TCS BROAD AS BARN

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WU2D:
The guys on the Old Military Radio Net (CW Edition) which meets on 3570 kHz at 9:00 PM EDT on Sunday Evenings, laid a challenge to see how many TCS stations that we could get on the air. I usually check in with an ARC-5 or my ART-13, both of which are somewhat tamed. If you have ever heard a TCS on CW, it is a scary thing with all of the relays banging. Actually the ART-13 was worse before I put grid block keying and a changeover timer in.

Well anyway I have a TCS station which has not seen much action on AM and has never been fired up on CW. Last time I had it on it had a snapping crunching sound coming out of the speaker even with the volume control all the way down, and this discouraged me from using the darn thing. Anyway, today I decided to see if I could get the station operational on CW.

The TCS Receiver that I have is broad as a barn. But everybody knows that. The ARC-2 XCVR's RX section is pretty much in the same league, as is the R-105/ARR-15. All of these rigs are wide, probably for good reason in wartime, when somebody calls you and he is a bit off frequency, you still hear him. I have used the R11 or ARC-5 Q-5er on these in the past to good effect.

First I had to clean and lube both the TCS TX and the RX. This did not take too long.

I pulled tubes in the Rx until the snap crackle pop stopped. I had to go all the way to the last tube, the output tube, the 12A6. I should have realized this since the volume pot did not effect the noise. This actually was a good thing. It meant that the problem was down to just three caps, the cap across B+ feed to the PA, the grid coupling cap or the 12SQ7 first audio/BFO plate bypass cap. It didn't think it could be the cap across the output transformer because the noise went away when I pulled the tube. 

Anyway, it turned out to be the Bypass cap. The 12SQ7 is in a reflex circuit. the tube functions as a BFO, AGC, Detector and first Audio Stage, all at the same time. How clever of those Collins engineers. Anyway, the bypass cap was for the BFO only so it was a 50 pf cap, a postage stamp type. It failed nicely. Breaking down; this cap was sending noise right into the grid of the 12A6 PA.

Now for selectivity..
I decided to try the modification published in Feb 2005 or ER, which adds a ceramic filter between the First IF Can (Z202 tap)  and the First IF grid (V205). The author K3HVG, warned that the gain would go down some. Anyway, I had a nice Murata CFS-455 in my junkbox which is a pretty large metal can filter and a bit wider than I thought would do much, but I put it in anyway. All I can say is wow! Not only was there not any perceptable derease in gain, but 75M AM is now listenable on a Saturday afternoon. The IF cans were re-peaked, but honestly, there was not much of a change. I can not believe how much of a difference this made. According to my measurements, I went from 16 kHz wide to around 7 kHz and sharp... No matching, no caps no transformers, no nuthin- just the filter.

Tonight I made several CW contacts on the setup and it worked great with only a slight hint of chirp and full break-in that really works.

Next I have to try a Q-Multiplier. See my TCS setup in the station photos section.

Mike WU2D

W2VW:
A good source for filters is castoff CB radios. There were some decent ones used years ago.

WA1GFZ:
Mike you could be breaking the law changing the design of that radio.

w3jn:
It is, after all, a Collins.  Are you calling Art Collins an incompetent engineer!?!

 ;D

Vortex Joe - N3IBX:
A "TCS" is what it is,like a BC-348, BC-312, etc. Yes, they are a bit on the broad side to say the least. I use one on occasion with it's companion transmitter and pray the band doesn't get too congested!
Regards,
           Joe Cro N3IBX

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