Problem with Vintage Pioneer receiver SX-1000TW Output

Have a vintage Pioneer SX-1000TX receiver that the right channel doesn't seem to be working 100%. The output is slightly lower than the left channel and "thin" as best I can describe it (e.g. lacking any body to the sound). I have the schematics and have done some basic trouble shooting but at a loss where to go from here as I'm still learning about electronic/circuits etc. Here's what I've determined so far.

1) Using a old audio signal injector, I believe I narrowed it down to the Main amp unit or beyond. Initially, I thought I thought the problem was in the control amp unit where the tone controls/preamp circuitry all lie, Since it seemed like the lack of gain and lack of "body" could be related to the bass circuitry but injecting the audio signal right at Pins 2 In on the Main Amp unit (post Control Amp Unit shown in the block diagram below) still shows a disparaging difference between the channels.I was afraid to put the injector any further down the circuit as it would getting more severe of a "pop" as I moved the injector through the circuit. It's an old tube EICO Model 324 frequency generator and I'm not sure if it has the proper protection necessary or not, so I didn't want to chance it any further.Given it was designed for old tube radio diagnostics and they have much higher voltages, it probably is, but would like a confirmation from anyone that knows how far down a transistor based circuit it's safe to attempt signal injection.

2) I believe I esr checked many of caps in the main amp section (2nd schematic below), at least those greater than 10uf. Also esr'd the big 1000uf caps that follow the main amp. They all seem fine though it was done with an in circuit measure, so I realize under some cases those readings won't be accurate.

3) I measured the voltages around the transistors in the main amp circuit. I color coded those voltage readings in Red on the 2nd schematic below. I find a few interesting things from these measurements which I can't explain:
a) the first transistor Q801/Q802 in the circuit doesn't seems to react the same between the left and right channels. The right channel seems "abnormal" to me that all the voltage all around 10 volts. On the left channel, the emitter has a much higher output from the base/collector.
b) The right channels has higher voltages in general across the circuit. For example Q812 has 26.3 volts at the base, yet driven from the same 40 volts that the left channel's Q811 gets, but it's pulled much lower to 22.1 volts. I want to understand what in the circuit of the right channel could be causing that large differential. The Cap C808 failing (even though esr'd ok), One of the resistors R814,816etc out of tolerance? Diode D802 failing? Or something else in the circuit affecting it?
c) The output of this main amp unit feeds two power transistors from pins 7 & 11. On the right channel, Pin 7 (which feed q2 on the block diagram) is a couple of volts higher on the right than the left channel, yet Pin 4 (that feeds Q4) a 10th of a volt lower. I don't understand the relationship between these two power transistors, but it seems the overall voltage output for the amp is higher on the right channel, yet the signal is weaker/thinner.

4) One last item of note - The power resistor R4 that follows Q2/Q4 on the block diagram seems pretty heavily out of tolerance. Should be 330K and measures about 220K. What is the effect of that lower resistance on the circuit?

Appreciate any specific response to my questions above, as well as anything else to check for, measure etc. Obviously I want to fix this but in truth I want to learn "why" things are doing what they are doing, and what the circuit purpose is. I'm really at a beginner level in a lot of this stuff, but looking to learn and improve my diagnostic and repair ability.

 

Chris, I will check that out but note that I inadvertently posted before I was finished editing. I just saved after your post so there is much more information there (and questions I have). Thanks!

 

The effective voltage across R29 (left channel) and R30 (Right channel) is zero. Not sure I'm surprised given a .5 volt or less at each of those connected transistor bases.

Unrelated - I had an part # error in my original text regarding item #4 and that 330K resistor - it was R41 not R4.

 

Meant 330 ohm (typo). I will check everything else you list tomorrow
and get back here. Thanks!

I'll have to check that resistor voltage again tomorrow as well . I'm pretty sure one was definitely zero. The other might have been very very small so I need to remeasure and se how close it was to your .014V target (where did you get that from by the way?). As long as I'm adjusting things, should I adjust the Center rail voltage first, prior to that Vr5/Vr6 adjustment or visa versa? Also what's weird is the the block diagram shows the powersupply as 78v, but as it goes to pin 6 on the Main amp schematic it says 82V (yet is actually reading 86.9v). Is that a typo on their schematics or are the power filter caps (C12/C13) somehow boosting the voltage? I get the powersupply board could have it's own issues not supplying the correct voltage (and maybe I should have started my diagnosis there, schematic for that below), but the question was really more how we go from a schematic 78v to schematic 82v without any obvious voltage circuit boost (at least not obvious to me) And if I adjust the center to 1V less then half the rail - should it be half of the actual ((86.9/2) - 1) or intended spec ((82/2) -1)? I assume it wants to be half (minus 1) of actual voltage, whatever is there.

 

I expect that it will, especially after what happened today (more on that following) but I am PT and about to retire in two months so I have plenty of time on my hands.

I verified you were correct about the mismarked collector/emitter values. just got them mixed when writing them.
I adjusted VR5/6 first to .014v. They were definitely lower initially. Didn't resolve anything but didn't seem to cause any harm either, at least initially.
I did check the resistors R804/806 for high/low. Both right on the money value wise.
I decided to swap the 801/802 transistors to see if the problem moved. Did that and was trying to verify if it shifted the problem to the left channel or not and check measurements and that's when the fuse blew. I had sound just prior and I was in the process of comparing left vs right sound to determine if it got better or not when it went.

Pulled the dead fuse out. Noted that it was a 4amp fuse yet the block diagram in the original post shows 1.5amp. I had a 1 and a 2 so I went with the 2amp. Blew again. Disconnected both the 40V and 78v/64V outputs from the power supply board and unit stayed on. Measured those outputs and got 40 & 64 Receptively (the 64V threw me, because I had been referring to a 78 and 82 V source from the schematic pins but I also realized that the schematic has 64V in parathesis, so I'm guessing it's really ok. Also measured the other Power supply outputs and they all are ok as well. I tried just connecting the 40 and then just the 64 but blew the fuse each time. At that point I realized (and should have about 5 fuses earlier) I needed to build a bulb limiter, which I did. Put both 40/64v connections back and bulb is glowing bright and hear a hum out the speakers as well.
Re-ESR'd both the power filter caps and the output caps and they check out ok. Tried to measure voltages but with the limiter circuit I'm assuming that radically affects all the reading due to the current change, as I only saw a volt or two on the 40 and 64 lines....

When I swapped the transistors, I'm sure they were in correctly. I also had a heatsink on the pins both when removing and soldering back though I did a real dummy move and left the heatsink across all three pins on Q801 when powering on. It was a brief moment of stupidity and I realized what I had done the second I turned it on - but the fuse didn't blow then. I turned off removed the heatsink and back on again, and it was shortly after that the fuse blew when I was comparing channels.

What kind circuit damage would that Q801 pin short do? That's certainly one possibility.
Another is some heat damage to either transistor, or some else close on the trace (maybe the connected collector diode?)
Another is the VR5/6 voltage adjustment pushed a partially faulty/failing output transistor over the edge. I guess I could pull them off and do a diode check on them to see if they are shorted or open?

Other thoughts to check what might be causing the fuse to blow?

 

Update - With unit on under bulb limiter, it's only the right channel that has a loud hum. Left is quiet (at least with volume set to zero.) Checked all 4 output transistors q1-q4. Right channel Q2 has a short between collector and emitter. Which certainly would contribute to the loud hum and I assume the blown fuse as well. Interesting that it was the right channel that I had the weak signal all along. Perhaps the Voltage adjustment to .014v just took that transistor over the edge? Or that in combo with a better Q802 transistor driving the amp? My heatsink short mistake would have no bearing, since that was on the left channel.

So next step obviously to get a replacement Q2, but anything else in the circuit that should (or even can be) checked with Q2 out before I risk a new one?

 

I have the original user manual which I just scanned and posted here http://home.comcast.net/~jimkarl/Pioneer_SX-1000tw_User_Manual.pdf

I'm in Maryland. Not really anyone local to my area, short of a radio shack though. I tend to buy most of my parts on ebay, unless it's a harder to find item. Since I buy stuff very onesy/twosey, it generally hasn't made sense to buy from Mouser/digi etc because the shipping usually costs as much or more than the part itself. But there have been some hard to find bulbs, for example, that I needed for a different receiver that I had to bite the bullet and pay some heavy shipping for.

I could get a NOS direct replacement (same part #) for Q2 off ebay, a pair actually for about $13.50 - one for the fix, one as a spare. It will take a while to get here but I'm not really in any specific hurry. I find that if the "alternative" replacement has some slightly different performance characteristics, and they often do,, then I need to match all the others as well (and the cost starts to rise) or things might be unbalanced/uneven. .As I'm just starting repairing things (this being about my 6th or 7th item) I'm starting to build up a cache of parts but unfortunately transistors aren't in there (yet). Have a few different diodes, but a wider kit range still on order. In truth, my goal for this is not a complete restoration on this particular item. It's really more to just get rid of any major operational problems (in this case, the weaker right channel), as these items are for resale and I will disclose what I have done to the unit and what I haven't or any known issues/concerns. So I have to be mindful about parts costs because they can quickly exceed the product value. I found that especially to be the case when I had leaky vintage power caps, especially on things like vintage radios. Those parts are harder to find and get pricey quick! Having said that, I've found myself "investing" in parts as I go. I generally try to buy a spare or in some cases a whole kit (e.g. I needed a 50V cap for this same receiver's power supply board last week (failed ESR and capacitance was way out of spec) and since I didn't have ANY 50Vcaps, regardless of uf value, I figured go ahead and buy a wide range kit of them, as somewhere down the line I'll need another, and likely a different value too. Anyway, the point being that if the replacement transistor fixed the weak channel problem, I would probably consider that a stopping point, barring no obvious other problem like noise etc.. If it was still weak or channel was noisy, I would be back to the control amp board and further part diagnostics/replacement. I appreciate you pointing out the troublesome 2SC458 transistor as well. That BC549B is certainly cheap enough that I can buy a small lot of them for the next time I might need them and once I can compare my channels again may find one channel still needs some work over the other but at the moment, it sounds like getting a replacement going is my top priority.

I will check out that transistor on the power supply board as well. I think I did the diode checks last week but I'll do them again just to be certain.

 

I agree with you. I'll replace both output transistors on that channel. I was also planning on getting those BC549B replacements for Q801/802 since you pointed out they are high failure parts and they are so inexpensive and I get get a bulk of them for future repairs.
I uploaded a pic of the amp board in case there are others you see and recommend as well. I have to be a little realistic that this isn't a restoration project and I can't replace every part that is old (otherwise the whole device should be replaced LOL). Where I can identify failed parts or significantly out of tolerance I need to replace them. Where they have known failures "histories" parts (but are currently still working and in tolerance) and the parts are so inexpensive and can get a bulk of them and will high a high reuse of the leftovers over time, I want to do those too. but I have to draw the line someplace so I don't want to replace parts that aren't bad yet because that isn't the end goal, at least for this particular unit.

The whole purpose of coming to this forum is to actually learn how I can tell a bad part from a good one, or a marginal one that should be replaced. Prior to this unit, I learned that I really needed an esr meter to check caps. Caps I thought were good from a capacitance reading, but really weren't and only the ESR meter could tell me that. I learned how to check diodes, and transistors the same way. That's how I found the short on this Q2 output one. I suspected a problem with Q802 originally, not because I knew what the expected B/E/C voltages should be but from a comparison of the other channel's transistor relative values. Though ideally I would be able to learn and understand what the expected values should be without a comparison part in circuit.
I also learned about the light bulb limiter (though built it a few fuses too late LOL).


In this thread - I've learned a number of things so far from you and all that information is much appreciated. What the target center voltage needs to be (though I'm not sure I got an explanation for the -1V below center, does that have something to do with a circuit voltage increase downstream that will get it ultimately closer to center?) How to check and set the output transistor current/voltage. I saw those variable pots on a number of other receivers I had and wish I had known then what I know now. Though I knew ohm's law, I learned a real practical application for it here - determine what the measure voltage should be when not stated on the schematic - buy using the target amperage value. When you told me that, it was like a lightbulb going off for me. Now I get how I can apply that to solve and debug next time.

There's still so much I don't know yet or understand. I'm going to check out that power supply right after I hit send here, but the moment, let's assume the 64V output with the power supply out of circuit is correct (it does after all match one of the two voltages listed on the schematic. My assumption is they listed the unloaded voltage as 64V and the loaded one as 78V). When the unit was "working" I was getting 86v, certainly well over both targets. Without anything obvious in the circuit to boost that voltage (power runs from the power supply, through the filter caps and to the amp board and also to the output transistors) the applied Ohm's law would teach me that either the resistance of the circuit changed, or the current did. Certainly connecting the amp board and those output transistors to the circuit adds some resistance. I guess it's possible some transistor in the circuit (amp board or possibly that then failing Q2 output one) is/was leaking current onto that path, which would drive the voltage higher. Of course, won't know for sure until I can get the Q2 parts and get it back up and running again. I'm hoping to see that 86V come back down to a more nominal 78v as the schematic specifies. That 86V was a red flag for me and like the Q802 transistor voltages, while I don't' understand the "why" or the cause, I know (or at least feel) something isn't right.
Another example is you mentioned the base voltages (.5v and .4v) looked low on the output transistor. How do you know what expected base voltages are when not printed on the circuit schematic?

 

It is a goal to make sure the device works well and doesn't come back. I did qualify "and the parts are so inexpensive and can get a bulk of them and will high a high reuse of the leftovers over time" which appears to be the case with the parts you listed. It looks like I can replace the entire board of transistors for less than $10 with lots of spares left over for next next time one comes along, and I see a lot of these older receivers, in fact this will be the third pioneer I've had/worked on. I was concerned this was going to end up being $30-$40 of transistors, with no spares for the future, in which case I would simply have to call it quits and sell it "as-is" because between that and what I paid for it I then wouldn't be able to get my money back out. But no worries here.

It would be the other way round. An unregulated supply always has a higher voltage when there's no load. The load current pulls the voltage down. So the schematic showing 78v (64v) out the power supply board is showing the unregulated (regulated) values? I also wanted to be clear when I saw 64V in measurement - it wasn't right off the power supply transformer - it was (one of the) outputs of the power supply board, with everything following (the amp boards and such) disconnected. I rechecked all of the diodes on that power supply board and they all seem ok. Re-esr'd the caps as well. The transistors on that board could have a problem I guess, and I could lift them out to do a diode check as well but I keep struggling with the fact the schematic says (64V) and that's what I'm seeing so why would there be concern?


When a transistor is operating in its linear region, i.e. partly conducting, it will have a base-emitter voltage of around 0.6~0.8V (for a silicon transistor).
Ahh - so that is target value for the circuit voltage there. So either via pot adjustment (if applicable) or part repair/replacement.

I've order all these transistors (all coming slow boat) but most had the data sheets attached to them so I've got those printed out and ready. A couple that didn't have the B/E/C printed on them but I can get the data sheets as well online. The challenge isn't the pin swapping but the upside down, back of board look and fix. That's how I got those Collector/Emitter voltages written down wrong initially I will definitely be plotting this out to be 100% sure all is right before proceeding.

Since all these parts will be coming in at different times, is there any safe way to test in stages? I had run a quick test last night under the bulb limiter and (with both channels in circuit but Right channel Q2 output tran removed since it's dead). Bulb went dim on power on and remain so for about 30+ seconds and then lit up brightly. Then right channel Transistor Q806 started smoking (or so I thought). Turn out it was just getting really (really) hot and burning the plastic sleeve on a part next to it. It had 75 or so volts coming in the collector, about 11 on the base and emitter. I did take that part out to test it, as I figured it was fried, but it actually still checked out ok. I'm assuming the lack of Q2 in circuit was casing it to heat up, or possibly some other circuit damage that happens as a result of Q2 dieing. My question is, if I disconnect Q4 will I get the same result (assuming it wasn't something else int he circuit that died)? Meaning would leaving Q2 in the circuit with no Q4 cause this effect? Do I need to remove the +75V from the board to test the amp board alone safely? In short, I'm looking for a way to put all these new opamp/driver transistors on the mainamp board and test first, without risking the more expensive output Q2/Q4 replacements, if there is a way to do that. If I make a mistake and burn out a some of the driver transistors, it's only pennies and I'll have lots of spares. That won't be the case for those output transistors though!

 

The 64V is printed on the first diagram in my post - At the bottom on the power supply block diagram right off pin 5 it says 78V and just the the right it shows 78V (64V). It is here that I initially measured 86V before the failure. After the failure and after disconnecting the main amp circuit from pins 5&7 and under bulb limiter connection, I measured 64V at pin 5. Reconnecting the main amp circuit as I tested last night but with Q2 out of circuit, it was measuring 75V. I could disconnect the main amp circuit again from pins 5 & 7 and double check, but I'm sure I it was 64V on pin 5 that I saw when everything was disconnected.

I'll get those power resistors but I was hoping for a more "baby step" test in which I could isolate, say, the first half of the circuit to see that those updates worked, without concerning about second half circuit center voltage adjustments, or needed Q2/4 in circuit etc. (but also not heating up Q806) so I didn't know if some of the pins 6-11 could be disconnected to allow the first part of the circuit to be tested in some fashion. Then move to the second half of the board and install and test those parts. Plus that Q802 was ordered first so should be here first and it gives me something to move on and verify until everything else arrives (which may be several weeks given where I bought it all from). That would also help me if something goes wrong since I would have some discrete changes between tests, rather than a flat out total replacement then test. But if that's the only way to do it, I'll just wait till everything gets here.