PIoneer SX-939 Part 1: Restoring the power supply and protect circuit

Service begins on this receiver with rebuilding the power supply and protect circuit. We also take a look at options for replacing the protect relay.

[@lobsteroverrun]

I just got one of these amps passed down from my grandpa. I’m planning to do a restoration, and, while I’ve worked with other vintage electronics before, I’ve never worked with vintage audio. this project looked somewhat intimidating. Your video here is IMMENSELY helpful, thank you so much for sharing your personal experience and tips for parts like the relay.

[@conservative599]

DO IT, follow Tony's advice and have fun and pride bringing these great stereo's back to life! Better capacitors and most other parts are still available, check ebay and other great old/classic parts suppliers.

[@NeilSnapePhotography]

Incredible amount of knowledge shared here. I don't and never will be able to do precision renovations yet watching this deserves respect and admiration of how you do work!

[@toddanonymous5295]

Tony, Once again you solved a problem that has had me pulling hairs. Intermittent protect trip. Problem solved thanks to you. Your videos are invaluable !

[@dennisfrancis1126]

Tony is truly the ‘Bob Ross’ of restoration and repair - and I mean this in the absolute sincerest and best possible way. Like B. Ross (who makes his incredible paintings seem easy as he’s doing them) Tony makes these difficult and intricate repairs look relatively easy as well. Always patient, meticulously thorough and of course a shear pleasure to watch. Thanks Tony for these!

[@xraytonyb]

Now if you can just convince my wife of all this ;)

[@johncunningham5435]

I love the current presentation the way you present it, even a season tech like myself, needs a brush up on new approaches.

[@ianide2480]

I love all the extra info, the more detail the better. Keep at it bud... Love your videos

[@TY-ob7fz]

Hi Tony. Enjoy so much seeing you doing these restorations. Would love seeing you restringing the dial cord ( sorry 🙃) and seeing the problems encountered and how to do it. Always watch these series multiple times and can’t get enough Thanks for your time amidst your busy schedule. Hope family is well and weather not too harsh.

[@71Fenderv22]

Nice talk on relays, I've never given them that much thought. I agree 100% about cleaning the contacts, I do the same thing with a piece of card stock or computer paper and IPA; previously trying very fine grit sandpaper on contacts especially high current circuits seeing them to only crud up in a matter of months or less depending on cycles/cycle count.

[@jasonthejawman5442]

Enjoy your work I learn so much

[@Charlee2015]

Great detail! Love your videos!

[@judges69]

Good info as always

[@Waltkat]

I have one of these beasts that was given to me and needs restoring. One channel is intermittent so it looks like I have a bit of work ahead of me. Hopefully I can get all of the parts through DigiKey. Thanks for the video.

[@conservative599]

HOT TIP: DigiKey ALSO 3RD PARTY reps both NTE ELECTRONICS from NJ, and TUBE DEPOT from Memphis. I just tagged them for many 1% old 5% value resistors in Metal Film, Mica RF caps, and Semiconductors for my 1944 Hallicrafters SX-28A restoration project using the DK site. Just make them as SEPARATE ORDERS - and shipping charges thru DK, it took their CS guys some hours late at night to break out my big order of Ilinois Capacitor 600V parts ect, and re-enter it for me! Also Mouser has/had older Semiconductor and general hobbyist parts selection, and huge ferrite beads for my Ham Radio projects...

[@Kennynva]

Do they make solid state relays for the relay in your video? I know it will cost alot more..but it probley never wear out..just a thought..these are used in refrigeration units as such..

[@guyaldrich5878]

I have a sx 838 that has no sound,I'm thinking it's that protection circuit because I do not hear the click after turning it on. Last time I used it was like 10 years ago,everything worked when it was shut off !

[@alexandermeyerzon7059]

Where do you suggest ordering replacement parts such as the transistors and caps for the protection board?

[@V109FM]

Great video, I'm servicing one now and the regulator transistors are running really hot, to the point the speaker protection is kicking in. I've re-capped the power supply and measured all the currents. Nothing is drawing too much current yet it's running extremely hot so I'm going to try to replace the transistors. Have you run into this problem?

[@antraciet]

18:42 I wonder what cross reference data bases you are using. The 2SC1384 you replaced with SK3024. They are a few databases on the internet but all of them give different results.

[@pioneermax1906]

Tony my favorite sounding pioneer receivers. Is the 30 series I think you said it’s yours as well what makes them sound better then other pioneer receivers just curious thanks

[@ewhibs]

Hi xraytonyb, I'm taking your warning to heart about the Pioneer SX-939 is not for the beginner. I'm a beginner and would like to try to rehab a vintage receiver. Do you have a recommendation of a vintage Pioneer model to start on? Marantz? Sansui etc. Love your clear and well done videos. Thx Ed

[@pdppanelman5889]

What you need is very high quality card that has china clay (kaolin) in it. Its a very very fine abrasive

[@plcamp1]

Relays are (generally) not able to open a dc fault higher than about 30VDC...you get an arc instead. Isn’t it better to instead use two back to back low RdsOn FETs, which can interrupt high voltages and do it faster and less expensively? And, if a relay is used, isn’t it better to have the relay open the amp connection AND short the speaker out? Given an arc is inevitable, it needs to be shorted to ground or it will kill the speaker anyway?

[@MarkTillotson]

I suggest the relay DC ratings are the useful ones to go by: If there is a fault putting DC on the speaker, the relay has to open and break DC, not AC.

[@MrPageplayer]

Tony, if you haven't already done so, could you explain your technique for dealing with wire wrapping/unwrapping, along with preping corroded contacts for soldering. Thanks for your hard work.

[@1959Berre]

Lately, I wanted to buy a wire wrapper tool. The price made me cringe. Those things are expensive!

[@MrPageplayer]

1959Berre, I've seen them on ebay recently for $10.

[@BobPegram]

That's one you twirl by hand like a tiny screwdriver. It's all you need. The expensive one is powered, so you don't twirl it with your fingers and exacerbate your carpal tunnel pain.

[@davidgranic2509]

EXSO Manual Wire Cable Wrap Unwrap Tool Hand Driver 22AWG 0.65mm 1piece 10 usd.

[@Zonfeair]

I am in the process of restoring the same receiver sx-939 and have many of the same issues you have where someone had gotten into the unit and did a poor job. Also soldered the fuses in. I cleaned everything up and used Deoxit on all of the switches, now they work great. One of the things I would have loved to see is how you removed the boards. How did you unwind and rewind wires? How did you keep track of where everything goes so you know how to put it back in? I did not remove most of the wires and just loosened them from the tie downs and flipped the boards over or on their side to work on them but it was a pain in the ass and soldering was very difficult. I have recapped everything and replaced most of the known bad transistors but not all. I am currently at a stand still. I checked all of the power transistors and they are good. It lights up and everything appears to work fine with the exception of two blown bulbs in the display which I am going to replace. After I had done all of the work the receiver still does not work. On power up it does not make any sound out of the speakers. I am new to this as I took it up for a hobby in my retirement and have been working at it for about a year and a half but never took on a job like this, just simple things like fixing speakers, appliances, small radios and the like. I could use4 some help if you have the time.

[@ranmacman]

I am in the exact same situation. I have not yet rebuilt my power supply board but I have verified that the board is not producing the proper DC voltages to the rest of the componentry (except the lights, mine lights up too but that's it!). I printed the Service Manual for the 939 (very faint, hard to read with old eyes!) but it unfortunately does not provide any info on the correct DC voltages one should have at the pins/outputs. I know a good tech could calculate the voltages based on the schematic but that's beyond my skills at this time. It doesn't sound (pun) like you are hearing the protection relay click after power up, so you may have a bad relay or a problem with the protection circuit board. Do you have any activity with the FM tuning meters when spinning the dial with an antenna connected? Just curious as I do not have any activity there. Have you tried connecting headphones? I believe the headphone circuit is signaled by the pre-amp section and not the main amplifier section, so you may get sound there. Hope this helps, Good Luck! Love the SuperBee!

[@randysalamante1306]

Hi there! Hope you can help me out on this issue. My SX 939 doesn’t make any clicking sound when it is powered on..everything powers up lights on but no clicking sound from relay I supposed after a few seconds and no sounds coming from the speakers at all. Hope you can give me guidance on this one. Thanks in advance

[@xraytonyb]

The protect relay will not come in if there is excessive DC offset at the speaker output of the amp, along with a few other causes. There are many things that can cause this, but usually it means that there is a faulty transistor or resistor, shorted capacitor, or other power-related issue. I would start by measuring the output section for DC and then trace your way back from there. Thanks for the comment!

[@mohimadvani6425]

how do you remove the wires? and do you mark them whilst removing? each board apparently has around 20 wires.

[@conservative599]

Great questions! I have been servicing Pioneer starting back in 1972, see my recent 10/29/21 post. On my SX-1010, and most Pioneer receivers, the wires are stiff solid conductor, tightly WIRE WRAPPED on the tall rectangular posts everywhere. TAKE SEVERAL CLOSE-IN PHOTOS, costs nothing these days. Also, for reference and better circuit understanding, DO make a list of all the wires on the board you are servicing, USING the usually informative and clear board markings. At least a post/terminal number will be silk screened on the board, and maybe a function name. My bigger concern is that with one or more STIFF wires on the post, you have to find a metric Unwrap Tool (has a hole and a spiral to catch the wire) or just unwrap using long nose pliers. With all that bending, often the wire will BREAK near the insulation. And the curly wire, if straightened to re-wrap it, will be work hardened and may fracture. My solution is to simply unwind and then CUT the wire just where it began the wrap. Then strip about 1/4" of insulation without nicking the wire, loop it around the post and SOLDER it securely. For looks, and less unwrapping "working" stressing the post PCB joint, simply cut the wire at the bottom of the wrap, and loop and solder as I mentioned here.

[@Justplast]

The relay on this is simple. I'm restoring Sansui QRX 5500 and the relay is in board and soldering so it's hard to change relay on this sansui

[@sdwputnam]

I bought my SX939 in Germany when in the Army back in 1974 and have enjoyed many years with my Bose 501's. Just like me, they are getting old and tired now. I am having trouble with sound not coming out one side on stereo/mono unless I turn up the volume then it will become intermittent then will stay on. Might the balance control need cleaning or might it be a more serious issue. I was wondering if it's worth repairing these days. Regards, SD Wood

[@conservative599]

Hi SD Wood, Please see my comments on the SX-838 fellow... Be glad you did not come home with the equivalent Sansui 5000 system! My 1974 SX-1010 has more trouble with the BLACK pushbutton switches, so a Deoxit or similar treatment there and the Volume control may help. Not easy to access inside all those slide switches, however! Try pushing in and out rapidly, and you may find the tarnished silver contact (dirty) one! Also try MOVING the speaker wires from A to B or C. Often these pushbutton slide switches get intermittent, on my rcvr also. Finally, it may be a tarnished Protection Relay contact. Remove the relay and pry off the clear cover. Burnish the contacts with rough paper and use contact cleaner. Follow Tony's advice there - NO abrasion of the contacts like the diamond file tools. You are demonstrating the theory of Sealing Current; in my old Analog Telecom days my boss/mentor at Lorain TE designed and patented a DC spike generator circuit that hit the phone line with 50-100mA when the line went off-hook. That cleared any oxidized connections!

[@conservative599]

And YES, it is definitely worth repairing! You have a top rated Tuner with a great preamp, big audio power with low distortion, and solid construction they can't afford to build these days! That FM tuner was so excellent that I used to receive 1-3 stations from Boston to Delaware at nearly every FM odd frequency channel, center tuned between the tic marks, just by rotating my FM Yagi 40 ft above our house in NNJ, when I was at home from college getting my EE degree.I foolishly bought (and sold) a Sansui 8 Deluxe, because it was QUIETER on distant FM in an A-B demo. Reason - only 12KHz FM Stereo spec!. BOY did they mock me for that back at the Pioneer store/shop I worked at summers, and bought the 1010 there for only $470! Also, at College I bought a pair of OHM F's, Walsh Driver speakers for $900 thru our ARC Radio Club president, it sounded like training elephants in my Dorm Room.

[@uglsas]

Hi Tony, I love your videos specially the way u explain electronics. I am also into vintage stuff specially Receivers from 70s. I have one problem with Rotel Amplifier RA-980BX from 90s. It is a problem with driver transistors 2SC4381&2SA1667 which get very hot - you cannot touch them i have measured and they have around 170 F. I have seen this video and was thinking maybe it will be god to upgrade them with some, never transistors which could better dissipate heat. Can you help me with a suitable upgrade? Thank you.

[@BobPegram]

One thing you didn't mention about relays was that they ring! If you remember seeing those ancient two bell alarm clocks, most likely in a cartoon, picture the relay arm moving like the clapper in those alarm clocks, The relay arm's just too fast to see. It's a problem for designers since waiting for the relay to stop moving is an eon or two for electronics. As you might imagine, the ringing adds wear to the relay contacts. These days you can actually see the difference between relay switching and solid state electronic switching. Compare how quickly brake lights and turn signals go on and off on a recent Caddy, with red LEDs, versus most other cars, including some with LEDs. Cadillacs can be almost shockingly faster on and off. Brakes and turn signals used to all switch with relays. Caddies aren't the only cars with this, but they were the ones I first noticed.

[@johnbecker370]

The problem with my SX- 939 is that after about 5 min or so it shuts off the sound. I've hooked up new type Plokauido speakers to it instead of my JBL 4311X speakers which I thought

[@markschmitt2641]

I just watched this video. you have a lot of useful information in here. I purchased my SX-939 when I was in the Army in Germany in 1974. It has the changeable plug for the AC input power. This receiver is fantastic. I love the sound. I have a set of JBL 4311 Control Monitors that I also purchased in Germany. Everythinf is working and sounding great. From time to time the protection relay will click out and then back in. From your video it sounds like it could be anyone of a couple of things. Right now it is not a big problem. When I turn up the volume to a nice listening level. (the little arrow on the volume knob points to 7 o'clock when the sound is all the way down. I turn it up to just about the 9 o'clock position) The protection relay will click out and then right back in. Sometimes I feel like a cap or transistor is a little leaky. What city are you located in or near? There is no way I will do anything more that clean the dust out it. After what you said at the beginning of the video. I'm taking your advice and leaving any kind of repairs to a professional like you. If I begin to have larger issues with this receiver I will contact you.

[@TheBigfoottrucker]

What would that cost em ? What city and state are you in ?

[@johnsweda2999]

You could use silver dip to clean your contacts just dip the paper in it and do what you're doing don't know if it leaves any coating behind don't think so! Maybe you're better off disabling the circuits completely it affects the sound quality top end amplifiers don't have it? I was thinking maybe you could put a bulb in its place so if there is a short that just lights the bulb wouldn't this work got any opinion on that. Will it be better to use solid-state relay then mechanical how come you don't do that? If you had a heat gun you could heat up the plastic and push it over the relay.

[@franksolem7094]

Love it when you are splitting hairs :)

[@HOGAR-ol1gv]

Buenas estimado justo lo que comenta en el minuto 23:00, tengo un problema con mi amplificador del mismo modelo, cuando activo el loudness o subo el volumen cuando hay bajos es como si se muteara o perdiera el sonido, le agradecería si me da una pista sobre si puede ser ese circuito que ahí comenta muchas gracias por compartir su conocimiento ❤

[@robertdestefano8327]

when I was a harmon kardon bench tech we saw the fuses soldered often the standard reply from the customers was they could hear that extra resistance. please dont yell at me I'm just the messenger.

[@xraytonyb]

3 watt SE amp with 0 gauge oxygen free "directional" speaker cables :)

[@joe1569]

Is there a list of old transistors to new ?

[@noahbrock5350]

search on audiokarma for SX-939 BOM

[@pdppanelman5889]

Card is white because has been bleached. There could be residue.

[@bspaipaoja1286]

มีอะไหล่ข่ายใหม่

[@len9518]

Are you using a different mike? Your voice is muffled...no highs.

[@mt2smooth]

Do you repair Receivers that's shipped to you. I have Pioneer SX-980 might need capped or ETC. I will E-mail you later. Thank you. Have Blessed day in Jesus Name.

[@BobPegram]

Man, it may sound really nice, but in many ways That Pioneer is a terrible design! It's not designed for serviceability. You can't get at the underside of the tuner board without removing it, which makes more to fix (replace properly) once you're done with the backside of it! On the opposite side of the metal panel the tuner board is above, sits a board that very much suffers from being upside down with little to no air flow! It's a wonder it doesn't fail more often or screw up the sound quality. The relay cleaning procedure made me laugh. Not so long ago I didn't know what old fashioned distributor points were. They don't need an electromagnet to close the contacts aka points, but cleaning them is almost exactly the same! Only thing is that the contacts are more delicate in a relay, too much or too harsh cleaning can destroy a relay. Otherwise I learned from this video; e.g. one source of audio Pops. I know you're busy, but keep making these videos, they help pass hard won knowledge to the next generation. I'm of your generation, but one can almost always learn. God knows, there are too many incompetents claiming to fix things while doing nothing of the sort.

[@musicstevecom]

Tony another great Video, I have some SX- and they get really hot, How hot did it get after installing the new MJE3055T and MJE2955T ???? after 45 years, can these boards handle that much more heat before breaking or melting? Recapping did not reduce the heat and instilling new old stock 2sd313 were still to hot. did you have time to think about it? The board get hot like 128 than rises to 158 degrees (top rt by C14) might think about lifting the heat sink off the board creating a air gap (hopefully reducing the heat going into the board) also thinking about taking out the 2sd313 for the 5.5vdc and do what you did, for the Led lights and adding it to the 5.4dc which would reduce the heat a lot and would be more efficient . also another option in better heat sink , I did some experiments and it would have to be BIG heat sink?