urnsing

[ve8xj]

Hello

  

[W2PFY]

How do you hook your VFO to the tuning units? A little background on the way I do it. I have a 50 ohm non inductive resistor in parallel with the output of the VFO to the crystal socket.

How are you keying the transmitter? I have mine set up so the VFO and the plate supply shut off at the same time.

what band does this happen on?

Reading Don's notes below, I should have said that I use a transceiver as a vfo.

If I remember right, the cathode of the 6V6 osc tube is keyed and if you have that keyed all the time, that could be part of the reason it is running away.

[k4kyv]

The oscillation is most likely in the first stage, the one that normally is the crystal oscillator.  That tube, a 6V6 IIRC, is not screened very well, but more likely the cause is the feedback circuit designed to make it oscillate with a crystal in place.

I would have to dig out my schematic to the '610 to be sure of the details but I believe that you need to bypass the cathode to ground at rf. That would disable the crystal oscillator function, but it's a real PITA to get beneath the chassis to wire in the bypass capacitor or to short out the rf choke.  Perhaps an easier approach would be to make a plug-in adaptor that would make the changes above the chassis, and the adaptor could be removed to restore the '610 back to crystal operation. I wired in the permanent conversion for a Heath VF-1 back in the 60s for a local ham but that was a long time ago and I can't recall or reconstruct the details without a schematic in front of me.

I would still advise a loading resistor (several hundred to a couple of thousand ohms, not 50Ω) at the output of the VFO if it has enough output to compensate for the loss, since the BC receiver audio tube used in the first stage is not shielded well enough to work as a stable rf amplifier without squirrelliness if the grid circuit is tuned anywhere near the same frequency as the plate circuit. With an appropriate rf driver unit, Terry's idea of the 50-ohm resistor would probably work without any circuit changes, but the typical 50's era ham VFO would not work into such a lo-Z load, and probably would not have the spare output to burn up in any value of a resistor. In my conversion, there was no resistor, but we lucked out and there was no oscillation when fed by the VF-1.

[ve8xj]

I
Cheers



  

[W2PFY]

Parasitic chokes ? Amazed that it doesn't have any

Who needs chokes when your at war? Oh, I forgot that it's over. I don't know if it would help much to have one in the final. The  250TH tube do not  seem to self destruct. But it would not hurt to put one on it. There must be examples of how to build one somewhere. Sometimes people sell them on ebay scrounged from military rigs and busted SB-220's etc. 

[Ralph W3GL]

   If'n it works, don't try to fix it!  

   The 610 final should work just fine sans parasite chasers as
    long as it is neutralized...   Example of good design. 

    As for connecting the external VFO, Don covered that mod
    very well.   Yes, the internal self-excited frequency swicher
    sucks big time with poor access, stability and hum in the
    signal but it served its purpose.

    The HT-4 conversion to BC-610 was done to fill a war time
     requirement with minimum lag time and achieved its goal...
     Hense, the minor mods in ongoing manufacturing runs...

     This was one of the systems I was trained on at Hollabird
     Signal Depot, Sparrows Point, MD in the early 40's...
     

[KM1H]

Back in the days of old, etc.....when I operated a bit at NAB Olathe, K0NAB, the club station had a 32V3 driving the BC-610 on 10/15/20M. Ive no idea how they did it but it worked fine and I believe they also reworked the 32V audio to drive the 100TH's as the D-104 connected to it.

[N8ETQ]

Hey Tim,

   I am familiar with the early models, but with the introduction
of the BC-610-E the game changed. Prior models were only "tested
and provisioned" for operation up to 8mc. With the "E" model the
SCR-299 became the SCR-399, the upper frequency limit increased
to 18mc and gone was that little POS tuner in favor of the BC-939.

I Can't see a properly biased class C 250TH "Take'n' off" ever.

I Can attest to the fact that it's hard to tell if it is biased
correctly. They make enough self bias to mask the fixed bias.

Does it key OK in CW mode?

What band is it doing this on?

Why don't we turn off the plate when un-keyed?

I use FT-243 rocks Vs the FT-171 originally supplied with No problems.

I used to drive mine with a Junkston 122 on 75.

There IS some sort of "parasitic choke" arangement under the
NUETRALIZING cap but when operating below 2mc you were instructed
to jump it out!  I added my own, both in the plate lead of the
PA and for good measure one in the common plate lead to the 807's.
Standard handbook crap, 47 ohm 2watt with 4 or 5 turns.

To verify the fixed bias you need to be unkeyed in CW mode.
If its still pulling plate current look at the 5Y3 on the "MOD"
deck, Or there is a big adjustable "R" somewhere to set if needed.

One thing I recall from the external VFO days was shorter the cable
the better. 2 feet was 2 much. (get it?)

My "E" is in the SCR-499 configuration and is so sweet, Tunes just
as nice as a DX-100. (the SCR-499 was the air-drop version of
the SCR-399. Didn't come with the truck or shelter) The "System"
does shut off plate voltage when unkeyed.



The following quoted comment concerns me in that you may have a
"Hammy-Hambone" modification in there.. You said:

"Thanks for the responses. Yes Looking at the schematic You are right the
way I have it set up the cathode was keyed at all times. Since my goal is
to have this strictly operated as a vfo driven rig I think that the mod
to the 6v6 is on the agenda for this weekend. "

[END]


If your using this on 75m   AM no "setup" is required.
If your using it on 10m PSK 31 your on your own.


GL and keep us posted.

73
/Dan

http://users.ameritech.net/dharlan/dans.htm

[W2PFY]

He found the problem. It was the continuously keyed 6V6. I guess he's going to lift that keying circuit above ground and key it with a relay along with his vfo. The best way to do this IMHO is using a 115 volt antenna change over relay with external contacts on it to key the VFO and the 6V6. I do this by supplying the relay 115 volts from the  output terminals on the mod deck.It a small strip with two terminals located over by the 2A3's. You would key the transmitter via the BC-614 if your using one in the usual way. When keying it to transmit, it will supply 115 volts to the terminals by the 2A3's.

If you not using a BC-614, you should be keying it from the 8 pin socket on the rear right of the mod deck. That's how I do mine.

Know what I mean Bert?