Pioneer SX 737 with FM tuner issues

Not much happening on F1, except that the hiss becomes even louder.
On F2 I hear some radio coming through, but it seems independent of the dialer.
If I only touch pin 1 of HA1137 (or pin 3 of F2) with the multimeter probe I hear radio coming through. But then again, turning the dialer knob has no effect on it.
The probe probably acts as an antenna. Does this mean that HA1137 is working and the problem should be in the circuit before HA1137?
 
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Your initial instinct was probably correct - Attaching the meter "antenna" to the decoder and getting some FM probably absolved the 1137. I'd be looking farther upstream toward the antenna, VCs, and their stages.

Do you have an oscilloscope available?
 
Re: starting at the t6 replacement question.

There is a parts 737 on the bay for $20, then there is a 737 tuner board out of Canada for $30. I might have one (T6 probably, maybe a board - but not likely a specific 737 board)

Either there is a part that failed in the RF/IF system up though to but not including the HA1137 detector chip or the tuning cap adjustments (LO or RF stages) are way off.

since pin 3 of the tuner board has 13v on it, the "front end" is powered.
they would be:
RF amp Q1 3SK45 ** caution this is a MOSFET (an early one) and very static sensitive **
Mixer Q2 2SC535
Local Oscillator Q3 2SC535
IF amp Q4 2SC461

The DC biases around q2, q3, q4 look reasonable - no gross faults there.


See if the zero beat "go the other way": set the SX-650 to 90, and check 100-101 with the battery radio to see if the 650 LO is capable interfering with the battery radio. If so, then try it with the 737.
Then we will at least know the LO is working or way WAY off frequency.
OTHERWISE, finding out the LO ISN'T working is golden...

Larry, etc: what about the idea of connecting a "radiating wire" between cf2 and the ha1137 on the SX-650 and getting it close to the 737's ha1137 pin 1 input. Can't be coax, I suspect it would be eaten by the capacitance. maybe a 10nf cap TO pin 1, if no result, pull cf2 (internal short?) and repeat.
In other words, an old "tuner subber"....
 
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I am leaning towards a RF section issue, but what exactly.
It could be a CF internal short, as well, so they can be removed one at a time and short pin 1 to pin 3 with a 10n cap.
Lets back up a bit here, test at the assembly level first before going to the component level.
I have in the past troubleshot instrumentation where I used part of a working second assembly to function as part or sub in for a section that was having a issue.
As an example use one receiver's good RF section, 650 IF output, disconnect from its IF input section and connect to problem receivers IF input section(737). Must isolate the 737 RF section IF output from its IF input section. You can just use a wire, through a isolation cap of 10nF, to make the link between receivers. I do not think that you need coax wire for this test. This method should determine if the 737 IF section is the problem assembly.
You can do the reverse as well, connect the 737 IF output to the 650 IF input.
 
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Thanks for the input. I will try some things tonight. Removing CF1 and CF2 for testing is not problem. However if I want to remove Q1, Q2 or Q3 for testing that becomes difficult, as they cannot be reached from the bottomside! The whole tuner board would have to be removed and also the tuner cord. Not really in the mood for that now :scratch2:

use one receiver's good RF section, 650 IF output, disconnect from its IF input section and connect to problem receivers IF input section(737). Must isolate the 737 RF section IF output from its IF input section. You can just use a wire, through a isolation cap of 10nF, to make the link between receivers.

This is a good idea. To get this clear: I solder the wire onto leg 3 of CF2 of the SX650 and onto pin 1 of HA1137 of the SX737, with a 10nF cap in between? Or should I solder the wire before the signal enters CF1 on the SX650 and onto pin 1 of CF1 on the SX737?
 
btw I have no oscillator, but found this one for not too much money: velleman oscillator pcsgu250.

would that be it good unit for this purpose? it might come in handy in the future as well.
 
Let me pull up both schematics to refer too.
On the 650 , lift leg on R11 (100 ohm) for the net(work) that connects to CF1.
This is your new IF source.
On 737, lift R15(100 ohm) on net that connects to T5. This will isolate the IF section from the RF section, on the 737.
As you can see they are very similar ckts.
Connect these two nets together (lifted legs on 100 ohm R's), with as short a wire as possible. This way the IF output from the 650 is input to IF section of the 737. We know the 650 RF & IF is op, so this tests the 737 IF strip, HA1137 quad detector, stereo PLL etc.
Good luck
Rick
 
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Ok, did it Rick and it works like a charm! Finally got some tunes out of the 737.
But no stereo light and also no sound with FM muting, only with FM muting off (button pressed in). But the sound is good.

So, that narrows the problem down to the sections before CF1. LO? RF? Mixer?

Also I tried to let the zero beat "go the other way". But that does not work on the 650 either.

What's next?
 
update, turned bottom screw of T6 and put the tuning meter in the middle, now even the stereo light is on!
FM muting works also now. Getting 2.4V on pin 8 of HA1156 as well.
 
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Great work!!
No stereo, 19KHz pilot signal too low, that is okay.
Would need tons of RF i/p to cross the switch threshold.
No auto-mute, signal too low also, that is okay.
Yes, narrowed down to a handful of components in RF Front End(FE) of the 737.
Anyone can be the culprit, but I doubt it is a coil, bias voltages look good, so that leaves the capacitors or signal flow through the conductors, even a bad solder joint, have you touched them all up in the RF FE?
Use some small cap's, of same value, to short out these, to check if they are opens, C1,2,7,8,9,12,15,16,17
making progress as we speak = cool = lots of signal
 
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I touched most of the components in the RF front end to check if something was loose. But I cannot see the solder joints on the bottom for this section. As stated earlier I would have to remove the tuner board completely in order to do this.

Tonight, I will try to short out the capacitors as you suggest and report the results.
 
I think that you are going to have to get that dial string/wheel off the var cap and take a good look on the foil side of the RF FE section, to inspect, touch up solder joints. It can be as simple as a bad solder joint.
Word of advise, wrap tape around the wheel to keep the string from falling off and keep tension on the dial string so that it does not fall off the idler pulleys etc.
I guess the best idea is to have the vc cap at one extent, before you take the wheel off that way you know exactly where it came off and where to put it back on.
Makes you wonder how anybody could make any money doing this for a living, in no time you have eaten up the current value of the item. Good thing our time is free.
 
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I tried it, but could not loosen the screw of the wheel. Can also not put too much force on it, because I don't want to damage the axis. Even if I would get that wheel loose, I would have to desolder alot of wires on the bottom in order to be able to turn the board to have a look at the solderjoints. But I probably need those wires for powering the board and testing. So kinda stuck here.

I did notice that touching anything with the probes on the antenna or around Q1 doesn't have any effect. But when I touch some leads of resistors and caps in the area of Q2 it results in louder humming. That leads me to suspect a problem around Q1.

Also it was suggested earlier to clean the VC, but it is not really dirty. And does the VC not also function as a dialer for the AM radio? I mean, AM radio works.
 
I recapped the whole tuner board on my 737. Didn't unsolder 1 lead. You have to undo all the tie wraps and make slack for the various leads (a couple WILL be tight as you approach 90*) but it can be done. Go very slow and you can get it up on it's side for work without removing anything.

Tape all of the pulleys that carry the tuner string, and tape the string to the chassis along the front as needed. Wrap the main pulley as much as you can with masking tape. But if it does come undone the instructions (diagram) is in the manual. It's not that hard.

The VC is what's known as an AIR GAP capacitor. FM and AM do not go thru the same plates. The narrow gap Large plate sections are the AM and the LARGE gap smaller plate sections are FM. Even a piece of dust sticking on a plate is sufficient to throw it out of alignment. Which is why some of the better units have a cover over them in addition to the cover being used as a shield.
 
Thanks Larry, when I have more time I'll give it a try.

In the meantime I also found this thread.

http://www.audiokarma.org/forums/archive/index.php/t-507581.html

Although my problem is a bit different, I do measure good voltages on Q1, like this guy. But still 3SK45 had failed.

When I tap on the top of Q1 the white noise also changes in volume. I suspect this to be the culprit. Any way I can be 100% sure it is broken before ordering a new one?
 
Taping on it could be simple microphonics!!
Out of the 3 semis, I'd pick the MOSFET, 3sk45 as the 1, but it is a crap shoot.
Hint, if you change 3sk45, wrap a few turns of very thin, un-insulated copper wire to short the leads, while handling, soldering. Take off wire shorts after all the leads are soldered and ground yourself too.
Nice to get input from someone that has actual worked on one, so thx Larry for helping out.
 
Hi guys, wanted to give you an update on the FM tuner issue. Sorry it took a while, but that's because I had to order the 3sk45 and it took some weeks to arrive. I tried it, but without success. I was pretty fed up with the whole trial and error method and spending any more hours on the thing, so I purchased a complete working tuner board through ebay. That one arrived this week. Put it in yesterday and wow.... the FM radio sounds amazing!! Really happy with my fully functioning SX 737!!
 
Thanks for the update. If you had discovered a damaged VC you would have been shopping for a replacement anyway. This method gets you there without all the troubleshooting effort. Unfortunately for the rest of us, we don't get to learn what the root cause might have been, but your unit is up and singing.
 
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