[Collins] [collins_radios] Re: help needed

[Dr. Gerald N. Johnson]

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
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>