[R-390] R390 RF Cap Confessions

[Cecil]

I suspect that's why every ceramic bypass and coupling cap in the SP-600 is the same exact value...not a design spec but supply chain efficiency.

Cecil

Sent from my iPad

> On Nov 30, 2014, at 3:48 PM, Roger Ruszkowski <flowertime01 at wmconnect.com> wrote:
> 
> 
> Fellows,
> 
> Not only have I been replacing the black or brown plastic tubular paper coupling and bypass caps
> referred to as black beauties, but some times I have installed new caps of different values.
> 
> Many of the caps were selected as from a preferred parts list of items in the supply chain
> and selected to maximize common values for procurement discounts  and simplify construction.
> 
> I have been using 0.001 UF for the 5000 PF by pass caps. 
> I use a 0.02 or 0.022 UF for C553 the blocking cap on the 1st IF stage.
> 
> I use 600 volt caps.
> I do not know why but I find the 600 volt caps have a better signal to noise ratio when I am done.
> Maybe its just the luck of the caps I was comparing back when, but the few times I have had
> a chance to install a 200 or 400 volt cap and then replace it with a 600 volt cap, the higher the
> voltage the better the signal noise ratio was. 
> 
> The difference is not dramatic use the lower voltage if its what you have.
> Use the original value if you feel the need.
> 
> But if you need to buy a dozen go for over 400 volt to 600 volt and bump the value up.
> I use a factor of 20 on the by pass filter caps and a factor of 2 for the inline signal passing DC blocking caps.
> I think on the inline blocking caps the higher voltage yields less popping so the meter needle has less jitter.
> You do get some stage gain so you need to be careful. You can get oscillations. You do let more noise pass 
> with the signal so consider if the extra noise is raising the noise floor or if you are working in a stage
> where the extra noise is still masked by other stages.
> 
> You get to back the IF gain down after you increase some inline values  
> 
> All subjective, as a sample of a few is not a significant statically meaningful value.
> 
> I can not say that my final signal to noise ratio has improved because I just replaced 
> a bunch of old poor caps or because I replaced a bunch of old caps with more filtering or both.
> 
> Exact before and after measurements of each cap was not done.
> And the few IF decks I have done is not meaningful as statistics.
> 
> The decks would not do 27 to 1 when I started and changing tubes
> did not get me to the signal to noise ratio. But after doing a change
> off all the usual suspects, I have been able to get every IF deck up to 27 to 1
> and the receiver up to a good solid 20 to 1 from end to end on any and every frequency.
> 
> Roger Ruszkowski AI4NI
> 
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