SX-737 volume drop on FM Stereo in left channel..

babeba67

babeba67

Active Member
Hoping for some help here...

I posted a couple of other times about this SX-737 that I just picked up. I have done a "dusting and cleaning" on the unit and sprayed all controls with Deoxit. The pots are much cleaner now and no issues with scratchy volume like when I picked it up.

The only thing I have now is a volume drop when listening to FM. If it's in mono mode then equal volume levels are in both R & L channels, if I push the button in for "stereo" mode then I get a significant drop in volume in the Left Channel. I have tried on both A and B speaker connectors on the back and the same thing is true for using headphones through that jack.

Any ideas?

thanks in advance!
 

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You may need an alignment of the stereo decoder section. Not for the average owner, but of course since it needs it anyway you could experiment a little.

Get the service manual.
 
babeba67 said:
Hoping for some help here...

I posted a couple of other times about this SX-737 that I just picked up. I have done a "dusting and cleaning" on the unit and sprayed all controls with Deoxit. The pots are much cleaner now and no issues with scratchy volume like when I picked it up.

The only thing I have now is a volume drop when listening to FM. If it's in mono mode then equal volume levels are in both R & L channels, if I push the button in for "stereo" mode then I get a significant drop in volume in the Left Channel. I have tried on both A and B speaker connectors on the back and the same thing is true for using headphones through that jack.

Any ideas?

thanks in advance!
Click to expand...

When the mono switch is depressed, the left and right channels are ADDED together, so the louder channel compensated for the weaker channel. This switch is right after the Tape Mon 2 input, and before the balance control.

This mono doesn't affect the stereo lamp, or turn off the stereo decoder, it is still lit, meaning the tuner IS putting out stereo.

There are several possibilities, starting with the stereo decoder chip which puts out the left and right channels (which is very likely to NOT be the problem), both signals go through transistors for amplification, and then some capacitors that could have failed by drying out, - then out of the tuner board to the function switch - where a dirty contact could STILL be hurting it.

There IS a way to troubleshoot this, with an RCA cable (or 2) and a capacitor to make a signal probe (another receiver would be nice, but not necessary), after which I will direct you where to probe, and compare left and right channels on the tuner board, starting with the output pins 11 & 13.

BUT FIRST, we have to see whether the aux input, tape mon 1 and tape mon 2 have equal input sensitivities. The audio does travel through a LOT of switches through that area, and a lot of them are resistant to deoxit.

PLEASE DO NOT mess with the tuner or decoder adjustments AT ALL, the advice above, about experimenting, no less the need for alignment, is absolutely wrong. I don't S.W.A.G. it, I look it up and think about it before posting.
 
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Forgot to mention...this ONLY occurs with FM. The Aux and tape sections are not affected and work properly.
 
babeba67 said:
Forgot to mention...this ONLY occurs with FM. The Aux and tape sections are not affected and work properly.
Click to expand...

That exonerates quite a few switch contacts, leaving just the FM source contacts suspect, while the common selector contacts that carry the aux etc signals are ok. We're making progress.

blhagstrom said:
Excellent, Mark has arrived.

Hope you can proceed with the instructions.

I don't have a 737 but I'll follow along with the schematics.

Service manual is here. http://akdatabase.com/AKview/displayimage.php?album=44&pos=73

Mark, I'm guessing the process you mentioned is to inject a signal at various places on the tuner board. Yes/No?

:lurk:
Click to expand...

Close, but not quite, first we will be looking at the raw signal coming out of the stereo decoder chip when a station is tuned in and then the signal coming out of the tuner board itself. Like Scotty said: "The more they overthink the plumbing, the easier it is to stop up the drain."

The first step involves using an rca cable plugged into a working tape monitor input, and then touching the center conductor of the cable to pins 11 & 13 of the tuner board - essentially bypassing the function select switch - and seeing if the audio levels are similar. This is actually done first because we don't need the cap to do the test. If you wanted to juggle two cables, and touch one to each pin at the same time to make the comparison easier, be my guest...

The tape monitor is used instead of the AUX, because switching to the AUX instead of FM could turn off the power to the tuner somehow (I haven't checked) and - silence - .....

The second step needs a 0.1uf capacitor in series with the center conductor of the cable, to block DC voltages that would make a huge POP AND block the DC that the DC resistance of the aux input may load down too much. the other lead of the cap becomes the probe, that is touched to the pins of the 1156 integrated circuit to access the raw stereo audio signal.
 
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Thanks for everyone's help so far. I used the RCA cable to test pins 11 and 13 like you suggested. The drop in volume occurs when touching the RCA pin to #11.

Next step?

Here is a photo...
 

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markthefixer said:
<snip>

The second step needs a 0.1uf capacitor in series with the center conductor of the cable, to block DC voltages that would make a huge POP AND block the DC that the DC resistance of the aux input may load down too much. the other lead of the cap becomes the probe, that is touched to the pins of the 1156 integrated circuit to access the raw stereo audio signal.
Click to expand...

Need the cap, 0.1uf or better - not an electrolytic, and 50v or more, any one of these from Radio Shack will work:
0.1µF 50V Hi-Q Ceramic Disc Capacitor Pk/2 Catalog #: 272-135 $1.49
1.0µF 250V 10% Metal-film Capacitor Catalog #: 272-1055 $1.59
0.1 uF 250v Metal-Film Capacitor Catalog #: 272-1053 $1.49
0.1µF 50V 10% PC-Mount Capacitor Catalog #: 272-1069 $1.49

then do this:
attachment.php


probe with the bare wire end, the ha1156 chip, pins 4 and 5

BE VERY CAREFUL NOT TO SHORT ANY 2 PINS TOGETHER.

They should be the same level.

After that we find 2 caps on the board, remembering that pin 11 has the drop in volume.
 
Great...Thanks Mark I'll let you know what I find out after picking up the cap and testing. I appreciate your help and detailed photos :thmbsp:
 
babeba67 said:
Great...Thanks Mark I'll let you know what I find out after picking up the cap and testing. I appreciate your help and detailed photos :thmbsp:
Click to expand...

The big question is whether or not the underside of the tuner board is accessible, or if it is mounted over a solid sheet of metal. I think it is accessible from the manual's description, but the circuitry we are interested in is between pins 11 & 13, and the 1156 chip across the end of the board, nearly on the opposite edge could overhang the chassis.

Also a picture of the end of that board, with the area of the pins through the area of the 1156 chip would be more useful for pointing.

Have you downloaded the service manual? The hi-res one is best. If you don't know where to get it, PM me with your email address and we'll correct that... we need some common points of reference as we get more detailed in this task.


The little blue cans with the bare metal tops near pins 11 & 13 in your picture are the likely culprits, with the one closer to pin 11 the likely culprit.

IF you think you can shoehorn in this capacitor in those two positions:
1.0µF 250V 10% Metal-film Capacitor Catalog #: 272-1055 $1.59
get two of them, and not only will they be useful for the test, they could be the solution. We replace both because the originals are prone to failure, plus then the sound, IF affected (improved bass slightly), will be the same for both channels. The difference in value from the original isn't a problem.
 
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Old thread but thought I would mention it. I had the same exact FM problem. Swapping the two transistors Q8 and Q9 solved it for me.
 
RyeGuy859 said:
Old thread but thought I would mention it. I had the same exact FM problem. Swapping the two transistors Q8 and Q9 solved it for me.
Click to expand...
I just tracked down the same issue to the same Transistors. What did you use as a replacement? I am assuming I can use this one from digikey KSA992FTACT-ND. I also figured out that on the phono input the channels have a volume difference, so I was just going to replace everything on that board.
 
jake0157 said:
I just tracked down the same issue to the same Transistors. What did you use as a replacement? I am assuming I can use this one from digikey KSA992FTACT-ND. I also figured out that on the phono input the channels have a volume difference, so I was just going to replace everything on that board.
Click to expand...
Most likely 992s. I don’t have the unit anymore to verify but i believe that was right. Glad you figured out the issue.
 
Replaced Em they worked great for a single power cycle and theb I could not get that 992 or any other to give me a voltage output to the right channel. I did have voltage on both the emitter and base but nothing on the collector. Gonna go ahead and replace those Sanyo light blue caps know for failing in the meantime while I stew on it.
 
Could be a shorted cap, I had one short on my tuner on my 737 so it’s not unheard of. Trace back the collector and see where the voltage stops or is getting pulled down at.
 
No voltage at the collector, can't find shorts after it either. It can't be the transistors cause I've tried like 5. Both voltages to the emitter and base are good, not exactly 11 volts but definitely in a margin of error.
 
Are you talking about Q8 or Q9? luckily for you they are mirrored circuits so everything on the Left channel you can compare to the right and vice versa to find the issue
 
Also be sure of your pin out, I believe 2sa725 is a BCE, 992 is ECB so they will be facing opposite of before
 
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