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



Many, many thanks to several list members who enlightened me on this
one.  Here's a summary:

--Early 75A-4 receivers (<2700 serial number) routed B+ through the
filter input coils.  It works, but is risky.  If something should happen
(short-circuit, bypass cap fails, etc.) where V5's plate draws too much
current, the filter input coil could be burned up, killing the
mechanical filter.
--An updated circuit (shown in my 1957 manual copy) uses an additional
inductor and two capacitors to AC couple the filter from V5.

I'm going to do this to my A-4, and would be happy to help anyone else
once I go through it!

Thanks!

VY 73, 
Chris KA8WFC 
Ann Arbor, MI


-----Original Message-----
From: collins-bounce@xxxxxxxxxxxxxxxxxxxxx
[mailto:collins-bounce@xxxxxxxxxxxxxxxxxxxxx] On Behalf Of Christopher J
Galbraith
Sent: Sunday, July 25, 2004 2:04 PM
To: collins@xxxxxxxxxxxxx
Subject: {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
1056).

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
C145!  

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
incredible.

(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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Nets: Tues: 3.805 Mc-2000 Central / Thur: 3.875 Mc-2000 Central
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1st Wed (of the month) AM Net 3.880 Mc-2000 local (ET, CT, MT, PT)
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