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Question about the function of a capacitor in a circuit
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fedaalis84:
I'm a newb here & I know some basic things about capacitors. But wondering specifically what this guy here is doing in this part of the circuit?

This is a schematic for the main board on the Sony STR-DH130.



Pretty sure this part of the circuit is part of the Protect system, to detect a voltage overload from the output transistors on the right audio channel.

Long story short, those two output transistors on the right channel shorted out (bringing the emitter resistor and an adjacent 100ohm resistor down with it). Anyway, when desoldering the emitter resistor, my dumb self also desoldered C761, a teeny... tiny SMD 220PF capacitor.

I also jacked up the trace from that capacitor to Q761 (the current detector).

After desoldering the output transistors, and even my SMD capacitor/trace mishap, the system will still start - and was not going into protect mode.

Fast forward to today, I installed a new emitter resistor, and also placed a jumper between R761 and Q761 EDIT: From the bottom of R761 to the base of Q761 (eventually once my SMD 220PF capacitors come in the mail, I'll get one of those bad boys in, too.)...

But afterwards (and checking my work with the multi-meter for any kind of weird solder bridges anywhere, everything checked out OK)

But now, the system will go into protect again after powering up.

So I'm wondering, is it because I still need to get this capacitor back in?

Thanks for the help!
xavier60:
It's unlikely that the missing capacitor is the cause. To be certain, solder in something else, no matter if it's larger than 220pF.
Is there DC on either channel's output?
What is the voltage on the collectors of Q762 and Q712?. Normally they should be almost the same as the negative rail.
fedaalis84:

--- Quote from: xavier60 on March 02, 2019, 01:49:06 am ---It's unlikely that the missing capacitor is the cause. To be certain, solder in something else, no matter if it's larger than 220pF.
Is there DC on either channel's output?
What is the voltage on the collectors of Q762 and Q712?. Normally they should be almost the same as the negative rail.

--- End quote ---

Thanks xavier60,

I'll try adding some random cap.

There should be no DC on the right channel's output, I have the output transistors out of circuit at the moment. I'll check the collectors on Q762 and Q712, but the left channel was not having issues before - so I think the protect has to be from my tinkering today.

Before, when it was going into protect when I had the right channel's transistors in, it would hit protect much faster. Now it takes some time, and almost seems like it won't. Then bam, Protect.

I might try desoldering the jumper if a random cap doesn't resolve the issue.

Thanks for the input!
fedaalis84:
Update:

Tried an unknown value SMD ceramic cap. Still goes into protect.

Tried unsoldering the jumper, still goes into protect.

BUT something that I hadn't considered:

I replaced the bad emitter resistor (which was previously behaving like an open circuit). So that could be causing protect to surface via some means that I have not identified.

Didn't have time tonight, but next I will try removing that & see what happens.

Couldn't be something else in the protect circuit that, because I have the right channel transistors out, is now being wonky because there's a connection via the emitter resistor that wasn't there previously?
fedaalis84:
I don't have a DC power supply, so I can really only measure voltages on top of the board, because I have to use the units power supply.

Anyway, on J469, which connects to the emitters of Q762, and Q712 (with a 68k resistor between either emitter and the junction)  I'm reading -53V DC, and drops down to almost nothing once it goes into protect.

Because the right side is open (transistors are out), this voltage must be coming from the left channel, right?

 :o

Stumped at why it's going into protect now, but didn't before I messed up C761 and the trace to the base of Q762.. if the excess voltage is coming from the left channel, which seemed fine before.
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