Ricebox as a VFO

[Tom W2ILA]

Whats the best way to use a ricebox output feed into a crystal socket for VFO op?
Things come to mind: Safety of the finals in the ricebox (although at a few volts out it doesnt seem like there is much risk of a problem).
Then there is safety of the Transmitter being driven by this VFO setup.  It seems like the very low ranges of the ricebox are critical.  You can go from a few volts out to a couple watts out with just a slight move on the Power-Output knob. 

Using a coax T off of a dummy load seems to make the adjustment much easier.  I need to get about 3v p-p to drive this TX.

Is there a better way?

73
Tom
ILA 

[WA1GFZ]

Tom,
I've done a directional coupler with the main path to a load. Also built a 3 DB pad that will handle 100 watts. You want the rice box to see 50 ohms either way.

[Pete, WA2CWA]

It might be helpful to the readers if you mentioned what rig you're using as an exciter. If you don't want to diddle with the internal ALC settings, you could probably provide an external voltage source through a voltage divider, using say a 5 or 10 turn pot, to the ALC line, for a "real fine" power output control.

[W2VW]

Best bet is to figure out a scheme to drive the driver in the boatanchor. You will run into problems with doubling and such using the crapstal socket. Chunk of coax into the rig terminated with a 50 Ohm resistor and voltage feed the grid of the driver stage through a small value disc ceramic.

[n2bc]

Is there a tranverter output?   Kenwood 850s and 940s vintage have a transverter output that can be selected via an accessory plug line, also kills the PA. Gives you a couple of mW output.

[VE1IDX]

I am interested in this idea as well. I have a Kenwood TS-820 that I want to use as a VFO for my broadcast transmitter.The oscillator section has been rebuilt with a homebrew crystal oscillator section before I got it and I want to bypass it and drive the 6146 driver directly.The TX is an RCA BTA1-S.I hope to actually power the beast up on the original freq of 720 KHz as a test by the weekend.I suppose I should use a dummy load for that or what do you guys think? 

[Tom W2ILA]

Thanks for the ideas,
Frank, I'll catch up with you at Hosstraders: not sure I follow.

The rig to be used as a driver is a TS-140 putting out near 2.000MHz.  The rig that is crystal controlled is an old Mackay 2017 reserve transmitter.

The transverter idea is a good one.  Simple and easy.  The 140 does not have a TV output but I have a TS-930s that does.

73
ILA

[WA1GFZ]

Tom,
The coupled port would provide the drive you need while the pass through would see the dummy load. This way the rice box would always be terminated while the coupled port could be anything and not bother the source. The same effect can happen with a power attenuator pad since say a 6 dB pad would have a 12 dB return loss even open or short. 12 dB return loss is about 2:1 SWR. I used a directional coupler on my TR7 to provide drive to a Raytheon BB 3 stage amplifier that only needed a couple milliwatts of power.
The whole performance and stabilty of a rice sollid state amplifier strip is based on a 50 ohm load. This is why they shut down at 2:1 SWR.
Say you want too drive a High Z load. A simple 50 onm termination will work but consider two modes. Can the resistor handle high power if the rice box goes high power. Also can the drive input go to a couple hundred volts peak to peak without blowing anything.
My case I was driving a 2N3866 which is a 1 watt bipolar so high voltage drive would have trashed it.

[WU2D]

I am interested in this idea as well. I have a Kenwood TS-820 that I want to use as a VFO for my broadcast transmitter.The oscillator section has been rebuilt with a homebrew crystal oscillator section before I got it and I want to bypass it and drive the 6146 driver directly.The TX is an RCA BTA1-S.I hope to actually power the beast up on the original freq of 720 KHz as a test by the weekend.I suppose I should use a dummy load for that or what do you guys think? 

I think the 520 and 820 both use tube drivers and finals. I would pull the finals and save the wear and tear and heat generation. I think you could simply jumper the driver output to the back. This may already be done in the 820. The transverter output is typically the driver output. The finals are biased into cuoff and are not used with this feature. It should drive 6146 perfectly.

Mike WU2D

[W2VW]

520 or 820 could also put resistance in series with the screen voltage to limit the thing to a half dozen watts.

[VE1IDX]

I think the 520 and 820 both use tube drivers and finals. I would pull the finals and save the wear and tear and heat generation. I think you could simply jumper the driver output to the back. This may already be done in the 820. The transverter output is typically the driver output. The finals are biased into cuoff and are not used with this feature. It should drive 6146 perfectly.

Mike WU2D

That is sort of what I had in mind.i don't want to pull the finals as I would like to use the rig normally at times as well. I was thinking about using the transverter output but haven't got around to looking at drive requirements of the RCA or the drive available from the TV port yet.Upon closer inspection it would appear that the TV port only has RF available on 10m.Maybe I could use the TUNE mode.  It has about 15-20 watts output and I do have a Bird 6 dB 20 watt attenuator and I know the carrier control will reduce the power out in the TUNE mode.Hmmmm...More digging to do.

[WA1GFZ]

100 watts - 6Db = 25 watts and 75 watts of heat in the pad. You will fry it.

[VE1IDX]

100 watts - 6Db = 25 watts and 75 watts of heat in the pad. You will fry it.

I meant to use the tune mode and carrier control with a MAX  of 15-20 watts out when using the 6dB pad.

[Steve - WB3HUZ]

Why not try the W2ZM approach? Change the driver stage in the RCA to a grounded grid amp. Drive it directly with the ricebox. You won't have to worry about too much power at 10-20 watts. Simple and effective.

[VE1IDX]

Why not try the W2ZM approach? Change the driver stage in the RCA to a grounded grid amp. Drive it directly with the ricebox. You won't have to worry about too much power at 10-20 watts. Simple and effective.

Never thought about changing it over to GG. That may be an idea that I will have to look into. Tnx. I really need to get that beast fired up and see what I have to deal with for proper drive levels etc. I may have time this week as I work evenings and should have the days to play.That is after I get the honey do list taken care of.