[Collins] 75A1 Modifications or enhancements

[Carl]

After recapping, replacing out of tolerance resistors, tubes, cleaning tube 
& socket pins, cleaning bandswitch, aligning, etc then you can decide if you 
want to go further.

While the A1 was ahead of its time the 6AK5 RF ( a 1943 release) amp has 
poor overload response and horrible AGC control. Replacing with a 6DC6 or 
6GM6 and applying the late A3 or A4 AGC values will make it easier to use 
without constantly riding the RF gain. If you want SSB there were several 
published AGC mods for that.

Swapping a 6SB7Y for the very noisy 6SA7 mixer will lower the ENR noise 
contribution to a fourth which will wake up 20M and above. Simply drop it 
in.

Realign when done.

The above require minimal work, no holes and are easily reversible.

There are other mods but they get more involved and may not be worth it for 
general, non battle conditions, use.

Most TV baluns offer no improvement below 5-10 mHz, there are purpose built 
ones available or you could wind your own in a few minutes.

Carl
KM1H


----- Original Message ----- 
From: "Dr. Gerald N. Johnson" <geraldj at weather.net>
To: <collins at mailman.qth.net>
Sent: Sunday, April 04, 2010 9:44 PM
Subject: Re: [Collins] 75A1 Modifications or inhancements


> Its single conversion on the lowest band, so make that work first.
>
> With it open, put a finger on the wiper of the volume control. If the
> speaker buzzes, the audio section is working. If not, be sure the
> speaker is hooked up and go from there. Check the power supply voltages
> with a meter, not a wet finger. Remember tubes are in sockets because
> they are expected to need changing. But when you change IF and RF tubes,
> you need to do an alignment which can get touchy with old ceramic
> trimmer capacitors.
>
> As for upgrades there probably were many published in the ham magazines
> back then, things like different mixers.
>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
>
> On 4/3/2010 9:41 PM, BUDW2GLN at aol.com wrote:
>> I just recently purchased an early serial (441) 75A1 and I am looking for
>> any information that I can gather in order to help me, after I bring it 
>> to
>> life  :-), perform ANY upgrades along with "things to be aware of" in an
>> effort  to help bring this beautiful Collins receiver up to par..... I 
>> have an
>> excellent Copy of the manual and a copy of the Sam photo fact......After
>> bringing it up slowly for most of the day the unit is dead.....I don't 
>> think
>> that I will have any problem with the general purpose troubleshooting
>> But...........If there is any downloadable information out there in that 
>> vast
>> hyper space :-) I surely could use it.....
>>
>> Thanks
>>
>> 73
>>
>> Bud
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