[Collins] [collins_radios] Re: help needed

[Glen Zook]

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
> ------------------------------------------------------------------------
>