Pioneer SX 780 - Weird channel balance issues.

cokedawgfer

cokedawgfer

^ no, not the white stuff
Hello Folks,

I recently purchased a SX 780 in really nice shape. It sounds great, but it has a weird issue with channel balance:

At first, I noticed that the right channel was sounding a little weaker on phono than the left - less bass, less punch, etc. - and a look at the VU meters confirmed it. The right channel was not putting out quite as much power as the left....sometimes. That's the weird part, sometimes.

To troubleshoot, I switched the speakers from A to B; this had no effect. I tried a different input (aux, as the tuner doesn't work well - a problem for another time), and it seemed that the issue increased a marginal amount, i.e. the imbalance between the left and the right was increased. I went back to phono, and the problem seemed to have fixed it self for a time and the VUs matched up, but there was overall a little less bass. Then I put on a different record, and it was back again.

I have fiddled around with different combinations of inputs and speakers (A vs. B) and have worked the knobs many times, and the issue is present most of the time, but sometimes it's not and the channels are in balance, albeit with a little less overall bass response, I feel.

What might be the cause of this? Is it simply that I need to really clean the switches? What else might be the problem? It's all original and over 40 years old, do I need to recap? I'm fairly new to this, so any help would be much appreciated!!

Alex

Edit: I should mention that i've had this thing for about a month, and the first two or so weeks, this wasn't an issue at all.
 
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You can input a known signal, then measure output. If you don't have a tone/function generator but DO have a CD player, you can burn a Test CD from this ISO disk image file (http://www.wwdsltd.com/files/AudioTestCDImage.iso). The CD track guide is here (http://www.wwdsltd.com/files/AudioDiagnosticTestCDKey.xls ).

If you haven't done so already, download the service manual. You can find it in the AK database (http://akdatabase.com/AKview/thumbnails.php?album=44) and other places.

The schematic in the download service manuals usually spans at least two pages and is poorly scanned. You can download my copy for a clearer look (http://www.wwdsltd.com/files/SX-780Schematic-Wide.jpg).

Study Section 8.4 on setting the vU meters. Instead of setting the meters, use the same setup to compare the output power (voltage) at a given volume for both channels and equal balance. That should help you quantify the differences. Come to think of it, while you're there you might as well set up the meters anyway.

You may find failed or weak components, poor solder joints, damaged foil traces, still dirty controls and switches, failing or dirty speaker relay contacts, or other problems. Knowing the (quantifiable) difference may be helpful.
 
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cokedawgfer said:
Hello Folks,

I recently purchased a SX 780 in really nice shape. It sounds great, but it has a weird issue with channel balance:

At first, I noticed that the right channel was sounding a little weaker on phono than the left - less bass, less punch, etc. - and a look at the VU meters confirmed it. The right channel was not putting out quite as much power as the left....sometimes. That's the weird part, sometimes.

To troubleshoot, I switched the speakers from A to B; this had no effect. I tried a different input (aux, as the tuner doesn't work well - a problem for another time), and it seemed that the issue increased a marginal amount, i.e. the imbalance between the left and the right was increased. I went back to phono, and the problem seemed to have fixed it self for a time and the VUs matched up, but there was overall a little less bass. Then I put on a different record, and it was back again.

I have fiddled around with different combinations of inputs and speakers (A vs. B) and have worked the knobs many times, and the issue is present most of the time, but sometimes it's not and the channels are in balance, albeit with a little less overall bass response, I feel.

What might be the cause of this? Is it simply that I need to really clean the switches? What else might be the problem? It's all original and over 40 years old, do I need to recap? I'm fairly new to this, so any help would be much appreciated!!

Alex

Edit: I should mention that i've had this thing for about a month, and the first two or so weeks, this wasn't an issue at all.
Click to expand...

It could be any number of things in the signal path causing an imbalance.
That includes the volume or balance controls themselves. The last time
I dealt with this, it wasn't a case of one channel being too weak, but rather
one was too high. The over all resistance of the left volume pot had changed
to where it was causing that channel to be too loud. It was simply assumed
that the right side was weak.

The only way to know for sure, is to isolate the where the imbalance is
actually taking place within the signal path. Signal tracing would have to be
done starting at the input, working through the various stages until an
imbalance between the channels is detected. I'm just assuming that you
don't have the tools and experience for this.
 
I have a multimeter and am usually pretty good at logically figuring things out, so I am guessing that's a good start? I'm assuming by signal tracing you mean using the schematic and checking voltages at various points through each, the left and right, channel's signal flow?
 
petehall, I tried that, but no difference. By the way, thanks for all the help so far, everyone.

Watthour, I will try that! I just downloaded the service manual, and will take some measurements later today.
 
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First do as suggested earlier and work all switches.

cokedawgfer said:
I have a multimeter and am usually pretty good at logically figuring things out, so I am guessing that's a good start? I'm assuming by signal tracing you mean using the schematic and checking voltages at various points through each, the left and right, channel's signal flow?
Click to expand...

Right, I'm not talking about checking DC supply voltages. I'm talking about the audio signal path itself,
Checking the level of AC signal voltage for each channel.

The ideal tool would be a scope. But you can still do it with an AC voltmeter.
The voltage readings wont necessarily be accurate. But all you're trying to
do is see if one channel measures higher than the other.

You will need something to inject a steady signal into the aux inputs.
Normally one would use a signal generator for this. But if you don't have that,
a player or computer with a recorded 1000 Hz test tone will work ok.
It has to be steady so that voltage readings aren't fluctuating all over the
place when trying to measure.

Make sure balance and tone controls are set at mid point.
Loudness switch off. Set the volume at about 1/4.

Set your meter on AC voltage, then connect the negative lead to ground.
You would need to start by measuring both ends of the entire pre-amp.
Meaning, take a reading at both the L+R center pins of the aux jacks. (both channels should read the same)
Then at the point where the signal enters the main amp for both the L+R Channels.
This would be the "base" lead of transistors Q5 and Q6 at the main amp. While the voltage
at the the input of the main amp will be different from what you get at the aux input,
the AC measurement at both channels should be about the same.


Why do this first?
Because you need to confirm that the imbalance is indeed taking place
in the preamp and not the main. Since the 780 doesn't have "Pre out" and
"main in" jacks, you cant simply plug another preamp source into the unit
to isolate which section of the receiver is causing the issue. "Yes its
likely to be in the preamp section, but never simply assume that".


Enough for now, later
 
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glen65,

perhaps you saw my post in the other thread. Anyway - I REcleaned and deoxited everything. Adjusted DC offset. Calibrated meters. Problem is lessened, but still there. I will do what you said above soon.

Thanks a ton!
 
Put a sine wave signal into both L/R Aux input and trace the signal through (all the way to the PA reading ACV on the DMM, to see where the signal levels do not track each other within a few %.
 
I own an SX 780 since new in 1978. It had been in long term storage. Got it set up and had the same issue with R L balance. L side was not outputting at a constant level - basically very little. I followed the advice here and bought DeoxIT F5 fader spray. After 3 sessions of spray and operate the balance pot many times, full function has returned. I did notice that after spraying it, the function returned after I let the unit warm up fully. So far, all is good.

Question: The front circuit board is very clean as the case covers it normally. The back board where the tuner sits is very dusty since that is under the top grill. Should I be concerned about this dust or just "leave it alone' as the unit otherwise plays perfectly. Thanks in advance
 
Dust will trap moisture, corroding the chassis and some metal parts, making the receiver look old and neglected.
Taking the bonnet off is fairly easy and some canned air or lite compressor air won't hurt anything.
 
Dear Audio Karma community,

Apologies for posting here, but this is the thread that fits my issue and I didn't want to create a new one.

My father in law asked me to take a look at his Pioneer SX-780 which was on protection mode. I have to clarify from the get-go that I am hobbyist and not a professional (I am in fact a mathematics university professor who studied physics in order to understand a bit of electronics!). I do have a good size of equipment to hopefully do repairs, such as the Fluke PM3082, HP 34401, Hameg HM 7044, and Leader LAG 120B (together with the Peak ATLAS semiconductor + ESR testers). My ultimate goal here is to try to fix it and hand him back a working receiver that he loves so much!

I noticed that the pins 2&3 and 8 & 9 of the right channel's STK 0050 were shorted. It also took with it the 0.22Ω resistor (R265) + 200Ω resistor (R259). I also noticed that its sibling, i.e., R260 measured 150Ω. I ordered a pair of STK's from ebay + 4 0.22Ω resistors, as well as the 200Ω resistors and I replaced them all (I believe R259 and R260 are fusible resistors, and I hope I ordered and installed the suitable ones, see CFM14JT200R). The unit was not in protection mode (success!), and as I was wrapping it up, I only paid attention on FM mode that the VU meters were moving, so I said to myself, at least it works. I should also mention that this same channel's STK got replaced some time back in the 90's, i.e., it had same channel issue. Just for the record, the 2 STK's now measure same voltages across pins (I believe ~1.16 or 1.17 volts on pins 1 and 0), so that tells me they are symmetric. As I was having the unit in our house, I put it on speakers and a turntable, and I noticed a slight imbalance that you can only hear when you go all the way to left and check it against right. I said maybe it is just the record (silly me!).

At any rate, I then decided to do a recap as he has this unit for more than 45 years. As I was going through the process (in fact I stopped at the vertical line # 7 on the main board layout), I wanted to do an actual check. I plugged a 1kHz (90mV) signal to both channels in AUX, and measured the output with my oscilloscope. There, I saw that indeed the right channel appears to be weaker.

As I was turning the bass/treble pots, the "weak" waveform started changing, and in cases, they got symmetrized. Kind of same effect I noticed when I was switching from mono to stereo (in fact in mono, both waveforms match but when I turned back to stereo, they were way far away...) Also, I noticed that the low pass filter has no action whatssover to the output (in cases it helps symmetrize the waveforms) which makes me thing there is a separate issue.

Reading this post, it seems that I need to clean all pots and switches with Deoxit D5/F5 (F5 for potentiometers and D5, followed by F5 for the switches) which I am planning on doing this w/e. If this doesn't fix the issue, it seems that there is an issue in the preamp section, and I was wondering where to look at.

Any advice will be much appreciated!
 
The mono/stereo switch S4 is before the preamp so if both waveforms matched when switching to mono it seems the mono/stereo switch needs cleaning along with the bass and treble.

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Thank you very much Mcdoug! I hope this fixes the issue! If successful, I will continue with the rest of the recapping process. Do you think that this will hopefully fix the low pass filter "inaction"?
 
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