resolved and the SX850 sounds terrific and works flawlessly. Power supply, protection and EQ boards were recapped and the small signal TRs on power supply & EQ boards were replaced. Bias & offset are spot-on. Since it works so well, I'm hesitant to go further, yet it's tempting to replace the other TRs and to rebuild the other, as-yet untouched boards. Maybe it's a dumb question, but how do you decide when to stop?
 
well, your flat amp has 2sa725's in it, they are replace on sight.(ksa992)

the 2sc1312's in the tone amp sometimes get noisy. (ksa1845)

the caps there are also getting old, and drying out isn't their only failure mode. They can just plain wear out.

What we call an electrolytic cap is the aggregation of about a page and a bit more of various chemical reactions to "look" like a capacitor of certain value.

Industrial spies lost the "and a bit more" when they sole a neat new formula at the end of the last century..

That resulted in the "capacitor plague" past the turn of the century. Cost a lot of people good computers.
https://en.wikipedia.org/wiki/Capacitor_plague

Don't say "chemistry doesn't matter".
 
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