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Yet Another SX-939 Problem

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oldhemi

Member
I just received my ebay receiver in the mail. Boxed ok and the unit is in very nice cosmetic conditio with the exception of a cracked glass - 2 cracks running from the 2 buttons to the tuning knob. I can live with that for now.
But the biggie, hence this post is that it blew the 5Amp fuse after 2 seconds of plugging it in. The lights came on, this is when I noted the cracked glass and then powered off.
I have been scouring the board capturinging everything posted about fixing 939's before I even bought this thing. Mostly from Markthefixer and some old ones from Echowars. I have no idea why I did this, its not like I suspected anything wrong with this particular unit. Maybe I am psychotic, um I mean physic.
So, armed with the schematic from the SM, I suspect that something aint right on the power supply board AWR-052 which contains 6 more fuses. Maybe it goes further? to the protection board -AWM-062.

Technical question:
So, where do I start some simple diagnosis on a power up problem?

I do not have a DVM yet, but I do have an old "analog"/needle multimeter and an idiot-proof Fluke which I have used for years for house and car current - its pretty useless for electronics due to its ability to only read volts - pos and neg - and ohms. Not indicative of any applicable skills: I have rebuilt cars and motorcycles front to back including wiring, wired several houses and due to living in Houston became somewhat adept at repairing A/C systems for both house and cars. My philosophy is, " If a man built it, another man can do the same"

Dear Abby question:
I just want to make a go-no go decision on whether to keep this sucker or send it back this weekend. The seller has responded with a vague I'll refund your money, not addressing the shipment costs. It also looks like English or at least writing is his 2nd language. I find it hard to believe that it only cost him $24 to ship from Las Vegas to Houston. I would have to travel 15 miles to an UPS store to ship it back.

In any case, I would like to try my hand at repairing this thing if it is feasible with my extremely limited soldering and electronic experience. Parts don't seem to be overly expensive just to get it powered up providing I dont run into broken PCB's and some unobtanium parts. I got $85 into it and should get at least that back from the seller. Hopefully, the return shipping also and call it a day. BTW, this is the 1st piece of stereo gear that I bought off of eBay that I have had a serious or unknown problem with - I guess that I was due.
It is such a nice looking unit with the exception of the glass, that its very tempting to keep it and fix it. But, there are more 939's and 1010's(for that matter) out there. OTH, I have 2 Sony TA-3200F amps that need work and fixing this unit could be the start-off to me fixing those also.

Finally, can anybody get me started on the diagnosis question part of my long winded(typed) post?
 
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OK, if those other fuses on the boards aren't blown (and even if they are) the thing that blows the BIG fuse is usually shorted output transistors: $3.83 each and you'll need at least 4, of 8 maximum for about 32 bucks. Almost all of the other transistors in it run less than a buck apiece, some as little as a dime.

The other thing that blows the main fuse is that the main caps could be shorted or the big rectifiers shorted - but that's far less likely. But considering the source anythings likely (including tampering). Vegas is a depressed area, with defaulted storage auctions that could be causing gear that was never intended to surface getting loose, guys pawning junk etc, so even the seller could be unaware of the issues.

The bright side is that other than the output transistors, the main capacitors are the only other pricey items inside with a few RARE technical exceptions for "unobtanium" bias diodes and the protection relay ~15.00?.

To do the repair you will need decent soldering and desoldering equipment, and a meter capable of testing diodes - which is very different from the ohms function. Between that and the ohms function we should get it going again.

If you want to supply the sweat, we'll (I'll?) supply the direction.

Look for flamed out components on the amplifier boards in the back by the heat sinks. But even that isn't a big deal.
 
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Mark! Thanks for the reply. Looks like someone else is up late at night!
Ratz, I was hoping for the power board, but skeered that it could be something up the line! I was kinda hoping that something shook loose because of the broken glass faceplate.

Dont know if get the required equipment this weekend. I know that my plumbing torch and Fluke were not going to cut it(joke), but I was hoping that I could use the old multitester for voltage readings. I wont update this thread until I go find me a meter with a diode function. I have always tended to buy TOTL or near tools and was thinking of buying online for a better deal. I think that my only options are RatShack(no way nohow) and Fry's this weekend.

Seller wants it back. Says that it was his "personal" unit for awhile. He did not say that the glass was cracked or not. So, I will try to get a handle on my skills, the internal cosmetic shape and then my desire to attempt fixing this thing.

The one deal that I want to make very clear to YOU is that I do not want to frustrate you in trying to deal with me. Your help is very much appreciated by ALL in the community. Dont want you to say, damn another idiot wasting my time. What i am trying to say is that this might be a long drawn-out affair IF I decide to go ahead. This goes for anyone else that might want to chime in!
 
If you get it working, it'll be a nice receiver to have. I bought one for my son and it has 2 broken bass knobs. It has plenty of power to drive your speakers. Cosmetically, my SX-1010 looks almost identical to his SX-939 with exception a few indicator lights and one switch lever. One thing I did notice is the tone control circuit board and circuitry is exactly identical in both receivers. I think you'll be pleased once you get it working.
 
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Allright, after some soul searching I decided to try my hand at this deal. Got a meter, got a solder sucker, got a 25 watt pencil gun and the old Weller biggun. Making myself a variac and Cap dis-charger based on all that I have read here. I got plenty of house electrics laying about.
I got some very old computers that I will use for the boards etc to practice my de-soldering skills on 1st.

Have to still print out the schematic and SM. But, I got it. Been reading on the net on some basic electronics, snagged as many relevant posts by all of the experts and am pretty much ready to go. If needed, I already have a Flickr photo site and hopefully can get my bride to upload photos.

So, where do I start?

Thanks to da boat a yous for volunteering and support!
 
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I finally took the top and bottom covers off last night. I have spent a ton of time searching for info on SX-939's. I cut and pasted comments, posts, and pictures into a Word document.
As Yogi said,"You can observe a lot just by watching."

1st thing I noticed was a sticker on the bottom from a Stereo Repair shop dated 2000. Not unexpected, but at least a "warning" that some things have been mucked with.

So, I am looking for burnt stuff since the original fuse blew out black, subsequent fuses just separated politely. I see that the AWR052 POWER SUPPLY ASSY has some discoloration and 2 of the capacitos have a brown gummy substance on them. I thought that maybe some thing leaked or someone spilled some beverage into the unit. My bride pointed out that the board is on the bottom, not the top. The unit would have to be upside down for that to happen. The board is stained a darker brown and not sticky. Maybe oil stain from leaky cap? So, my beverage guess is likely incorrect. Observed that there is a fair amount of clear "glue" near one of the suspect caps. I compared it to the excellent picture from Stratdeluxer. His re-capped board and cap have the same " glue" glob.

Seen a wire used as a tie-wrap to hold a wire from the FM antenna heading to the FM board on the top - not factory.

I see that 5 of the 6 fuses on the AWR052 POWER SUPPLY ASSY are soldered onto the clips. WTF? I then see a post with the same question and a response that the factory did that. I'm ok with that. Will trace the non-soldered fuse to see where that heads.

I flip the sucker over and study the top. Ha, I see 2 broken wires sticking in the air on the AWE041 PCB/MOS FET(C),TUNER. A closer look at the SM's foil side picture and it looks like this is a "connector" on the tracer's. There is no part missing. (For those following along it is above pin 7 above the 2 resistors.) Maybe added by a previous tech to repair a broken trace? Then separated/cut by another tech? I dunno, but it dont look right to have 2 1" long yeller wires sticking in the air.

I see that 2 diodes look different and obvious replacements under pin 54 on the AWH030 PWR AMP ASSY.

THEN, I spot a broken wire sticking out off of the heat sink. The heat sink has 2 screws that hold down 2(each) wires. Ha, this cant be good. Perhaps this snapped off during shipment due to old age or previous tech work? I look at the foil of AWH030 PWR AMP ASSY. I see the connections go a diode. But I cant see any diodes. I magnify the foil and see - to my horror - that it is the infamous STV-4H ! I had collected all of the info that I could find and echowars handle is all over these post concerning the care and handling of these boogers because they are unobtainable! I start looking for this thing. Supposed to be mounted on the heat sink. Looked for a picture of one, found another post from echowars that described it as horseshoe shaped. I take a close look at what I thought was just a junction type thingie that has thermogrease under it and the light bulb went off. So, this is what this diode looks like and I am now in deep doo-doo before I even have a chance to de-solder, and "meter" stuff.

Looks like that I will have to beg, borrow or steal one of these. Maybe ebay or buy some unit cheap for parts. From what I have read, they seem to be in a lot of different brands from around 1973-80.

So, I am pretty sure from what I have read that this diode could have taken out more goodies and this could be the problem that is causing the main fuse to blow.

Since I have seen a couple of problems/repairs on other boards and the difficulty of finding these diodes, do I have a "parts" unit on my hands.

Or, shelve it while searching for the diode.

Or, can I disconnect the problem area and work/test the FM, preamp side of this unit. Maybe as simple as removing the 2 pre/power jumpers? Possible to run the unit (short tests) w/o this diode?

The main point of fixing this thing is for education to hopefully get me to a fairly decent point of expertise to manage the repairs on some of my other old stuff. I aint ready to give up.

Just need some opinions from my betters at this point.
 
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DOH!!! I been all over that site the last couple of daze cutting and pasting parts descriptions etc for my Word Document! For sure I did not type the number in because it did not show up in the saved "popup" when I typed it in just now.

BIG thanks! $25 plus shipping is fine with me as opposed to the alternative! I really hope that they REALLY have them in stock. They ought to because there are so many models using them. I am going to stop on my info gathering of Pioneer units that use them. I started doing this compiliation in case I had to snag a POS off of ebay for cheap. My eyes have had it for today looking at "foil side" and schematics looking for that booger.
New parts have to be better than old parts. Unless they are made in China........

Echowars, Markthefixer and a couple of the other "usual suspects" - I am STALKING y'all on the net!

Anyone want to give me any advice or techniques on removing the board for some soldering when I get this part in or for some de-soldering if I get some direction for testing? I really dont want to make more work for myself breaking stuff.
 
Board does NOT 'get removed'. The metal chassis piece behind the board gets removed, and the heatsink assembly unbolted and flipped over the top of the board so you can get to the back of it.

This requires care so you don't break those lovely solid-core wires that Pioneer used.
 
Ordered 2 of them - just in case - S&H and Sales tax came to 38.41! Still happy that they are available, but UPS prices and sales tax are a bummer. I'll mosdef keep this in mind when I order parts from Mouser, digi etc.
 
Sorry about the delay in posting - I've been in mortal combat with some rather tricky and nasty gear (NO steve, NOT your 1280). Transistors that test good, but won't work in the circuit. So they can't ever again.... met up with a large hammer and an anvil.

Don't power up until we can get that blown up amp channel isolated, and the power output transistors (the big silver ovals on the heatsinks - INSULATE the empty sockets) on the channel with the broken stv-4h out of circuit (ohm them collector to emitter - they should show zero ohms...) because THAT broken wire fault on the stv-4 causes carnage. Look at the amplifier board for obviously burned parts.

BUT, the good news is you've run into one of the most expensive parts already, other than a little under 4 bucks each for the outputs, the rest of the transistors on the amplifier board are under a buck each. The big can type capacitors can be pricey, but we'll keep our fingers crossed.

Once we can power up the unit we can see what the power supply needs fixed. Or we can do it the slightly harder and somewhat more reliable way of replacing all the transistors, diodes and electrolytic capacitors on the power supply - while checking the resistors for correct values while the rest of the parts are out of circuit and cannot interfere with the readings. Should be under 15 bucks - maybe lots less...

PLEASE SAVE THE BROKEN WIRE STV-4's !!!!!!!!!!!

You MIGHT have seen my thread where I manage to reconnect them. Very non-cost effective though.... on and off that's 2 evenings of work...

Yes the Power supply board can look cooked, and the hard brown goo IS glue... leaking caps look more like barf.

The yellow wires are part of the 4 channel adapter circuit, pin 7 is the 4ch mpx out for an outboard 4 channel decoder. The broken yellow wires are open to allow the awm-063 muting circuit assembly to have an effect - if they were connected, that circuit would be bypassed. Basically it's the raw fm detector output that is eventually sent to the stereo decoder chip on the tuner board.

Worst case, we shotgun (fix NOT destroy) the power supply and amp... use the sx-1010 parts lists as a starting guide, it'll save you a lot of typing, then compare to the Bill of materials in the 939 manual to get the differences.
Shotgunning saves a lot of wear and tear on the receiver, as it's not being re-opened many times, and not being run spread all over the bench - THAT takes a lot of past experience to keep the damage to a minimum.

But as it is you'll be pulling a lot of parts to either test or replace....
Post the parts lists before ordering, we'll (i'll?) see if you're heading in the right direction.

BTW, I'm rather impressed with your research....
 
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EchoWars: Board does NOT 'get removed'. The metal chassis piece behind the board gets removed, and the heatsink assembly unbolted and flipped over the top of the board so you can get to the back of it.

This requires care so you don't break those lovely solid-core wires that Pioneer used.

Thanks for that tip, I now understand the process.
And yes, I am worried about breaking those tiny solid core wires. This does not look like the easiest board in this unit to work on.

markthefixer:
Sorry about the delay in posting - I've been in mortal combat with some rather tricky and nasty gear (NO steve, NOT your 1280). Transistors that test good, but won't work in the circuit. So they can't ever again.... met up with a large hammer and an anvil.

No need for apologies to me! I will work on this at your leisure!

Don't power up until we can get that blown up amp channel isolated, and the power output transistors (the big silver ovals on the heatsinks - INSULATE the empty sockets) on the channel with the broken stv-4h out of circuit (ohm them collector to emitter - they should show zero ohms...) because THAT broken wire fault on the stv-4 causes carnage. Look at the amplifier board for obviously burned parts.

1) Do you want me to remove all 8 output transistors or just the 4 for starters?
If so, I need to know how to remove the coiled wires on the 3 legs. I am pretty sure that I DONT remove these factory coiled wires. I have peeked under a board and see that the pins are soldered on and that it would be reasonable - in this example - to desolder the pin. I am assuming that the output transistors are also soldered and bolted on.
What are the steps involved?
2) I dont know what you mean or the method involved on "INSULATE the empty sockets". Please explain.

BUT, the good news is you've run into one of the most expensive parts already, other than a little under 4 bucks each for the outputs, the rest of the transistors on the amplifier board are under a buck each. The big can type capacitors can be pricey, but we'll keep our fingers crossed.


They might have been replaced. I see solder on the coiled wires - dunno if that would be factory. They are not bulged or leaky and are labeled Nippon Chemi-Con.

Once we can power up the unit we can see what the power supply needs fixed. Or we can do it the slightly harder and somewhat more reliable way of replacing all the transistors, diodes and electrolytic capacitors on the power supply - while checking the resistors for correct values while the rest of the parts are out of circuit and cannot interfere with the readings. Should be under 15 bucks - maybe lots less...

Thats a pretty slick trick on testing resistors.

PLEASE SAVE THE BROKEN WIRE STV-4's !!!!!!!!!!!

You MIGHT have seen my thread where I manage to reconnect them. Very non-cost effective though.... on and off that's 2 evenings of work...

Will save it/them. Only 1 is broke - for now.
Which leads me to my next observation. Pins 3, 14 and 15 on this Power Amp board (AWH-030) have had the wires soldered. 14 and 15 look like maybe factory due to the size of the 2 wires? 3 looks like the wire was rewound and then soldered - kind of funky compared to the rest of the pins. 2-3 of the pins located under the fuses on the Power Supply board have solder on them also. I just want to remember this when i get deeper into diagnosis.

The yellow wires are part of the 4 channel adapter circuit, pin 7 is the 4ch mpx out for an outboard 4 channel decoder. The broken yellow wires are open to allow the awm-063 muting circuit assembly to have an effect - if they were connected, that circuit would be bypassed. Basically it's the raw fm detector output that is eventually sent to the stereo decoder chip on the tuner board.

Thats good news. And, we dont have to talk about it no more............

Worst case, we shotgun (fix NOT destroy) the power supply and amp... use the sx-1010 parts lists as a starting guide, it'll save you a lot of typing, then compare to the Bill of materials in the 939 manual to get the differences.
Shotgunning saves a lot of wear and tear on the receiver, as it's not being re-opened many times, and not being run spread all over the bench - THAT takes a lot of past experience to keep the damage to a minimum.

I have copied capacitor lists from recent posts on this unit. Yours and PatA's.

But as it is you'll be pulling a lot of parts to either test or replace....
Post the parts lists before ordering, we'll (i'll?) see if you're heading in the right direction.

Roger that. I think that I (we?) am/are a long ways from that. I am being pretty cautious because I have never done this, never seen this stuff done and I am trying to eliminate mistakes by asking questions and receiving patient replies from guys like you.

BTW, I'm rather impressed with your research....

Thanks! Lets see how impressed you are when we get into the testing/fixing!
 
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The output transistors - the oval things, are mounted in sockets (no unwrapping, it pulls out/apart), and secured with screws:

it is a stack:
transistor, mounting screw, thermal transfer grease, mica insulator, thermal transfer grease, heat sink body, socket that the two transistor pins (base, emitter) mate into, it has a projection that fits into a mechanical recess in the heat sink to facilitate alignment - to PREVENT ANY of the pins or the mounting screws that make the third electrical connection ( to the transistor's collector) from short circuiting to the heat sink.

One set of transistors has their case at +55v. and the other set has their case at -55v - connected to the BIG capacitors... the ones that EAT screwdriver tips for breakfast. Thus they should not touch anything.

Try to save the mica insulators - it's just good form, and practice... the to-3 case insulating micas are still available but other shapes AREN'T.

Once the transistors are removed from the sockets the sockets are no longer mechanically secured to the heat sink, so they just flop around and thus because of the +/- 55v MUST be insulated.

Re: 4 or 8, the ohmmeter test will tell if only 4 (one channel ) or 8 (both channels) are shot.
 
Be aware that most Pioneer's of this vintage had the mica insulators attached with a thermal glue and cannot be saved. The insulators will have to be replaced if that's how this one is done, and the old glue removed from the transistor and heatsink with acetone.
 
I had to stop working on this unit, Mom in the hospital in Connecticut, I'm in Houston. Looks like a trip home is coming up.

I woke up this morning with a hot wife on my hands - the A/C is broke. It also overheated the 70 amp breaker which proceeded to try to weld itself to the buss bar. Pry bar and big pliers got it out, but it took the plastic holder nubs also. I had to go get some 2pole minis to get some extra space for a new breaker. SquareD Homeline Minis have a design/manufacturing bug in them. I had to remove some plastic on the breaker to get them to snap in. Echowars posts on transistor and relay modifications were my inspiration.

A/C has two capacitors in there, one is a start only and one is a dual line -start and keep running. Once I got the new breaker in, the fan ran, but the compressor motor tries to start but dont. Tomorrow, 2 new caps from the A/C supply store and hope that the motor is OK.

There is a resistor across the start cap. By reading all of y'all past posts, I would have never known that it was a bleedoff deal or how to read its value. Now, I get to practice my new still-crappy soldering skills if the new cap comes w/o a resistor. I also learned how to drain a cap with a light bulb and not have to do the old screwdriver trick. What I am saying, is that by reading posts here and then Googling the net, I have now have an inkling of what some of these parts do - electrically -, and how how to test some of them. I know that this is nothing as complicated as a piece of stereo gear. But, I can now look at a A/C Condenser schematic and easily read it and test it. I been shotgunning and getting lucky in the past. Now I understand.
Thanks to everybody so far. Back outside to finish hosing off the fins. At least I got the mosquitoes to keep me company.

markthefixer, I'll post once I have the trannys out of the sockets and my readings. It looks like 2 of them have been replaced on the same side of the board that the broken STV-4 diode. I dont see any burnt stuff on the Power board. I do see some replacement caps though.
I aint doing anything else until I post the values and hear from you. The board is loose and the heatsinks are laying on top of it. No noticeable broken wires - yet.
 
Too hot to sleep:
Removed the good STV4 diode. Managed to break the OTHER wire from what was the single broke wire STV4 diode by touching it very carefully!

Looking at the back of the unit, I numbered the transistors on the heat sinks 1-8
Trans 5-8 are on the right heat sink w/broken wire STV4 diode.
I was able to save all of the Mica Insulators. Easily popped them off of the heat sink - one came with transistor. Is that good or bad?
Also noted that some previous tech scratched a B or D on the heat sinks above the transistors that correlate with the 3rd digit of the transistor.

I referenced Echowars thread and markthefixers post#11 for guidance:
http://audiokarma.org/forums/showthread.php?t=43186

Set my Sears meter to Diode/Continuity Check and my $4.00 POS to DIODE Check
1,2,5,6 are 2SB541 PNP NEC's
Red lead to Collector, Black Lead to Base/ Black to Base, Red to Emitter
CtoB BtoE
1 541 548
2 528 540
5 537 546
6 518 521

3,4,7,8 are 2SD388 NPN NEC's
Black lead to Collector, Red Lead to Base/ Red to Base, Black to Emitter
CtoB BtoE
3 526 532
4 525 532
7 534 541
8 512 518

I THINK that these are all OK values but?
BTW, my $4 meter's values are all about 30mV less across the board.

NOTE:#6 trans rang the buzzer on my Diode/Continuity test when I used the Red Lead on the Collector and the Black lead on the Emitter by mistake. Just for the heck of it, I ran the same test on the rest of the 2SB541 PNP's. None of them beeped. The Meter manual Continuity Section of the Combined Cont./Diode Check function says" if the resistance is less than 30ohms, the audible signal will sound"
Cause for worry?

Did I run the test that I was supposed to?

I have both heat sinks removed. I have a small cotton towel sitting under the 8 plastic sockets. Wires and body's are not touching anything but a towel. Is this ok?

If so, whats next?
 
You need to run collector to emitter readings on ALL of these transistors. If there is ANY conduction in that mode, the device is bOrked.
 
I have had a lot of personal "stuff" hit me all at once which has caused me to put the SX-939 project to the side. I thought that I would have to take a week or so to take care of the "stuff", but mo' "stuff" keeps getting added to the heap. Y'all know how this goes.

I do not see me extricating myself for another 2 weeks or so. One of my "stuff" items has caused me to re-locate my work area for a bit while I fix the area. I will be buying me a Hakko Solder station next month, since my old equipment is not specialized enough for me. I am also going to try to come up with a Lazy Susan deal (got a piece of plywood for bolting the receiver onto) as MTF had posted in the epic "Fixing a SX-1010 with salt damage in Canada" thread. I missed that thread while I was cutting and pasting MTF and Echowars pertinent posts! Lot of info in that thread and the OP sounds like he had the same amount of "expertise" going in as I do. :scratch2:

I have received the 2 new STV-4 diodes from Pioneer. Neither of them have any outside covering - just bare wires. Not sure if I should remove the covering from the broken STV-4's wires and put them on the new wires without breaking the new ones! I am planning on either using my unbroken STV-4 Diode and keeping the new one as a backup if I break the old one.

Email Reply from Mark:Sounds like an excellent plan,
yes, save the other stv-4 as backup, and hope you don't need it.
yes re-use the fabric covers, I do... the key to those diodes is to avoid at ALL costs, bending the first 1/8 inch of the leads, then keeping the bends gradual, AND NOT repeatedly flexing them.

This goes for the old stv-4 as well. I hold both the leads stationary at the body, gently with a non-serrated needle nose pliers over the fabric when forced to bend the wires to keep the flexing & bending away from the body of the diode.

Mark, you will be getting a snail mail in a couple of days!

Hopefully, you guys will still be willing to help when I get back into a more steady mode.
 
Make sure the new diodes are installed with the correct orientation...they are diodes, after all.
 
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