[ARC5] RAK-7 caps - newest radio adventure

[Bill Cromwell]

Hi,

Thanks for the input on this. Apparently there is a difference between 
these paper caps and the 'other' paper caps. In any event I intend to us 
my Heathkit cap tester om them. It's not all inclusive but seems (so 
far) to show me leaky caps and an approximate value of capacitance. 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. I did measure the resistors and found most of then far 
out of spec.

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

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

73,

Bill




On 08/24/2015 11:47 AM, Kenneth G. Gordon wrote:
> On 24 Aug 2015 at 9:34, Bill Cromwell wrote:
>
>> Somewhere over the years I acquired some bad info about the caps in the
>> RAK/RAL radios.
> You may have gotten some of that from me, Bill.
>
>> I have a RAK-7. I have the BAMA manual for a RAK-5. It
>> was my understanding that the RAK/RAL used no paper caps and the caps
>> should never be a problem.
> Well, I didn't say THAT, exactly: what I said was that all of the caps, except
> those between stages, are oil-filled and very seldom give trouble.
>
>> Wrong! I took a closer look at the manual
>> because the performance of my RAK has deteriorated so badly. There are
>> 29 paper and oil caps in the beast and it's power supply. They are
>> around about 60 to 70 years old!!! I'm just happy I didn't let the smoke
>> out!
> Well, I have never had one of those oil-filled jobs short out, or even go
> leaky...yet.
>
>> Maintenance and TLC start with replacing those caps and checking all the
>> mica caps.
> I strongly suggest you test the oil-filled caps before "shot-gun" replacing
> them. I think you will find that most, or even all, are just fine.
>
> The mica caps (if they really ARE micas) are, routinely, leaky. As I said, you
> can tell this by listening to the receiver without an antenna connected. If you
> hear "static" one or all of those are bad. I have found so many of those bad
> over the years that I routinely replace them now.
>
>> That will be followed by checking all of the resistors for
>> out-of-spec.
> THOSE are far more likely to be out of spec than the oil-filled caps, Bill. FAR
> MORE likely.
>
> Good luck.
>
> Oh: BTW, the only real differences between the RAK-5 and the later models
> is the antenna input setup.
>
> Ken W7EKB
>