[Collins] [collins_radios] Re: help needed

[Dr. Gerald N. Johnson]

http://www.danssmallpartsandkits.net/ has the exact Johnson trimmer cap 
used for the later neutralization, part number 160-104. Dan just 
returned from a winter vacation.

73, Jerry, K0CQ, Technical Adviser to the collins Radio Association

On 1/31/2012 5:01 PM, Glen Zook wrote:
> As far as I know, there was never a Service Bulletin issued, at least
> for the civilian market. Obviously, since retrofitting kits were
> supplied to the military, there had to be some documentation. However, I
> have never seen such a document! What I have done to convert the ceramic
> trimmer to the air variable is to find an air variable capacitor pretty
> close in value and then make an insulator to fit the space occupied by
> the ceramic trimmer from a small piece of "perf board". Then, I replace
> the 2 disc ceramic capacitors and the resistor to complete the modification.
>
> As for the ceramic trimmers: I have been able to "save" the vast
> majority of them by very carefully removing the rotatable portion, the
> wafer, and the insulator/holder and VERY carefully cleaning them. Often,
> the wafer is stuck to the rotation portion and that requires a very
> steady "hand" using either a single edged razor blade or an Xacto knife
> with a #11 blade to separate them.
> Glen, K9STH Website: http://k9sth.com
> ------------------------------------------------------------------------
> *From:* Dr. Gerald N. Johnson <geraldj at weather.net>
> *To:* Glen Zook <gzook at yahoo.com>
> *Cc:* Richard Knoppow <1oldlens1 at ix.netcom.com>;
> "collins at mailman.qth.net" <collins at mailman.qth.net>
> *Sent:* Tuesday, January 31, 2012 3:51 PM
> *Subject:* Re: [Collins] [collins_radios] Re: help needed
>
> Is there an organized service bulletin or letter or collected history of
> that modification?
>
> Its a bridge neutralization circuit where the 1000 pf bypass (C62 in
> 32S) s part of the bridge. In my early 32S-1 manual that capacitor is
> listed as 1000 pf +80 - 20%. That is a lot of variation besides the
> variation in tubes to adjust for and my experience with capacitors of
> that tolerance is that they will vary THAT MUCH over their operating
> temperature range. How can that keep the PA properly neutralized.
>
> As for the neutralization capacitor itself, the 8-50 trimmer has 10 pf
> in series so the effective range is 4.44 to 8 pf. In the later circuit
> the 10 pf is replaced by 1000 pf, so its not affecting the range of the
> air variable which is specified as 1.8 to 6.7 pf.
>
> Today the many 8-50 pf capacitors in these radios are beginning to fail
> and the modern tiny ceramic or film variables made can't possibly handle
> the RF currents on the PA grid, so what is one to do when a cap fails
> and a replacement isn't in the parts box?
>
> And wasn't there a feed through bypassing the PA grid bias end of that
> tuned circuit prone to fracture in early transmitters also?
>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association
>
> On 1/31/2012 1:58 PM, Glen Zook wrote:
>  > The problem with the final amplifier tubes in his 32S-1 sounds like
>  > either 6146B/8298A or 6146W (with a code date of after mid 1964) were
>  > used in the transmitter at one time. Unfortunately, all the 32S-1 and
>  > 32S-2 transmitters, as well as the earlier versions of the 32S-3 and
>  > 32S-3A transmitters and the earlier versions of the KWM-2- series
>  > transceivers, used a ceramic trimmer as the final neutralization
>  > capacitor. When the 6146B/8298A or 6146W tubes are used in those
>  > transmitters there is well over a 95% chance that the ceramic trimmer
>  > WILL burn up! Collins had to redesign this neutralization circuit to
>  > utilize an air variable capacitor. There were 3 other components changed
>  > in the neutralization circuity as well. However, the easiest way to tell
>  > if the transmitter can use the 6146B/8298A or 6146W tubes is to look at
>  > the neutralization capacitor in the final amplifier cage. If it is a
>  > ceramic trimmer then do NOT use the later tubes. If it is an air
>  > variable, then it is OK to use the later tubes.
>  >
>  > Collins changed the neutralization circuit in those units that were on
>  > the production line and all subsequent units. Collins also supplied a
>  > modification kit to the military to change the neutralization circuitry
>  > in the units already in service to facilitate the use of all of the 6146
>  > family of tubes, thus allowing the use of the 6146B/8298A and 6146W.
>  >
>  > Unfortunately, if the neutralization capacitor is burned, the only "fix"
>  > is to replace it. Since it is a "pain in the posterior" to change this
>  > capacitor, I strongly suggest rebuilding the circuit to use an air
>  > variable and change the other components as well. Although a specific
>  > value air variable is specified by Collins, just about any air variable
>  > that comes close to the specific value and that will physically "fit",
>  > can be used.
>  > Glen, K9STH Website: http://k9sth.com
>  > ------------------------------------------------------------------------
>  >
>
>