Any RF amp mods for the TCS-12 receiver?

[KF9CM]

According to the maintenance manual the TCS-12 Receiver, The sensitivity is 5 micro volts @ 6 Db's. I have heard of changing to a 12SG7 or a 717A to improve sensitivity. using the 717A would require a 75 ohm resistor in series with the filament.  Has anyone come across any other mods for this Receiver?

TNX Gary KF9CM

[WU2D]

Gary,

I have found the receiver to be a good performer in terms of sensitivity. It has an RF amplifier and two IF amplifiers - that is two more gain stages than a Sky Buddy!

The radio is bang your head stable - what a great mechanical design. By sharing the same tuning components as the companion transmitter, it is ridiculously solid.
 
With separate RF and AF gain controls, it is far easier to control for CW reception than a BC-348 or a Command set.

The AGC is very good on AM (if the caps are still good) and the two pole CW/MOD switch lends itself to modifications with the AGC between modes.

Everybody likes the full Audio of the TCS which is quite good on AM. You could increase the audio gain and lower distortion some by substituting a 12L6 for the 12A6 in the output or by bypassing the cathode with a 20 uF electrolytic. The 12L6 has more than double the transconductance at 7000 and it runs on lower voltage. This tube really peps up command sets too.

The weakness of the TCS design is the very wide passband. This can be fixed by employing a Q-Multiplier or inserting a 455 kHz Ceramic Bandpass filter between the tap of Z-201 the first IF transformer and Pin 4 of the first IF. The bandpass filter idea was written up in Electric Radio and it works great. An old Heath Q multiplier would perform wonders with this radio both on AM and CW.

MIke WU2D

 

[WB3JOK]

I lost my TCS-12 many years ago (left it at a friend's house, I moved away, then he died unexpectedly and his family threw out all the "old junk" ) Still have the transmitter though.

...but I have a vague memory that the IF was 465 KHz, not 455?
 
-Charles

[WU2D]

I lost my TCS-12 many years ago (left it at a friend's house, I moved away, then he died unexpectedly and his family threw out all the "old junk" ) Still have the transmitter though.

...but I have a vague memory that the IF was 465 KHz, not 455?
 
-Charles

Charles - It had to be 455 kHz - the ceramic filter trick would not have worked if it was 465 kHz! I'll bet you could tune the Xfmrs to 465 though.

Here is a little Q-Multiplier circuit that will steal a little power from the TCS. A 12SQ7 will work too if you wire it right. You can put it outboard or inboard like that famous TCS noise limiter module kit. As a matter of fact, that module would make a good foundation for this thing.

Mike WU2D

[KF9CM]

I tried swapping out the RF input tube (V201,12SK7) with the 12SG7 and gain was phenomenal, there was also an image problem with strong signals. So I swapped out the 1st IF tube(V204,12SK7)and that seemed to do the trick. As far as the selectivity goes,  I have several small Chrystal lattice filters that I'm going to try. Hopefully by this Saturday I can check into the AM swap net on 3885. Incidentally I was wrong on the sensitivity when I said 5 microvolts, It's about 1 microvolt average throughout the range.


TNX Gary KF9CM

[Carl WA1KPD]

I tried swapping out the RF input tube (V201,12SK7) with the 12SG7 and gain was phenomenal, there was also an image problem with strong signals. So I swapped out the 1st IF tube(V204,12SK7)and that seemed to do the trick. As far as the selectivity goes,  I have several small Chrystal lattice filters that I'm going to try. Hopefully by this Saturday I can check into the AM swap net on 3885. Incidentally I was wrong on the sensitivity when I said 5 microvolts, It's about 1 microvolt average throughout the range.


TNX Gary KF9CM

Did yo also use a 12SG7 on the first IF?

[KF9CM]

I just used it in the 1st IF and kept the 12SK7 in the RF. I wonder what replacing all the 12SK7's with 12SG7's would do?