[Milsurplus] Unk crystal adapter

[Kenneth G. Gordon]

On 14 Sep 2017 at 18:55, Hubert Miller wrote:

> BTW, this brings up another question on my mind. I brought back a
> BC-610 plug in crystal oscillator unit.

If you mean a BC-610 TU, it is not, strictly, a crystal-oscillator unit. It is actually a VFO with 
a switch to allow it to be used as a crystal oscillator. Very strangely, the VFO is actually 
unusually stable too, and keys quite well. Very odd, in my experience. Especially since the 
VFO tube is a 6V6GT, which is a beam-tetrode. Beam tubes, generally, make very poor EC 
oscillators, although the number of circuits using them or a 6L6 are extremely common. 
Personally, I've never understood that. Instead of the 6V6, a 6AG7 (which didn't come along 
until 1947, as I remember it), or another power pentode, like the 837, would have been a 
much better choice.

Converting the VFO tube to a 6AG7 makes a pretty impressive, easily done and reversible, 
improvement, actually. The VFO tuning cap needs a vernier though.

> When I cast eyes on it, it
> occurred to me it might make with little change, a QRP type
> transmitter.

Yes. Such a circuit was the subject of at least one article in QST magazine a long time ago.

> One knob on it actually looks "Japanese", like off
> something WW2 Japanese, and might come in handy for otherwise
> unobtainable replacement of one missing.

Actually what that is is a "minaturized" version of knobs on the SX-28. The BC-610 is 
actually a "militarized" version of the pre-war Hallicrafters HT-4 ham transmitter. Thus the 
Hallicrafters knobs.

> But - I want to ask first, is
> there already still a plentiful supply of these BC-610 TU's, like
> enough for existing BC-610s ?

I don't know. I see them offered for sale on eBay once in a while for around $50 each. I had, 
at one time, a complete NOS BC-610-I model, with all tuning units (including the BC band 
ones) and plate coils (including the BC band ones) and vacuum capacitors. Took me a long 
time to assemble the entire thing. I did not have the BC-614 speech amp/control box though. 
Sold it to a fellow ham in Missoula, Montana for about $600 when our family desperately 
needed the money. Wish I still had it.

BTW, the BC-610 was notorious for outputting TVI. Some of us discovered that the primary 
source of that TVI in the BC-610 was the parallel 807 driver stage. Once that was tamed, 
the transmitter became very clean.

I and another AFMARS member used modified BC-610-E models as SSB linear amps for 
SEA phone patching. Mounted first one, then two (in push-pull) 304TLs in the final amp 
compartment, installed a bias regulator (circuit "stolen" from the Wilcox 96), set operating 
parameters at Class B, and drove them with a modified SB-101 (in my case) and a KWM-2A 
(in his case).

Output was 2600 watts CW...

The power supplies in the BC-610s were very robust...

Ken W7EKB

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