Pioneer SX-580 Receiver Repair - Page 1
Forum section: electronics / repair

author: Smoky
author_rank: Frequent Contributor
timestamp: March 21, 2020, 04:48:44 am
content: Here's a prime example of a well-used Pioneer receiver, mechanically and physically!The mission here is to get it back up and running and learn how to do an AM and FM alignment.The exterior has been painted black and the veneer is pealing on the top so I'll be doing a little woodworking too.I took a look through the perforated vent on the top of the case, and all I can say is, I own a Shop-Vac!With speakers A & B selected and the volume at minimum I read -30V on the speaker terminals, yes, -30 volts not mVIf I can fix this, I want a badge! or at least a ribbon, as in, PBRUltimately, this isn't going to be a "beauty" queen. Just a solid performer!

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2020, 05:15:20 am
content: You have busted power transistors. It means you may have busted driver stage as well. find the schematic and check all the transistors and diodes in the power amp section.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 21, 2020, 05:33:31 am
content: Thank you Andy.Here are the power transistors. I checked beforehand and they cost about $8 a piece.I can't wait for the day that I own a curve tracer!

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2020, 06:15:50 am
content: OK, so those are not "power transistors." They are complete amplifiers in one single package for each channel. They are not repairable and a curve tracer will not help either. You will simply have to replace them. There should be a few caps/resistors outside of them in each channel. Check and replace them as necessary.They are pretty cheap also.Edit:Here is a typical schematic with the STK0029. As you can see, it does not have any output caps on the speaker lines. One less thing to worry about. You should not hook up speakers as they will burn up. Power it up and make sure the +ve/-ve supplies are as per the circuit diagram for this model as the loss of either will produce DC on the speaker lines.http://www.electronica.ro/audio/STK0029.shtml

author: Smoky
author_rank: Frequent Contributor
timestamp: March 21, 2020, 06:24:06 am
content: Thanks Andy!,Here is the datasheet of a STK-0029. I do find it a bit confusing as to how to test them. It looks like a complicated circuit.I like the way the heat-sink bar was able to be removed. It makes great access to the board's components.If I do a diode test from pin 2 to pin 3 (with the + probe on pin 2) I get continuity. My guess is that I shouldn't.

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2020, 06:28:45 am
content: All you can do is to check the voltages on the pins as per the schematic, if you can get the Pioneer circuit for the amp.

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2020, 06:30:15 am
content: Manual is here:https://elektrotanya.com/pioneer_sx-580_sch.pdf/download.html

author: Smoky
author_rank: Frequent Contributor
timestamp: March 21, 2020, 06:36:47 am
content: Thank you Andy!Here's what I'll do first. I'll recap the power supply as planned and check other components on the PS board.Next, I'll install the heat-sink bar and the old power amplifiers and do voltage tests.My thinking is, no fuses were blown in the receiver, but I may be confusing a failed power transistor's collateral damage and not these power amplifiers.Good stuff!

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2020, 06:44:38 am
content: You say "failed power transistors." In this case, there is none. They are all in the STK IC. But, you do have some drivers Q204/203 if I see it right on the schematic on Electrotanya.Anyway, check all nearby components. The fuses would have blown if the speakers were connected. That is the kind of protection these had at the time. But often they don't and the speakers burn up!

author: Smoky
author_rank: Frequent Contributor
timestamp: March 21, 2020, 07:11:52 am
content: Here's where I believe is the epicenter of corrosion or where a liquid was spilled. It's right at the base of the main filter capacitors. Those caps are coming out next which will open things up even more.I literally chipped away a layer of hard goop. I wonder if a glue was used to hold the big caps in place. If so, it made components trapped underneath it to corrode. Look at the tarnish on those jumper wires.Time to clean this mess up

author: Smoky
author_rank: Frequent Contributor
timestamp: March 21, 2020, 11:46:36 am
content: Guys, the area is cleaning up well around those large filter capacitors. I replaced the cruddy jumpers with new silver-coated ones and I replaced that red wire that was burning up against that cement resistor. The sheath of the wire is rock-hard! I used a red striped Teflon wire of a larger gauge for replacement.Here's the only odd-ball of the night, I pulled that little resistor next to the large 150 Ohm cement one. That little resistor that was trapped under the glue and being cooked is apparently R410.I got a reading of around 67k Ohms. But if you look at the schematics, it's written as a 100 Ohm resistor.I'm thinking that I should replace it with a 100 Ohm as written in the schematic. It looks like it was stressed and may be out of whack. What would you do?*Sure enough!that resistor should be 100 Ohms. I found another 100 Ohm resistor on the other side of the board that's just fine. I'll be testing and replacing resistors, if need be, as I go.One a side note, notice how none of the components are numbered on the PC board. One must keep a close eye on which part goes where and the polarity of the capacitors

author: andy3055
author_rank: Super Contributor
timestamp: March 21, 2020, 05:16:22 pm
content: You seem to be making great progress. Go ahead with the cleaning up as you have started. If those large caps are bad, there is a good chance that it contributes to the DC on the speaker terminals. Just check all the components around the area and you may end up with the STKs being good after all. If not you will need to replace whichever one is bad.Those old amps/PCBs unfortunately did not have a silkscreen like these days. Only way is to follow the diagram closely which is what you are doing now.Good luck and keep us posted.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 22, 2020, 12:32:24 am
content: The two large original 6800uf 35v 85c filter capacitors tested low in capacitance. One measured 5855uf and the other 5535uf. I ordered two 8200uf 50v 105c Nichicon LGY capacitors to replace them. From past experience, I'm guessing they'll measure around 7500uf on the Sencore. I will update the actual values when they arrive Wednesday.*Here are the caps. The new 8200uf LGY's settled down to ~7300uf!I'm now in the area of the diodes. Out of these four diodes, two have corroded legs.The schematics identify these diodes as 30D2 (GP25-D). These are 2.5 amp diodes and I believe they are the 200v version.What I could find are the Vishay RGP25M diodes.They are the 1000v version. They are available from Digi-Key and Mouser.BTW, all of the large capacitors in this receiver are glued down. I mean "really" glued down. Expect to spend some time carefully removing them.The small electrolytic capacitors that I've removed so far have tested low in value by at least 50%.As for the film capacitors, I've been randomly pulling them to test on the Sencore meter. Here is a .068uf "K" 10% film capacitor (C170). It not only measures good, it's under 3% with zero leakage!There are also two 47uf at 25v "CELL" electrolytic capacitors by Nippon Chemi-Con (C206 & C207). Could this mean low-leakage or long-life?There are also about 15 small orange electrolytic capacitors in a variety of values marked "LR."

author: andy3055
author_rank: Super Contributor
timestamp: March 22, 2020, 02:47:59 am
content: Yes, those diodes should work fine. That glue is like the kind of stuff you use to patch bicycle tubes with. Over a period of time it gets very dry and hard though.Pay special attention to the electrolytics and before you put the STKs back in, check the voltages on those points where the legs go.

author: mzacharias
author_rank: Frequent Contributor
timestamp: March 22, 2020, 02:17:48 pm
content: The STK packs can be tested for shorts between pins 2 and 3 and between 8 and 9.Original STKs of this type are long discontinued and any new ones you find are Chinese FAKES.These COUNTERFEITS rarely if ever perform to original spec. They have inferior internals; I have seen severe notch distortion right out of the box, and I have seen total failure at less than 1 watt output power.Look for good used pulls, although even these would be hard to find except in a working receiver you might buy on eBay. The 0029's were used in various models.

author: poot36
author_rank: Frequent Contributor
timestamp: March 22, 2020, 10:35:49 pm
content: You can build a replacement STK pack with better components. Link here:https://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/page-21#post-11612519and here:https://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/Video of one of them being built:Good luck.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 23, 2020, 12:33:12 am
content: Poot36, You blew my surpriseI sent an email requesting just the custom PC boards because the SX-580 has such a huge heat sink that I'm going to bolt the transistors directly to it. I won't need the aluminum plate.I also plan to remove the 10-pin connector on the main SX-580 PC board and use the pass-through holes for jumpers instead.It should look real clean.Anyway, I was able to get another 20 capacitors replaced last night. I match all pairs. You can see the black 10-pin sockets that will be replaced with jumpers. I'm also modifying the heat sink bar so it can just lift straight up and out for easy repairs in the future. The area around the diodes looks much better too.WooHooI made a special cradle to accurately solder all components to the board. The board is also spaced a distance up and away from the transistors too. I selected the lower 150 Watt ON Semi bipolar transistors over the suggested 200 Watt versions. This Pioneer is only rated at 25 Watts per channel anywayInstalling the modules is a cinch inside of a Pioneer SX-580. The heat sink comes in and out in a minute when 6 to 8 screws are removed. All that needs to be done is to line up some holes by eye and through-bolt the parts to the heat sink. I used stainless fasteners with just lock-washers. For extra insurance, I put a dab of paint on the threads to keep the nuts from loosening. You can also see where I de-soldered the black connectors from the main PC board. Now the cement resistors and jumper wires will get attached with ease.Make marks on the heat sink with a felt marker where pins 3 and 8 align on the main PC board. That way, the 0.22 Ohm 5 Watt cement resistors can be installed pre-bent to make their connection easy.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 30, 2020, 02:17:06 pm
content: Guys, this is almost as good as a classic Sherlock Holmes murder mysteryWhen putting together the new Power Pack modules it is imperative that no continuity exist between the new transistors and the heat sink they are bolted to. Special plastic shoulder washers and isolation pads are required and are used. I had no continuity and the transistors remained isolated from the heat sink after they were bolted in.Get this! As soon as I soldered the new jumper wires of pins 1 and 2 of the new Power Pack module to the main PC board of the SX-580, my DMM rang out its "continuity" tone for transistor Q1's collector to the heat sink!The same thing happened when I soldered the new jumper wires of pins 8 and 9 which made the collector of Q4 ground to the heat sink.Both the left and right channels respond the same.Remember now, the problem with this receiver originally was that I was reading over 30VDC at the the speaker terminals.This leads me to believe that the new modules are indicating that there still lies a problem/short in the main board of the SX-580.Could I be right about this?Up to this point, no electrical power has been applied to this receiver.Here are the schematics to the SX-580 and the modules.I'm guessing that the problem is beyond the new Power Pack modules and more towards the direction of the speaker terminals.

author: andy3055
author_rank: Super Contributor
timestamp: March 30, 2020, 04:54:43 pm
content: So, did you check the power supply voltages (that come to the holes where the jumpers go in) before you connected the module to the main board? If not, remove the jumpers and check that first. If there is a short anywhere it will blow some fuse which is better than blowing expensive parts. Make sure you have the -ve and +ve 28 volts (or whatever the rails are supposed to be) are all good.You should also check if there is a short to the ground from the "power in" connections on the module. Check them on both directions.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 30, 2020, 05:08:50 pm
content: Thanks Andy!I took "actual" voltage measurements at the holes after the new electrolytic capacitors were installed, no Power Pack modules were installed.*The voltages printed on the schematics are in parenthesis:Pin 1, -28.8 (-1.2v)Pin 2, -29.1 (-28v)Pin 3, 0 (0v)Pin 8, 0 (0v)Pin 9, +29.1 (+28v)Pin 10, +42.9 (+1.2v)The schematics read that I should have -1.2v at Pin 1 and +1.2v at Pin 10 but I imagine that would be with the Darlington Power Packs installed. Should the voltage read the same at Pin 1 and Pin 10 no matter if they were installed or not? If Pin 1 and Pin 10 should read -1.2v/+1.2v no matter if a modules are installed or not, there's a problem right there. No kidding, what would make the -1.2v/+1.2v shift to -28.8 and +42.9? In the schematics you can see the +41.7 rail nearby in the circuit.I just tested Pin 1 and Pin 10 on both new modules. I do not get continuity to the heat sink or to the chassis ground in either direction.I believe the new modules are good to go.All fuses check good.I'm in the process of lifting legs of several diodes in the area to see how they test out of circuit.I love this stuff, we're going to fix it!*Update* I spent a little more time this afternoon going around the main PC board of the SX-580 and here's what I discovered. With the new Power Packs detached, I did a continuity test from the six holes of their connection to the main PC board to chassis ground. Get this, two holes showed continuity, Pins 2 and 9, the same two holes that give me proper voltage when the receiver is on! Could this be normal after all?

author: andy3055
author_rank: Super Contributor
timestamp: March 31, 2020, 01:00:10 am
content: Yes, checking the diodes etc. is a good idea.Check Q 303 and 304.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 31, 2020, 01:26:30 am
content: Thanks Andy!Q303 and Q304 test good and so does a slew of diodes that I removed from the circuit.I think what holds key here is what effect on the published voltages does the Darlington Power Packs have when they are removed from the circuit.The notes on the schematic read that the voltages are taken with the power on and with no input to the receiver. It's just sitting idle.I am trying to get a hold of someone I know who owns a SX-680 that may be willing to pull his Power Packs and measure the connector pin slots. The SX-680 is almost identical to the SX-580 except that it uses the STK-0039 Power Packs.

author: andy3055
author_rank: Super Contributor
timestamp: March 31, 2020, 02:52:46 am
content: Those voltages may be correct. If you check the diagram you see pins 1 and 10 (shows as 0!) are the collector and the emitter of that 1st transistor in the package. Since you did not check the original STKs, why not hook up one of them and see if you still get the DC on the speaker terminals? Since you have replaced some caps etc., I am thinking they might still be good. I know it is a pain to get it on and off the board

author: Smoky
author_rank: Frequent Contributor
timestamp: March 31, 2020, 11:55:13 am
content: Andy,After reading 30 more pages a second time on AudioKarma, I came across a post where a 1K Ohm resistor is placed across Pins 1 and 3 and Pins 8 and 0 on the main PC board to check voltages without the Power Pack modules installed.This is good news!With the 1K Ohm resistors in place, here are the voltages.*The voltages printed on the schematics are in parenthesis:Right channelPin 1, -5.1 (-1.2v)Pin 2, -27.4 (-28v)Pin 3, 0.0 (0v)Pin 8, 0.0 (0v)Pin 9, +27.4 (+28)Pin 0, +5.2 (+1.2)Left channelPin 1, -4.9 (-1.2v)Pin 2, -27.4 (-28v)Pin 3, +0.2 (0v)Pin 8, +0.2 (0v)Pin 9, +27.4 (+28)Pin 0, +5.3 (+1.2)I went back to each position 3 times. The voltages remained the same and didn't budge.To me, this is good and going in the right direction.I have one transistor on order, Q502. It was the only odd-ball component that seemed to fluctuate when I tested it in circuit earlier in the capacitor replacement phase. It is called out in the schematic as a 2SC945A but a 2SC1815BL was installed in this receiver. I ordered another C1815BL. We'll see if a new one has any effect on the voltages when it's installed.I will leave everything as it is until then. Also, to maintain the accuracy of future voltage tests, I'll monitor Diode D401 so that it measures 45.6v (as per schematic) by adjusting my Variac.Any comments on what you see in the numbers?

author: andy3055
author_rank: Super Contributor
timestamp: March 31, 2020, 07:08:19 pm
content: That method of inserting a resistor to check the voltages is great. Those numbers look ok to me. There are slight differences but I think those might be dependent on the many other components in the circuit. If your measured numbers in parenthesis were way over the specified, I would have worried.What is the next step you plan after you replace the transistor you have ordered? I wonder if we can keep the output transistor legs off the board and power it up. This way, we keep the power transistors safe. It might be easier said than done though. Just a wild thought!

author: Smoky
author_rank: Frequent Contributor
timestamp: April 01, 2020, 02:48:06 pm
content: I pulled that transistor (Q502) and it does test bad on the workbench. When I touched it with a wooden stick while it was in circuit, I could see the front panel lights go dim and flicker.*7 April 2020The new transistor finally arrived! It looks different than what was posted on Ebay (no bent leads) but it fit perfectly.Now after I powered the receiver back up I noticed that the "tape monitor" and "source" LED's would not light up. I started checking voltages and found no power at the base and the emitter of Q401 (2SD525). Q401 tested good out of circuit so I pulled Zener diode (D408) for an out of circuit test too. It is marked as an MZ-140 (WM-140) and it turns out to be a 14v Zener. It tested having continuity both ways using my DMM set for a diode test.I then gave it a test using my power supply set to 18VDC along with a 1k resistor. As you can see, the DMM measures around 115mV across it. I just read that a Zener diode will read a lower voltage if it is shorted:After the new Zener diodes were installed, the LED's came back to life on the front panel and the voltages within the receiver went back to normal!Next was to connect the new Power Pack modules. Guess what?I was able to dial in 14 mV DC of Bias on both modules as required. When I put together the RCS16 Circuit, I distanced the board from the transistors, doing this gives great access to the emitter resistors from above with grabber hooks when making the Bias adjustment.Here's the surprise of the night though, I turned the receiver up-side down to measure the "DC Offset" from the soldered-side of the PC board and here's what I got (also at the speaker terminals):Left channel:Pin 3, 231mVPin 8, 234mVRight channel:Pin 3, 44mVPin 8, 44mVQuite a disparity between the two channels. Funny thing is, the service manual doesn't explain how to make adjustments between the channels. I come to find out that the lower-end Pioneer receivers have no adjustment for "DC Offset."But wait...

author: Smoky
author_rank: Frequent Contributor
timestamp: April 19, 2020, 03:16:53 am
content: It was hinted to me that the problem with the "DC Offset" is due to two of these 2SA798's. They are two transistors in one and they leak as they get older.I was told to buy a handful of KSA992FBU's and solder two together by joining the emitters. Will this work?So I made a little test pad since I don't own a breadboard yet and soldered it into the PC board holes of the removed 2SA798's. Both emitters of each pair of new resistors are tied to one green wire. I plugged in the first four KSA992's:Here is the result from the Right Channel (55 mV) and the Left Channel (~38 mV).Notice how the Left channel came down considerably from around ~230 mV!For the next hour or so, I'm going to play "musical chairs" with 25 new transistors to see how low, and even, I can get the "DC Offset"

author: andy3055
author_rank: Super Contributor
timestamp: April 19, 2020, 05:00:20 am
content: Those "dual" transistors will have the same variations of gain etc. due to temperature variations as they are in one enclosure so to speak. This configuration is called a "long tailed pair." If you are using 2 individual transistors in each case, you may have to keep them close together by some means. I think you could use a piece of heat shrink so that the casings are in contact and any temperature variation is equal in them. If one of them is warmer than the other, they will heat and compensate each other. Not sure if what I say is making sense!Make sure to tie the emitters together before the 13k resistor. Below is a nice write up on the "Long tailed pair."https://www.electronics-notes.com/articles/analogue_circuits/transistor/long-tailed-pair-circuit.php

author: Smoky
author_rank: Frequent Contributor
timestamp: April 19, 2020, 05:25:12 am
content: I see exactly what you mean Andy.Here is a closely matching pair. Not the lowest mV but even.Most of them seem to linger around 50-ish mV until I decided to touch one of them with my finger. Immediately, the mV's dropped like a rock!I then pulled out two of these Radio Shack clip-on heat sinks for one transistor while gripping the other with my finger. This gave me the best overall readings.Here's the weird part, if I grabbed two transistors of the same pair with my fingers, the mV's would climb.One other thing, while on the Variac at 117 VAC, I could get a pair down to 5.8 mV. But when I plug the receiver straight into the wall outlet in my house (~122 VAC) the same set of transistors is at 28-32 mV.I will try a variety of heat sink combinations. Btw, inside of a Sencore oscilloscope I can see how they bonded two transistors with epoxy onto a tiny bar of aluminum.Good stuff!

author: andy3055
author_rank: Super Contributor
timestamp: April 19, 2020, 05:43:44 am
content: When you touch them with your fingers you will be introducing noise because this is a high gain stage. Best is to use a piece of Aluminum or heat shrink and hold them together. Keep the leads as short as possible to minimize pick up of stray signals. I would not use any kind of glue but you could use some heat sink compound between them so that any imperfections get filled up with it that will make better heat transfer and keep them at the same temperature.

author: Smoky
author_rank: Frequent Contributor
timestamp: April 19, 2020, 05:53:01 am
content: And yes, the emitters were tied together under the sockets. The green wires tie the emitters. The black wires are the bases, and the white and violet colors are the collectors. The heat shrink identifies the pairs.I will be able to put every transistor into its corresponding hole on the PC board once the pairs are selected.I bet a lot of the noise is from the 6" length of wire I used too. I still see variations between transistors though.

author: andy3055
author_rank: Super Contributor
timestamp: April 19, 2020, 05:55:45 pm
content: Transistors will have differences, no doubt. If you have a transistor checker that shows you hfe etc. you will be able to narrow down some more.

author: aqibi2000
author_rank: Regular Contributor
timestamp: April 19, 2020, 06:11:03 pm
content: Quote from: andy3055 on April 19, 2020, 05:55:45 pmThis is where the curve tracer mentioned earlier would be handy

author: Smoky
author_rank: Frequent Contributor
timestamp: April 19, 2020, 06:57:56 pm
content: I tell you, behind the scenes, I've been trying to find a curve tracer. Someone locally has a Tektronix 575 but he stopped communicating with me by e-mail. I'll get one soon enough though.Anyway, I'm going against what's recommended and went ahead and bonded two transistors to a small bar of anodized aluminum. I used Super Glue Gel and it grabbed like crazy. I still have some of the black epoxy from a previous project and I may dip these assemblies in itBtw, I was thinking, would anyone know if there is a newer SOIC package that is similar to a 2SA798 with matched gain built into it? Why I ask is, it wouldn't be too hard to produce a small board that pins could be added to and inserted into the main Pioneer PC board.I'm just a weekend-warrior when it comes to electronics so I'm just throwing stuff at the wall to see if it sticks!

author: Smoky
author_rank: Frequent Contributor
timestamp: April 19, 2020, 07:22:47 pm
content: The reason I off-set the transistors on the bar is because it allows the legs to align to the board with just slight bending. I clipped one emitter leg and swung it over to the other and it will be silver-soldered at that joint.Does it matter which side of the board the transistors are mounted on?Here is a transistor being mocked-up.*Update*I installed the new transistors and plugged the receiver into my home's wall outlet (~122.5 VAC). Here are the "DC Offset" results:64.9 mV at the Left Channel speaker terminals62.3 mV at the Right Channel speaker terminalsVoltages at the new transistors:Right Channel:B +0.2 VDCC -29.4E +0.7C -28.1B +0.2Left Channel:B +0.2 VDCC -29.4E +0.7C -28.1B +0.2They are identical. But, the collectors should measure a little lower in voltage per the schematcs (-26.5 VDC and -25.3 VDC).So I put the receiver on the Variac and lowered the AC voltage until the voltages at the transistors aligned to the schematics. When the voltages at the transistors aligned to the schematics, the AC voltage at the Variac was at 111.8 VAC output.But this is the most important part:With the voltages correct at the transistor's collectors (-26.5 VDC and -25.3 VDC), and the Variac at 111.8 VAC, the DC Offset dropped to 33.4 mV for the Right Channel and 35.8 mV on the Left Channel!Guys, we need to find a way to lower the voltage to the transistors while the Pioneer receiver is plugged into a 122.5 VAC outlet....and I make many edits to my posts, so please check them regularly

author: andy3055
author_rank: Super Contributor
timestamp: April 19, 2020, 10:12:39 pm
content: If the space is there, mounting under the board is no harm as long as you don't mix up the legs looking at it from the wrong side!

author: feedback.loop
author_rank: Frequent Contributor
timestamp: April 19, 2020, 10:25:03 pm
content: Quote from: Smoky on April 19, 2020, 06:57:56 pmDo you really need a curve? Perhaps some particular parameter should be matched.How about a combination of a programmable power supply to sweep through some range you want, a meter with a computer interface too, and a little program, say, in Python?

author: Smoky
author_rank: Frequent Contributor
timestamp: April 19, 2020, 11:01:46 pm
content: Thank you Feedback.Loop!I'm not too familiar with the proper testing of transistors, so for the moment, I used the Pioneer receiver to determine a matched pair, at least in the "DC Offset" category.I purchased 25 of these transistors for $5 from Digi-Key. The four I ultimately installed, tested the best that way.The DC Offset voltages remain stable so I imagine that's a good thing?Anyhoo, I hope to own one of those nice oscilloscopes like you have that does Frequency Response too

author: Smoky
author_rank: Frequent Contributor
timestamp: April 19, 2020, 11:15:27 pm
content: Guys, as you can tell, this receiver was a total basket-case, and it's not even close to being over!There is another problem that surfaced when the new Power Pack modules were installed. There was little volume coming out of the Bose speakers that were attached at the time.The good thing was, I was able to hear FM Stereo. Music was coming out of the Left and Right channels but at about .01 Watt per channel.I was also hinted that there is an issue somewhere in the muting circuit.I'm going to start digging in to that area to find the problem. I see two 2SK117's, and plenty of other things to check and to take voltage measurements at.Let's keep it going

author: Smoky
author_rank: Frequent Contributor
timestamp: April 20, 2020, 02:08:41 am
content: You're not going to believe this!If you go back to reply #10, remember resistor R410 trapped under the glue while being cooked by that 150 Ohm 5 Watt cement resistor? Well, that R410 is the 100 Ohm resistor that drops the voltage to those dual-transistors!!! When the R410's value went haywire due to the heat, the voltage rose on that rail and did the damage to those dual-transistors!I just measured the voltage at the new 100 Ohm R410 that I installed during the re-capping process. It measures -29.6 VDC. I want to bring it down to -26.6 VDC as noted on the schematics.I measure -31.1 VDC feeding the other end of R410(-28v Rail, higher due to the voltage at my home's outlets ~122.5 VAC).So we need to go from -31.1 VDC to -26.6 VDC.If we can adjust this voltage, it will correct the voltage going to four very important transistors in this receiver, and in turn, the DC Offset should drop to the 30 mV range.This is good stuffI'm guessing the current is around 15 mA.So would the resistor be around 300 Ohms?I just found a 1/2 Watt 330 Ohm resistor in my stash that measures 319 Ohms. I'm calculating that it should bring the voltage to -26.3 VDC. Should I try it?

author: andy3055
author_rank: Super Contributor
timestamp: April 20, 2020, 03:18:55 am
content: At this moment are you connecting the speakers? Providing there is no damaging DC on the speaker terminals, I would put it on the Aux input and feed a 150 mV signal and see if there is sufficient volume. If there isn't, then you still have a problem in the power amps. If there is good volume, then you can look towards the Preamp area.If you don't have an audio generator, you can use this site:http://onlinetonegenerator.com/You can take the output from your sound card and hook up to a pair of RCA jacks.

author: Smoky
author_rank: Frequent Contributor
timestamp: April 20, 2020, 03:38:09 am
content: Thanks for hanging in there Andy!After the new dual-transistors were installed, there is still ~60 mV DC of noise at the speaker terminals.I do get FM Stereo from both channels but at a very low volume (~.01 Watts).I'm told there may be a problem in the "muting" section.I first wanted to address the higher than normal voltages at the dual-transistor's collectors. I want to take this step by step.

author: Smoky
author_rank: Frequent Contributor
timestamp: April 20, 2020, 04:06:17 am
content: You Guys are not going to believe this!I changed out the 1/2 Watt 100 Ohm R410 resistor for a 330 Ohm type (319 Ohms actual).Can you see what I see?It dropped the voltage rail to absolute schematic specification!!!-26.6 VDCI also mounted it on the foil side of the board to be away from the heat of the big 5 Watt cement resistor it was next to.

author: andy3055
author_rank: Super Contributor
timestamp: April 20, 2020, 05:37:56 am
content: Great job! The numbers might change when you find the issue prior to the power amp section.

author: Smoky
author_rank: Frequent Contributor
timestamp: April 20, 2020, 06:06:15 am
content: Andy, would you mind taking a look at these IC's? They are supposedly matched double PNP transistors in a 6-TSOP package. I'm not sure the gain is adequate though compared to the 2SA798 dual-transistors. Anyway, I see little PC boards are available for them that can be adapted to the Pioneer receiver by using jumpers:https://www.digikey.com/product-detail/en/nexperia-usa-inc/BCM856DS,115/1727-5391-1-ND/2676874I'm not too sure these paired-up KSA992FBU's are the best option with 60 mV of noise at the speaker terminals.You know me by now, I'm going to chip away at this until I get silence

author: andy3055
author_rank: Super Contributor
timestamp: April 20, 2020, 08:17:26 pm
content: The specs are quite close but check this out! You may be in luck here:https://www.ebay.com/itm/2SA798-Original-New-Mitsubishi-Transistor-A798/121668651619The seller is Dalbani corp. a well known supplier and they claim these are original Mitsubishi products which is great. Years back I have bought parts from them and they are reliable as far as I know. Here is the we site:http://www.ebaystores.com/dalbanicorporation/sch/_i.html?_nkw=2SA798&You might as well get the originals and avoid the glue/thermal binding process. Also they will be matched. At $8.95 each and free shipping, you can't beat that. They dare not claim original if they are not as eBay will be behind them for sure.Good luck.Edit: They even have the STK 0029 for $15 and free shipping!http://www.ebaystores.com/dalbanicorporation/sch/_i.html?_nkw=STK+0029&

author: Martin Hodge
author_rank: Unknown
timestamp: April 21, 2020, 02:58:09 am
content: I made a little smt adapter board to replace the 2SA798's with 2SA1873's in my SX-750. They have worked great ever since! And I have some extras...

author: Smoky
author_rank: Frequent Contributor
timestamp: April 21, 2020, 03:40:02 am
content: ...and after two long years of electronics, I finally put together my first 8 Ohm dummy load!Tomorrow I'm going fasten Hard Maple cleats to the 3-1/2 pounds of aluminum to suspend it off of the table

author: Rollin Hand
author_rank: Regular Contributor
timestamp: April 23, 2020, 01:33:08 pm
content: I need 4 pc boards ship to 33183.Can you tell me costs?Thanks

author: Smoky
author_rank: Frequent Contributor
timestamp: April 23, 2020, 03:12:54 pm
content: Rollin Hand, I changed my mind and decided to buy two 2SA798's so I didn't buy Mr. Hodge's boards.I didn't want to add any more "custom" parts at the moment.If the new 2SA798's are still noisy, I may try the 2SA1873 option, but then again, the noise may indicate just another issue somewhere else in this receiver.*Update*I swung over to Audiokarma for a little while regarding the SX-580. The SX-580 is back up and running with the help of Mr. Merlynskihttps://audiokarma.org/forums/index.php?threads/a-pioneer-sx-580-with-low-volume.912613/The problem was with diode D403. It was intermittent. It controlled the voltage of the collector of Q502. I replaced several other 1S2437's too with 1N4148's and the voltages were put back on track! All I can say is, check all of the diodes within these old receivers, especially in the power supply area. But, really, you should replace every glass diode that you see.The final waveform of the receiver's output, just under the point of distortion, read 13VDC, ~23 Watts per channel.I may fiddle with other 2SA798's to get the DC Offset down in the low 20's or single digits.The 8 Ohm dummy load is done too!Thanks again for all of the help everyone

author: dkrietz
author_rank: Newbie
timestamp: November 07, 2020, 09:02:38 pm
content: Hi Smoky, I recently purchased a SX-580 and have determined one of the STK-0029 Power Packs is faulty. I tried to go the quick and easy route of ordering one online, though I had read all the warnings....needless to say it's not great.Technically it does produce sound and it's not terrible, but there is clear distortion I can both hear and see on the oscilloscope. And I'm confident it is coming from the STK-0029 because the other channel is working great and I can swap the packs and the distortion follows.Anyway, lesson learned. I see you went with the STK replacement boards and now that it has been a few months for you I wanted to ask how those are working out?I also wanted to verify the modifications you made. I saw in the post you:1. opted out of using the aluminum heat sinks and just mounted it directly to the existing sink2. swapped the 200 Watt ON Semi bipolar transistors for 150W3. added the 0.22 Ohm 5 Watt cement resistorsWere there any other changes you either needed to make or simply chose to make that I missed or weren't mentioned previously?I really appreciate everything you've documented so far, it has made it much easier digging into mine since this is my first attempt at working on vintage audio gear.

author: Smoky
author_rank: Frequent Contributor
timestamp: November 07, 2020, 09:53:42 pm
content: Absolutely worth it!Not one hiccup.Do this to make the install even easier. Follow the traces on the Pioneer's main board from where I soldered down the cement resistors. There you will find jumpers installed by Pioneer at the factory. Remove the jumpers that Pioneer installed and mount the cement resistors in those locations.That way, you can use all "wire" jumpers from the new power pack modules down to the main PC board.It was a great project

author: SpecialK
author_rank: Regular Contributor
timestamp: November 09, 2020, 03:36:39 am
content: Does the SX-580 have speaker protect relays? I recently picked up an SX-3600 and was surprised that it does not.

author: Smoky
author_rank: Frequent Contributor
timestamp: November 09, 2020, 04:32:00 am
content: The SX-580 has no relays.I was told not to increase the capacitance of the bulk filter capacitors because that could cause/worsen a "pop" in the speakers when the receiver is turned on.Other than that, the "muting" circuit quiets the receiver when the selector switch is turned. That's about the extent of my knowledge of the circuits.