[AMRadio] stock or mod

Donald Chester k4kyv at hotmail.com
Wed May 31 14:02:03 EDT 2006

>From: W5OMR/Geoff <w5omr at satx.rr.com>
>>  Rather than trying to re-design or rebuild the 75A-4 into something it 
>>was never meant to be, I prefer the other alternative: get another cool 
>>radio! Of course, I paid a fraction of the going epay-high price for mine 
>>or I wouldn't even be able to comment from experience. Seems like a lot of 
>>folks have paid big bucks for them because someone said they should.
>That's my thoughts.  I think the 75A-4's price is artifically inflated.  
>They weren't/aren't -that- good of a reciever.

I have two 75A4's.  Bought them in the early 80's.  Paid $200 for one and 
$100 for the other.  In stock condition, the receiver has excellent 
stability, sensitivity, selectibility and dial calibration.  But the audio 
output seems an afterthought, and in the AM position the sound coming out of 
the speaker or headphones leaves a lot to be desired.

However, the 'A4 can be converted to as good an AM receiver as its 
mechanical filters will allow, with a few REVERSIBLE mods.

Change the coupling caps between the diode detector and firal AF stage to 
increase bass response (just solder caps with 10X stock capacitance right 
across the original ones without removing them, unless they appear to be 
leaky).  Clip out the 510 pf AF by-pass capacitors Collins added to later 
serial # models to stifle the higher frequency audio response.

I replaced the audio chain with an outboard 50's vintage 10-watt "hi-fi" 
amp, using plug-in adaptors to replace the 12AT7 (or is it a U7?) and 6AQ5, 
with NO wiring changes underneath the chassis.  I also added 4 additional 
mechanical filter options using an outboard mechanical filter box.  This 
requires a plug-in adaptor to replace one of the mechanical filters in the 
receiver, plus a couple of easily reversible wiring changes underneath: 
removal of the 100pf resonating caps (the filters in the adaptor box each 
has its own adjustable trimmer caps), and  I MAY have had to slightly rewire 
the agc line around that filter position, but don't have the schematic in 
front of me.  BTW, I did not completely remove the resonating caps, just 
clipped one lead on the "hot" side, and pushed the caps out of the way where 
they won't short to anything; they could be restored to the circuit simply 
by pushing each cap back into position and resoldering that clipped lead.

The key to modifying vintage equipment is to make no mods that can't be 
reversed, and to keep good documentation so that you, or a subsequent owner, 
may rewire the equipment back to stock if desired.  Also, keep on hand any 
of the original components that were removed.  On my 'A4, I run all outboard 
leads through the ventilation louvres.  No need to drill any holes in the 
front or back panels of the  receiver.  At hamfests, I  have seen otherwise 
mint mid-30's National HRO's mutilated with extra holes drilled right in the 
front panel!

Electric Radio sells (or at least used to sell) a compendium of mods and 
upgrades for the 75A4.  Many of the mods are from CQ, QST and 73 magazine 
and some of these seem questionable to me, but the book also includes all 
known Collins-issued upgrades, many of which actually improve performance, 
such as hum reduction.  An ESSENTIAL  modificiation takes the B+ off the 
mechanical filters, since the stock circuit could  result in destruction of 
every one of the filters if the plate of the i.f. tube, or some associated 
component, were to short to ground.

The mods I have put in my 75A4 were added incrementally over the 20+ years I 
  have used the receiver, but I could put it back into stock condx in less 
than a day.

Don k4kyv

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