{Collins} 75A-4 help reversing mod/repair

Hi gang,

I could use some help in reversing a mod I've found in my 75A-4 (s/n

In the process of trying out an R-390A mechanical filter made to plug in
the A-4, I noticed there was B+ on the filter input (zap--ouch!).

A look at the schematic revealed C144, a 100 pF DC block.  No problem, I
thought, just replace the cap.

After an hour+ of looking at the rig, I couldn't find C144, L32, or

After tracing some things out, it appears that R26, the 2nd mixer (V5)
plate load, has been connected directly to the filter pin 1 "bus" (and
this node is longer are connected to ground), and C144, L32, and C145
have been removed.  In this scheme, it looks like V5's plate is fed
*through* the input coil of the mechanical filter!  The rx actually
works like this with the stock 3.1 filter.

Here are my questions:

(1) Anyone see this before?  I'm just wondering if it's a standard mod,
wiring mistake, or just a way to get the rx to work in lieu of a proper
repair (i.e. L32 shorted and the owner didn't have a replacement)?
There is definitely past repair work evident in this spot.

(2) Where are L32, C144, and C145 supposed to be located?  These happen
to be some of the only components *not* shown in Figure 5-3.  From the
schematic, it looks like C144 is connected directly to the filter switch
section S-2-A.  I don't see any obvious place where L32 and C145 would
go--maybe a terminal strip was removed, too.  Geez, if someone has their
A-4 opened up and wouldn't mind taking a digital photo, that would be

(3) Where is the filter socket pin 1 "bus" ground point supposed to be?

(3) Is L32 exactly the same type as L24 (molded type in this one)?
Anyone know of a good source for these?  It seems 2 mH molded inductors
aren't as popular as they were in 1955.

THANK YOU for any help getting these info gaps filled in!

VY 73,
Chris KA8WFC
Ann Arbor, MI

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