SX-1980 Power Supply R209 running hot

omri617

Super Member
Been running the receiver with no issues for few months and I decided to have a look and inspection on the supply board (awr-154) .
Noticed that R209 (39ohm 1/2W) is running extremely hot it's untouchable by finger.
It gets 8V from Q205 but drops down the voltage to 1.5V which by my understanding should be 5V going to APC board
could be C213 causing this?
Thanks
 
It's possible, but IIAC you have a new C213 in there and that reduces the odds greatly.. This circuit DEPENDS upon the current draw to get the correct voltage.
If C12 (just a "plain jane" cea cap - use upw or whatever) in the APC assembly is original it could be leaky (electronically) - there really isn't much else except the APC logic chips to draw on that line.
8v to 5v across 39 ohms means about a 77 mA draw and about 1/4 watt of heat to be dissipated.

8v to 1.5v across 39 ohms is 166 mA, and about 1 watt of heat.
at 1.5v I don't think the APC logic chips have enough voltage to operate.
166mA at 1.5v is a quarter of a watt, so whatever is pulling the extra current probably isn't getting very hot.

Please talk to me more here before tearing into the can of worms called the APC.
 
Thats the thing - there is no C213 . Instead there is a diode with it's Cathode towards R209. and looks original . if I can read right it's 0578S.
I remember we talked about this loooong time ago :)
I disconnected velvet wire going from Supply Board AWR-154 pin 15 to APC board - no change.
Still Getting the same 1.5V on R209 .
Did not touch APC .
 
Ok, I found the 2014 thread, or part of it... Is R209 a 39 ohm resistor now or 22 ohms? How hard is it for you to get zener diodes in your area? I couldn't find the discussion where it was decided to leave it be.

Specifically 1/2 watt 5 volt zeners?

What have you noticed about tuning operation and the APC circuit / LEDs?
 
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try clipping the diode lead and see what the voltage comes up to. There are power bypass caps on the 5v in the APC box.
 
Not the best idea for the long term, for troubleshooting - maybe. Rising voltage that high shows that lower power draw chips were used.

A 5v zener (1/2 watt) would be a good idea, also we can calculate a greater resistance to accommodate the reduced power draw and not get things as hot.

Measure the voltage across the 39 ohm resistor AT the resistor's leads so we can calculate the current draw of that 5v line.

The numbers on that diode weren't helpful (not your fault) in figuring out what it WAS, but the 5v 1/2w zener is the most logical choice - and I suspect it was - and failed.

Remember, a zener pulls as much current as it takes to get the voltage across itself to the "knee" point. The less the client circuit draws, the harder the zener works and the hotter it gets.
 
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Zener diode is in and a new 39ohm resistor and Voila! Voltage drops from 8V to 5V as should! Mark you are an asset :)
 
Well, I fake it well.


What's the tuner doing now, and if it isn't "doing" - what is the +13v supply running at?
 
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