[Milsurplus] TCS Transmitter: OK Smart People; Why?

[Ray Fantini]

Huge rocks were common in prewar designs, look at the monster rocks they used in the TBX. Many of the early oscillators used awful amounts of feedback thru the crystal. Currents that wood smoke an modern crystal in a second! Also maybe it was easy for the manufacture to deal with a huge hunk of quartz then a small one and it had the added benefit of allowing you to open the crystal and grind it down to change the frequency a little. Broadcast transmitters like the Gates BC-1 series and the RCA MX all used huge crystals in ceramic or Bakelite ovens that were way large. They had to keep 30 cycles +/- of their assigned carrier and maybe in primitive crystal making large equaled more accuracy?

The TCS always sold for more money than the ARC-5 stuff so if you were building up a rig as a Novice it was way better to start with an ARC-5 also there was tons of information on how to modify and use the ARC-5 on the Ham bands, many of the fifties and sixties editions of the ARRL handbook had a sections on building an operational Novice transmitter from an Command transmitter that included an AC power supply and installing crystal control. Its pure speculation that the TCS may have enjoyed some use in marine use but as a Ham radio I have always found it under power on AM and maybe tried to use it once or twice on CW but found the keying relay and lack of side tone a bit much and in VFO though it would change frequency from the banging of the relay, but I was never much of a CW operator anyway.  Would have died a Novice or Technician if they never reduced and eventually dropped the CW requirement so perhaps my CW impressions are tainted.

The external antenna loading coil was always a bit of a mystery to me also being that when it was set correct for the transmitter it would tend to desense the receiver, maybe that's why they had all that IF gain? 

Ray F/KA3EKH



-----Original Message-----
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of David Stinson
Sent: Friday, February 12, 2016 10:25 AM
To: milsurplus at mailman.qth.net; boatanchors at mailman.qth.net
Subject: [Milsurplus] TCS Transmitter: OK Smart People; Why?

I think I know why an outstanding transmitter like the TCS, with both MO and Xtal oscillators, never got real traction as a Novice rig (other than the high price at the time).

Those familiar with crystal-controlled transmitters of the 40s and 50s will recognized this pentode oscillator circuit:

http://home.netcom.com/~arc5/TCS/TCSXtal.jpg

The values in black are nominally the ones found in the many flavors of this circuit.  The values in red are those found in the TCS transmitter.  The TCS values bias harder and reduce the screen voltage considerably.  As a result, crystals like FT-243, CR-1, HC-6 and even large-blank FT-241 will not reliably function or fail to oscillate altogether.  However- I have a couple of pre-war, Collins-made large-blank transmit crystals:

http://home.netcom.com/~arc5/TCS/TCSTXXtalCollinsPreWar.jpg

With these installed, the oscillator is perky as a pug.  With parallel resistors installed to bring the cathode down to 500 Ohms and the Screen dropper down to 22K,  all properly active crystals I've tried work well and give good drive.

The engineers knew what they were doing and what the result would be.  Why did Collins do this?  Were they trying to ensure only crystals bought from them would work?  Is this why it's easy to find TCS Receiver crystals, but Transmitter crystals are scarce? 
Curious. Thoughts?

GL OM ES 73 DE Dave AB5S