75A-2 question

[Opcom]

The 75A-2 here seems to have a discrepancy or two but it looks original. It's the first one that is puzzling.

1. On the AVC rectifier V8, pins 1 and 2 seem to be connected. The schematic shows a 100pF cap between those. Why would someone do that?

2. C81 was changed from 0.1 to to 4.8uF by adding a 4.7uF cap across it, giving a nice slow timing. It makes the fading effect less annoying. On CW this extra cap would maybe be bad but for the natural sound of voice and music it is fine. I like a slow releasing AVC.

[N2DTS]

Delayed AGC?

[W3RSW]

Common mod for SSB some years ago.
In mine someone made the fast and slow avc mod switchable by lifting the lid and flicking a small switch.

[Opcom]

Well so far I found only the S-meter needed a zero adjustment and there was an intermittent bad connection from the SO-239 to GND making the signal sometimes cut out. It was the mounting screw that has a solder lug going to the RF input transformer switch. It's an estate radio and is very very clean. It's also been aligned just recently as it passed through the hands of the local radio club and they always fix the radios to get the best $ for the widow etc. (Maybe the screw was just tight enough then? -no telling with things like that) It's super quiet and very sensitive, the guy assured me it didn't need any alignment and I think he's right. I don't want to touch it. Not broke don't fix.

I don't want to do any mods on this set beyond adding a wire or something, meaning to make use of the NBFM socket. I had one of these in 1983 and someone had done a bunch of hackery to it for SSB. I hated that because the job was sloppy and there was no diagram to what had been done.

I have one of those synchronous detectors from Sherwood Engineering already set up for the SX-28. A guess is that it is only the levels of the signal that are different between the various receivers having a 455KC I.F. A panoramic adapter would be useful as it would with any set that's going to be used a lot. I let all my old tube ones go but one of those SDR things would be interesting for it save having a noisy PC in the shack.

As of now it's in the rack and connected for regular use in place of the GSB900DX.

[W3RSW]

One of the neatest uses for the FM adapter socket is to acquire a Universal Service Co. Product detector model A2-3. This little gold box comes with a 12au7 det. And its own 6au6 BFO.

It is a mostly seamless installation sitting upright on the socket with an extension shaft for BFO tuning coming out through the top of the 75 A  2 panel at the Collins emblem location.

If you don't change sidebands often and don't wish to drill a 1/4 inch hole in the front panel where the emblem was you can simply use a shorter shaft and place the small knob inside the cabinet. Lift the lid to tweak it about 1/8 turn each way from center for USB or LSB.

In the US co. Instructions is a simpler mod to get a longer AVC time constant.
A 2 mfd cap is run to ground from the top of the CW-am-fm switch where two of the contacts are connected together near the top of the switch.  That was the mod as found in my 75-a2a and is switchable for original and long Avc.