[Heathkit] SB-610 Pot Question - Puzzled

[Garey Barrell]

Carl -

I don't have any specific information, but pots were never precision 
devices!  :-)

There is one specific problem that I'm aware of, and that is if there is 
an ON-OFF switch, the pot cannot go to minimum resistance without 
turning the switch OFF.  The Drake TR-7 suffers from that problem in 
that you can't turn the AF Gain all the way down without switching it 
OFF.  According to Bill Frost, the reason (apart from the decision to 
put the power switch on the AF Gain control) was that the original dual 
pot w/switch assembly was put together from "parts" rather than 
purchasing a manufactured part.  I learned that pots that are "designed" 
to operate with a switch have a slightly different track that allows to 
you get minimum resistance BEFORE the switch is activated.  Most pots of 
that era have a "rivet" at each end of the track, and don't allow the 
wiper to actually touch the rivet.  So on a pot with that high a 
resistance, I suspect the coating that is bonded for the track varies 
slightly over the length of the track, and that the stops (usually a tab 
punched out of the housing) is far from precision.  I think I wouldn't 
worry unless the "end offset" was over a few percent of the total value.

By the way, I use Caig CaiLube/MCL spray for carbon pots and it does a 
great job.  Cleans them and leaves a nice smooth feeling lube.

73, Garey - K4OAH
Glen Allen, VA

Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>



carl lowry wrote:
> I refurb a lot of SB-610 Scopes & have a Control Pot Question.
> Four of the Pots are 1MEG LINs. Most all of the pots are sticky or some
> won't even turn, therefore i disassemble all, soak them in 90 proof
> denatured alcohol, relub with Mobil-1 Synthetic Grease & reassemble.
> All turn Fine & my final results show a sharp trace & good movements of the trace line.
> Noticing even before i disassemble them that on the 1MEG LIN pots
> i get a variety of readings on my ohm meter.
> Today i metered four pots & got the following variety of readings:
> 1.   57 to 1114 ohms
> 2.   105 t0 1110 ohms
> 3.   30 to 1.5857 ohms
> 4.   53 to 1.2235 ohms
>
> Could one of the readers please explain to me the variety of readings and why ?
> They appear to be the same manufacture, vintage, etc .
> The 500 ohm on/off Intensity pot gives me the following:
> 102 to 635 ohms
>
> Is there a low or high limit i should strive for before i disgard a particular pot ?
> Thanks for your kindness in responding.
> 73s
> Carl, k4zrd
>
>
>