Inelco Transmitter WUVT

[Chris P.]

Does anyone have any info on an Inelco model 5-621? I spotted this in the trash and figured it would have a few good parts, but after looking it over, it might be fun to get it operational again. Its missing a power transformer, and it looks like it has had a few mods under the chassis. The crystal is 640 Khz which was the frequency for Virginia Tech's radio station. Does anyone have or know where I could look for information or a schematic? Thanks

[John K5PRO]

Yes, I have a schematic somewhere. We bought several of those from Herbach and Rademan surplus house in Philadelphia in the late 1970s at WUVT. They were probably used by RCA or one of the other receiver manufactures nearby. Unfortunately, the modulator was only capable of running about 30% modulation with a tone, and they drove the production test stations for aligning receivers. We were unable to get them to do decent AM at that time, so we abandoned them. I am surprised that they are just getting tossed from WUVT. I have several LPB transmitters from same, that are capable of 100% AM, but both had dead transformers also.

 

[Chris P.]

Hi John. Thanks for the info.

I saw this sticking out of the top of a dumpster beside Lee Hall in May, and it looked interesting so it came home with me. If you could find a copy or scan of the schematic that would be very helpful. I plan on playing around with it some. I'm not sure what I will eventually end up doing with it.

[John K5PRO]

I guess it was one of the forgotten pieces that was stored up in the FM transmitter shack on the roof, or in the first floor janitor's closet where AM64 used to run carrier current rigs. I thought those Inelcos were long gone, there were a couple of them years ago there. Serial numbers 1 and 2 i think. As far as I know, they were custom built for a receiver production line and had anemic audio problems, RF wise they did put out a carrier.

I will have to dig around to find the old blueline schematic, you can have one if I locate it.
73
John
VA Tech EE class of '78, WUVT chief engineer '76-78.

[Chris P.]

I found a transformer in my junk box, replaced a few bad electrolytics, and a broken resistor(I guess from being thrown in the dumpster). I connected a saltwater dummy load and the thing actually works. There was a lot of 60Hz hum being picked up in the audio. I noticed the input transformer was mounted directly below the power transformers, so I unbolted it, let it hang, and now the hum is gone. The most I got out of it was almost 20 watts according to the meter. It seems to work and sound fairly good.

I think it would make a nice QRP transmitter.


VT CSES '09

[WD5JKO]

Was that transmitter loaded into the dorm power grid (carrier current), or into an antenna?

During my college stint, at Ferris State College in Big Rapids, Mi., I worked at campus station WFRS on 560 Khz AM, carrier current. There were 17 dorms where each had a transmitter in the boiler room. Most were made by LPB (low Power Broadcasting) using a pair of 6AD10's. There were a few home made units too using an 807 modulated by 6L6's.

I had a 1st FCC Fone, and for two years I had to maintain all that stuff. Lots of interesting experiences.

All that old stuff is long gone now at Ferris. I left there in 1976.

Jim
WD5JKO

[Chris P.]

I think it was loaded into the power grid.


I have been playing around with this thing and I'm not sure how to tune the rf drive. Also, the plate dips fine but max output does not happen at the same time.

I whipped up a schematic this evening of the modulator and amp. There have been resistors and caps paralleled here and there and I showed that in the schematic. One example of this is where capacitance was added in the pi network. Can anyone tell me how the variable inductor and cap on the 6CL6 is supposed to be tuned?

I would have drawn the schematic for the power supply, but it is a crazy thing with 3 rectifiers, 3 pass tubes, regulator, voltage reference, and error amplifier.

[N2DTS]

Quite standard stuff.
The 6146 is not neutralized so you will get peak power at someplace else than the current dip.
Not a big deal, especialy on the broadcast band frequencies.

As far as drive goes, just look in the back of the handbook for grid drive requierments for a 6146 in plate modulated service, 8-10ma?

They also have info on how to neutralize the 6146.

It should sound quite good without a driver transformer, if those mod tubes stay in AB1 long enough to give 100% modulation....

Brett

[VE3GZB]

I spotted this in the trash

Wow! I wish there were such trash bins near me!!! LUCKY!!!!

[Chris P.]

Thanks Brett.

I don't have a handbook, but I was able to find the info online.

About 2.5ma is what most sites said for a 6146.

Checking voltages, I found the screen of the 6BQ5's were 250v while the plate was 245. The screen of the 7199 was about 15 volts higher than the plate. I'll need to fix that.

[N2DTS]

Well, the screens run off the plate voltage, with just 100 ohm dropping resistors, and that was the way the thing was built, so maybe its not a problem.
Under load, when the screens draw current, the voltage goes down.

Did you try feeding audio into it and see how it sounds?

looking at the design, it should sound good if the mod iron is good, and the mod tubes work well.
I always disliked sweep tube rigs as there are no spec's on running sweep tubes in audio service.
My G76 had a 6dq5 modulated by a pair of 6dq6's in some sort of zero bias triode connection using the screen grids as the grids, try and figure that one out...
It worked though...

Brett

[Chris P.]

Yes, I have fed some audio into it, and it does sound decent.

Now I am chasing a 60hz hum problem in the carrier. When I pull the plug, the hum disappears as the filters drain. I pulled the modulator tubes and the hum is still there. If I set a receiver close by and pulled the 6146, I can still hear the hum. So it must be in the power supply? I tried adding more caps but it didn't seem to help.

[W2PFY]

Try reversing the phase (AC Power). You may have to unsolder to flip them.

[John K5PRO]

Sorry Chris, I haven't forgotten, just been slammed with too much going on. Seems like you already figured it out. I do have the blueline schematic for that thang here, just which drawer? Like i said, we could never get high level plate modulation from it, only seems to do 30-50% without serious distortion, if i remember correctly. That was in 1976 when I bought several of those TX from Herbach and Rademan in Philly.

[John K5PRO]

I found the schematic for that Industrial Electronics (INELCO) transmitter. It has a penciled comment saying "expect 20 watts output with 100% modulation, due to 250 volts DC". V2 the 6CL6, was called the count down stage. The power supply appears complex. It had automatic modulation control and automatic RF power control, both taking feedback from the output and rectifying RF. Send me a PM with your full address and I can try to send a copy. Or I will scan and email it, but its pretty ugly already.
John

[Chris P.]

Thanks John! I'll send you my address.

According to the tube specs, the 6BQ5's should be able to modulate it 100%. Maybe there is an impedance mismatch in the mod transformer?


I tried reversing the phase, but it didn't help any. I ended up staring at it for a while trying to think what else to try. I noticed a clipped off lead on the output of the pass tubes that had a piece of spaghetti tubing like that was on the other filter caps. Hmmmm. So I wired in a 120uf cap and the hum is gone! I guess I'll find out if that is correct when I see the schematic.