[ARC5] Looking for info on BC-474 conversions

[Ian Wilson]

Well, I have made progress. Reverse engineering the changes that
had been made were difficult because the designers of the BC-474
receiver liked to pack stray Rs and Cs into coil cans. This makes it
much harder to map the physical layout to the schematic.

Discovered that the original modifier of the receiver to 6v indirectly
heated tubes had wired up two stages backwards. I have cleaned all
this up and now we are getting close to being a receiver again .. about
300uV input at 3MHz for 0.7V audio at the diode detector.

The original modified used a 6SN7 as a diode/BFO, and a 6A8
as an IF amplifier with the triode grid used only for BFO injection .. so
I am missing some active devices. I am going to cheat and use
solid-state diodes for audio (and later, AVC) detection. This will free
up half the 6SN7 so I can get out of the receiver at a somewhat lower
impedance level than the 1M diode load!

Things were looking rosy until I increased the B+ from 90v (the lower
limit of my regulated PSU). Something went pffft and the waveforms
went to hell. It recovered somewhat after removing power, but I need
to find the problem and fix it. Since this radio has been hacked beyond
recognition, for once I have no qualms about replacing old caps with
anachronistic ones.

OK, so the end result of all this will be a BC-474 with non-original
front panel, non-original tube lineup, CW only, 80m only. I really (really)
hope that this is what W6NT (SK) was trying to create!

73, ian K3IMW

this is what my mentor


On Sun, Aug 17, 2014 at 7:24 AM, Ian Wilson <ianmwilson73 at gmail.com> wrote:

> Time for my annual time effort to do something with a partially completed
> massive
> conversion of a BC-474 (inherited from my SK elmer but no idea if it was
> his conversion).
>
> The receiver has been converted to use indirectly heated 6.3v tubes (at
> one point I thought
> that the receiver had been replaced with an AVR-20, but the layout is
> consistent with the BC-474
> receiver. The transmitter has been rebuilt (incompletely) for something
> like a 2E26 or 6146 maybe.
>
> Occasionally I drag this out of the garage and attempt to reverse engineer
> the receiver conversion.
> There are parts that just don't make sense to me, but a lot of thought and
> planning went into this
> effort.
>
> Anyway, wondered if anyone recalled seeing a conversion article on this
> set, in particular one
> which made changes of this magnitude.
>
> 73, ian K3IMW
>
>
>