Variable Resistor V Semi-Fixed

mbz

Member
2013-07-05 1:12 am
Service manual has a number of variable resistors (for bias, dc offset) marked
as semi-fixed.

Had planned to use Borne multiturn VR's but read a post where guy did same
on a Pioneer SA-7800 and the amp ran way too hot, he reverted back to original
and all was well.

How does a semi-fixed behave differently to a VR?
If the VR failed open circuit then it might cause a mess.
Apologies for the dumb question.
 
Service manual has a number of variable resistors (for bias, dc offset) marked
as semi-fixed.

Had planned to use Borne multiturn VR's but read a post where guy did same
on a Pioneer SA-7800 and the amp ran way too hot, he reverted back to original
and all was well.

How does a semi-fixed behave differently to a VR?
If the VR failed open circuit then it might cause a mess.
Apologies for the dumb question.

I've been repairing broken audio and video gear over 40 years and do not hesitate to make some changes (improvements) but semi fixed pots only get changed if they fail and that is a rarity. IOW don't fiddle with it if it isn't broken. Just enjoy the music.

 

mbz

Member
2013-07-05 1:12 am
Should have tried gooo..gle,
"There are two general ways in which variable resistors are used. One is the variable resistor which value is easily changed, like the volume adjustment of Radio. The other is semi-fixed resistor that is not meant to be adjusted by anyone but a technician. It is used to adjust the operating condition of the circuit by the technician."

ie, semi-fixed because they are set then remain fixed. Means any trimmer is ok.

Again, apoligies for the dumb question.
 
Thanks for your response stratus46, no substitute for experiance. I was kinda hoping for an explaination of the different behaviour etc... or are semi-fixed and trimmers the same...

Electrically there is no difference. Pots on the front panel are more durable while the more fragile internal trimmers have a limited adjustment life.

The goal should always be to calculate the fixed resistors such that few to no trimmers are needed.