VD1222 device. Resistor?....Diode?

[62vauxhall]

Wanted to adjust the idle current of this Akai receiver I was given. Rotating the pots (they seem fine) had no effect. According to SM, the objective is 40mA (not mV) and I only get 20mA.

It was suggested that these VD1222's are failure prone but how do you test them? Diode check function is OL both direction and ohms are 4M for one and 7M the other.

What further confused me is the VD1222's schematic symbol is 3 diodes in series and the parts list description is thermistor.

I comprehend the intended function of regulating current based on heat and what I've read about thermistors says that's exactly what they do - increase and decrease resistances as they heat and cool.

But why the triple diode symbol on the schematic?

Additional reading (and there's not a lot to read) about VD1222's called them "double diodes" and substituting two IN4148's in series is acceptable.

Does that sound right to you guys?

 

[orange]

Warren/Dr. Audio suggests in this thread on AK that it might have caounterfeit transisitors but I think there is a lone of logic you might follow seeing it was a search hit for VD1222 and the same temperature regulation is mentioned...

http://audiokarma.org/forums/index.php?threads/need-troubleshooting-help.335735/

 

[62vauxhall]

Had already seen that thread. One of the ones suggesting IN4148's.

Since I have IN4148's I'm going to give them a go when home tonight. See if that restores any adjustability to the idle pots.

 

[Gepetto]

https://retrovoltage.com/2014/11/25/vd-1221-repalcement-substitute-diodes/

 

[8991XJ]

VD1222 are not checked, they are replaced with a pair of 1n4148s in series. Seems the VD diodes are failure prone and one used one 4148 for each diode built into that unit. More common replace on sight are the VD1221s. Always.

 

[Gepetto]

Wanted to adjust the idle current of this Akai receiver I was given. Rotating the pots (they seem fine) had no effect. According to SM, the objective is 40mA (not mV) and I only get 20mA.

It was suggested that these VD1222's are failure prone but how do you test them? Diode check function is OL both direction and ohms are 4M for one and 7M the other.

What further confused me is the VD1222's schematic symbol is 3 diodes in series and the parts list description is thermistor.

I comprehend the intended function of regulating current based on heat and what I've read about thermistors says that's exactly what they do - increase and decrease resistances as they heat and cool.

But why the triple diode symbol on the schematic?

Additional reading (and there's not a lot to read) about VD1222's called them "double diodes" and substituting two IN4148's in series is acceptable.

Does that sound right to you guys?

Diodes act like an NTC to a certain extent. That is why they may be comparing these to a thermistor. In a bias circuit, the warmer they get, the lower the forward voltage drop of the diodes which tracks the behavior of the base emitter junction of the power output transistors. This keeps the bias current from running away when the output devices heat up.

 

[laatsch55]

NTC????

 

[Gepetto]

NTC????

Negative Temperature Coefficient (NTC)...

 

[62vauxhall]

The IN4149 pairs are in and nothing bad happened sparks & smoke wise - DBT went totally dark after initial power up.
But, no go on achieving increase/decrease of bias current. Still 20mA no matter where the pots are.

Oh well, 15mA or better is supposed to be OK. Odd that those pots don't make one whit of difference.