Re: {Collins} AM filter in 75S-3c - C92?


Also, make sure you use very fine wire in all connections. That was my mistake the first time around and things did not work correctly at it works great on all modes.


de Mike

----- Original Message ----- From: "Glen Zook" <gzook@xxxxxxxxx>
To: "Rich Hallman - N7TR" <rich@xxxxxxxx>; <collins@xxxxxxxxxxxxx>
Sent: Saturday, February 04, 2012 8:55 AM
Subject: Re: {Collins} AM filter in 75S-3c - C92?

Several things:

First of all, if your receiver was manufactured after about 1 June 1968, T-4 is not part of the AM filter. In those receivers, and in the receivers with Service Bulletin #2 applied, T-4 is moved to the 2nd mixer, V-4. The only i.f. can in the AM filter is now T-5. If you are removing T-4 along with T-5 from the circuitry and if the receiver is one of those with T-4 moved to the 2nd mixer, then you can definitely have all sorts of problems! You need to verify where T-4 actually is in the circuitry in your particular receiver.

When another filter replaces the i.f. cans used in the receiver for the AM filter, those i.f. cans are completely removed from the circuit. Therefore, it does no good to adjust them because they play no role.

There are a couple of possibilities with why you are getting such attenuation. The first is that the filter is not wired correctly. Check to make sure that the "high side" is connected properly on both ends of the filter and that the ground connections are properly made.

Then, there is also the chance that you have a bad filter!

Glen, K9STH Website:

From: Rich Hallman - N7TR <rich@xxxxxxxx>
To: collins@xxxxxxxxxxxxx
Sent: Saturday, February 4, 2012 9:40 AM
Subject: Re: {Collins} AM filter in 75S-3c - C92?

Also...I noticed after doing the mod for the AM filter that sensitivity
was very low. 50uv was about s2. This was for LSB and AM. When I
removed the changes for the AM filter and back to stock, sensitivity was
back to normal.

Do I need to retune T4 and T5 after adding in the AM filter mod?

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