BASF D6075 idle common problem

[Poskok2010]

aka NEC AUA-8000E aka MCS3865.
Found two similar treads:
https://audiokarma.org/forums/index.php?threads/help-with-idle-current-adjustment.578811/
https://audiokarma.org/forums/index...rent-question-nec-aua-8000e-amplifier.893200/

but with no solution, couple of guys tried also to revive the treads with same problem, so i assume that is general issue with those amplifiers.

Anyway just after a startup on test points (two per channel) i have on: Right Ch 1,8mV and 0,9 mV
Left Ch 1,2mV and 0,4 mV

after one hour of amplifier idling i get:
Right Ch 11mV and 10mV
Left Ch 8mV and 4 mV
here is a pic of schematic with my measurments in green letters (red are original values)

https://ibb.co/s2LCdvy

just load full resolution and after right click on it save as.... and watch it in full size.

All voltages are almost ok except the rails to power transistors. They should be +/-42V but they are in real +/-46,4V on right channel and +45,7V/-45,9V on the left one. I assume that also the voltage on D203 (lover 10% than rated) is messing with idle current.
Somebody on another thread mention lazy outputs. How can i test their laziness .

Any other ideas or suggestions?
Will be very grateful on any help, thanks.

 

[elnaldo]

I don't see a problem yet. both channels a bit low, they should be adjusted to 14mV when the amp is stabilized. Try to get as close as you can to 14mV and report back. The difference you can have in the same channel is caused by unmatched transistors + unmatched emitter resistors, and if within a small %, that's not a big problem. Adjust to 14 or 15mV and check the difference.

Regarding the "laziness", you are checking it right now. Given the same bias at their bases, one transistor conducts a bit more, one transistor (the lazy one) conducts a bit less. At high power, unmatched transistors can cause an amplifier to blow, since the current is not shared equally between the transistors, one transistor takes more current, overheating and conducting even more. But a small difference between transistors is normal, there is a tolerance when matching transistors for this application.

Don't adjust the bias current with the unit connected to a DBT, you need the full voltage to do this adjustment.

Power supply voltages are a bit high, like 10%. If you have a voltage selector, check the setting, and if possible adjust it. If set to 110V, change it to 120V. If set to 220V, change it to 240V.

I'd follow Avionic's advice on those linked threads.

 

[Poskok2010]

Thanks on lazy explanation.

Regarding the bias setting, like on the other threads the trimmers are at maximum , so i can set it higher. The amplifier is connected on variac so he is getting exactly 220V (power grid in my area is 240V). The BASF doesnt have voltage selector like NEC has. He is fixed on 220V.

 

[petehall347]

if its going to be run on 240v set bias at 240v . it will likely need turning down a bit now

 

[Poskok2010]

plug it on 240V, now the rails to power transistors are +/-50V, idle is
R:8,7 mV and 6,7 mV
L: 12,8 mV and 10,5 mV
not much of improvement, The output transistor are still almost at room temperature after one hour warm up , the emmiter and base resistors are checked and in great shape and value.

 

[elnaldo]

Are your emitter resistors 0.47 ohm? If so, it seems simply that the 100 Ohm trimpot doesn't have enough range to adjust. You could try replacing it with a 200 ohm trimpot, or add a 56 ohm resistor in series with the 100 Ohm trimpot. Or, adjust both channels near 8mV. If the amp is working and you don't hear any distortion, you could simply adjust it "cooler" and close the cover.

You have a working amplifier now, priority is to keep it working. Don't risk the output transistors while measuring, don't do any movement or anything you don't trust 100%. If in doubt, let it alone and just enjoy playing music.

 

[Poskok2010]

Yep, the meter show me on emitter resistors 0,6-0,61 ohm each so i guess they are OK. I did not well write up there trimpots are on minimum O ohm.

 

[elnaldo]

Bias current should increase if the trimpot resistance increase, if I'm reading the schematic the right way. Have you tried setting the trimpots at mid position? If not, set them to the mid position with the unit unplugged, connect your meter, power ON the unit and read the values. If too far from specs, readjust.

 

[Poskok2010]

Yes sorry you are right.
You think it is OK to raise the factory trimmer and set the bias on factory recomended value or is better to set both on 8mV?
thanks

 

[elnaldo]

Factory setting of course. I wouldn't argue the engineers who designed the amp.

14mV on a 0.47 ohm resistor makes 30mA (14/0.47), an average value perfectly normal. If you feel it too hot , you can set it to 25mA (approx 12mV on the resistors) or 20mA (approx 9.5 mV). But 30 mA is not a high or "hot" value. Probably the manufacturer has already set it on the safe side.

(I'm assuming the test points are across ONE 0.47 ohm resistor.)

 

[Poskok2010]

So i wil change trimmer with 200 ohm.
Why is that happening.......... components decline during time? Forgot to mention my first idea was to recap it and i did that allready.

 

[Poskok2010]

Right Ch 14mV and 12 mV
Left Ch 14mV and 12 mV
Transistors on room temperature. Playing low allready for couple of hours.

I will be happier if I find the problem why cant set up without rising the original trimpot value. 100Ω to 200Ω, one channel trimpot is on 112Ω resistance and another on 120Ω.

 

[Poskok2010]

Just to inform the google searchers for BASF D-6075 VU meter repair / replacement, and that is also purpose of this forum.
It is common problem since the aluminium plate with dial, after years of use unglue from back plastic and bend or destroy the needle of VU metre, and small spring under the needle.
I find allmost the same replacement:

https://www.tme.eu/hr/en/details/v-22/analog-panel-meters-others/monacor/

They are 250uA, 750Ω, dimensions: 55x47.5 mm. They work perfect after some adjustment with trimpots.


From old meter i took aluminum dial plates, open new ones remove plastic dial plates from them and insert aluminum plates from old meter, you just insert it, no need for glue.