[Milsurplus] Wanted: BC-610 Tuning Units & Coils

[Kenneth G. Gordon]

> > I have seen both the TUs and the coils on eBay at irregular intervals.
> Definitely watching eBay.   There was a TU for 80 listed several days 
> ago, but it was in Italy and the shipping cost was rather steep.   
> Mulled over the purchase for a few days and when I went back it was gone.

Well, those I have seen are in the U.S. I'd wait if I were you. They show up.

Here is a plate coil:

253287780540

And here is another one:

122542637945

BTW, the BC-610 coils are a standard Bud coil. Like this one:

142578140698

And:

253287780529, and 201613042105 and 182927951819

Here are some tuning units:

322920472531

Crystals: 112377785784

Etc.

> I'll check that before I apply power. This one sat in a barn and had a 
> hefty layer of dust/dirt on every horizontal surface so it's taken lots 
> of cleaning to get it into a condition where I believe it's clean enough 
> to power up.  Incidentally, I think the dirt acted as a protective layer 
> -- the internal surfaces are in pretty good shape.

Good.

> Curious about this conversion. I've read about the challenges of 
> conversion away from the 250TH and have heard of successes on that 
> front, but  never really came across any details about what the 
> successful conversions  entailed.  For now, trying to stockpile 250THs 
> and not considering conversion but it would be a good thing to know 
> about, just in case.

We had to increase the value of the neutralizing capacitor, and (of course) find a source of 
the filament voltage/current for the 304TL, but it went forward with no serious issues. I 
always wanted to install two 304TLs in MY '610 since the final tank circuit is already set up 
for push-pull. Adding a second 304TL on the other end of the tank circuit, then 
cross-neutralizing would give you a very capable transmitter.

We also 1) installed a "bias isolator" (shunt regulator) using an 811A in the bias circuit to 
hold the grid bias fixed at the "projected Class B" level, and 2) disconnected the 807 driver 
stage (which BTW is the source of ALL the TVI from that rig), and 3) drove the grids of the 
304TL(s) directly by a hand-wound added link on the tuning-unit grid coil.

This "bias isolator" circuit was taken from the Wilcox 96A.

As I mentioned, both our "E" models were junk when we got them, so we weren't particularly 
concerned with "restoring" them. We (W7DPG and I) were more concerned about having 
enough output power to effectively 'phone patch into Vietnam and other places. They 
worked very well for us. Power input was 1 amp at 3600 VDC in CW. We measured about 
2600 watts output in CW, but never held the key down very long.

I learned much later (like 40 years later) that our linearity was probably not "wonderful". IMD 
from the 304TL is not up to modern standards. But no one complained.

And both Bob and I got commendations from General Creighton Abrams for our work.

> > MV rectifiers.
> This 610 already has 3B28s installed, but I'll also check out that 
> overload relay before powering it up.

Great. Sounds like your '610 is not all that far from being operational.

Good luck, and carry on. Please keep us informed of your progress.

Ken W7EKB

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