Pioneer SX 737 with FM tuner issues

[pioneersx]

Hi guys,

I have a pioneer sx 737 with FM tuner issues. There is no sound when the selector switch is set to FM. Only faint hiss. When I push the FM muting button the hiss becomes much louder, but no reception of radio stations.

AM tuner does work. Nothing happens on the signal meter and tuning meter.

The voltages on the power supply board are according to the service manual.
Everything else works fine and sound is good on aux/phono.

So the problem seems to be on the tuner board.

Any help would be appreciated!

I measured voltages on the pins of the tuner board and on the HA1137 and HA1156 chips.

HA1137:
pin 1: 2.0V
pin 2: 2.0V
pin 3: 2.0V
pin 4: 0.0V
pin 5: 1.6V
pin 6: 5.7V
pin 7: 5.5V
pin 8: 5.6V
pin 9: 5.6V
pin 10: 5.6V
pin 11: 13.4V
pin 12: 4.6V
pin 13: 0.0V
pin 14: 0.0V
pin 15: 5.0V
pin 16: 0.0V

HA1156:
pin 1: 13.6V
pin 2: 3.1V
pin 3: 5.3V
pin 4: 11.0V
pin 5: 11.0V
pin 6: 13.6V
pin 7: 0.0V
pin 8: 0.0V (according to manual this should be 2.3V)
pin 9: 2.3V
pin 10: 1.6V
pin 11: 2.2V
pin 12: 2.4V
pin 13: 2.4V
pin 14: 3.3V

Voltages on pins of the tuner board:
pin 1: 0.0V
pin 2: 0.0V
pin 3: 13.3V
pin 4: 1.3V
pin 5: 4.0V
pin 6: 5.5V
pin 7: ---
pin 8: 5.5V
pin 9: 0.0V
pin 10: ----
pin 11: 0.0V
pin 12: 0.0V
pin 13: 0.0V
pin 14: 4.1V
pin 15: 13.3V
pin 16: 11.4V
pin 17: 11.4V
pin 18: 1.57V
pin 19: 0.0V
pin 20: 13.4V
pin 21: 4.1V
pin 22: -----
pin 23: 2.5mV
pin 24: 0.2mV
pin 25: 11.2V
pin 26: 10.6V
pin 27: -----
pin 28: 0.1mV
pin 29: 0.1mV
pin 30:-----
pin 31: -----
pin 32: 0.0V
pin 33: 1.3V

 

[Watthour]

HA1156 Pin 8 is the FM Mute input from the switch, apparently to force MONO mode when mute is active. The lack of voltage there is probably not causing the unit to not receive FM.

The critical 13V supply on Pin 3 of the FM board seems to be present, so the board should be powered.

You mentioned that the AM tuner works but nothing happens with the signal meter. Is this true in AM mode as well?

What kind of FM antenna is connected? The unit will at least need a 30" wire connected to one of the FM ANT inputs, and a dipole would be better.

Is the tuner VC clean? Do any of the plates of the VC appear to be touching each other?

 

[markthefixer]

did it ever work for you, then just stop?

 

[pioneersx]

I bought the unit like this, so for me it did never work.

I had an antenna connected from another stereo, which works good on other receivers. So that should not be the problem.

In AM mode I also see nothing happening on the meters.

Is there anything strange on the voltages I posted?

 

[rcs16]

AM tuner does work. Nothing happens on the signal meter and tuning meter.

Why is there no signal indication in "AM"? That has to be investigated first! as it maybe a clue as to no FM at all. Check/replace C5 (220uF/6.3V) on AWX-071.
HA1137 pin 9 is not correct, at 0V, it should be the same as pin 8? ~5.4V, so something seems wrong with HA1137 or the quad coil. Inspect the quad coil for damage, possibly someone was in there with a metallic tuning tool and damaged it.
Check L2 for an open ckt?
HA1137, pin 13 is suspicious at 0V, indicating that there is no signal being received or detected. I think this should show some output when a strong signal is being tuned, regardless if the quad ckt is op or not. I'd have to investigate the ic op more thoroughly.
The signal meter does not move at all? as you tune through the dial?
Can't see it being a bad ecap, but anything is possible.
If L2,T6 look okay, I'd say to remove the HA1137, put in a good machined pin socket and try an new HA1137. If changing the HA1137 gets it going again, correct voltages, follow the quad coil adj procedure to center the tuning meter or adj so that pin 7 and 10 voltages are equal or you get 0V between them(centered meter).
Adjusting for lowest THD is a problem without the proper equipment, but at least you should be able to get it going again. There are some threads on the proper tuning tool to use, IIRC 5/32" hex nylon is what is needed. The cores are easily damaged!!

 

[pioneersx]

I checked again pin 9 on HA1137 and it reads 5.6V. Sorry for this confusion, I corrected this in my initial post.
On pin 13 I read 2.3mV. What should this be?

The quad coil looks undamaged.

....I found out that the signal meter is working on AM! However very slightly, because reception is not that great.


Do you still suspect HA1137 for the FM issues?

 

[rcs16]

With no IF input signal to HA1137, you get no DC signal voltage out of pin 13, so for you it is a bit of a crap shoot, having no RF instrumentation.
Check bias voltages on Q1-Q4 in FM mode, check against schematic, report any inconsistencies.
If you have no IF into HA1137, (which is what tuning to a quiet part of the dial is doing) then you should be able to adjust the quad ckt, per SM, for tune meter center, that will show if the quad ckt is working, at least.
Rick

 

[pioneersx]

Below the measured values of Q1-Q4 and in brackets the values from the schematics.

Q1
pin D = 0.3 mV
pin G2 = 0.1mV
pin G1 = 3.2V (3.1V)
pin S = 11.6V (11.4V)

Q2
B = 2.3V (2.1V)
C = 13.5V (12.1V)
E = 1.6V

Q3
B = 7.1V (6.6V)
C = 13.1V (12.4V)
E = 6.4V

Q4
B = 2.0V (2.0V)
C = 13.1V (12.6V)
E = 1.4V

adjust the quad ckt, per SM, for tune meter center

Can you explain how I should do this and what you mean by SM?

 

[larryderouin]

SM or S.M. = SERVICE MANUAL. Link HERE

 

[pioneersx]

If I turn T6 I can indeed center the tuning meter. The FM muting is pressed in like described in the SM, but I only hear loud hiss. No matter what frequeny I dial to.

 

[larryderouin]

Take a Battery powered FM Radio and tune to 90. Tune the 737 to roughly 100-101(roughly 10.7mHz difference) and listen for a ZERO BEAT (in FM the noise will stop and you'll get a null or lack of noise)on the 737, if the oscillator on the 737 is working. Rock the tuner knob back and forth slowly between 100 and 101 until you hear the null on the 737. It should be close to 100.7-101.0 The battery radio has to be fairly close (within a foot or so of the tuner board).

 

[pioneersx]

I tried it but could not find the zero beat on the 737. For reference I also tried it on my good working sx-650 and there indeed the zero beat was at about 101.0.

Can I conclude that there's a problem with the oscillator on the 737 then? Which part on the board is that exactly?

 

[rcs16]

Not necessarily that it is the LO, it can be other things too that have the same issue of no quieting.
q3(2sc535) is the LO, local oscillator, but the bias voltages are okay, could be a open vc4,tc4,c8,c17,c18, need to test them, usually using a LCR meter.
If something is wrong in the IF section or HA1137 you will get the same issue of no null or quieting.
Like i said trial and error with no instrumentation.
Could test the HA1137, by adjust the bottom slug of T6 to see if you can null the tuning center meter. need the 5/32" nylon tuning tool for that. see sm step 13 on page 14.

 

[larryderouin]

rcs16. See post 10. He already tested for centering of meter.

 

[rcs16]

rcs16. See post 10. He already tested for centering of meter.

Thx, i missed that one!!, so what's next? it is hard to troubleshoot a RF section with no RF SG and scope. Trial and error method of troubleshooting is frustrating to say the least.

 

[pioneersx]

The tuner section doesn't look like it has been tampered with. So I suspect that it suddenly stopped working because of component failure. But which one?

The most easy thing to do is replace HA1137 and see if I get anything going. What do you think?

Also the adjustment hex of T6 seems to have become very brittle and not working very smooth. Maybe try to replace T6 as well? Is there a replacement available for T6?

 

[rcs16]

Exactly, which one.
If the quad coil adjusts for center then at least prt of it is working and bias V's are correct, but anything is possible, as other internal ckts could be faulty, no sig meter for instance.
Well I did suggest to socket and replace HA1137, but I have no solid proof that is it = trial and error.
Just thought of another idea, try shorting out each CF,ceramic filter, F1,F2, one at a time, with a 1nF ceramic cap, to see if it makes any diff. Solder in, so that you can fiddle with the dial to test and see if you can quiet/null the FM or pick up a signal. I do not like the idea of of 12.6V on F2 however. Have to take a look at the CF spec to see what the breakdown V is.

 

[rcs16]

Is the tuner VC clean? Do any of the plates of the VC appear to be touching each other?

 

[pioneersx]

Exactly, which one.
Just thought of another idea, try shorting out each CF,ceramic filter, F1,F2, one at a time, with a 1nF ceramic cap, to see if it makes any diff.

Can you be more specific about how I should do this? Do I have to take a CF out or leave it in? To which pins (CF has 3) do I solder the ceramic cap?

I do not like the idea of of 12.6V on F2 however. Have to take a look at the CF spec to see what the breakdown V is.

12.6V is what is written in the SM.

 

[rcs16]

Short the two outside CF pins with a 10n capacitor, short leads, starting with F1 first. Best if done on the pcb bottom, foil side. The middle lead is supposed to be ground, so you can verify this as well.
I have never seen a CF fail, has anyone else? Probably fail open ckt.
I realize, F2/Q4-e is 12.4V, as it is supposed too be. I just made a note to myself that I see 12.VDC on the CF, which is unusual, as I would capacitor couple it, so Pioneer saved on 1 cap. The 12.5V may be stressing the CF and causing it to fail prematurely, well earlier than usual, 30+ years ? Shorting the CF's with a cap can let the signal pass around a potential open ckt. We are grasping at straws here without a scope.