6m Larcan Conversion

[vo1ks]

CB stuff works on an entirely different set of rules, half of them are magic.

The sync compression is due to non linearity in the amplifier. I think there were amplifier problems, possibly FETs only partially working. Simply being able to bias to 500mA does not guarantee the FETs are good. The multiple curves given show expected frequency and gain response with only one transistor turned on at a time. It is useful for troubleshooting bad FETs.

Take a look at this picture.
http://members.rennlist.org/warren/failedFET.jpg
It is from a different type TV amplifier, but similar to the MRF151 used by Larcan. There is clear internal damage to the FET, but it could be biased to the proper level and was making power. The best way to determine whether a FET was ok was to briefly increase the bias to about double the normal level. If it wouldn't go much higher it was an indication of internal damage and was changed out.

The long traces on the circuit board are transmission line transformers. Changing certain capacitor values could help improve the low frequency response to be more acceptable at 6 meters, or to allow the high band module to work at 2 meters.

 

[vo1ks]

Someone offered me a couple of 1kW UHF television amplifiers. I suspect they will work from 470MHz to at least 800MHz, and likely cover the 430-450MHz band as well. Is there any interest in high power UHF, or better to pass on these?

 

[Captain Kilowatt]

Hmmmmm......decent power is hard to make up there cheaply and easily. Lots of 70 cm EME activity. Antenna gain is easier to come by as well due to size. I never even considered UHF modules.......................until now. Might be some interest out there but not likely nearly as much as 6m. Maybe more then 2m if the modules do not need to be retuned.

 

[avistar23]

if they are a strait hookup to 70 cm, im interested in details .

 

[vo1ks]

Let me make a few calls and see if there are better specs available. Maybe I can borrow one to run a few tests and take pictures.
All I know is each module is rated 1kW and requires 28VDC @ 80A. They use oddball output connectors too...

 

[avistar23]

wish it was 48 volts, but details would be good im interested, either gonna go QRO on 2 or 70cm, cant decide.

 

[vo1ks]

Why just one? Go QRO on 6M, 2M, 1.2M, and 70cm!

 

[Captain Kilowatt]

Why just one? Go QRO on 6M, 2M, 1.2M, and 70cm!

LOL! I am envisioning one rack with identical modules and one common power supply for the 50 volt stuff. Man that would make a nice setup. Too bad the UHF stuff is 28 volts. That just means another power supply but that is just a slight annoyance.

 

[vo1ks]

The upper half could be Larcan 6M, 2M, and 1.2M with 48V power supply. Band switch by changing out modules. The lower half is UHF with a separate 28V power supply.

 

[Captain Kilowatt]

Since my 3000 watt 50 volt/60 amp supply runs on 240 VAC if I plug it into a regular 120 volt outlet can I get 25 volts at 120 amps from it?


Don't laugh.


I know of a case where a fellow was messing around with an avionics radio that required 24 volts at whatever amps. He wired up an adapter to plug his mega amp 12 volt power supply into the 240 volt outlet to get 24 volts from it. :blink: He didn't get 24 volts out of it but he did get lots of smoke. He also got a costly education and was nominated for a possible future Darwin award because of it.

 

[vo1ks]

The Larcan transmitters did not have significant voltage regulation on the 48VDC bus. Essentially they used very large capacitor banks after the rectifiers. If you dropped the input voltage to half the PA voltage would drop as well.

 

[vo1ks]

Has anyone made progress with the 1kW modules?

I'm contemplating building a dual band amp, two modules in slide in Larcan drawers, power supplies, etc all mounted in a single equipment rack. I've dug up coaxial switches, cables, etc and basically need to reconfigure the rack and glue it all together. It could be 6/2-meters (or 222MHz) or even both modules put in parallel for 2kW as a mono band amplifier.

Wondering what others have done with the amplifier bias during receive mode. Have you snipped the wire on the board or shut down the power supply? I'd like something that could be brought out on the rear panel DC connector or even via the RF input connector to allow the module to be removed easily.

 

[vo1ks]

Dug out the manual and schematic diagram. The long pins on the DC connector supply power to the bias / VSWR board. Removing 50VDC from that pin would remove the 39V bias to the transistors should effectively shut things down. Anyone able to confirm this in operation?

 

[Captain Kilowatt]

I haven't done much of anything with it yet but hope to do so over the winter. I plan to switch the bias line on/off during T/R. I just do not like the idea of constantly switching the power supply ON/OF so many times. Usually if something is going to go POP! it will happen when components are stressed like on power up especially under load. My biggest hang up is something to house it in. Those modules are really long and almost need a deep rack to mount in. I have a large table top type rack/box but it is not deep enough. Everything sticks way out the back. I had contemplated building a nice cabinet out of oak plywood and shielding it inside or at least shielding the component side of the module. Still may go that route. Also have the skeleton of an old RCA BTA1-S AM broadcast tx to use as an aluminum panel donor. Just have lots of screw holes to contend with. I have a pair of SPDT DOW RF relays to take care of the switching and a 50 volt 60 amp blade server switching power supply for it but the cabinet is the biggest hold up right now. Little things like RF output metering, and supply current/voltage monitoring can be added later. I have external RF power metering good to 2 Kw at 50 MHz and DC metering is simple to install.

 

[Captain Kilowatt]

Dug out the manual and schematic diagram. The long pins on the DC connector supply power to the bias / VSWR board. Removing 50VDC from that pin would remove the 39V bias to the transistors should effectively shut things down. Anyone able to confirm this in operation?

I can't remember where I saw it but perhaps it was the Minuteman Repeater site but yes, the 39 volt bias is derived from the main 50 volt supply so killing the 50 volt line would also kill the bias. On the same site it was mentioned about leaving the 50 volts on and lifting the common bias line during receive.