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2385 and dead FM

The Fuxtor

The Fuxtor

Addicted Member
Working on this 2385, was working great and then suddenly the FM went silent? Went thru the power supply already and checking the 13.5v supply , I'm only getting 7.8v. I disconnected the power feed to the P200 board and rechecked the voltage and then it reads proper voltage ( 12.8v). So I gather it's either A- power supply issue that shows up when a load is attached or B- something dragging the power supply down on P200 board.
Am I able to power the P200 board with a bench supply for testing purposes??
 
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Don’t bother with the bench supply yet — a resistance check answers the same question with the receiver unplugged and nothing at risk.

With the 13.5 V feed still disconnected from P200, meter from the board’s supply input pin to ground, both polarities. Then compare with the board’s supply traced on the schematic:

  • Hard short or a few ohms either way → something on the board is shorted.
  • Hundreds of ohms to a few kΩ, higher in one polarity than the other → that’s a normal semiconductor load; the P200 board is unlikely to be your problem.
 
txturbo said:
Don’t bother with the bench supply yet — a resistance check answers the same question with the receiver unplugged and nothing at risk.

With the 13.5 V feed still disconnected from P200, meter from the board’s supply input pin to ground, both polarities. Then compare with the board’s supply traced on the schematic:

  • Hard short or a few ohms either way → something on the board is shorted.
  • Hundreds of ohms to a few kΩ, higher in one polarity than the other → that’s a normal semiconductor load; the P200 board is unlikely to be your problem.
Click to expand...
From supply pin to ground on the P200 board I read 1.8k ohm both polarities.
Same test on power supply reads 520k ohm/ 480k ohm reversed .
Power supply test points are from ground to 13.5v output pin.
I'll add, I have almost completely redone this power supply. I did discover the board error for a 330uf cap ( 470 uf in sm) that shows backward polarity.
 
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The Fuxtor said:
I did discover the board error for a 330uf cap ( 470 uf in sm) that shows backward polarity.
Click to expand...
Are you referring to C855 and C862. (see below)
If yes, the board layout in the SM shows the correct polarity with + connected to GND because they are used for the negative voltage.
Does your board show differently than in the SM?
1791998511739.png
1791998587604.png
 
dankik said:
Are you referring to C855 and C862. (see below)
If yes, the board layout in the SM shows the correct polarity with + connected to GND because they are used for the negative voltage.
Does your board show differently than in the SM?
View attachment 3856374
View attachment 3856379
Click to expand...
No, C853.Screenshot_20261014-115016.png
I had gone with the silkscreen originally before discovering this error..
This was after the FM had gone out tho..
Not my pic, taken from another AK post
 
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Well, I gotta go deeper in this problem..
I've disconnected both 13.5v feeds. The voltage has bumped up there, but I'm getting bad voltages on the transistors in the positive npn side. Q854 is getting super warm. Base is only at 6v?
 
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