SX-737 Power Supply voltages

[Rico126]

Evening all. I am having some weird voltages on the PS board (AWR-057) on a newly acquired SX-737. At the start the relay wouldn't click. I checked around the PS and found a bad 2SD313 and a bad 2SC945 after pulling them and checking them on my Peak analyzer. I replaced these and triple checked that they are installed properly. Now here are the voltages coming off the power supply; expected voltage in parentheses:

Pin 12 (-13v) -13v
Pin 14 (35v) 13.4v
Pin 16 (30v) 12v
Pin 17 (23v) 9v
Pin 19 (13v) 3.8v

I have 48V going into Q1 Collector, but Base should be 35.4, but it is 14. What have I missed?

 

[merlynski]

Look again at Q2, it is either saturated, shorted C-E or miss-oriented. What is the voltage on the base of Q2?, should be about +14.7vdc.

 

[Rico126]

Base of q2 is 5.5v, emitter is 4.9, and collector is 14.1 This was replaced yesterday with a KSC1845. It is an ECB Orientation which I verified again by the silkscreen marking on the board. I pulled this out earlier and retested it on my analyzer. To me it is something right there that I am missing

 

[merlynski]

Emitter of Q2 should be +14vdc because D9 is (should be) a 14 volt zener diode. I assumed it was ok. Sorry about that. Back to basics. If Q1 and Q2 were fried D9 was probably damaged too. Power down and lift one end, test with diode test on DMM both directions. Test Q3 and Q4. R6 (3.3k) is the pull up resistor for that zener. Measure it and R4, R5, R7 R8. Check C16 for being leaky.

You listed "pin 12 (-13v) 13v"
Is that a typo and it is -13v? D8 is a 13 volt zener, but if a positive voltage tried to appear there it would be forward biased at 0.7v (or toasted).

What is the voltage on pin 7? Sch shows -38vdc.

 

[Rico126]

Pin 12 was a typo, -13 is correct. I corrected this in the original post as well. Pin 7 if I remember correctly is correct to the schematic. Q3 and Q4 were replaced yesterday as well as c16, as it was swelled. I'll check resistors tomorrow, none look out of the ordinary.

 

[Rico126]

Bah. No work on it tonight. Late day of work then a craigslist pickup has me too tired to see the small stuff. I am hoping that by tomorrow night the receiver will have fixed itself.

 

[Rico126]

Looks like D9 is toast.

 

[merlynski]

Looks like D9 is toast.

Ah Ha! progress

 

[Rico126]

Yep. of course I do not have one in stock here. Why must mouser charge $8 to ship anything...

 

[merlynski]

If you have a 13v zener you could use that and a regular diode in series to get 13.7v, or two and get 14.4. That would get you in the ball park for testing and troubleshooting purposes . . .

 

[Rico126]

I have neither the 1n5244 or the 1n5243. Just have to wait it out. May go with Newark, they are closer but usually don't have everything I need. Will. Heck later

 

[merlynski]

I hate it when that happens. 100 of either one is less than $5 from Mouser, makes me think I should stock up.

 

[Rico126]

The zener, and the rest of the stock, showed up today. I hate to buy stock just to make the shipping worth it, but alas $30's worth of parts to get a $.25 diode. Anyway the zener was the answer. A few seconds of putting it in, plug into DBT, and the voltages were spot on and the relay pulled in. Good times. Thanks Merlynski. Sad thing, if I had them in stock before it would have been replaced along with the transistors, but then we wouldnt have this thread to refer to in the future either.

 

[merlynski]

You are welcome! Glad it worked out