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SX-636 no longer working properly after using DBT

V

voxswain

Member
To preface this, I am new to audio repair and definitely did not think I was going to be getting into a full repair with this receiver. I'm handy with a soldering iron and have a multimeter, but I don't have a huge amount of experience or knowledge with electronics. I'm excited to learn, but please don't assume I know anything!

I recently built a dim bulb tester using this guide that I was planning to use for testing during the recap of my Pioneer SX-636 receiver, which was previously working perfectly other than some hiss from the headphone out that I was hoping would be fixed with a transistor replacement. After building and testing the DBT with my multimeter (and ensuring that it works as expected), I powered up the SX-636 with it once to ensure that it worked. All appeared well, the bulb lit up, and the receiver turned on. I later hooked the receiver back up to my stereo system (still waiting on parts for the recap), but upon turning it on, noticed a terrible hum from my speakers.

I immediately disconnected my speakers and tested each speaker output, and I'm seeing a DC offset of about 24 volts from both A channels and about 15 mV from both B channels. I also opened her up and took a look inside, but didn't see anything obviously wrong. This is the extent of the troubleshooting I've done so far, and I'm not quite sure where to go from here. I would appreciate any advice or resources any of you have that I might be able to use on my first repair. Thank you!

(Side note: It has been suggested to me, and I realize, that it would probably be better to find someone in person who has experience and is willing to help, but I don't think the guy who owns the only local repair shop would be interested in helping from my previous experience with him. Suggestions on how to find someone locally who might be willing to help would be appreciated, but my primary goal here is really just getting my receiver up and running again.)
 
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Hi Voxswain, welcome to Audio Karma

What wattage and type of bulb are you using? around 75 watt should be more than enough to kick it out of protection.

Good job on building the DBT. Many new techs, myself included, miss this step. I'm not sure if the DBT is to blame but we will be using it as we service this unit. I'm not sure why one speaker channel has a high offset and the other does not. I suspect that we have a problem with the negative voltage.

If you have the confidence to work on your own gear lets get the ball rolling lets get some voltage readings of your power supply. Fuses are healthy and correct spec? Be very careful not to short any pins on the power supply board - all work will be done on the DBT. If you are using probes on your multimeter wrap them in electrical tape and expose just the very tip. Be sure to get the service manual, I found one here: http://www.kallhovde.com/pioneer/sx-636sm.pdf

Below information is not for the 220v model. If you are on 220v AC then consult the service manual for the correct info.

Notice the pins have a voltage next to them in the below photo, please take note of the voltages on the pins compared to ground (chassis is fine). Pins 1, 3, 4, 5, 7, and 8 will be AC I believe, the rest will be DC. Voltages may not match because of the DBT but should be in the ball park.

upload_2020-5-28_12-48-48.png
 
MIJ_Pioneer said:
Hi Voxswain, welcome to Audio Karma

What wattage and type of bulb are you using? around 75 watt should be more than enough to kick it out of protection.

Good job on building the DBT. Many new techs, myself included, miss this step. I'm not sure if the DBT is to blame but we will be using it as we service this unit. I'm not sure why one speaker channel has a high offset and the other does not. I suspect that we have a problem with the negative voltage.

If you have the confidence to work on your own gear lets get the ball rolling lets get some voltage readings of your power supply. Fuses are healthy and correct spec? Be very careful not to short any pins on the power supply board - all work will be done on the DBT. If you are using probes on your multimeter wrap them in electrical tape and expose just the very tip. Be sure to get the service manual, I found one here: http://www.kallhovde.com/pioneer/sx-636sm.pdf

Below information is not for the 220v model. If you are on 220v AC then consult the service manual for the correct info.

Notice the pins have a voltage next to them in the below photo, please take note of the voltages on the pins compared to ground (chassis is fine). Pins 1, 3, 4, 5, 7, and 8 will be AC I believe, the rest will be DC. Voltages may not match because of the DBT but should be in the ball park.

View attachment 1880912
Click to expand...

I used a 65W incandescent that I had laying around for my initial testing, but I've got a 100W clear bulb coming in the mail that I'm planning on using the majority of the time. Fuses all look good, although one appears to be adhered to its holder somehow (glue?). I think this receiver has had some previous work on it, because I've noticed a few unwrapped wires that were soldered instead of re-wrapped. Here's the voltages I was able to read off of my power supply:
  • Pin 1 read out at 20.5V AC
  • Pin 2 had -28V DC, vs -31V expected
  • Pin 3 had 7V AC
  • Pin 4 had 6.5V AC
  • Pin 5 had 20V AC
  • Pin 6 had 28V DC, should have 31V
  • Pin 7 had 30V AC
  • Pin 8 had 29V AC
  • Pin 9 had no charge
  • Pin 10 was 9V DC and should be -41V - looks like you were right about the negative voltage.
  • Pin 11 had no charge
  • Pins 12/13 both read about 13V and should be 13.5
  • Pin 14 tested out right at 6V, as expected
  • Pin 15 had no charge
  • Pins 16/17 both read a clean 30V, right where they should be
A couple questions:
  • How are you able to tell which pins are AC vs DC? I figured it out because my MM just wouldn't read if I were on the wrong setting, but is there a better way to tell?
  • How much variation is acceptable in the voltages I read? Obviously that 9V vs -41V on pin 10 is a problem, but is .5V, as in pins 12/13, enough of a difference to cause problems?
  • I see that expected voltages are labeled for some pins, but not all of them. How might one figure out what the unlabeled pins should test out at?
I was also saved for the first time by my DBT! One of my MM probe clips fell over and bridged 2 pins, and I wouldn't have noticed had the bulb not blinded me. I guess I see the value in it now ;)

Thank you so much for your assistance and the warm welcome! It's exhilarating to feel like we're making progress :)

One more (semi-unrelated) question - is there a way that I'm supposed to tag someone so they see my response? Or do you see it when I hit the "reply" button?
 
markthefixer said:
2sc1451 in one of the power amps. (known troublemakers)

They just LOVE to do things like this. ANY excuse at ALL.
Click to expand...

Thanks for the suggestion. This is the kit that I ended up buying to recap (and hopefully fix my headphone out). Hopefully it'll come with some replacements for the ones you mentioned! If not I'm sure I can pick up some replacements from Mouser. Is it common for there to be modern equivalents to these older parts that aren't as likely to go haywire?
 
petehall347 said:
check d7 and r1 if ok suspect c7 c8 .
also for fun check f4
Click to expand...

What would be the best way for me to figure out which components are referred to by each part number so that I can test them? I don't see the numbers printed anywhere on the top, but maybe I should flip the board over and see if they're on the bottom? Looks like some weird plastic posts holding this one down. F4 is good, fortunately.
 
You probably put in a transistor the wrong way. Swapped pins or something. There've even been cases of people using the wrong polarity (PNP vs. NPN). Check that first before doing anything else. Blanket replacement of components usually is a Bad Idea™. Except in cases of Known Troublemakers™.

BTW- 65 W bulb for your DBT is big enough. Most amps draw 20 W or so. Only for troublesome amps that won't start properly due to rush-in current a heavier bulb is called for. I almost never use it and am at a 60 W one most of the time.
 
Kits are only as good as their documentation. Finding the "code" that tells you what board and circuit position gets what replacement part is CRITICAL.
Sometimes in a GOOD kit the author (not he whom "Borrowed" the freely shared list) will reverse engineer the component, and coupled with thirty years of hindsight about the design and trade-offs used, makes a choice that is not intuitive, of a more appropriate component.
Kits must include documentation of the lead assignments for transistors, because only rarely do modern replacements conform EXACTLY to abandoned lead layouts - and voila, the transistor goes in wrong (5 chances out of six) and lets the magic smoke out.
The DBT wattage can be increased by connecting in another lamp of appropriate wattage in parallel with your DBT lamp - just like a "candelabra" type light fixture.

Flipping the board: the plastic posts can be gently squeezed on the end to compress their diameter to allow the plastic to slip out of the hole - warning: plastic can deteriorate to where squeezing can crack them. Appropriate modern replacements CAN be found, but it will sure as %$#$ slow you down.
We do NOT unwrap those wraps. The harnesses can be loosened to gain slack.
At the worst the pins holding the wrap are unsoldered from the board. I don't have to do that very often.
The better the tools, the easier the job, things that cause you to fight your tools just plain suck the fun all out of this effort. I bootstrapped myself tool- wise over the years.

Re: figuring out the components.

get the service manual which will have schematic diagrams, and they will identify the board (sub assembly) you are working on, Then find the foil pattern diagram - that will identify, say q4, as to location and lead arrangement.
2 things to remember - the foil pattern will be BACKWARDS of what you see of the components upon the NON-foil side of the board- because they are intended to be used FROM the FOIL SIDE.
also the pc board itself will have symbols (like +'s for capacitors when that matters) the diode symbol's arrow direction, and the transistor's emitter, base and collector leads from the symbols.

Oh!! INSULATE all but the tiniest part of the tip of your dmm probe, it WILL slip when you shift your eyes to read the meter. It happens to ALL of us.
And also on the subject of insulation - gonna stick a screwdriver in there to adjust something? INSULATE IT TOO. There is a long litany of times where a small screwdriver is threaded past exposed connections to make adjustments, it hits something and suddenly it's another week or two and a bunch of parts to get back to where you were just seconds ago. Talk about a time warp!!!. Insulated (office transparent tape!!! ) screwdriver shafts are CHEAP insurance.

Yeah, my head isn't yet hammering me tonight.
 
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markthefixer said:
And also on the subjetct of insulation - gonna stick a screwdriver in there to adjust something? INSULATE IT TOO. There is a long litany of times where a small screwdriver is threaded past exposed connections to make adjustments, it hits something and suddenly it's another week or two and a bunch of parts to get back to where you were just seconds ago. Talk about a time warp!!!. Insulated (office transparent tape!!! ) screwdriver shafts are CHEAP insurance
Click to expand...
Or get a non-conductive adjustment tool.
 
I have been working on a SX-636. Giving it a shakedown tonight finding the function selector still needs more work. It is a nice sounding Pioneer.

The white pins can use pliers to help squeeze the top part together and try to pop each corner loose. Be careful the 45 year old plastic is brittle and it is easy to snap them off. Can also crack the board too. My PS board two are missing the top part. Reworked mine did not undo any of the wires except the ones to the filter caps when they got replaced. Can pop some of them them loose and work on the board at an angle. Tone board take the knobs and front face off and undo things to get the tone board free. Also work some of the wires out of the retainers to allow more freedom of motion with some boards, particularly the tone board. Amp and tuner boards can be worked on installed.

If you need to get to the output transistors, you can take the back screw on each side holding the heat sink to the bracket and loosen the screw on the tuning cap(right) side of the chassis. That should let you tilt the heat sink forward making it easier to unscrew those four transistors. The transformer blocks the other screw but loosening the other side I was able to tilt mine.

As far as recapping, there are some good lists on this site to use that Markthefixer and others contributed to with part numbers to order from Mouser or Digikey. Here is one thread to look at:
https://www.audiokarma.org/forums/i...-sx-636-rushing-sound-on-left-channel.517531/

With any list, check you unit and make sure things match. Some caps get missed, some values that were shown in the service manual get changed during production. Check unit board by board and any others like the one on the signal strength meter.
 
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If you want to get someone's attention you can quote something out of their post in your post or you can like their post. That puts a red flag up on the Alerts at the top of the page. Also something will pop up on their screen telling them they have an alert if they are still on the site. Doing either should get their attention and they will likely check the thread.

Also under Inbox there is an option to start a conversation. That is basically how you can private message with another member on the site. It nice to have most conversations public. I have made use of many past threads helping fix units I have been working on.

You have a number of transistors in these old Pioneers that tend to act up with age. 2SA726, 2SC1451, 2SA725, 2SC1312. The first two types should be on the amp card where they can cause the most trouble. Some tend to be worse than others.
 
Something like this is a good way to test every transistor to verify the pinouts against the symbols on the PC board. I've been checking every transistor before installing it.
transistor tester.JPG

Also I've used these hemostats for all sorts of tasks including gently squeezing those pins that boards are mounted on, and just all kinds of other holding and light squeezing, it's a great tool for working on an SX-636:
hemo.JPG

I found the wires on my SX-636 PS board to be short on slack so getting those loose enough to flip the board might be a challenge, but others report doing it. I made the unwrapping mistake to remove the board which was really nice for replacing components but I had to resolder the wires back on the right pins without much slack to work with, which was a PITA, but it worked - not recommended though!. :rolleyes: nooby mistake I learned from

GOOD LUCK AND YOU'RE GETTING SOME GREAT ADVICE HERE. I still can't believe how great my SX-636 sounds after using MTF's list and getting lots of help here. Great project. Nice simple receiver, no relay to worry about, fun to work on.
 
PE9ZZ said:
You probably put in a transistor the wrong way. Swapped pins or something. There've even been cases of people using the wrong polarity (PNP vs. NPN). Check that first before doing anything else. Blanket replacement of components usually is a Bad Idea™. Except in cases of Known Troublemakers™.

BTW- 65 W bulb for your DBT is big enough. Most amps draw 20 W or so. Only for troublesome amps that won't start properly due to rush-in current a heavier bulb is called for. I almost never use it and am at a 60 W one most of the time.
Click to expand...

Hmm, to be clear, I haven't replaced any components yet. I based my bulb pick off of the listed 140W power consumption in the service manual - did I make the wrong choice here? The 65W white bulb I'm using right now definitely works fine, but I've got a 100W clear one coming in the mail that'll hopefully make it a bit easier to see the filament.
 
I've managed to get the board off of the plastic posts with some needlenose pliers and patience, but the wires feel like they're still holding it down pretty tightly. It's possible that the wires are just stiff, but can anyone confirm how it should feel? The last thing I want to do is break anything else o_O

Edit: Yeah, it's in there pretty solid. The wires on 2 of the 4 sides make it pretty tough to "hinge" on one side, so I think I'm going to have to desolder. Fortunately someone else already unwrapped the wires, so I don't have to feel bad about unwrapping them myself :) Maybe it would make sense to replace the wires with longer ones so that I don't have to do this again. Is that a thing that people do?
 
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Alright, got it up enough to test things. Lesson learned here: check the bottom! I did still have to desolder a few wires but I was able to unclip a few ties on the bottom that made it much easier to pull the board up. I tested R1 and D7, and I think both were good. R1 was spot on at 470 ohms, D7 was about 0.5V 1 way and 0V with probes reversed - that's indicative of a good diode, right? Also, would someone mind explaining the insulation bit a little more in depth? I understand that lack of insulation might cause you to accidentally bridge two contacts, but why does it matter if you're testing things with the receiver off?
 
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