Yamaha RX-A380 Protection Circuit

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#21
gsanc said:
I first tried to measure the resistance, but my multimeter show 20 M ohm which is huge compared to the 33 k ohm in the schema. Could the measurement be wrong and including many other elements around R1045 ? When I compare with R1044 (other front channel), multimeter shows something closer around 30 k ohm as expected.

I measure 6.8V now across R1045 measured across C1038 since they are mounted in parallel.
Click to expand...

gsanc said:
Should I cut out R1045 to be sure of the measurement ?
Click to expand...

I believe this is one of those occasions when it's good to measure out-of-circuit. This might be a culprit. You can lift just one end of the component if that's easier.

Another tip is is to measure resistance with both polarities; if the readings differ, there's likely a shunting semiconductor influencing the reading or perhaps stored charge on a cap.
 
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#22
BSST said:
Thanks, Wakh.

The only thing I would add in clarification is that the base voltages at Q1005 and Q1006 will be negative by a few tenths of a volt due the bias currents flowing in R1017 and R1045. Ideally, they would have equal voltage drops, leading to 0V at amp output; there's likely to be some output offset error.
Click to expand...
counted the currents and adjusted the voltages
111.png
 
#24
In the event that R1045 measures OK out-of-circuit, reinstall and probe around the Q1006 base, D102, R1045 node. Try to measure on the component leads of these parts--- perhaps there's an open PCB trace at this circuit node.
 
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#25
Hey everyone,

Back to business after a few days off !

I managed to get the mainboard off the case and measured more or less everything I could !

all tested 2 bads R.jpg


The 2 only bad measurement I have are in the center/left part around R1045/R1052 ! I had some 33k/1W resistors, so I tried to replace R1045 ! Bad luck, the resistor shows a correct 33kOhm measurement when measured alone off the mainboard.

Stil replaced It anyway with another brand new (and verified) 33k resistor, but same problem when measuring resistor "in place", slow growing and stabilizing around 22k when the working channel equivalent resistor stabilizes around 33k. Could be related to the missing transistors at the end ?

R1052 is surface mounted ! It makes it more challenging to remove and measure (should be ok) and put back in place !

Any other suggestions ? Capacitors (I don't have adequate equipment to measure them) ?

Thanks 😉
GS.
 
#26
Unless you have solder tweezers, I suggest not removing any surface mount parts just to measure them—- too easy to damage. Just report voltage measurements until suspicion of a part becomes inescapable.

I’d like to discover why output is stuck near -5V. Would you report voltages re ground at:

Output at R1150 or equivalent
Base of Q1006
Base of Q1005

Voltage across R1038
Voltage across R1081
Voltage between base of Q1033 and cathode of D1008

Voltage at cathode of D1008 re ground.

Thanks.
 
#28
Hey,
To keep you informed ! I ended changing C1031 and now the amplifier starts with no more voltage measured on speakers output !

Yay... but... the sound is pretty bad, like saturated sound but all time.... Tried with different sources, different speakers, same...

Well... maybe something else is broken somewhere else 🙁

Thanks !
GS
 
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