Pioneer SX-939 Recap

Bass blaster. It really depends on where the transistor to be replaced is. There is a list for the 636 in this link. Copy it and compare it to your unit. Some had minor changes to value. Increase in voltages is fine. Then check on Mouser for availability but don't purchase yet. Put the list up on a separate thread for your 636 and have the list vetted. There may be some small changes due to obsolte parts, etc. Once it's been vetted, THEN go get the parts. http://audiokarma.org/forums/index.php?threads/pioneer-recap-lists-sticky-suggestion.457663/
 
FM MUTE will only work effectively with an antenna connected, and with you able to get both weak and strong signals. FM MUTE is a cut off for weak signals and inter-station hiss, not a mute switch that you would associate with dropping the volume so you could say answer the phone. It will mute any signal that is below a set threshold when on and also inter-station hiss. It is not adjustable.

I agree with rcs to check voltages on the Power supply against the scat 1st and address any problems there. Think of the power supply like your car engine. If it's NOT running on all 4-6-or 8, it's gonna run like shite. When doing one of these it's best to do the power supply 1st, then the Main amp, then start at the inputs and work towards the amp. Replace any problematic transistors (2sa726,& 725's and their complimentary partner if any). Trouble shoot the protection board, (look at sx-1010 rehab projects also as the 939 and 1010 share the same protection board. Everything there that applies to the 1010 applies to the 939.)

When checking the offset and bias on the 939 while on a DBT, when finished, turn them DOWN all the way. You'll fry the amp if you don't due to the increased voltage. then readjust when on full line voltage. Set it for 25ma as per the 1010 update. 50ma is a mistake the PIONEER Engineers made in their math and Mark The Fixer found it and explained why it was high. I would get new pots for the bias and offset. Like RCS said the old ones could be and more than likely ARE flaky.

Check the old pot with 2 dmm's set up on the correct pins. Rotate slowly and check operation one side against the other. Shoot some DeOxit F-5 (F-5 is the Faderlube not F-4) into the pot and work it back and forth 50 times and then check again. If it's now running right keep it for a spare (sealed in a plastic bag). Do the same for the new one. Note that some PIONEER Volume pots do get offset. See if you can get inside (take it apart) and check the wipers for play on the shaft. You might be able to work out the play, or not. The only place to get factory pots is a parts unit. So you're kind of stuck trying to get and keep them
working. A lot of manufacturers used the same brand parts as pioneer and have the same problems. So it's not an isolated problem. A lot of people just chalk it up to the age of the unit.

Many thanks larry and rcs! Lots of good info.

I need more info on how to check/adjust the offset and bias. I am assuming these settings are performed on the 4 pots on the power amp, according to page 19 of the service manual? (using 25mV, instead of the written 50mV).
What is a DBT?
I'm going to just get new pots for the bias/offset, rather than test/clean the old ones. Is it a good idea to get all new pots for the power amp and tuner boards?
Regarding the DeOxit F-5, can I use CRC QD Electronic Cleaner? I've got that on hand.

I still think it was a good idea to do the recap first, since those really old caps are pretty much guaranteed to be bad.
 
DBT ( DIM BULB TESTER) a series wired light bulb between the wall plug and you unit. Easy to build, cheap too. It's basically a load limiter. For the 939 a 100W bulb will do nicely. It should glow bright for a second and then dim down. Any shorts created will cause the bulb to glow very bright as it's taking a substantial load off the unit and helping to preserve the transistors. Google for building instr.

Yes the 4 pots on the amp board. Change ONLY THE AMP BOARD POTS Do NOT change anything on the tuner
CRC Electronics cleaner is fine. If you have the CRC 626 Electronics lube use that after the Cleaner as it washes out the original lube.
 
Is the DBT a must have? I do have the parts for it already, just wondering if it is needed.

I was already leery about touching anything on the tuner board, which is why I haven't.
 
The DBT could save your amp from smoking. You do not need it for checking only if you are going to try to adjust those old bias pots.
Before you do any pot adjusting/changing just measure the ops(output stage) offset voltage and bias as per the manual.
 
Well I did all I could do with it, and finally sent it out to a pro to be fully restored. Still waiting to get it back.
 
I'm really not sure why KSC2073 keeps being mentioned as a replacement as it is only a 1.5A, 25W part. The original 2SD313 is a 3A, 30W part. Use something beefier :thumbsup:.
 
MJE15032G is what I've seen mentioned and what I use. MTF has been seen to say that it handles the heat better amongst other things. On another note, isn't the KSC2073 NLA or have I missed something?
 
The MJE15032G is a high speed device (30 MHz) vs the original 2SD313 (5 MHz). It does make a great driver/output transistor in some circumstances though.

https://audiokarma.org/forums/index...eries-pass-transistor-in-power-supply.688206/

The KSC2073 is still available from Mouser and others but as l said l wouldn't use it to replace a 2SD313.

Fair point. My only reason for recommending was the fact I've seen them used in a lot of BOM lists as regulators. Used in the SX-1980 for example on the +/-80V supplies but perhaps that was due more to their voltage tolerance...
 
My only reason for recommending was the fact I've seen them used in a lot of BOM lists as regulators. Used in the SX-1980 for example on the +/-80V supplies but perhaps that was due more to their voltage tolerance

Fair enough, maybe they are more suited to some supplies than others.
 
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