[ARC5] BC-454-B, tough problem

[Kenneth G. Gordon]

On 18 Nov 2016 at 13:04, Ian Wilson wrote:

> I've been resurrecting a BC-454; everything worked FB except for the local
> oscillator stage (triode part of the 12K8 triode-hexode).

Hmmm....innersting...
 
> The triode plate voltage appeared to be absent.

Well, as I remember it, it is pretty low to begin with: something like 35 VDC at most.

> This part of the circuitry
> is a little obscure, since there is a 200k plate series resistor that is contained
> in the RF oscillator subunit of the plug-in coil thingy. Also, the plate supply
> for this stage is routed through the capacitor cover.
> 
> OK, I now know more about the LO stage than I ever wanted to. Removing
> the capacitor cover (which involves approximately every screw plus removal
> of 2 of the IFTs and most of the tubes)

Yup. Good thing everything is held in by 3-48 screws, eh?

> and disconnecting the bare wire going to
> the variable cap showed 8 ohms to ground. A strange number. The almost-short
> turned out to be one of (there are two!) the air-variable trimmer capacitors that
> are in parallel with the oscillator tuning part of the 3-gang capacitor.

Yes. One you adjust when aligning the rig, the other hidden near the front is to be set and 
forgotten.

> I can't see the moving parts of this trimmer, but turning it a few times must
> have broken up the whisker (guessing that's what it was). I left the trimmer
> off its previous position. All working now.

Well, you may wish to set it where the manual says to. Otherwise alignment may suffer a bit.

> All the coil cans contain more than just inductors. Worth studying during these
> dark hours while awaiting Armageddon, perhaps.

Indeed. 

In one of my hacked up R-25s (1.5 to 3.0 MHz), I found a no-solder joint in the oscillator 
coil-can. It had been painted with that red lacquer that indicated that it had been checked 
when the receiver was built, but whoever it was missed the no-solder joint. Soldering that 
brought the receiver back to life.

I find it interesting to find 70+ year old construction errors... :-)

Also, in at least one case, I found that the slugs in the coil-cans were all loose after 70+ 
years. I flooded each can with melted bees-wax and that cured several instability issues.

Ken W7EKB

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus