Pass F5 potential bias issues

[JOHNNYCR100]

Hi gents a question if I may please.

I have just rehoused my trusty F5amp in a new enclosure but wanted to rebuild it from scratch just in case there thermal issues ( my old case was just an open backed heat sink)

My new case has smaller heat sinks but the case is massively thick alimumim so rebiaseing may be in order I think?

However.....when I look at potentiometers p1 and p2 from the front of my cviller PCB I can't rember which way ( clockwise or anti clockwise) was reqiured for 0 ohms to get started? ( my bad!)

R4 is in parallel with P2 /R3 is in parallel with P1...but when I put my ohm meter across Legs 1and 2 of P1 and turn multi turn pot I don't see any change in resistance at all..I would expect to see some change in resistance?

SO ....can any please tell me the definiteve position or across which component to place an ohm meter to measure across P1 or P2 to actually see the resistance change?.

Please excuse my ignorance as to a simple pot resistance mesurment but I don't understand why is not so simple and as past posts seem to testify my ingnorence her will cost me my amp!

Thank you all
Johnny

 

[Zen Mod]

ref. to which exact schematic ?

 

[JOHNNYCR100]

Hi zen mod, your quick!!

Ref fig 6 - F 5 power amplifier on the pass labs site ( according to the solder splattered paper here in front of me!)

Regards
Johnny

 

[Zen Mod]

ref to fig.6 in http://www.firstwatt.com/pdf/prod_f5_man.pdf

P1 is in parallel to R3 - set to zero

P2 is parallel to R4 - set to zero

both resistors are 2K2

be sure that you're looking at proper position parts - dunno which nomenclature Cviller boards are using

 

[JOHNNYCR100]

Hi Zen mod,Thank you for looking ( I only have my I pad just now..can't do that cut and paste thing like on my pc!)

Yes that's the diagram I am useing, however it's a good point you make, I was assuming cviller board followed that designe ( but assume makers an *** out of U and me) if you see what I mean sir.

But looking at my pcb tracks It looks to follow the diagram, but why can't I simply measure the resistance change across the pot when I adjust it...it's not rocket science ( unless it is?)

Thanks zen mod
Johnny

 

[JOHNNYCR100]

Sussed it out eventually

No it's not rocket science.....disconnect the pcb from the power supply then try it! ( ohm meter just loves to charge those caps) ...looking at the component side facing you go between pins 1 and 2 on the pot..turn anti clockwise to reduce the resistance to 0 ohms ( or as close as you can get.),.start from there..till biased/ cooked.thanks ZEn mod for the reply, it's 3 am here!! Now I can get on with my ironing!!

Thanks ya all
Johnny

 

[AudioSan]

measure 0 ohm across R3/R4. not at the pot.

 

[JOHNNYCR100]

sorry for late reply

hi Audiosan,

sorry to be dense! but to be explicit one probe of the ohmmeter across R3 and the other across R4..--trim pot for 0 ohms----that's whats required..correct?

Sorry for newbee question but have seen disastrous issues with "turn the pot left" (when its inverted and is turned the opposite direction !)...have to be 100%

regarda

johny

 

[JOHNNYCR100]

Woops..sorry about that!

Sorry audiosan , my preavious question is silly...I did not have the circuit diagram in front of me at the time

So if I put me old ohmmeter across R4 to measure P2 ..what stops me just seeing 2.2K as that's what R4 is and I am right across it?
( or would the fact there in parallel just demonstrate a change in resistance as twiddle the pot on way or the other?

Sorry cant try just now as I am at work

Thanks sir

Johnny

 

[Russellc]

Sorry audiosan , my preavious question is silly...I did not have the circuit diagram in front of me at the time

So if I put me old ohmmeter across R4 to measure P2 ..what stops me just seeing 2.2K as that's what R4 is and I am right across it?
( or would the fact there in parallel just demonstrate a change in resistance as twiddle the pot on way or the other?

Sorry cant try just now as I am at work

Thanks sir

Johnny

The setting on your multimeter, it should be set for dc, not resistance. You are measuring the voltage across the resistor, not its resistance. You should probably re read something like 6L6 "illustrated F-5 build" http://www.diyaudio.com/forums/pass-labs/188691-illustrated-guide-building-f5.html

Russellc

 

[Zen Mod]

if he needs to measure initial zero of trimpot , DMM must be set to resistance

of course , with power off

once when power is on , there is nothing to measure with ohmmeter

 

[Russellc]

if he needs to measure initial zero of trimpot , DMM must be set to resistance

of course , with power off

once when power is on , there is nothing to measure with ohmmeter

Mind drifting, from title I thought he was setting bias...awake now

I remember being very confused when I built my F-5 about pots, at time I wasnt sure if "0" meant all the juice came through or none. I fidgeted until I finally put them in middle and used variac. Somehow smoke stayed in...

Russellc

 

[AudioSan]

Sorry audiosan , my preavious question is silly...I did not have the circuit diagram in front of me at the time

So if I put me old ohmmeter across R4 to measure P2 ..what stops me just seeing 2.2K as that's what R4 is and I am right across it?
( or would the fact there in parallel just demonstrate a change in resistance as twiddle the pot on way or the other?

Sorry cant try just now as I am at work

Thanks sir

Johnny

yes. the pots are conected in parallell with R3/R4. so the pots position will decide the resistance you measure across the resistors.