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Pioneer sx-737 no fm stereo

V

vmedia

Active Member
Hello.
I believe I have scoured every sx-737 thread and have hit a bit of a dead end on this challenge.
I have a 737 with a recapped power supply/phono board and everything is working as it should except the tuner is not producing a stereo signal. Reception is decent (only the best tuners get great reception in my workshop) but no stereo separation. The lamp has been replaced with the proper bulb (I have tried disconnecting the bulb based on some advise on a similar thread) - tried multiple antennas (properly connected)

I have cleaned every switch and pot to no end (mono/stereo, selector, etc)

I have cleaned and oiled the tuning cap.

When I was first investigating, once or twice the unit kicked into stereo when I was poking around T6. I touched up solder joints in this area thinking there might be a cold joint or something loose. No change. I can no longer get stereo to occur no matter how much I tap or prod. Possibly a faulty T6?? I did spray a bit of CRC in there and gave it a twist or two (bad idea - perhaps I broke something inside T6?)

I have installed a socket and swapped the IC HA-1156 with a known working sx-636. Both ICs work in the 636, same problem in the 737 regardless of which IC is installed.

I have replaced Q10 - no change, put back the original (just in case, I will replace with the newer part eventually) Based on yet another thread, I have noticed that the voltage at PIN 8 of HA1156 is too low with q10 in place. When removed, the voltage is correct. I believe this is part of the mono function and not stereo separation but I am not sure of that.

I'm wondering if a faulty HA-1137 might have something to do with this?

Any advice would be most appreciated - I have found a replacement board, but I'm in this game to figure out exactly what is causing this vs. a wholesale swap of a main board.

Thanks in advance.
 
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What was done about C38, C39 & C40? Were they replaced, and with what part numbers. They are the dreaded CSSA caps, the sky blue Sanyo e-caps that morph into resistors and solid short circuits.

What about C27? is it new?

Try swapping the HA1137 between units (socket it on the 737).

check the L2 connections.

before swapping boards, try swapping the 1137 AND the L2 AND the t5/t6 transformer. Those are the major elements of that stage.
 
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Thank you Mark.
I will answer what I can now and report back once I have time to complete all of your suggestions.

C38, Nichicon .33uf 35volt electrolytic (too tight for film) C39 & C40. Wima film caps 1uf and 0.47uf respectively (63 volt)

C27 - New 10uf Panasonic electrolytic (35 volt)

I have installed a socket in place of the HA1137, but in the process, one of the legs of the IC broke off (groan) - I will likely still swap from the 636 or source a new replacement since I'll need one anyway.

Checked the L2 connections and touched up solder joints.

If a new 1137 does not do the trick, I will swap L2 and t5/t6.

More to come...
 
markthefixer said:
What was done about C38, C39 & C40? Were they replaced, and with what part numbers. They are the dreaded CSSA caps, the sky blue Sanyo e-caps that morph into resistors and solid short circuits.

What about C27? is it new?

Try swapping the HA1137 between units (socket it on the 737).

check the L2 connections.

before swapping boards, try swapping the 1137 AND the L2 AND the t5/t6 transformer. Those are the major elements of that stage.
Click to expand...


At this point I just pull them and swap the blue sanyo caps. Tested them initially but after just how many were always bad figure there's no point anymore, in the bin they go.
 
The early stages of sky blue Sanyo cap failure MAY be especially upsetting when they are part of a phase lock loop circuit.
 
Also if you haven't, replace the 2sc1451's on the driver board with KSC3503's. They often go bad and cause all sorts of issues, just watch the pinout.
 
Apologies for taking a while to respond - work and life got in the way.

I have replaced all blue caps - no change.
Swapped HA1137 no change to 737, but the 636 does not receive stereo with the other units IC- so I deduce this is part of the problem.

I swapped t6. No change. 636 receives stereo with both parts.

C27 is new.

Swapped L2 no change. Both parts work in 636.

I see that getting to the solder pads of t5 in the 737 will be complicated. Shrouds are in the way (?)

Anything else to check before I have to get serious about t5?

@Eldin thanks for the tip on the 2sc1451’s
I assume this was a general comment and not specific to the tuner issue?
Thanks.
 
@vmedia

Yep, just a side comment! For some odd reason I get a ton of 636's and 737's my way, replacing those often fixes background static/hiss.


Did you check the voltages on HA1137 in the 737? Just looking at the schematic, replacing the 2SA726's wouldn't be a bad idea either (on the radio board). They get to be noisy, and can be replaced with ksa992's.
 
@Eldin Thanks for your response.
Speaking of voltage readings, the HA1156 has a low reading on Pin 8 (.027) I believe this is part of the mute circuit and was mentioned above.
Here are the readings on HA1137
1) 2.17
2) 2.18
3) 2.18
4) 0
5) 0.3
6) 4.9
7) 4.5
8) 5.6
9) 5.6
10) 5.6
11) 13.2
12) 3.7
13) 0.5
14) 0
15) 4.9
16) 0
 
Just looking at the datasheet of the HA1156, Q10 is part of the mono circuit. You should be able to remove it for testing as pins 8/9 are connected by the cap for a low pass filter.

Pin 2 on HA1156 is the input from HA1137, you said all the voltage except for pin 8 are ok on ha1156 right?

If you can use the tuning needle/strength meter try and tune for one that has good reception. Check pin 6 on ha1156 as that pin is what drives the stereo indicator. Curious if the stereo bulb is burnt and the signal is getting lost later down the line.

Since all the other caps were checked, take a look at C36 that goes between pins 3 and 11 too.
 
I had removed Q10 (and replaced at one point) and the voltage came back on Pin 8. I will remove again in the morning just for grins.

I've been monitoring Pin 6 (15 connection point on board) and it stays the same regardless of the strength of station. I have tried disconnecting the bulb (which I believe is good as it lit a couple times early in my investigations - when I tapped t6. This no longer happens) I disconnected and measured voltage on pin 6 - always stayed at 13 volts.

I checked C36 - reads 0.047
Interesting note - the schematic shows this as R36, but the drawing is of a 0.047 cap.

Here are the voltage readings on HA1156 (with Q10 in place)

1) 13.08
2) 3.04
3) 5.06
4) 11.14
5) 11.2
6) 13.18
7) 0
8) .027
9) 2.28
10) 1.54
11) 2.15
12) 2.31
13) 2.31
14) 3.22
 
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Good morning. I removed Q10 and, with the good HA1137 now receive VERY intermittent stereo indication (stereo light and sound) in 2
Or three spots on the dial. Q10 tests good, and I previously replaced with a new part, so I’m thinking it was the combination of a bad IC and this transistor that caused this condition (and possibly a cap or two that have been replaced)

I’ll put in the new transistor to see if this changes anything (and if the voltage stays correct on pin 6)

I feel like this unit is in need of a serious alignment. Are there any first steps I might take to rough it in? I’m a total novice on aligning tuners and don't have all the devices needed to do a thorough job.
 
When the components are reinstalled and unit is operating, there are a couple things which can be checked without having access to the full instrumentation needed for a proper alignment.
 
@Watthour Q10 is a bit of a mystery, but I believe I'm at the point now where I can try those things? I am receiving stereo, just not very strong or often across the dial.
 
Q10 could be removed from the circuit to disable automatic muting, which might indicate if IT is the cause for the "stereo" lamp not being consistent.
 
Sorry. I probably wasn’t as clear as I could have been in earlier posts. Q10
Has been removed. Stereo comes on now,
but Not very often at all.
Even though the transistor checks out as good, I am replacing (I already did this once before)
 
@Watthour Yes. C40 has been replaced.
I read the sx-880 thread. I wish the 737 had a VR that directly impacts the stereo decoding...
I think my next step is re-trace my steps to make 100% sure I haven't overlooked something.
 
vmedia said:
@Watthour Yes. C40 has been replaced.
I read the sx-880 thread. I wish the 737 had a VR that directly impacts the stereo decoding...
I think my next step is re-trace my steps to make 100% sure I haven't overlooked something.
Click to expand...

Watthour said:
The SX-737 has the same VCO adjustment for the HA1156, identified as VR2.
Click to expand...


I think a blind spot has had light shed into it...
 
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