Variable Resistor V Semi-Fixed - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 26th May 2016, 04:31 AM   #1
mbz is offline mbz  Australia
diyAudio Member
 
Join Date: Jul 2013
Default Variable Resistor V Semi-Fixed

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.
  Reply With Quote
Old 26th May 2016, 04:41 AM   #2
diyAudio Member
 
Join Date: Nov 2009
Location: Los Angeles
Quote:
Originally Posted by mbz View Post
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.

G
  Reply With Quote
Old 26th May 2016, 06:09 AM   #3
mbz is offline mbz  Australia
diyAudio Member
 
Join Date: Jul 2013
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...
  Reply With Quote
Old 26th May 2016, 09:00 AM   #4
mbz is offline mbz  Australia
diyAudio Member
 
Join Date: Jul 2013
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.
  Reply With Quote
Old 27th May 2016, 07:02 AM   #5
diyAudio Member
 
Join Date: Nov 2009
Location: Los Angeles
Quote:
Originally Posted by mbz View Post
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.

G
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Semi-Fixed Bias? needtubes Tubes / Valves 22 12th August 2014 10:37 AM
Benefit of fixed resistor LPad over variable LPad? midrange Multi-Way 3 31st January 2014 09:26 AM
CD fixed and variable outputs ngovanhinh Digital Source 6 25th April 2012 08:25 AM
Generic snubber values: fixed-/variable-voltage regs hollowman Power Supplies 44 28th April 2008 11:09 PM
Can a fixed and variable LPF be used at the same time? GuyPanico Subwoofers 2 8th April 2005 06:53 AM


New To Site? Need Help?

All times are GMT. The time now is 09:16 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki