[Collins] 75A-3

[Dr. Gerald N. Johnson]

Two key items in that SB. Lowering the gain means a lower impedance 
plate load. 100K in parallel with 4.7K changes the resistive load 
(neglecting the impedance of the mechanical filter which is significant) 
from 4.7K to 4.49K a change of 5% which is smaller than the resistor 
tolerance. Not a significant change. 10K in parallel changes that part 
of the plate load to 3.2K a more significant change. Then a quick google 
search on the resistor part number 745-1128-00 pops up several manuals 
showing its 10K which is a popular value in radio design, probably also 
in the 75A-3 parts list. Where its R2, R11, R21, R25, R29, R54, R77 in 
the main receiver and R305 in the FM module. If not the most popular 
resistor part number in the radio, very nearly so. And all these say 10K 
1/2 watt 10% carbon composition and it would not have been made by IRC 
because even in those days IRC were film resistors and didn't have the 
same RF characteristics and fuse operations as carbon composition.

Tube stage gain is a function of the product gm * Rl where gm is the 
transconductance of the tube and Rl is the plate load impedance.

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

On 2/2/2012 11:14 PM, Joe Walden wrote:
> Hello again from Joe w5jdy in Okla...    Got most of the kinks out of the
> not-working 75A-3 receiver I received as a gift..  After restoring the radio
> according to the schematic I have it working  pretty darn good....  I am now
> attempting to install the Collins mods according to their Service Bulletins..
> Everything has gone smooth up to Bulletin #1 date 1-22-54 step #3 where it
> says to connect a (100,000) 0hm resistor between pins 1 and 2 on the bottom
> of the mechanical filter adapter..  In the explanation below that,it says-
> The (10,000) ohm resistor,not a (100,000) ohm resistor serves to lower the
> plate load of V18..
> My question is:   There is already a resistor part #81 in the radio and it
> is a 4700 ohm resistor..  Do I remove this resistor and replace it with a
> 100,000 or a 10,000 ohm resistor.....  Thanks for your time..  Joe W.
>
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