Hafler DH-200 bias issue

[The Fuxtor]

Been refurbishing one of these, one channel will not bias lower than 300ma? Other channel all good?? 275ma is the target..
This unit had a bad channel, but that was fixed and that channel received all new resistors and matched transistors. I couldn't have only one channel with new components, so i replaced the resistors on the other channel so that they were matched metal films thruout.. New caps were added too and a new trimmer for bias. The first channel that was fixed, bias' nicely , but the other channel only after replacing resistors will not bias properly now? the resistors were the last components added on this channel whereas the bias issue came to light, was biasing nicely before...
Time to double check values maybe??

 

[mondialfan]

Time to double check values maybe??

Yes! If it was working before and now its not working then return to not only inspect the components you changed but the general area you performed the work. Look for errant solder splashes or cold solder joints. You may also want to check the bias pot as they tend to go bad over time.

 

[The Fuxtor]

Yes! If it was working before and now its not working then return to not only inspect the components you changed but the general area you performed the work. Look for errant solder splashes or cold solder joints. You may also want to check the bias pot as they tend to go bad over time.

the trimmer was replaced and working great before, thinking I will end up finding a lifted pad, or like you say a solder bridge maybe.... hopefully get to this tomorrow to investigate

 

[The Fuxtor]

Ok, got it! I did the old "replace a 22ohm with a 22k ohm" trick.....
Got another nicely refurbished Hafler to play with!!

 

[soulstreamerAK]

You mean you just mistakenly put in a 22k instaed of a required 22ohm? Interesting.

Also, how does one obtain matched semiconductors like power transistors?

 

[The Fuxtor]

You mean you just mistakenly put in a 22k instaed of a required 22ohm? Interesting.

Also, how does one obtain matched semiconductors like power transistors?

You need to buy a large batch, and match them as close as possible for hfe. Cheap eBay component testers will display the hfe , but to do it more precisely you can put a simple circuit together. I found an easy method using a 9 volt battery and a few resistors from YouTube!