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Author Topic: COLLINS 75A2 RECEIVER- ADVISE NEEDED  (Read 3004 times)
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N2IDU
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« on: February 19, 2011, 03:46:07 PM »

Greetings from 'Old Shincracker, Vt.

I've have recently acquired the above captioned receiver. It bears serial no# 1601. I have inspected the underside parts and it would appear that it is completly stock and in original factory condition. I can find no evidence of anything being repaired or appearing out of place. It came with the crystal calibrator and the NBFM installed. It fires up and receives well on all bands and modes. The AM audio is certainly not hi-fi and seems a bit restricted but I undertand that is normal for them as the audio section and ALC could be improved. I have a 32V3 which I would like to pair it up with.  My questions are as follows:

(1) Other than replacing the original electrolitics and paper caps, are there any known weak spots or issues to check and take corrective measures with this model of receiver? I do not have available any bulletins or changes recomended by Collins, if any exist.

(2) I am aware of the deadly mica cap issue known to exist with the 75A4. Is there a similar or like issue with this receiver as well?

(3)  Lastly, are there any modifications to generally help improve its audio other than the usual tapping off the detector followed by a hi-fi amp and speaker? 

Thanks,
N2IDU (Peter)

 

   
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w3jn
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« Reply #1 on: February 20, 2011, 12:39:02 AM »

There are two different versions of this receiver - one has a 6AK5 and 6BE6 in the front end, and the second has a 6DC6 or 6CB6 and a 6BA7.

The PTO endpoint WILL be out of adjustment.  And I suspect you'll find some out of tolerance resistors and a few bad caps.  There aren't "deadly" caps per se in this radio, or at least not so they've reached the Collins Collectors lore as the 75A4 has.

It has perhaps one of the worst stock AVCs out there.  You'll notice the S-meter deflects backwards on weak signals and doesn't deflect forward again until a signal reaches S-3 or so.  I was never able to figure out why that is - I've had 3 of those damn things and each one did the same thing.

Don't even bother messing with the audio, the output transformer's response is communications quality.
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N8ETQ
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« Reply #2 on: February 20, 2011, 07:21:08 AM »


Hey Peter,

     Johns comments are exactly correct. I'm fighting with a 75A-3
now. AGC SUX. Look here to see the list of items that need to be
done.

http://www.collinsradio.org/archives/Service_Bulletins_and_Information_Letters.aspx

    From your serial number most of these Service Bulletins are
probably installed but check all tubes for the correct type.

    The one I have ALWAYS has -4.5 vdc at the diode load even
with no antenna. Noisey POS indeed. Some stages are prone to
oscillation.

I love my R-388!

GL
/Dan

* 75A-2_3_SB_1.pdf (275.19 KB - downloaded 83 times.)
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K5UJ
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« Reply #3 on: February 20, 2011, 07:29:49 AM »

There were 2 or 3 articles in QST in the 1950s on improvements to the 75A2.  I think there have also been some articles in ER over the years--they might sell an A line compendium with them.  Collins did put out some change notices but I think the QST articles I recall seeing covered them.  If you go to the ARRL website and search the QST index for 75A-2 or 75A2 you should hit them.

I found with my 75A3 that the AF stage driving a speaker was pretty hopeless and poked around looking for the best place to tap and eventually found JN's advice to tap the AF pot wiper was the best place.  Get a 0.1 uF cap of adequate voltage in series with the wiper and bring that out in a shielded cable to a p.p. amp and the audio will be much better.  With no load on the AF stage, the transformer isn't happy so you'll have to pull the single ended AF stage tube and to shut up the transformer unless you also want to have a speaker on it.  While you have it open you can also put a 10 pF high v. ceramic cap on the plate of the last IF tube and bring that out in the same manner to an oscilloscope for mod envelope viewing.

Rob
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N8ETQ
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« Reply #4 on: February 20, 2011, 08:39:01 AM »



  The Pappenfus article is here:

http://www.jptronics.org/radios/Collins/75A2/75a2.modifications_qst.pdf

  as well as a lot of other gud info.

/Dan
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N2IDU
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« Reply #5 on: February 20, 2011, 10:46:21 AM »

Wow! Fantastic stuff you fellows sent me. Just what I was looking for.
Keep it coming!

Thanks,
Peter N2IDU
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Dave K6XYZ
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« Reply #6 on: February 21, 2011, 09:57:26 PM »

I have an A2 and an A3.
Both were pretty terrible sounding and were almost unuseable until I put in K3ICH product detector and AVC mod that really works.
This is a no holes mod that was in Electric Radio July 97'.
The S-Meter works as it should and the AVC prevents overload and distortion and has a nice hang time. Both receivers AVC act like a Drake on SSB, CW and AM.
Highly reccommended.

My A2 is the early model with the metal escutcheon and was pretty clapped out and neglected when I rescued it in 90'.
It had not operated since the mid 50's and took some playing around with to get it going properly....now its my favorite AM receiver.
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KM1H
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« Reply #7 on: February 22, 2011, 10:14:09 AM »

After restoring I put my A2 on the shelf as I wasnt interested in doing a lot of mods just to have another single xtal filter radio and I dont have enough BC-453's to go around.

OTOH, I do use an A3 that came with the AM and CW mechanical filters and made a few AGC and audio mods so thats its tolerable on AM.

The A4 is another story and Ive had that since 65, is loaded with mods and is my primary 80/160 CW DX digger and slaved with a TS-950SD which has upstaged a TS-940. The 950 RX is better than the 940 but the 2nd RX feature sucks so the A4 is used mostly for that. The 950 is also excellent on AM with a few tweaks and Ive been getting into a bad habit of using it a lot lately Roll Eyes

Carl
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