Collins 75A3 AM Filter Build

[k7iou]

I see after a Google search there are aftermarket filter adaptions for the "B" type AM filter that mention they work better than the original 6kc filter.
Anyone have experience with this? If so please share here!
73 de k7iou

[W3RSW]

Way too much better.  +\- 16k bandpass. Hi Fi. 
0.004uf / 400 volt cap across filter terms. 10 k resistor to other pin on input side. 15k to other pin on output side. Twist the cap and resistor leads to slide fit into existing terminals.

[k7mdo]

I used the same approach but I first cut a small piece of lexan to the same footprint size as the narrow filter, then I glued in pins of the same size as those in the narrow filter ... turned out to be small brad nails.  This gave me a removable base to wire the cap and resistors to...

It worked neat.  I also tried small pots for the resistors for tweaking them right in place.  Not much improvement though.

T

[N8ETQ]

    Yo',


       You may want to see if this applies to your A-3.
Collins referred to these as "Low Loss" filters and a few
people just swapped in the filters without doing the rest
of the job. If your A-3 has a 6DC6 in the front end AND
is in the serial number range indicated in the SB. you
will want to see if the Whole job was completed.

GL

/Dan

[k7iou]

This is what the previous owner did to mine for AM.
He took a double coiled slug and trimmed the coils to 455 Khz and adjusted the slugs for the 455 khz. He used two xtal holders for the pin insertion and soldered it to perf board. It's seems to work good but I don't have a way to check it for bandwidth other than operation. I might clean it up a bit though. But......

[k7iou]

    Yo',


       You may want to see if this applies to your A-3.
Collins referred to these as "Low Loss" filters and a few
people just swapped in the filters without doing the rest
of the job. If your A-3 has a 6DC6 in the front end AND
is in the serial number range indicated in the SB. you
will want to see if the Whole job was completed.

Yes my serial is under 700 and has this mod performed. I printed the QST July 1955 that had this mod in it. Thanks for posting it here!
de k7iou

[WBear2GCR]

One way to do this with passives might be to put together an elliptical bandpass filter.

They tend to have very sharp slopes and 30-60dB drops in the skirts, although they do come back up about 50% after that...

Another approach might add some high voltage transistors (to take advantage of the PS voltages already present) and use them as emitter follower buffers, in and out.

[WB2CAU]

This is what the previous owner did to mine for AM.
He took a double coiled slug and trimmed the coils to 455 Khz and adjusted the slugs for the 455 khz. He used two xtal holders for the pin insertion and soldered it to perf board. It's seems to work good but I don't have a way to check it for bandwidth other than operation. I might clean it up a bit though. But......

It looks like the guts of an IF transformer sans the shielded enclosure.  I used an IF transformer in place of a mechanical filter on my 75A4 25 years ago since I didn't have the wide AM filter.  It worked but the insertion loss was much less than the mechanical filter so signals (and noise) were considerably higher with it switched in.  If I had planned to keep the A4, I probably would've incorporated an attenuator into it to match signal levels with the narrow mechanical filter. 

[VE3AJM]

After years of looking, I was lucky enough to find a 6kc AM filter for my A3 at a hamfest in a box under a table. I had to tear into the innards of the filter to rebuild it, but it works great now. I had tried some of the alternatives that are out there, but I found them and the associated bandwidth to be too wide for me. Not useful in my case in using the receiver up at 7290 or 95, or on 80m at nite dealing with slopbuckets who were close by. Yeah, the bandwidth was wider, but too wide. You could copy 2 or 3 QSOs all at the same time. The A3 will never be a HIFI rx in any case.

I had used the KIWA filters and the KIWA MAP-1 sync detector for AM with good success having a 3.4 and 6.8 kc bandpass, but those don't seem to be as available as they used to be.

Al VE3AJM

[k7iou]

I am surprised at how well this homemade coil seems to work. The last couple days I have been using the 75A3 on 40M and if a side bander comes along side usually #1 on selectivity removes the interference. I considered buying the original 6kc filter but I prefer 8-9kc for enjoyable AM listening.

[Dave K6XYZ]

I use a 4.7k in parallel with a 4.7mh connected across the adjacent pin sockets. Two networks.....One network for the front and one for the rear sockets.
A 510pf connects the networks together.

I found that stagger tuning the IF really helps.
The procedure is in the rear of the manual.