[Collins] 30L-1 Repair

[Carl]

----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at netins.net>
To: <collins at mailman.qth.net>
Sent: Friday, July 04, 2014 5:10 PM
Subject: Re: [Collins] 30L-1 Repair


> 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.


** The symptom points to an input circuit problem. Any disturbance in the 
actual grid or parasitic circuits wouldnt surface on 20M but rather 10M.

 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.

** While the 811A is a 811 with a more robust anode it is still capable of 
good performance to 6M in a well designed circuit. I regularly convert 
various 572B amps to 6M and it is simply a 811A with a thick graphite anode.


>
> 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.

** The input circuit was poorly designed as was the norm in the 50's and the 
specific length of coax was adopted by many.

>
> As noted recently on this forum mica capacitors are pretty good but can 
> fail.


** Thats fairly common when the bandswitch is set to a different frequency 
than the exciter. Those micas arent built to be part of a 50-100W dummy 
load.

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

** While the 30L1 isnt the easiest amp to work on it is still rather basic. 
If not already done its a good time to replace the various electrolytics and 
check the other high failure items as mentioned on several web pages.

Carl
KM1H


>
> 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
>>
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