Broadcast transmitter control (RCA)

[KA3EKH]

I have been working on converting a RCA BTA-1MX for use on 160 and have finished moving all the tuned circuits up to 160 with no issues, the migration to 1,850 was easier than I thought it would be although I am still playing around with the output tank and configuration for a T/R relay but the question now is the control systems. The transmitter was originally designed for continues operation with a very simple plate control system. The original logic determined that after the filament was on and the filament time delay relay was happy and none of the overload brakes were tripped the plate contactor would engage.  Control of the plat contactor is easy enough by using the original HV contactor interlock (TB9 and 10) and I can turn the HV power supplies on and off by controlling that line but the new question is the T/R relay. Would assume that the T/R relay has to engage before the HV contactor is engaged and must disengage after the HV contactor has disengaged. I will have a external relay that will control keying that will mute the receiver and switch in a antenna relay that protects the front end of the receiver but was thinking of using the external keying relay to control the T/R relay and having a second contact on the T/R relay that controls the HV contactor so HV can only be supplied after the T/R relay completes  closing but that problem would be that when the T/R relay disengages the HV contactor will be disengaged but there may still be HV in the filters that will arc the relay contacts, and maybe destroy the receiver front end. Using a microcontroller I can build time delays and rules to avoid this and control the HV contactor in such a way that it only engages after the T/R relay transfers and when the key is let up disengages the HV contactor before the T/R relay transfers back. I would assume that anyone using an old broadcast transmitter has run into this issue and wanted to know what they did to resolve this. Don’t want to use a modern PCU to control a fifties vintage transmitter but may have to.
Ray Fantini KA3EKH

[KB2WIG]

R,
 FWIW,
Cheep and dirty protection..... you can always  have some type of crowbar across the receiver; maybe an Amperite thermal relay? I don't know if they have short time delays. Most of the stuff I've seen has a min. time of a few seconds....


klc

[N8ETQ]

Hey Ray,


   VHF and UHF guys use this sort of thing all the time to get there
GaAs fet preamps "outta the way" of the 8877 amp. They call It
a Sequencer, Its in most of the handbooks and you can buy kit versions
from DEMI. Down East Microwave Inc.

http://www.downeastmicrowave.com/PDF/trs.PDF

   8 channels in 4 steps the preform step 1 to 4 with PTT and
steps 4 to 1 when PTT drops.  Timing is set with a cap and they
work well.

   It uses an LM324 to fire the relays in sequence.

/Dan

[K6IC]

K4KYV, Don,  published his clever four-relay (IIRC) sequencer in ER Mag.

See this link:
http://amfone.net/Amforum/index.php?topic=6211.0

As the article bears a copyright,  am not sure that this info made it to the AMWindow tech pages.

Personally,  I'd advise against too many ICs,  especially an MCU for this process.  Think that hard-wired relays are more bulletproof.

73  GL  Vic

[KA3EKH]

Finally decide to use a commercial time delay relay to control the plate supply contactor so the plate contactor will only engage one half second after the keying relay and a 24 volt DC relay with a 470 uf capacitor across its coil diode isolated to control the T/R relay so the T/R relay stays engaged for about three quarters of a second after the plate contactor disengages. All in all its only three relays that will be located on the controller panel external of the transmitter. There is also one additional relay that will remove the antenna form the receiver and mute the receiver that will be located on the controller. Will be taking pictures and posting them of the new controller soon, just did not want to use something digital or store bought!
KA3EKH