CA-1010 Unable to adjust Class A bias, Normal OK

[Yamaha5000]

In class A, measure +/-B Vdc supply (red/white) wires to power amp boards, expect ballpark +/-22Vdc.

 

[Yamaha5000]

In class A, measure +/-B Vdc supply (red/white) wires to power amp boards, expect ballpark +/-22Vdc.

Measures +/- 22V

 

[Yamaha5000]

CA-1010 Unable to adjust Class A bias, Normal OK

Normal is adjustable 25mV on both channels
Class A is adjustable on one channel
Class A is not adjustable on the other channel, it stays at 25mV and adjusting VR302 makes no difference.

 

[Yamaha5000]

Avionic says: Check D306 and TR311 and TR312

Pulled the board one more time and D306 and TR311 and TR312 test fine

 

[mbz]

A problem with VR302?
Measure DC voltage at TP2 to GND then adjust VR302, does the TP2 voltage change? what is the maximum voltage you see at TP2.
Wise to have a second MM connected between TP2 and TP1 to monitor bias.

 

[Yamaha5000]

VR302 Tested and working fine

Does change but minimal
TP2 to GND 41mV

 

[mbz]

Check voltage at FR306 OR D306 anode, as marked, which ever is easier.
Vary VR302, does the voltage change (maybe 100-200mV?)

 

[Yamaha5000]

Vary VR302, does the voltage change (maybe 100-200mV?)

Present change is less than 10mV

 

[mbz]

Present change is less than 10mV

At FR306 or D306 anode? Also what is the max voltage you see at that point, expect about +1.2Vdc at D306a and about 0.65V at FR306. Either way the bias control appears not ok.

 

[mbz]

Might be related to that "Diode/Link" issue J304??? What diode (Vf) did you use for replacement

 

[Yamaha5000]

Check voltage at FR306 OR D306 anode, as marked, which ever is easier.

D306 Anode 1.21v
That side of FR306 .07v

 

[Yamaha5000]

Might be related to that "Diode/Link" issue J304??? What diode (Vf) did you use for replacement

4148

 

[Yamaha5000]

Might be related to that "Diode/Link" issue J304??? What diode (Vf) did you use for replacement

Left and Right Channel boards are identical

 

[Yamaha5000]

Either way the bias control appears not ok.

That is the title of this thread.

 

[Yamaha5000]

At FR306 or D306 anode?

Were you asking me to adjust VR302 while taking this measurement?

 

[Yamaha5000]

D309 Cathode .04v
D309 Anode 0.00v
D308 Cathode .00v
D308 Anode .03v
D310 Anode .62v
D310 Cathode .07v

 

[Yamaha5000]

0.65V at FR306.

On the other side of FR306 it is 0.70v

 

[mbz]

On the other side of FR306 it is 0.70v

OK, looks like I marked the wrong side of the FR. So we have +1.2V and 0.7V at TR316 b,e.

Were you asking me to adjust VR302 while taking this measurement?

Yeah, the idea was to see if bias voltage responded to VR adjustment, possibly pointing to TR316 (under the lower class A voltages).
Check if D306 anode responds to bias adjust, should be adjustable by 100-200mV... Assume you have resistance checked VR302 and that you get full range resistance of 0-2kohms between wiper and non-connected leg.

 

[Yamaha5000]

OK, looks like I marked the wrong side of the FR. So we have +1.2V and 0.7V at TR316 b,e.


Yeah, the idea was to see if bias voltage responded to VR adjustment, possibly pointing to TR316 (under the lower class A voltages).
Check if D306 anode responds to bias adjust, should be adjustable by 100-200mV... Assume you have resistance checked VR302 and that you get full range resistance of 0-2kohms between wiper and non-connected leg.

I removed VR302 and bench tested it

 

[Yamaha5000]

Yeah, the idea was to see if bias voltage responded to VR adjustment, possibly pointing to TR316 (under the lower class A voltages).
Check if D306 anode responds to bias adjust, should be adjustable by 100-200mV...

Will do