|[AMRadio] Babcock MT5-A & LW-51|
CL in NC
mjcal77 at yahoo.com
Mon Sep 18 12:11:00 EDT 2017
Well, got this little vintage transmitter going. The info on the web you can find is for the MT5-B, but I opted to get the manual from Surplus Sales of NE. I know folks have mixed opinions of this company, but, I have had all good luck with them. They have a lot of manuals, many that nobody has, they make clear and sharp copies, and comes in a plastic binder, check them out if you have an odd piece of gear.
The MT5-A is different than the B, and I was able to put this one back together and get it going. The oscillator works as a Colpitts in 2 positions and a Pierce in the other two, and for some reason, a previous owner cut one turn off the oscillator plate coil, making it not come to resonance on 20 meters. He also either replaced the tank coil for 80 and 40, and used link coupling with a piece of fabric covered wire, the only non-plastic wire in the unit, or modified it in some other way, the schematic shows the tank tapped, not linked, and 20-10 meter tank coil is tapped.. I redid his 3 turn link, which had the cold end going to the output and the hot to ground, the math called for 4-6 turns to match a 52 ohm load at that diameter so put 5 on it with teflon wire, and went from a couple watts to 28 out. It is the king of possible harmonic operation though. The crystal selector is also the band switch. You can operate 80 and 40 in position 1, 40 in position 2, 20-10 in position 3 and 4, but, in all positions you get multiple dips and it will tune up fine and radiate on a harmonic in a flash. The manual tells you where the dial should be for the band of interest, and it is accurate. It is carbon mic only, but puts a hefty amount of voltage on the mic circuit, and has some really clean sounding audio, even with a beat up T17 mic. The mic audio stage is balanced input and output to a pair of balanced input 6AQ5's, plate modulating a single 6146. Ought to be fun to try on 10 meters when the band is open too.
I still need a picture of the top and bottom of the chassis if anybody else has one of these. The plate tank circuit, and even the plate tune capacitor, while working as the manual describes, looks like it was all changed from the original. I had to guess about the wiring of the oscillator tank and PA grid circuit by looking at the remains of clipped off components and wires underneath and putting the missing parts back in, as it logically seemed they would go. If you even listen to the Old Friends Net on 3715, Tuesday mornings at 9AM or Sunday's at 4PM, you might hear this one in operation. If anybody else has one, or intends to restore one, let me know.
I also got the LW -51 6 meter transmitter going, running it off of a Multi Elmac P2V power supply. Another hard to find information about rig, I got the manual for it off a site called 'Ebaman', but this was apparently a later model with a couple differences. Still don't know if this came with a bottom cover, there is no indication of it ever having one, or a way to attach it if it did. I chased my tail on getting that one going. I was using a crystal that works just fine in the Lafayette, Ameco, Gonset, Polytronics, and Heath Seneca 6 meter rigs, but the LW-51 didn't like it. Could only get low drive and 5 watts, until I tried another crystal and realized how much time I had spent for nothing. Well, trying to get a lot out of that one crystal had every thing optimized, so the new rock put the rig at it's best. We also have a 6 meter AM net that meets down here on Tuesday nights at 8PM, 50.4MC. We have had as many as 16 checkins, but not so much lately. Heck, even six has people hooking a computer up and typing instead of actually hamming, if this keeps up, I'm going to model trains. I sent an email to a fellow that has a site with info about the LW company, but it was 8 years old, haven't heard anything back, either it went to spam or he is dead.
Charlie, W4MEC in NC
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This page last updated 24 Nov 2017.