RCA transmitter move from 160m to 80m

[n5ama]

I'm considering buying an RCA BTA-1M transmitter (pair 833 modulated by pair 833) but it's currently on 160m. I need it to work 80m because of antenna issue constraints on my property plus more hams on the air on 80m.

It currently has the typical RCA plugin module for frequency. I planned to modify the crystal holder by replacing the crystal and driving it with a Collins type PTO. I've had difficulty making this work in the past. The oscillator unit doesn't want to move up in frequency to 80m.

The tank circuit will have to be modified way beyond its normal upper range. Is there anyone with experience making this modification work without gutting the oscillator and final circuits?

Thanks,
Tom N5AMA

[n5ama]

I found out how to make the 80m conversion!! Just simply buy a transmitter that has been converted by a broadcast engineer.
:<}
The transmitter is the 250w versus the 1kw but it is lighter (not as much stress on my 78 year old back) has a buffer amplifier between any low level vfo and the 807 driver and is very clean.

The downside is a 1600mi one way drive from Houston to Valencia, CA. This will take 5 days for the round trip drive and packing onto my truck. The upside is I wasn?t having much luck on details of a clean conversion to 80m and would probably take 5 days plus to make the conversion.

The 250w versus 1kw won?t require any additional wiring for its AC supply and the transmitter was on the air daily until recently. Also, it won?t require the heavier duty casters I would need for the KW rig. LOL

73,
Tom N5AMA 

[W3SLK]

I'm not a broadcast engineer nor do I play one on TV but it seems to me the critical parts are there in order to make a transition to 75M with -1MX, provided the neutralization has been properly conducted. The manual lists what capacitor combinations are required to move operation from 540Kcys to 1600Kcys. I think if you look closer at the math involved, you should be able to extrapolate the necessary combinations to get it in the 75M phone band. But keep in mind these units were designed to work on one frequency only for its foreseeable lifetime. If you want to move it up and down the band, that is going to require some cutting and 'blasting.' For some reason I recall the RCA BTA-250K being able to go up to 4.0Mcys. The -250L will not though. Regardless, there is sufficient inductance there to facilitate the change. It's just changing out the capacitors in the driver/buffer stages and possibly the final that will need to be looked at.
Good luck!

[n5ama]

Hi Mike, thanks for the input. I had a 40s vintage 250K until about 2 years ago. Great transmitter but way heavier than the 250M. The 250K used a pair of 810s versus the 250M using 813s both in the final and the modulator with 807s everywhere else. I won?t be moving much in frequency (probably less than a one Kc, so outside of touching up the plate, I shouldn?t have any major issues.

Tom

[W3SLK]

Ok I stand corrected, the BTA-250K will go to 2.0Mcys. which made it a cinch for 160M operation. Using half the inductance in the driver and final stages can get you to 75M easily. My friend had a -250K but it came with only one 810. It was no problem to drop another 810 in there. I'm going on memory but it and the -250L were modulated by 828's. The 250K was to be the driver for a BTA-1K. It was advantageous that if you were a station that had to reduce power at dusk, you could switch over to the driver and it would carry the night programming for you. I loved those boxes because they were very art deco. They were designed with the presence of mind that they would be showcased for a tour of the station which happened quite often back in those days!

[n5ama]

Mike, spot on the 810s and 828s. My copy of the 250K was the nighttime TX for KBIX in Muscogee, OK and I'm not sure if the hooked it up their day rig or not. Great looking rig but very heavy. My guess is RCA hired ex-military engineers that designed armored personnel carriers during WW2. 3/16 steel on the cabinet and there was a lot of cabinet. The iron was as big and heavy as some of the 1kw stuff and was mounted on the vertical chassis which required a hydraulic jack and some large inclined boards to lower for removal prior to moving the transmitter.

I'll attach a few pics but I haven't had any luck attaching more than one at a time.

[n5ama]

Another 250K pic.
N5AMA

[n5ama]

and another.
Tom

[W3SLK]

Nice photos Tom. I have to tell you this funny story involving my friend's, Duane, KK4AM,(nee KN0R) when he bought his 250K. We both worked together in Lynchburg, VA and he found out about it being available from a local AM station. So we hopped in his 'Lil' Ram', (Mitsubishi) pick up truck to check it out. It was in the basement of the station. While scoping it out, he decided to remove the iron. He said, "You know, I think we can get this out and in the truck." We began tilting it over and I told him that if it gets on the floor, we would never get it up again! He hired a moving company to get it and it took 5 'gorillas' to haul it out of that basement. I laugh everytime I think about having that box in the back of that little pick-up truck!