Yamaha RX-730

[mbz]

Yeah, replace D15. It diode tested ok but under load it's not ok

 

[ve ze]

It'll take a few days to get the parts.

 

[mbz]

Thinking about your power amp issue.
Previously said Q24 looks kinda ok but voltages are bouncing about a bit which will affect bias.
Consider replacing Q24 2SA1145ecb 150V 50mA 200MHz 2.5pf
maybe KSA1381ecb 300V 100mA 150MHz 3.1pf

 

[ve ze]

I'll look into that. So far the parts due for replacement are IC7, IC8 (this one looks dodgy too), D15, C164 and Q36 (if needed). That's what's been ordered and shipped by DigiKey.com. Arriving in a few days.

 

[ve ze]

These've been replaced IC7, IC8, D15, C164.
What's next? Power up with DBT? The Right ch power still disconnected by the lifted legs of the jumpers J51 J121.

 

[mbz]

These've been replaced IC7, IC8, D15, C164.
What's next? Power up with DBT? The Right ch power still disconnected by the lifted legs of the jumpers J51 J121.

Yeah, I think so. First sort out power supply, look at right channel later.

DBT power up, bulb dims?
Measure Vdc at "+" pin of D15, repeat for "-" pin

 

[ve ze]

The results of that are exactly the same as before, the bulb dims and the Vdc on + and - is almost 0. Measured as in #138.
And measured directly between + and - gives me 24 Vdc.

 

[mbz]

measured directly between + and - gives me 24 Vdc.

Means D15 is working and you have +/-12Vdc on each rail, probably ok since on DBT

Vdc on + and - is almost 0. Measured as in #138.

What are you using as GND? I like using any black speaker connector, provides quick/easy/reliable connection

 

[ve ze]

Sometimes I use the chassis, sometimes the inner heat sink. I'll be using the black speaker connector now.
What's our next step? Should I reconnect the jumpers and enable power to the right channel?

 

[mbz]

What's our next step?

Confirm "D15" power supply, measure Vdc at Q36e, repeat at Q37e expect +/-9.4Vdc

If that is ok then reinstall right channel jumpers then DBT power up, quickly measure Q26e voltage repeat for Q26c.

Most probable cause is failed short output or driver transistors.
Typically all transistors should be diode tested. Inspect all resistors for stress.
Then I would prefer that you pulled the outputs and connected diodes between the b-e, this should reduce further damage and allow reduced stress fault find. But I sense you want a quicker resolution so go with voltage measurements.

 

[ve ze]

Q36e : +9.26 Vdc
Q37e : -9.3 Vdc
After jumpers reinstalled Q26e and Q26c does not go above 2.4 mV.
Neither does the equivalent for the L channel Q25. Both Q26 and Q25 show the same results.

 

[mbz]

After jumpers reinstalled Q26e and Q26c does not go above 2.4 mV.
Neither does the equivalent for the L channel Q25. Both Q26 and Q25 show the same results.

I'm assuming DBT is bright, the right channel short is dragging down the power supply to both channels
Expect Q26c = +1.2Vdc Q26e = -1.2Vdc

Q32 does over current detection
Please measure output (Q102, Q104) b/c/e voltages to GND
Looking for Vbe = 0.6V abouts and Vc >> Ve
Suspect one is failing under load (Vc ~ Ve) and Q32 is then dragging down the power supply

edit need to step out. Otherwise measure b/c/e voltages for drivers Q28, Q30 looking for same symptoms Vc ~ Ve

 

[Soundscape]

edit need to step out.

Gotta light?

 

[ve ze]

Q102 e: 2mV; c: 2.5mV; b: 0.3mV;
Q104 e: 2mv; c: -1.6mV; b: 0.2mV;
Q28 b: -2.5mV; c: 3.5mV; e: 0.4mV;
Q30 b: -2.5mV; c: -3.5mV; e: 28mV;

 

[mbz]

Wanted to see about 0.6V between Q102e and Q104e, so as to turn on Q32, nada...

Q30e at 28mV is significantly higher than anything else, where did it come from?
Plz recheck Q30e is it "minus" 28mV in which case suspect Q104 c-b breakdown however these voltages don't indicate a serious problem

Maybe fault in sensing circuit? b/c/e voltages to GND for Q10.

 

[ve ze]

Q10 e: +9 to +6 mV; c: +0.5mV; b: +6 mV; Is it supposed to be e=b= 51V; c=0V ?
They all start higher V then drop, including Q30 that you asked to recheck. Q30e: +25mV to +5mV

 

[mbz]

Q102 e: 2mV; c: 2.5mV; b: 0.3mV;
Q104 e: 2mv; c: -1.6mV; b: 0.2mV;
Q28 b: -2.5mV; c: 3.5mV; e: 0.4mV;
Q30 b: -2.5mV; c: -3.5mV; e: 28mV;

Q30e: +25mV to +5mV

Odd that Q30e is the high point and +ve voltage, probably indicates solder bridge.

With amp powered off measure resistance between one main cap terminals then reverse probe polarity and repeat. Then do same 2 measurements on other main caps. Measuring resistance between +/- rails and GND. The resistance readings should ramp up as the MM current charges up the cap. A constant low reading 0-50ohms would be a bad sign, note probe orientation if you get a bad result.

 

[ve ze]

The solder points on Q30 look clean to me.
When you say main caps, which are those?

 

[mbz]

Measure between red and blue marked, repeat between red and green.
Then reverse probe orientation and repeat, so 4 measurements, expect readings to climb

 

[ve ze]

Blue - Red:
+ to - : 0.3 ohm
- to + : 0.5-0.3 ohm

Green - Red:
+ to - : 0.3 ohm
- to + : 0.3 ohm