The R114 Mod.

[Mk121]

Wow NZ8N that's great information!! That's exactly what I was looking for. Yes I do have a scope so I will be able to adjust everything like you described. Awesome little tip about putting that resisrlt across c90.
For the VR pot , one leg goes to ground, one leg to the diode that I add and what about the other leg? Or do I have it wrong?
This will be a very nice little project for me this weekend. Thanks for writing everything up and the research.

 

[NZ8N]

You could tie the wiper leg to the non grounded leg, if you wish. But, one of the side legs go to ground.

 

[Mk121]

Just wanted to give an update. I put r104 back and removed r105,like you said. I could not get the vr pot to work. So I thought just for the heck of I'll try it at r104. Bingo it worked, direct swap. Also I did the resistor across c90 and really like how it turned out. What's the capacitor to tap into for a ranger ept3600 board to do this amc mod? I'm thinking c133. Thanks

 

[NZ8N]

Yes, it is C133 on th eEPT3600 board. A 10k is what I used, unless it was one with the add-on amp on the bottom cover plate (like a Ranger 2970, non DX, which I have), if it is the high power radio like a 2970, I use 22k for that. I originally tried taking the cap out and changing the value to a lower one, like a 4.7uF, but the pattern came out "funky", so I added the "bleeder" resistor across the cap, and that did the trick. I can't say what the new chassis SMD radios are like, as I got out of the business 19 years ago. If I had a schematic, though ......

 

[NZ8N]

R104 should have been the resistor from the base to ground, a 1.5k (Brn/Grn/Red). R105 (10k, Brn/Blk/Org) should have been from the base to 8v TX. Cobra 2000 GTL has a variable 500 ohm pot going from ground to the base, through a 1N4148 type diode.

 

[Crambone]

R104 should have been the resistor from the base to ground, a 1.5k (Brn/Grn/Red). R105 (10k, Brn/Blk/Org) should have been from the base to 8v TX. Cobra 2000 GTL has a variable 500 ohm pot going from ground to the base, through a 1N4148 type diode.

I know this is an old post but does this mod really do anything? I just purchased a 2000 gtl that has it installed and listening on a receiver it sounds identical with mod in or out of use?

 

[loosecannon]

Crambone,
what NZ8N was talking about was the stock AMC configuration in the single conversion 8719 chassis (142GTL/Washington) and the fact that they don't have a variable AMC pot inside (some versions do, some don't).
he was comparing it to the 2000GTL which has a variable AMC pot.

the part numbers he was talking about (R104, R105) would not be the correct numbers in a 2000GTL.

hope that helps clear things up.
LC

 

[NZ8N]

The Madison had a fixed 1.5k resistor there, too, I changed them out for a 500 Ohm pot and a 1N4148 to match the C148/2000 configuration. And any other single or double conversion MB8719 chassis (Radio Shack, Teabury etc...) that came across my bench.
Also, while you are at it, I came up with this mod to cause the Sample and Hold Cap that is common to the AMC and ALC circuits, to release a lot faster. The cap value is fine for instantaneous peaks, but holds too long. This makes it release faster, makes it a quasi syllabic speech compressor. You do need to readjust the AMC pot and ALC pot, though.



Just solder a regular 22k resistor across the 2 traces.

 

[nomadradio]

It will look like a sideways fat Christmas tree on the scope pattern,

Way cool!

The way we would do it 40-plus years ago was to show the customer the waveform on the 'scope, how it sounded on a monitor radio, and the average reading on the Bird wattmeter.

Correct ALC setting, if you start with it wide open is to crank it slowly while "Foooour" or your favorite long syllable modulates the radio. Advance the trimpot until the wattmeter reading takes a dive off of a cliff, down to around half what it was wide open and you're there. This would disappoint a lot of folks, especially when sideband was still new to CB, and AM operators just wanted a wattmeter to max out, no matter what. But the flattened peaks on the 'scope and the distorted audio from the monitor radio usually did the trick.

Sure had to convince a lot of AM operators that sideband is just different.

And for the skeptical ones I would ask who he wanted to talk to. The wattmeter? Or to someone who can understand what he said.

Just a choice.

73

 

[brandon7861]

Working on my Uniden Washington (8719), I ran into problems trying to put it back to stock (keeping the clarifier unlock though).

The plan was to put everything not related to the clarifier back to the way it was, which I think I did, but it didn't work.

This radio had R104 clipped, TR32 (mic shunt) cut out, VR6 (am power) extended out to the back panel with a pot and the PS putting out nearly 18v. I turned down the PS to 13.5v, resoldered the leg of R104 and replaced TR32. I then turned the mic gain all the way down and set the AM TX final collector voltage to 6.45 (roughly half of the supply).

Problem is, if I turn the mic gain all the way up and talk on the louder side, I still get boxcars. Is that common on these radios? This one does not have an AMC adjustment inside.

The ALC (VR7) does nothing to my SSB output either and I don't see anything else clipped (besides D36 and R187).

Before I dive into signal tracing something I have absolutely no experience with, is there something else common to these AMC/ALC mods that I am overlooking?

Thanks.

Edit: I have a theory, but it will have to wait until tomorrow to be checked. When I replaced the missing TR32 (2sc945a), the first 945 I found was a 945B, and it went in without any thought. The gain difference may be the issue. I will start looking for a 2SC945A.