[Collins] 30L-1 Repair

[Dr. Gerald N. Johnson]

The cathode circuit and band switch is indeed suspect, but so are the 
tubes and the grid bypassing. These tubes from the late 1930 have 
relatively long leads on the grids and the grounded grid circuit depends 
on getting those grids effectively grounded. A change of grid bypass 
capacitor type or lead length may contribute to the problem or 811A made 
primarily for audio applications may contribute. 20 to 30 MHz was high 
enough in the 30s that the power rating of the 811A was already reduced 
from its low frequency rating.

I note in the Collins 30L1 manual that they want a 20.5 foot long piece 
of RG58C/U between the exciter and the linear. Rumor has it that had 
some effect on linearity, but it also might have a big effect on match, 
being about 1/2 wavelength at 20 meters in polyethylene coax, not foamed 
coax. And will improve the SWR seen at the exciter just from the loss in 
that coax. The round trip loss would be about 0.8 dB reducing the 
reflected power a bit. And interacting with the input tune circuit to 
affect the match seen at the exciter end of that coax.

As noted recently on this forum mica capacitors are pretty good but can 
fail. Check to see if the whole slug in in the coil, if it got broken 
off or a wrong slug inserted it won't tune properly either. 
Unfortunately the parts list doesn't give the measured inductance range 
of the coil to check for the proper slug.

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

On 7/3/2014 3:30 PM, Tom wrote:
> Hi All,
>
> Posted here before about this but have a follow up question.
>
> I've been told that the problem is not the tuned input ckt. but the 811
> tubes. I fully admit that I'm not an expert on RF Amps, BUT it has good
> output on all bands except 20M. The tuned input on 20M will not go below
> 4 or 5 : 1 , all other inputs go to 1:1. I kind of think that if the
> problem is isolated to just the 20M tuned input that the logical
> "suspect" is one of the caps in the tuned input ckt. or am I incorrect
> and the 811's are the suspect? I "do realize" that the 811's are easier
> to change out, but it still doesn't compute in my ole brain.
>
> Thanks for everyone's feedback on this.
>
> Tom W1ALZ
>