Luxman M-2000 can't adjust bias

[david winter]

Hi,

I'm trying to find out why the bias on both channels can't be adjusted.
The amp was recaped, I cleaned the relays which work fine (the first clicks within one second after powering up, followed by the second relay (which turns the VU lights on) 3 seconds after).

The voltages going to both output stages are correct: -56V/+56V and -64V/+64V.
One stage has a shortened tantalum in the bias circuit (the 47µF/6.3V), but each output stage, even connected alone, has the same issue: I just can't adjust the bias to 60mA and read 0.28mA.
Has anybody had this trouble ? What am I missing ?

Thanks.

 

[sanjib]

Is your DMM fuse ok?

 

[david winter]

Yes of course. I thought it could have turned bad and made sure it gave correct values.

 

[tube-a-lou]

I'm following this thread, thanks for posting. I been having the say trouble with bias
in my amp just like this.

 

[sanjib]

Can you measure the voltage across R311a and R311b?

 

[david winter]

0mV.
With the ampmeter I had around 0.28mA wherever I set the bias trimmer.

 

[sanjib]

Check the biasing diode D203

 

[david winter]

Seems ok. Drops 1.914V at 12V with a 10K series resistor.
Of course Vf drops when touching it.
The bias circuit has a minor change vs the schematic.

 

[sanjib]

Have you checked all the transistors? DC offset is fine?

 

[david winter]

Yes I can adjust the DC offset without trouble on both output stages, which both have correct voltages at their connectors.

 

[sanjib]

I would check all the transistors and diodes starting from biasing transistors and upstream.

 

[sanjib]

Can you post a pic of the board?

 

[mbz]

Have you changed the bias transistor, maybe the different Vbe is causing issues...

Near zero current flow might point to one of the resistors R304,5, 311, 312 drifting high. Resistance check, incircuit should be ok.
Monitor dc voltage at pin/connector 14/2, this should vary (mV) when the VR is turned, expect about +1.2Vdc at pin 14/2.

 

[sanjib]

Do you have a little fuse placed between the bias measurement points?

 

[david winter]

No fuse, but I found the culprit on one block: the 2SC945 bias transistor had an almost shorted section droping Vce to 0.6V .
Can't understand how I could have missed that.
I'll check the other block tomorrow. Thanks for your hints.

Edit: couldn't resist checking the other 2SC945 which sure enough had the same problem.
Second block adjusts find now.
Can't figure out what can have caused the same trouble on both blocks but I can only suspect bad aging of those transistors (which I have already experienced like many others)...

 

[Binkman]

Can't figure out what can have caused the same trouble on both blocks

Many Lux's have small solder joint pads and with age the constant heating and cooling ages the solder.. usually noticeable if grayed out and under mag glass might look like cold soldered, too. I suppose lack of ventilation could also cause increase transistor failure too. many posts with transistor replacements on lux's.

 

[david winter]

This one had already been servied a long time ago due to a fried output stage, and although the solders were already refreshed on the daughter boards, I had to secure a number of others as well.
I sure had to refresh many, many others, and not just on Lux's. Pioneer also just to name one.