[ARC5] BC-458 as C.E. 458 VFO

[Mike Morrow]

Bob wrote:

> Reviving a C.E. 458 VFO, which is a BC-458 with some mods.    
> 
> Anybody here know what the downside is to removing the osc filament
> leads from the cathode circuit?  Specifically, separating the 1626
> pin 7 from pin 8, then grounding pin 7.  Next abandon the filament
> winding in T-53 (labeled "B" on the schematic) by clipping wires
> both into and out of it, then discard C-20, the 3,000 mfd
> electrolytic.  Finally, connect pin 2 of the 1626 osc tube to
> 12.6 VAC.

That will work, bacause that's what most ham hackers did.

There is no "C-20" or any other electrolytic cap in one of these transmitters.  That must have been a ham mod.

The purpose of T-53B and of connecting the 1626 filament to the MO RF circuit was to eliminate any effect of heater-to-cathode capacitance variations which would affect oscillation frequency.  That was done by making both sides of the heater (pins 2 and 7) and the cathode (pin 8) at the same RF potential.  Without this, changing the 1626 will likely somewhat alter dial calibration.

In the T-15 (0.5-0.8 kHz) and T-16/ARC-5 (0.8-1.3 kHz) units, this circuitry (including T-53B) does not exist...connections are identical to what you describe.

But personally, I'd leave this subtle original A.R.C. design feature intact.  Some effort and expense was taken to implement it in the higher-frequency transmitters.

Mike / KK5F


	
        
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