Pioneer SX-850 Restoration

Discussion in 'Pioneer Audio' started by Ross Henning, Jun 3, 2018.

  1. Watthour

    Watthour Electron Rancher - JS3600

    Are you confident in the polarity of those voltages at Q6 and Q8? They seem backward, but the rail voltage is so low that biasing is possibly way off.

    Also, are we certain that the correct output trannies are in the correct sockets? I would suspect the previous owner may have been in there swapping hardware.
     

     

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  2. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I believe you are correct, I mixed up Q6 and Q8 when I wrote down the measurements. I also went back and double checked the output transistors and verified that I have the correct replacements in the correct sockets.
     
  3. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    Is it all likely that power supply voltages or some other external issue could cause the biasing to be off enough to throw this thing out of whack like I'm seeing? I've not done anything to the power supply or any other board as of yet.
     
  4. 1MEGΩ

    1MEGΩ Active Member

    Messages:
    184
    Location:
    Mamou, Louisiana
    You can't get the correct voltages at low AC input. It screws up the biasing.You need to apply 120 volts AC to get the right voltages through out the receiver.
    Are you using a light bulb in series with the AC power? If not, then you need to do that. I'd start with a 75 watt bulb. That way it won't damage anything in the 850. With a 75 watt bulb you can get no more than .65 amps to the power supply. Put all the transistors back in the unit. Power on and bulb will go dim if all is ok. If the bulb stays bright, let us know.
     
  5. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I don’t have a DBT currently. I purchased a really nice Variac that I’ve been using. I’ve read conflicting information on the DBTs. I’ve read that it would limit the voltage, and I’ve read that it would limit the current. If the bulb is acting as a resistor, would it allow the receiver to power up to full voltage with the current limited, or would it act the same as the variac and limit the voltage as well as the current? If it will limit the current without limiting the voltage, I can see how it would be different than the variac, and extremely useful.
     
  6. 1MEGΩ

    1MEGΩ Active Member

    Messages:
    184
    Location:
    Mamou, Louisiana
    Right now, limiting the voltage and the current is exactly what you need to do to avoid damaging the unit. A variac will limit the voltage but as you increase the voltage, you increase current as well. The bulb will limit both. A good working SX-850 will operate with a 75 watt bulb in series with the AC power. However, as you turn up the volume, the voltages will drop and the unit will stop working. Lower the volume and power will return and it will work again. Trust me, I have been doing this for almost 50 years now.
     
    Last edited: Jun 12, 2018

     

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  7. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
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    Alright, I’ll stop by HomeDepot on my way home from work and pick up the supplies to build a DBT and see what I find with that. Thanks.
     
  8. 1MEGΩ

    1MEGΩ Active Member

    Messages:
    184
    Location:
    Mamou, Louisiana
    This is my DLB on Variac I built back in the late 70's or early 80's Maybe you can build yours this way.
     

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  9. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I built my DBT and gave it a whirl. Incandesent bulbs in higher wattages are hard to find, at least at Home Depot. Most of them are now “equivalent” with halogen lamps inside a traditional bulb shape. I bought a 75 watt work lightbulb that appears to be a regular 75 incandesent with a silicone sheathing around the bulb. Naturally it doesn’t work. I tested it and its an open circuit, so either there was a bad bulb in the batch, or someone returned it and Home Depot put it back on the shelf without checking it. Fortunately, I still have a few bulbs at home up in my cabinet. I didn’t have a 75 watt, but I had a 60 watt. With the 60 watt bulb in the DBT, firing up the SX-850 results in the filament glowing at a mid brightness for a moment when the receiver is first turned on, followed by a quick dimming. About 3 or 4 seconds or so after this, the bulb brightens for a moment and then dims back down. Unfortunately, there’s still DC at my outputs, so despite good signs with the DBT, I’m not out of the woods.
     
  10. 1MEGΩ

    1MEGΩ Active Member

    Messages:
    184
    Location:
    Mamou, Louisiana
    It will work just fine even with a 60 watt incandescent light bulb! It is dimming, so that is a very good sign. Just make sure the volume control is set low, like no more than 8 o'clock.
    Can you be a little more specific? Are you using the variac? If so is it all the way up on voltage or at least 120 vac? Is the protection relay clicking in? How much DC is on the outputs and where are you measuring it? Did you readjust the amp?

    I have a Doctor appointment today, so my time will be limited at least until tomorrow.
     
    Last edited: Jun 14, 2018
  11. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I didn’t have much time to do any thoughrough testing last night. I ramped the Variac up the first time to make sure I didn’t have a solid strong light from the bulb. After that, I ran it with full 120VAC. The protection relay is not kicking in. I don’t remember the exact voltage on the outputs, but I will go back and get measurements and repot here tonight. I do remember that it was tens of volts, not mV. I have not readjusted anything yet. When I tried at low voltage, the pots made no difference. That may or may not still be true, but I have a feeling that it probably still remains true. There’s something just not right, although I’ve not been able to find any components that test bad, or are installed incorrectly thus far.
     

     

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  12. 1MEGΩ

    1MEGΩ Active Member

    Messages:
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    What I would like to know is, where are you taking the DC offset voltage readings? What pins on the power amp board? Something is squirrely here.
     
  13. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    Okay...not sure what the heck I measured last night, because they weren't tens of volts. I put the meter on the points described in the manual and was able to get both sides down to 20mv on bias and the protection relay kicked in. The only thing I'm not able to get right is the DC offset on one side. One side I can get extremely close to 0 volts, the other side I can't get any closer than -154mv with the potentiometer pegged to the counterclockwise position. Is there something I should check here? I could replace R12 with a smaller value (I tried a 10K resistor in parallel with the 9.1K that is there and got it below -100mv). I'm not sure if that's the right thing to do, or if there's a component I should look to possibly change (maybe a transistor that will bias a little more a different direction than the one that is in there...possibly Q2)?
     
  14. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    Okay...nevermind that last question. It turns out I had one of those "Doh!!!" moments. My Dad calls them "HUYA" moments (Head up your posterior). Looking at the back of the circuit board, I found a point where I had unsoldered a resistor, when I was checking all of them, that I failed to re-solder. Once I fixed that joint, I was able to align the amp perfectly. It sounds great. It might not have quite the emphasis on lows that my SX-980 has, but it sounds very clean and "open" if that makes sense. That's without anything else done to it. I'll go through it and replace all of the electrolytics, noisy transistors in the flat amp, etc. This one should turn out really nicely when I'm done. I'll come back and post progress. I'm beginning to wonder now if I'm going to be able to let this one go to my son, or he's going to get the SX-880 I rebuilt after all! :) Daddy may have to keep this one...lol
     
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  15. 1MEGΩ

    1MEGΩ Active Member

    Messages:
    184
    Location:
    Mamou, Louisiana
    When you stated you had "tens of volts" at the outputs and a dim bulb, I knew there was something squirrely because if in fact you did have dc voltage on the output , the bulb would have stayed bright! Glad you got it working.
     
  16. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I've now gone through the flat amp and the tone amp and replaced all of the noisy transistors, electrolytics, "greenies", and tantalums. I've also replaced all the transistors and electrolytics on the power supply board. I don't know if it's my imagination or not, but I would swear the thing sounds even better now. Here's a question for you expert guys: I've been looking at someone else's restoration of an SX-850 here: http://marantzhallo-fi.blogspot.com/2017/07/pauls-pioneer-sx-850-receiver.html. This guy replaced all of the rectifier diodes on the power supply board, and he removed the bypass caps. He added one film cap back on to the 1/2 wave rectifier (two diodes) that feeds 23.8V to Q8. He replaced the diodes with some ultra fast recovery Schottky diodes which I've read switch more quietly. Perhaps that's why he removed the bypass capacitors? He claims that this was a very worthwhile upgrade. For grins, I've ordered a bunch of both of the diodes that he used. I had thought about making some little boards like he did to mount them on, but I don't see why I couldn't just solder the new diodes in place of the old ones, and keep the bypass caps as they are, or possible replace them with better film caps? Anyone want to weigh in on whether that would be a good or bad idea?
     

     

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  17. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
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    Tonight I replaced the main filter caps, recapped and replaced transistors on the protection board as well as the phono preamp (equalizer) board, and replaced all of the bulbs with soft white LEDs. I added .1uf bypass caps to the new filter caps, along with the bleeder resistors. The only thing left to do is recap the tuner board and align the tuners (and when the come in, replace the diodes on the power supply board). The bad thing is that in order to recap the tuner board, I'll have to take the dial cord loose. Has anyone else done this with an SX-850? Is it a pain to get back on? Here's photos of the power supply board after the recap as well as the front of the receiver after replacing the bulbs.

    Oh, I almost forgot one other question I have. With 120VAC input to the unit, I'm only getting 43 volts rail voltage. According to the schematic, I should be getting 45, and the rebuild that I referenced above also shows 45 volts. Could the current diodes somehow be bringing that voltage down a little somehow? Just curious if there's anything I can do to bring it up to spec.

    More to come....

    IMG_0377.jpg
    IMG_0379.jpg
     
  18. Watthour

    Watthour Electron Rancher - JS3600

    43V is probably within the ± tolerance. I doubt that the diodes are at fault since a typical silicon rectifier has a forward voltage drop of around 0.65V on its worst day. That doesn't account for a 2V difference, but a meter calibration (either in 1978 or today) might make that difference.

    As for the dial cord, wrap removable (blue) masking tape around the cord and drum, then remove the drum. If it goes very badly, the cord stringing diagram is in the manual.
     
  19. 1MEGΩ

    1MEGΩ Active Member

    Messages:
    184
    Location:
    Mamou, Louisiana
    You don't have to take the dial string loose to recap the tuner. I have done it many times. See my posts #122 and #129 in this thread.
    http://audiokarma.org/forums/index.php?threads/recapping-sx-850-for-my-son.815516/
     
  20. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I used Watthour's suggestion and wrapped the drum. I recapped the board, then did a full AM and FM Tuner Alignment. It sounds great. I do have some weird intermittent problem where the left channel gets grounded out somewhere possibly in the "Adapter" switch. When I first got the tuner board back together I had lost the left channel and started checking all of my work. After wasting probably a half-hour on that, I figured out that it wasn't just the tuner. All inputs were affected. I started looking for the issue, and after flicking switches on the front, I'm pretty sure it was the adapter switch that I flicked a few times and it came back. This has happened twice now, so I'll have to keep playing with it. If I can determine for sure that it's the adapter switch, I have a spare I can take off a parts board I bought on eBay. Otherwise, this one is finished and working well. I got another one in today, and I have a question that I will post in the next message rather than ask about it in this post.
    IMG_0385.jpg
     

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