After rebuilding all the boards but the tuner on my SX-737 with great help from Markthefixer , the tuner has decided to quit working . Go figure eh :sigh: .

It seemed to happen slowly over a day or two . I was losing signal strength till there was none at all . The stations would come in , barely , FM Stereo would light up , and i would get a thup thup thup . Like a helicopter almost . Now as a scroll across the dial , I get no sound and the tuner needle stays dead center , no movement . Also , the signal meter stays at zero , no movement .

I did a search , but couldn't come up with anything specific to the 737 . What I did find was a " similar " problem on a 727 . It was suggested that it might be the multiplex circuit . 'There are a group of capacitors in the detector and mpx circuit that are prone to failure .'

Could this be my problem ?:scratch2:

Maron should you find you need any parts I have a 737 I am parting out. BUT condition of any electronic parts is unknown since I never plugged it in.

Thanks Mark , I'll keep that in mind .:yes:

Ok, first big question, is how is AM behaving? Is it working at all?

We need to read the voltages on pins 19, 20 and 21 of the power supply. They have to be 13.5 volts or so.

power supply pin 21 runs the stereo lamp, which connects to tuner pin 15, which (15) will go to a lower voltage (but not zero) to turn on the stereo indicator lamp.


power supply pin 20 runs through the function switch to tuner pin 3, the FM front end power, which can affect the signal strength with bad contacts conveying power. Pin 3 is live only on FM, Is tuner pin 3 13v or so on FM?

power supply pin 20 also runs through the function switch to tuner pin 4, which when switched to AM, turns off (the 13v does it) the FM detector chip (which drives the tuning and signal meters). Is tuner pin 4 13v or so on AM?

power supply pin 20 also runs through the function switch to tuner pin 33, which turns OFF the stereo decoder on AM. Is tuner pin 33 13v or so on AM?



power supply pin 19 runs directly to tuner pin 20, conveying most of the power needed by the am and fm tuner circuits. Almost ALL of the tuner circuits for AM and FM are live ALL the time - even in phono, with the exceptions noted (mostly) above.

The power that goes in on tuner pin 20 comes out on pin 25, and goes to the function switch, which feeds it BACK into tuner pin 26 ON AM to turn on the last few AM tuning circuits to bring AM live.

Tuner pin 25 is at the tail end of a lot of circuits, if something happened on the board, the voltage coming out on pin 25 would be affected. What is the voltage on tuner pin 25?

Next, if necessary, we will be probing power pins on the tuner's integrated circuits. But my bet's on the function switch starving the FM front end, the FM detector chip (tuning and signal meters) is always on, but supressed on AM by

The AM is doing the same thing .

I turned the radio on just now , after the unit had been powered off for about 12 hours . The tuner worked for about 10 seconds and then , thup thup thup . No sound again . The stereo light never came on though .

I will get the unit opened up and take those measurements shortly .
Thanks Mark . :yes:

Pins 19 , 20 and 21 on the power supply board (AWR057) all read around 6.5 volts :tears:

All subsequent pins on the tuner board read around 6.5v except for pin 25 which reads 5.1v .

What was done with the power supply card? Caps only? Read and post pin 14's voltage - there's a point to it.

Also read and post the Q3 (the second 2sd313, by the edge of the board) and Q4 (right next to q3) voltages.

We fix the power supply and the tuner should be fine.

If it was just new caps, an old beat up transistor could have been stressed by the new caps.

Pin 14 reads 34.5v

Q3 and Q4 test bad .

So , do we need to order new transistors ?

What was done with the power supply card? Caps only? Read and post pin 14's voltage - there's a point to it.

Also read and post the Q3 (the second 2sd313, by the edge of the board) and Q4 (right next to q3) voltages.

We fix the power supply and the tuner should be fine.

If it was just new caps, an old beat up transistor could have been stressed by the new caps.

Pin 14 reads 34.5v

Q3 and Q4 test bad .

So , do we need to order new transistors ?

Yes, but I'm forced to assume that the D9 zener diode voltage of 14v is present at the base of Q4, (and infer from pin 14 being 34.5v that D9 is generating it) and that there is sufficient voltage at the collectors of Q3 and Q4 (R3 not blown or C13 not shorted) to run things because the requested "Q3 (the second 2sd313, by the edge of the board) and Q4 (right next to q3) voltages." were not posted. There is a method to my madness.:D

A lot of folks have remarked that they re-read the posts several times, always spotting new stuff and gaining other insights.

I can't shake the deja-vu here, I thought I had posted this list already.

DID you get spares when you ordered the transistors? While Q3 is pretty stringent, Q4 could be quite a few of them. If Q3 isn't heatsunk, the:
512-KSC2690AYS to-126 ecb 120/a160 1.2a 20w 155mhz 35-320hfe $0.40
will do in a pinch. The tuner doesn't draw that much current.

Q3 2sd313 > 512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54 would be my usual recommendation.

while
Q4 2sc945 > can be a 512-KSC2310YBU to-92 ecb 150v .05a .8w 100mhz 40-240hfe $0.11 ea although the
512-KSC2383YBU to-92 ecb 160v 1a .9w 50mhz 160-320hfe $0.17
would be my usual recommendation.

between the gain of the KSC2690. the low draw of the tuner and the gain of the 2310 being able to drive 50mA and dissipate .8w, things should work fine.

Hope this made sense, it's been a long day......