Schematic Help

Hi John, regarding your question (Why 36V AC going in and 48V DC out of the the bridge rectifier on filter caps?) you have to go a “little deeper” into understanding the voltage types and the theory of it. So the AC voltage has a sinusoidal nature meaning it has 2 cycles (a positive and a negative cycle) Your multi-meter has measured an RMS (Root Mean Square) value of 36V (Attention: NOT a Peak to Peak value). A Peak to Peak voltage value (P-P) can only be  seen/measured with an oscilloscope and will always be higher than the measured RMS value of your multi-meter.  So on paper,  one can calculate the P-P  AC voltage value:  in your case 36V (RMS) x square root of 2 (that is: 1.414). So, 36×1.41= about 50.9V (P-P) Now, during the full wave bridge rectification you’ll loose about 1.4 to 2V due to the voltage drop in diodes and that will result a voltage of about 48V DC. Please note that all of the discussed, measured and calculated voltage values are only “approximate” since we are dealing with an unregulated voltage source, so as the mains (120V AC) varies at the primary of the transformer, so will the secondary voltage and in turn your DC voltage will vary too. Also when you put the voltage under load, the measured DC voltage will also vary. Now, one of the filter capacitors property (at this point) is to store part of the energy in them so that the output DC voltage can remain at a more steady value under load. In situations when the load becomes too high, the charged energy in the caps will begin to drain quicker than they can recharge/recover and that is when we will start to see voltage ripples (with an oscilloscope).  On the other hand, you also say that you do not have any voltage at the output of the top rectifier (SR 3AM-8). Well, if you’ve measured 36V AC at the input of the bridge you should in theory have voltage at the output unless the bridge has blown in such a way that is completely open or when measuring the output voltage you “perhaps” forgot to switch the range of your multi-meter from AC to DC. Also try to measure the voltage straight on the capacitor terminals instead an also verify that the center tap wire of that RED-RED winding (the WHITE wire) has a sound connection to ground.

@ovi4 Thank you for the information. Turns out you are right I forgot to set my DVM to DC lol. Funny how something so simple can screw things up. You might not remember you helped me with building my first project two years ago Mr. Carlson’s Capacitor leakage Tester. I took your advice and bought a second DVM Bryman BM 869S which has a setting that does both AC and DC at the same time. I should have been using that instead of my Fluke which needs to be switched when switching from AC to DC. Guess I will be using that more often. I also found that even though I replaced all of the caps and most of the Transistors that I have a problem on the Power Amp board. One of the capacitors I replaced pulled up the solder pad and was not making contact with the board. Also with the two transistors 2SC1451 which I found out are problem transistors in most Pioneer SX-600 series receivers and higher models. Both are dead when I tested them so that is causing my protection circuit to not let the relay close. I ordered new replacements and hopefully once I replace them the unit will work and all I will have to do is go through all the adjustments. If all goes well I will post some pictures when it is done Thank You for your help.

@john-barron I’m glad that I could be of help and yes I do remember helping you (some years back) 😉.  I would like to say a few things here: The SX939 is without doubt amongst the nicest receivers from the so called “golden era” of audio equipment (around 70’s ) but, it is or can be a very challenging project to restore even for the experienced people. It is really heavy and hard to maneuver machine.  Most of the wiring is actually wires wrapping around fixed square looking pin/posts and you’ll need to have/buy a specialized tool (in case you haven’t already got one) that you can still find today (perhaps E-bay or something) to nicely unwrap  and then re-wrap the wires the same way it was done originally(should you need to). It is important that the wire wrapping around the square pin/posts is re-done as tight as possible (as per factory) especially in a high current/voltage areas such as the the power amplifier boards because any loose wire wrapping can cause high resistance, therefore drop in voltages and sometimes strange crackling sounds at the output. Make sure that you take plenty of clear high res. pictures or videos of all wiring before you touch anything to avoid big disappointments later and I really mean that. The reason I’m saying is because for instance on the output amplifier amplifier the driver transistors and the final output transistors are configured in complementary pairs (one PNP with another NPN) and looking at the part numbers they are very similar and if you need to take them out for testing and not careful enough and forget their original positions you could potentially put them the other way around and that’s when a big bang happens bringing some unexpected destruction with it. Another thing is that in most of the older audio equipment of that era the complementary transistor pairs are actually paired with each-other for specs (such as amplification) and if you somehow mix-up an NPN from the left channel with the same part number NPN from the right channel the end result such as the final amplification and or the differences in temperature for each transistor (under load conditions) would be very different and you’ll scratching your head (Why is that?). The sound signature of one channel over the other could become different as well. So again, be very careful. Another area to be careful of is the radio’s flywheel pulley but especially the black string. If that comes off (for any reason) it can be a nightmare to put back properly (And don’t ask me how do I know that…🤪). Again, the schematic is massive to look at on a computer screen and it is best (in my opinion) that you print out enlarged parts of the schematic area that you work on. This way you can mark the drawing with various notes for you to remember, highlight certain components etc (very handy!). Now going back to your “Relay Not Closing Problem” that can be caused by a bunch of factors other than the 2SC1415 being dead as well. So  considering what you’ve said (changed all caps) I would be extremely careful  regarding the protection board (Protection circuit AWM-062). Make sure that is you’ve changed any caps there they need to have the exact same capacity values and that you didn’t somehow mixed them up when fitting the new ones as that board deals with some time constants that are governed by capacitor values !. Also the Relay itself (being old by now) needs to have the contacts cleaned or needs changing (sometimes). Good luck with your repair John.

@ovi4 Oh No I wish I had gotten this information about the out put transistors before I took them out to check and reseat them just two days ago. Thankfully I have not put them back in yet. They did all check out ok but now that I got them all mixed up I am going to have to figure out how to match them and put them back in their proper order. I did know that they were paired pnp and npn but did not know that they had to be in complementary pairs. And now after reading your advice I did some research and also found out that 4 of the pnp transistors Q1 – Q4 I replaced on the same board AWH 030 also need to be matched for gain. Dam I bought a rebuild kit off of Ebay and they gave no information about transistor matching. All they did was supply most of the failure point components and A992 pnp transistors for replacement for those 4 transistors with no information as too how to put them in. So I am going to have to take them out and check them for gain to match them in pairs as well. I also have a lot of them in stock so maybe it would be a good idea to check them and get pairs that are as close as possible with the highest gain.

As for all the other components Caps etc I made sure they were exact replacements or better then specs. Many of the small electrolytics were replaced with WIMA caps of same uF and voltage rating which I found on AudioKarma were a good thing to do that would improve the quality of performance and sound. I am also going to check and clean that relay too.

As for the rest I remember you telling me about taking pictures and that is standard procedure for me in all my projects. I also purchased several size wire wrap tools as I found that was the best way to take out the boards for cleaning, inspection, replacement of components. One thing though it is obvious that this unit was worked on before as some of the components have been replaced and wiring colors do not match. I also see that some of the wire pins have been broken off which I had too solder directly to the board to make a few of the connections. But hey for $20 bucks I still think it was a good buy as for the overall it is in pretty good shape once I cleaned it up. I’m just hoping it will work and sound great when I am done. Thanks again.

Do not worry too much, like you’ve said,  just match them again with a tester . But… keep in mind that over time the Hfe value would degrade due to the transistors aging, so what was once a very well matched pair (back then when the manufacturer built the unit) it might not entirely hold true (today) when you’ll be measuring them. Either way, some negligible tolerance between the matched pairs is to be expected. Say for instance you’ve found one NPN to be 115  and his potential PNP (complementary pair) is found to be 118  or maybe 113. Then, so be it ! Regarding purchasing a repair/upgrade kit of the E-bay. Well, there is a vast number of fake replacement parts (it is a well known fact) and you could end up getting them and again, you would be scratching your head as for why things don’t work as they should ?. I’m not saying that they are all fakes cos they are not but,….I’ve been burned quite a few times myself (in the past) and I’ve now learned my lesson. So, be careful. I would say, for important parts such as power transistors, high quality caps i would always go with a tried and trusted component distributor.  If you can lo longer find original parts there is always an equivalent somewhere (is just a matter of choosing it right). As a general idea on how to tackle the repair/restoration of your Pioneer SX939 unit there is someone on YouTube namely “xraytonyb” that has presented the restoration process of such a unit in great detail (in 3 video parts i think). I suggest that you take some time to watch those (in case you didn’t already) The videos are very informative and pretty well put together with quality filming and sound as well, detailed schematic explanation on certain parts of the unit etc. Good luck.

I did a lot of work on stereo receivers back in the ’70s and 80s.  One of the biggest problems that I ran into was loose wire wraps.  They would look tight if you wiggled them, but they would oxidize and turn into a resistor.  If I got a system in with problems, the first thing to do was to solder the “wraps”.  The repair rate was about 94%.  I also found that a lot of the cross referenced semiconductors did not work well in these systems.

I don’t know if any of this will help, but who knows.

Jim B

@ovi4 That’s a very good answer.