[Hammarlund] HQ-129x recap

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "William A Kulze" <wak9 at cornell.edu>; 
<hammarlund at mailman.qth.net>
Sent: Monday, August 08, 2011 10:11 AM
Subject: Re: [Hammarlund] HQ-129x recap


> On 8 Aug 2011 at 13:03, William A Kulze wrote:
>
>> Back in Jan I picked up an early model(red letter) 129x 
>> and have begun
>> a recap. Someone previously had done a partial job of it. 
>> The can
>> electrolytic was 3 sections of 20mf, 2 at 250V and one at 
>> 25V, with a
>> separate 8mf/250V cap for one of the 10mf sections. My 
>> question is, is
>> having 20mf of filtering ok on this instead of the 10mf? 
>> I've heard
>> some things of over filtering and that some issues can 
>> arise
>> concerning charge/discharge times and in-rush current at 
>> power up. I
>> can get the 10mf caps if I need to, but I'd like to know 
>> what problems
>> can come up with 20mf instead.
>
> I think if you examine the original electrolytic caps 
> carefully, you
> will find that their specs are something like "10 mfd 
> +100/-80
> percent", or something very similar.
>
> Therefore, using a 20 MFD cap in place of a 10 MFD cap 
> could still be
> within spec.
>
> I wouldn't worry about it.
>
> What you are referring to is that higher capacitance in a 
> capacitor-
> input filter can raise the peak current both at the 
> rectifier and the
> power transformer to unacceptable levels. However, the 
> increase in
> capacitance must be VERY substantial in order for this to 
> adversely
> effect the power transformer or rectifier tube.
>
> In your case, as I said, I wouldn't worry about it.
>
> Ken W7EKB

     Something else; since the rectifier is a vacuum tube 
the slowly heating filament will limit the turn-on current. 
The original values for the filter caps were probably chosen 
on the basis of cost and physical cost. As pointed out above 
the original tolerance of the caps was really a minimum 
value with the maximum up to double the case value. I rather 
think that going considerably higher would not hurt 
anything. The second cap follows the filter choke which will 
limit inrush current all by itself. Actually, I think you 
will get better filtering with no drawbacks by using larger 
caps.
     Note that modern electrolytics have much closer 
tolerance than the old ones.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com