[ARC5] 10 meter BC-454 - yet more

[Kenneth G. Gordon]

On 5 Feb 2013 at 23:00, Dennis Monticelli wrote:

> Ken,
> 
> I am thinking that the broad peaks may be nothing more than a
> consequence of removing turns in the front end.  That reduces the L to
> C ratio and thus the Q of the tuned circuits.
> 
> Dennis AE6C

I suspected that too, but remember that the C has been reduced to one-plate in each section, 
so the C/L ratio should be close to original.

However, since my last post, I have discovered a couple of other things.

First of all, the gain in dB through the RF amp is not, as I first thought, close to 20 dB: it is 
12.2 dB. This is much too low.

Secondly, I discovered that C-39 120 pFd, the capacitor that is supposed to be installed in 
parallel with L-2, the RF amp plate coil, and to (obviously) resonate it somewhere in the 
center of the band, was missing.

Thirdly, I opened up the coil box, removed L-2/L-3, opened it up and plugged it back into the 
receiver (powered off) and checked the resonant frequency with my Millen GDO: it resonated 
at 25 MHz, or at least 4 MHz below where I think it is supposed to be.

Since L-2 and L-3 are interwound, I was not sure which tuned circuit I was finding, so I 
soldered C-39 back into the circuit. Resonant frequency dropped to below 16 MHz, so it 
seems clear that the RF amp plate coil still has too many turns on it.

However, I also suspect that that broad, low peak is simply a result of the mixer-grid tuned 
circuit approaching as closely as possible to the TRUE desired resonant frequency, that is, 
that the peak I am finding is simply C-4F and C-4D "bottoming out" at their minumum or 
maximum capacitance.

I first have to figure out where the mixer-grid tuned circuit is actually resonating, then figure 
out how many turns to add or remove from the its coil to get it to tune properly.

After I have done all this, the gain through RF amp should be up to where it is supposed to 
be.

Then, and only then, can I attack the audio stage issues....which at this point I think are not 
going to be important any more.

Thanks for all the help and suggestions, folks.

I will repeat, if this is what it takes to make a usable 10 meter receiver out of an ARC-5, it is 
just not worth the effort. Anyone who tried this "back in the day" would have had to have been 
literally desperate, broke, and far smarter and more capable than the average ham.

Ken W7EKB