A-550 zero bias issue

zaibatsu

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I've got a problem with an A-550, the relay clicks on, however the bias measurement is 0 at any pot setting. It plays music but there is a lot of distortion, presumably because of the bias issue.

The problem is the same on both channels, which makes me wonder if the problem is something common to them, or if the same part/s have failed on both channels.

Hifiengine page:
https://www.hifiengine.com/manual_library/yamaha/a-550.shtml

Schematic for power amp (one channel):
1fUW9yf.png


Appreciate any suggestions!
 
I have the same issue with a denon POA 6600a where i replaced the blowed power transistors . I checked every components ( resistors. diodes, transistors) and everything is OK.Probably the issue comes from a bad solder .
 
If the VR has no effect on the bias then possible causes are,
- no/very low current flow through VR
Measure TR205 b/c/e voltages to GND, maybe it's not turned on
Verify VR is not open
- Bias transistor faulty
Measure TR207 b/c/e voltages to GND
- a "forward" transistor is faulty
Check TR215, 217 prefer by measuring b/c/e voltages to GND, alt diode check.
 
The problem is the same on both channels, which makes me wonder if the problem is something common to them,
Only common denominator. Power supply! Verify that you have the unregulated raw rail voltage( +/- 42vdc) on the output transistors collectors pins.As well as the regulated +/- 20 volts at the emitters of TR301 and TR304 in the power supply.
 
Thanks guys. I just measured everything.

There's clearly something very strange going on in the amp section.
The PSU section and rail voltages seem OK, but I listed everything requested anyway.

-Measure TR205 b/c/e voltages to GND, maybe it's not turned on
Verify VR is not open
- Bias transistor faulty
Measure TR207 b/c/e voltages to GND
- a "forward" transistor is faulty
Check TR215, 217 prefer by measuring b/c/e voltages to GND, alt diode check.

TR205 B/C/E:
-35.5 / -19 / -36.2 (collector constantly falling, E/B stable)

TR207 B/C/E:
-15.1 / -13.5 / -15.0 (all are varying)

TR208 B/C/E (the 2nd bias comp transistor):
14 / 16 / 14 (varying a lot, started around 8 and increasing)

TR215 B/C/E
-20 (constantly falling) / 36.2 (stable) / -19.5 (constantly falling)

TR217 B/C/E
-20 (constantly falling) / -30.8 (stable) / -19.5 (constantly falling)

None of these values are stable, and TR216 / TR218 on the other channel are a similar mess.

Only common denominator. Power supply! Verify that you have the unregulated raw rail voltage( +/- 42vdc) on the output transistors collectors pins.As well as the regulated +/- 20 volts at the emitters of TR301 and TR304 in the power supply.
- Rail voltage looks OK on all output transistor (x4) collectors (+/- 38Vdc on DBT)
- TR301 emitter is 21.0V
- TR304 emitter is -21.9V

These numbers were all stable.

Could be and "open" emitter resistor or bad solder connection on said emitter resistor.
- Emitter resistors (x4) all test OK (~0.5Ω, labeled 0.47Ω)
- Solder joints all look fine
 
Have you replaced any parts on the two amplifiers yet ?
 
Does it look like any parts have recently been replaced ? ie shinier solder connections than others.newer components than the rest within?
 
To the 2nd, no, it definitely all looked stock and untouched.

To the 1st - guilty - I tried changing some transistors in one channel to try fix the bias issue, they are the pictured ones:
bias.png

TR208 2SC2240 -> KSC1845
TR210 2SA999 -> KSA1015
TR212 2SC2320 -> KSC1815
TR214 2SA999 -> KSA1015

It didn't seem to change anything. I still have the old parts if needed.

Just to confirm, the relay still clicks on mains and it plays music, but there's a bunch of distortion.
 
To the 2nd, no, it definitely all looked stock and untouched.

To the 1st - guilty - I tried changing some transistors in one channel to try fix the bias issue, they are the pictured ones:
View attachment 1033867

TR208 2SC2240 -> KSC1845
TR210 2SA999 -> KSA1015
TR212 2SC2320 -> KSC1815
TR214 2SA999 -> KSA1015

It didn't seem to change anything. I still have the old parts if needed.

Just to confirm, the relay still clicks on mains and it plays music, but there's a bunch of distortion.
I thought this issue was in both channels ?
 
TR215 B/C/E
-20 (constantly falling) / 36.2 (stable) / -19.5 (constantly falling)

TR217 B/C/E
-20 (constantly falling) / -30.8 (stable) / -19.5 (constantly falling)
- Rail voltage looks OK on all output transistor (x4) collectors (+/- 38Vdc on DBT)

The collector of the drivers are connected directly to the rails
 
TR215 B/C/E
-20 (constantly falling) / 36.2 (stable) / -19.5 (constantly falling)

TR217 B/C/E
-20 (constantly falling) / -30.8 (stable) / -19.5 (constantly falling)

Should be very close to +/- 1 volt.
 
Make sure you have a good positive ground connection to the common junctions of D207/209 and D208/210.
 
TR215 B/C/E
-20 (constantly falling) / 36.2 (stable) / -19.5 (constantly falling)

TR217 B/C/E
-20 (constantly falling) / -30.8 (stable) / -19.5 (constantly falling)
These should be less than a volt.
 
I'm thinking a ground issue on your amplifier sheet... Cracked trace or cold solder connection?
 
- Yes, issue is the same on both channels
- Ground connection at common junctions of D207/209 and 208/210 look good

What do you mean by amplifier sheet, the clipping of the schematic?
Any ideas where this bad ground might be?
It seems the output stage voltages are OK, but everything before it is measuring oddly.
 
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