SX-838 FM radio troubleshooting.

tutor_turtle

tutor_turtle

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I have a SX-828 on the bench with a strange problem with the FM radio.

It picks up weak stations good enough, when I tune to a strong station, the signal strength meter pulses and bangs against the max stop.
The sound is distorted in sync with the pulsation. I get a very weak stereo light when tuned to a strong station.
I'm not well versed in diagnosing radio problems, but I'm pretty sure the issue is in the front end.

I can't make heads or tails of what I'm seeing on the scope.
Instead of a continuous train of signal, I'm seeing bursts of a signal, all the way back to the tuning capacitor.

The entire unit has been recapped.
Unfortunately, I did not test the radio thoroughly before starting, so it may already had an issue.
 
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The first thing I would do is clean the tuning capacitor shaft ground contacts (between the sections of plates). Poor connections to the shaft can cause a sneak feedback path between RF amplifier stages, that can cause all sorts of weird symptoms.
 
Here is a good description of how to do it in case you haven't found it yet.
dr*audio

Thread 'How to clean a tuner (analog)'

1. Vacuum out all the dust you can from the interior. Don't touch the fins on the tuning cap!

2. Blow the dust out of the fins with compressed air. Don't touch the fins.

3. If you examine the tuner cap, you will see brass contacts at each point where the shaft passes through the metal housing. (Including where it passes through the internal walls.) These make the ground contact and they get dirty. Buy some NON - RESIDUE contact cleaner, and inject it into all these brass contacts, then work the tuner back and forth across the dial at least 20 - 30 times.
4. Then apply just a drop of...
  • Like
 
AE Bob said:
The first thing I would do is clean the tuning capacitor shaft ground contacts (between the sections of plates). Poor connections to the shaft can cause a sneak feedback path between RF amplifier stages, that can cause all sorts of weird symptoms.
Click to expand...
I owned an SX-838 for many years that would rumble at the low end of the scale, only when the tuning knob was in motion.
Never was able to make it go away.
fromnowon said:
Here is a good description of how to do it in case you haven't found it yet.
dr*audio

Thread 'How to clean a tuner (analog)'

1. Vacuum out all the dust you can from the interior. Don't touch the fins on the tuning cap!

2. Blow the dust out of the fins with compressed air. Don't touch the fins.

3. If you examine the tuner cap, you will see brass contacts at each point where the shaft passes through the metal housing. (Including where it passes through the internal walls.) These make the ground contact and they get dirty. Buy some NON - RESIDUE contact cleaner, and inject it into all these brass contacts, then work the tuner back and forth across the dial at least 20 - 30 times.
4. Then apply just a drop of...
  • Like
Click to expand...
Makes sense. This was a smokers rig. The silver front panel was an even shade of amber before cleaning.
 
Had to unsolder the RF cage to get cleaning access. No joy.
For some strange reason the +8 volt supply to the stereo lamp get loaded down when a stereo station is picked up.
 
I'm feeling a bad HA1156.
Do you have a good one?
Maybe try it with the FM mute off? I don't know it will change my mind.
 
The 1156 chip grounds the stereo lamp circuit when it detects the 19khz pilot. It’s probably shorted internally and grounding other signals.
Swapping the chip is the fastest conformation. Not sure what other troubleshooting options there are?
Maybe pull the muting transistor and try that? I don’t have a scat in front of me.
 
tutor_turtle said:
All the caps are new. I could had got a bad one, unlikely (or put it in backwards, more likely)
Click to expand...
If it was backwards it would have probably let you know by now, but worth a check.

@zebulon1 Symptom is
tutor_turtle said:
For some strange reason the +8 volt supply to the stereo lamp get loaded down when a stereo station is picked up.
Click to expand...

I don't think a shorted HA1156 would drag the supply down. Super simple 1/2 wave rectifier with a cap filter powering one function lamp except when on FM and a stereo station is tuned, then add the stereo lamp load.

Is the stereo lamp original or an exact replacement?

Monitor the voltage on pin 21 of the AWR-055. Set the function selector to aux and then connect pin 15 on the tuner board to ground through a 10 ohm resistor and see what the supply voltage on pin 21 does.

838ps.jpg
 
I don't know :dunno:
Simple check. Disconnect the lamp and see?
Don't know why didn't think of that first?
Yes; Some of these Pioneers require the proper stereo lamp to work correctly. Not sure if this is one.
 
mrk229 said:
If it was backwards it would have probably let you know by now, but worth a check.

@zebulon1 Symptom is


I don't think a shorted HA1156 would drag the supply down. Super simple 1/2 wave rectifier with a cap filter powering one function lamp except when on FM and a stereo station is tuned, then add the stereo lamp load.

Is the stereo lamp original or an exact replacement?

Monitor the voltage on pin 21 of the AWR-055. Set the function selector to aux and then connect pin 15 on the tuner board to ground through a 10 ohm resistor and see what the supply voltage on pin 21 does.

View attachment 3727765
Click to expand...
Yes, this is all original, and I should note, I referenced the wrong model, I have an SX-828, same basic specs, but one year earlier, a lot changed in that one year.
The same schematic for the stereo lamp was retained for the 838, but the op amps were TA7060-P, not HA1156's
 
It's all tided together with transistors, the mute and indicator circuit.
Does the meter work on AM?
Disconnect the meter from the circuit and try it. Disconnect the lamp and try it.
Maybe meter the output of T2 at pin 2 while tuning it.
Something is shorting. The stereo indicator is a symptom not the cause.
Blanket fix?
I think I would replace (First Q7, 8, 9) then, 10, 11 and 17 with 992's.
 
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tutor_turtle said:
I listed the wrong model, I have an SX-828, it uses TA7060-P op amps in the front end.
Click to expand...
Well that makes a big difference. :whip: :mad: .

You did list it right in the first post. I was looking at the thread title. (might want to change it)

Where are you measuring the voltage droop/sag for the stereo light on the 828?

The voltage at the red arrow will float higher when the lamp is off (no load) I expect if you are metering at the red arrow point then it is normal.

Based on what I see here it appears that D1 and C9 are not on a PC board. Has C9 been replaced?

What is the voltage at the red arrow point with the lamp on and with it off?
828lamp.jpg
 
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zebulon1 said:
It's all tided together with transistors, the mute and indicator circuit.
Does the meter work on AM?
Disconnect the meter from the circuit and try it. Disconnect the lamp and try it.
Maybe meter the output of T2 at pin 2 while tuning it.
Something is shorting. The stereo indicator is a symptom not the cause.
Blanket fix?
I think I would replace (First Q7, 8, 9) then, 10, 11 and 17 with 992's.
Click to expand...
AM is working fine. Even the lower power FM stations come in ok, but never light the stereo beacon.
KSA992's I have by the 100's. Sorted by gain (for use when differential pairs are needed)

I have a lot of hours into this restoration, 9 out of 10 condition, sonically it came out super nice.
I really would like this last issue resolved.
That you for all your help.
 
tutor_turtle said:
AM is working fine. Even the lower power FM stations come in ok, but never light the stereo beacon.
KSA992's I have by the 100's. Sorted by gain (for use when differential pairs are needed)

I have a lot of hours into this restoration, 9 out of 10 condition, sonically it came out super nice.
I really would like this last issue resolved.
That you for all your help.
Click to expand...
It is possible it needs an alignment tweak. What kind of signal levels are you seeing on the lower power FM's. Do you need to turn the muting off to receive them?
 
mrk229 said:
Well that makes a big difference. :whip: :mad: .

You did list it right in the first post. I was looking at the thread title. (might want to change it)

Where are you measuring the voltage droop/sag for the stereo light on the 828?

The voltage at the red arrow will float higher when the lamp is off (no load) I expect if you are metering at the red arrow point then it is normal.

Based on what I see here it appears that D1 and C9 are not on a PC board. Has C9 been replaced?

What is the voltage at the red arrow point with the lamp on and with it off?
View attachment 3727925
Click to expand...
Full recap, checked polarity.
Voltage is 7.99vdc, off station.
With the lamp trying to light, voltage sags to 2.2V on the cathode & anode side of the diode and on the transformer side of the fuse. (blue wire)
Lamp is dimly lit with 2.2v across it.
 
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