CV-591A TMC SSB BOX

[WU2D]

Hello Gang,

These things have apparently become collectable since I got mine for $25 a few years ago. It is a handy and effective SSB converter and it looks sharp. Yes, it works on AM as a slicer.

I have been using this SSB adapter with my R390A religiously since the early 1990's. I figured that it was time to pop the top and change out a few caps and give her a "tune up".

Having no manual (as usual) and only a schematic to go on, I popped it open. The AGC function never worked at all and I was going to take care of that and I figured on replaceing some paper caps.
I was surprised to find that V8, a 6J6 had been pulled and it had a note next to it "pulled".

I started with the input stage, V2 a 6BA6 controlled gain IF amplifier running at 455 kHz. I say controlled gain because V1 a 12AU7 acts as an AGC amp and Detector to control just this one input stage. This circuit allows the converter to work with a wide IF level variation.

I found out why I had no AGC. Beyond the obvious of changing out the timing caps C13 and C14, C2, the input coupling cap was missing alltogether! No AGC - guaranteed! Well after fixing this, I had to hook it up and try it with the R-390A. Hey it worked! I now had AGC fast and slow. But it really did not do anything dramatic on all of the signals I detected. This may be a cool TMC circuit that really doesn't do anything useful.

I also noticed a high frequency buzz coming through when I turned on the BFO and I wanted to get to the bottom of that.

The converter box takes 455 kHz and converts it to a lower IF - but what was the lower IF? They are not using mechanical filters, just a magic box between the first converter and the product detector. This magic box swept out to around 17 kHz! My buzz was expalined. The UPPER/LOWER crystals confirmed my suspicion - they were using an IF of 17 kHz! Were they nuts?

The sideband appears to be selected on the first converter stage (6BE6), before the filter and both sidband selction and tuning can be accomplished remotely. That explained the pulled tube It was a reactance modulator which responds to an external +/- voltage. It forms a VCO. This tube was apparently pulled to lower oscillator noise, save a little current and possibly to improve stability.

The second 6BE6 which is a product detector has a baffling circuit consisting of two fixed tuned oscillators hooked to it along with the IF input. Can someone explain what TMC is doing with the two oscillators?
After the product detector there is another magic filter box. I did not sweep it but I have to assume that it is a sharp audio bandpass filter.

From there it goes into a nice little amplifier consisting of 1/2 of a 12AU7 and a 6AQ5. The gain of this stage can be lowered by introducing feedback, by throwing an internal switch - OP-AMP gain control! This feedback improves the audio response as expected and lowers the noise level.

The power supply is overdone - first class with a 5Y3 and two chokes and an OA2 regulator.

The only mod that I made on the box was to add a mute relay circuit to kill audio along with the R-390A mute function.

Comments Please!

Mike WU2D


   







 

[w3jn]

This belongs in the Tech session.  This forum is for completed projects only.

The low IF is 17 KC because it's easy using L/C circuits to get good selectivity.  The 6J6 is a reactance tube, as you noted, intended for remote tuning..  Why they included this I haven't a clue, but most of the CV 591s I've had have had this tube pulled.

BAMA has a manual at http://bama.edebris.com/manuals/military-kg7bz/cv591a/

I don't see two oscillators associated with the 2nd mixer?  Only one, 1/2 of a 12AU7.

73 John

[WU2D]

Hi John,

The problem with 17 kc is that it is audible to us younger guys- Hi. 50 kc might have been a better choice, but you are right, it would have taken a real IF filter or a couple of hi-Q transsformer cans to handle the selectivity.

The Second Mixer tube V4 appears to have a feed from V5A (1/2 12AT7) called the second oscillator and from another oscillator called the sideband tone generator V9b (1/2 12AU7) via a 47 pF cap.

I'm missing something.

Mike WU2D 

[k7yoo]

Not having the book in front of me --but using one of these critters-- I believe that there is both a manual variable oscillator --(It works as passband tuning on AM) and crystal controlled oscillator for USB/LSB selection. This would enable some sort of remote operation. I think the 455khz IF sig is converted to 17Khz, filtered, and then converted back to 455Khz. It should be mentioned that the not so common B&W RX sideband adapter used the same 17khz IF and functioned nearly the same as the TMC unit. I suspect a little engineering crossbreeding here-as they were both out of the northeast. This unit looked just like their xmt SSB adapter and had a speaker and audio amp built in. I think that the early phone company SSB carrier equipment used LF IF filtering. When I was in high school one of my friends homebrewed a transmitter using this scheme so the IF is not as bizarre as it seems--just awkward. These units can be tweaked to work on 500Khz IF's for 51J's. I need to go read the manual and see if I am correct--let me know if you need a copy.
Skip