[ARC5] RAK-7 caps - newest radio adventure

[Kenneth G. Gordon]

On 24 Aug 2015 at 12:40, Bill Cromwell wrote:

> Hi,
> 
> Thanks for the input on this. Apparently there is a difference between 
> these paper caps and the 'other' paper caps.

Yes. As I mentioned, I have never seen similar caps in any piece of mil gear I 
have seen...which doesn't mean they don't exist.

> In any event I intend to use 
> my Heathkit cap tester on them.

That is exactly what I use too. Although I have far fancier and more modern 
cap testers, the Heathkit C-3s I have and use are very fast, accurate enough, 
and take up less bench space. I use them in preference to every other such 
tester I have.

> It's not all inclusive but seems (so 
> far) to show me leaky caps and an approximate value of capacitance.

Yes. Exactly. I can easily read the Heathkit to within 10% of the cap's true 
value. And the leak test is very sensitive. I also have an excellent ESR meter, 
but seldom use it. I also have a digital auto-ranging type, and a new 
fancier-yet one, but seldom use either of those.

> I 
> have shotgunned the caps in several command receivers and since I got 
> the cap tester I went over them and found many of them did not show 
> *any* leakage.

Until you apply the full 250 VDC to them for a few minutes. In my experience, 
THEN they short out. Especially, C-16A/B/C, the .22 mfd job. I have yet to 
find ONE of those which was any good.

In my testing of caps in my ARC-5 receivers, I always find at least 80% are 
already leaky, even some few which are never connected. Many are just plain 
shorted. But then again, all of my receivers are pretty much junque anyway.

When I replace those caps with new modern ones, the operation of the 
receiver is vastly better.

> I did measure the resistors and found most of then far 
> out of spec.

Well, I have yet to bother to check any resistors in my ARC-5 receivers. I 
suppose I should do that.

> My understanding of the failure mode for the mica caps is the "whisker 
> growth" - metal migration inside the cap.

Yes. Also known as "silver mica disease".

> It is my intention to test all 
> of those too. I think the statistics I looked at to arrive at my best 
> guesses of what to expect came from several sources.

Yup. Same here.

> I remember how that radio used to perform and it is now way down the 
> hill. More later.

The RAK, when operating up to snuff, exhibits "single-signal" selectivity over 
its entire frequency range, and sensitivity is excellent. The Navy's original 
method of measuring sensitivity as shown in the service manual is 1) hard for 
us moderns to understand, and 2) appears to show sensitivity which is not 
very good. But this is very misleading.

In my experience, the sensitivity appoaches modern values of less than 1 
microvolt. I have yet to actually measure it, but have wanted to for some time. 
I'll eventually get a "round tuit".

Ken