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Question about the function of a capacitor in a circuit

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fedaalis84:

--- Quote from: xavier60 on March 03, 2019, 05:05:42 pm ---
--- Quote from: fedaalis84 on March 03, 2019, 04:00:09 pm ---So I'm still seeing something strange with the main power rails.

On R545, the resistor for the positive power rail, I'm reading about -20VDC. On R546, the negative power rail, I'm reading -62VDC.

R545 should be giving me positive 62VDC, correct?

Also, something weird... The positive leg of the bridge rectifier diode reads a steady 62VDC even after the AC is turned off by the relay in protect mode. It doesn't drop, it just sticks at 62VDC... Weird, no?

--- End quote ---
R545 must be open. The positive rail capacitor is staying charged because no current is being drawn from the rail. R545 and R546 function as fuses.
Don't bridge out R545. replace it with the same value or something close and hope that there is no permanent overload present.

--- End quote ---

Thanks!!

R545 is reading at 0.00 ohms when I check resistance, is tha4 because it's still in circuit?

Also, R546 is registering at 0.7 ohms, when its rated at 0.47, that seems pretty far out of spec?

I'll try maybe replacing them both.

xavier60:
The charged up rail capacitor is likely to be causing a false reading, It will need to be discharged before you start handling the board to avoid further damage. Don't directly short out large capacitors. use a low value power resistor or anything with suitable resistance.

fedaalis84:

--- Quote from: xavier60 on March 03, 2019, 05:18:33 pm ---The charged up rail capacitor is likely to be causing a false reading, It will need to be discharged before you start handling the board to avoid further damage. Don't directly short out large capacitors. use a low value power resistor or anything with suitable resistance.

--- End quote ---

Definitely the case. Reading high voltage on the capacitor side of the resistor & very little on the other.

Replacements on their way.

I think it might have been my fault. When I was measuring voltages on IC700 I think I may have briefly shorted pins 14 & 15 together...  :-[  probably why R545 is toast.

So I don't think that's what's causing the protect.

fedaalis84:
Found the source of the protect.

Whenever I have the system set to output *just* to speakers A, or just speakers B, it will short out & go into protect - and I get DC on both output channels. If the system is set to output to both speakers A and B, it will not go into protect - output on left & right look good with +- ~50ish mV; and the relays also are closing the circuit successfully to the speaker outputs. Also does not go into protect when not outputting to any speakers.

 :-DD

Something any electronics repair guy/gal worth his or her salt would've caught a LOT earlier... But - hey, this is my FIRST ever attempt at electronics repair . So if I can get this old hunk of junk up and running, I'm happy - and it was a success.  :-+

Got some new relays on order. New 0.47 ohm resistors for between the filter caps and IC700... And still waiting on heat paste for the right-channel transistors, and a 100 ohm resistor to replace one that went bad on the right channel, and she should be up and running...

Wish me luck!

xavier60:
I'm not sure about all of that. With one of the 0.47Ω open, that is a reason to have a DC protect detection.
Some thought needs to be given to the possible reason for the output transistors failing. Like being overloaded by driving too low impedance or shorted speaker lead.
The circuitry(2 transistors and resistors) that sits between IC700 and the output transistors  has the function of controlling the idle bias current through the output transistors. This circuitry needs to be carefully checked on both channels for cracked joints and damaged transistors.
A problem with this circuitry can destroy the output stage again.

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