Going with .01 or .0047 disc makes a much better bypass. I use parallel
.0047 and .001 6KV discs at both places as the idea is to keep 160M and HF
RF as well as harmonic RF out of the PS. Alpha is almost unique in using
sufficient C in the bypass, most others go the cheap route with 500 or
1000pf and are known for short cap life when there is a lot of 160 use.
Carl
KM1H
----- Original Message -----
From: "Robert Harmon" <k6uj@pacbell.net>
To: "Carl" <km1h@jeremy.mv.com>
Cc: <amps@contesting.com>
Sent: Sunday, March 13, 2011 7:25 PM
Subject: Re: [Amps] doorknob cap suppliers ?
Carl,
I located the 4 bypass caps and the VHF choke you mentioned, I will replace
these items as you suggest.
The bypass caps as you know are two .002 disc ceramics in parallel, I think
I will use .004 doorknob caps to replace
the .002 ceramic disc pairs. And also install the wirewound resistor to
replace the VHF choke.
Checked my bandswitch over and I don't see any evidence of arcing yet, I
will dope them as you describe with Q Dope.
Where are the steel tube clips you are referring to ? I will check them
also.
Thanks for the tips Carl, much appreciated !
Bob
K6UJ
On Mar 13, 2011, at 12:33 PM, Carl wrote:
> The 4 bypass caps down by the plate choke have been known to go and I
> always replace that little "VHF" choke with a 10-15 Ohm 20W real
> wirewound.
>
> Another common failure point is the bandswitch at the wafers to support
> screw on the right hand side. On 160 it will arc from the solder terminal,
> thru the nylon washer to the support screw. If they look OK I give that
> area on both wafers a good coat of Q Dope (liquid polystyrene). Never
> tried Corona Dope, dont know its RF ratings.
>
> Another nasty spot is the cheap steel tube clips, they rust bad on the
> inside.
>
> Alpha wants $300 per wafer but it is only $250 or so for a complete switch
> thru Mouser. I just got thru with a 76PA complete switch meltdown 2 weeks
> ago for a customer. A 76PA is also my 2nd station amp which I obtained as
> a meltdown in more than just the switch.
>
> Carl
> KM1H
>
>
> ----- Original Message ----- From: "Robert Harmon" <k6uj@pacbell.net>
> To: "Carl" <km1h@jeremy.mv.com>
> Cc: <amps@contesting.com>
> Sent: Sunday, March 13, 2011 3:00 PM
> Subject: Re: [Amps] doorknob cap suppliers ?
>
>
> Carl,
> Yes that is what is in there, two 2000pf 6kv discs in parallel.
> I'm going through and upgrading caps in the amp, already replaced the
> electrolytic filter caps in the power supply,
> thought I might upgrade these two disc ceramics while I was at it.
> It makes sense to replace old electrolytics but these two disc ceramics
> look just fine, maybe I should just leave them alone.
> (They just don't look beefy enough with their little twisted together wire
> leads, hi,hi)
>
> Bob
> K6UJ
>
>
>
> On Mar 13, 2011, at 10:22 AM, Carl wrote:
>
>> The 76 family uses two 2000pf 6KV discs in parallel. If they look like
>> the wax has boiled out I replace with 3 1000pf 6KV discs but Ive never
>> found one yet that shows leakage at even 3000V.
>>
>> Carl
>> KM1H
>>
>>
>> ----- Original Message ----- From: "Robert Harmon" <k6uj@pacbell.net>
>> To: <amps@contesting.com>
>> Sent: Sunday, March 13, 2011 10:20 AM
>> Subject: [Amps] doorknob cap suppliers ?
>>
>>
>>> I am upgrading my Alpha 76CA plate capacitors and I'm looking for two
>>> 2000pf doorknob caps
>>> to replace the two pig tail caps. Found used ones at RF parts. MGS
>>> doesn't show any of this value on their site,
>>> Found exactly what I want on the HEC home page for new ones but can't
>>> find a supplier.
>>> Any recommendations on suppliers ?
>>>
>>> thanks,
>>> Bob
>>> K6UJ
>>> _______________________________________________
>>> Amps mailing list
>>> Amps@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/amps
>>
>
>
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