[Amps] Replacing padder caps on an Alpha 77DX/SX

[Jim]

I just rebuilt a Ten-Tec Titan 435 that had horrible drift issues on 160 
meters. The RF current in the load cap on 160 is very high!

I used strings of 100 pF 3KV CERAMIC disk caps. *No more drift.* Mouser 
part number is: 80-c330c101JHG. Replace a 500 pF doorknob with 5 of 
these in parallel.


Don't waste your time with doorknobs, they cant handle the current. I 
just ripped the original doorknobs out of the Titan 425 and replaced 
them with strings of these. 20 second key-downs now no issue on the 
Titan 425 on 160 meters.

I would stay below 1.5:1 SWR to keep the voltage down. With high SWR, 
the voltage *can be* quite high on the loading caps.

Don't use mica caps either. I tried them. They just blew up.

Some info in the archives of the amps reflector about this subject. Do a 
search from contesting.com amps archives. There also some info on W8JI's 
site, www.w8ji.com

Email me privately and I can send you some pictures of what I did in the 
Titan. I'll try to post the pictures on my photo site later this week 
and share the link.

Thanks
73
Jim W7RY

On 3/17/2019 3:31 PM, Gary Smith wrote:
> Folks,
>
> I have been having drifting issues on 160
> only, not 80M or above, with my late model
> SX. I was thinking the 160 toroid
> transformers were overheating but the tape
> is not brown and I've been assured the bit
> of bubbling at the ends is from normal
> warming, not excessive heating.
>
> 160M & 80M transformers L7 & L5
>
> http://doctorgary.net/toroid-1.jpg
>
> http://doctorgary.net/toroid-2.jpg
>
> Upper band transformer L6
>
> http://doctorgary.net/toroid-3.jpg
>
>
> I tested the plate caps with my LCR and
> all tested as WNL. I cannot do a leak down
> test with this LCR meter.
>
> As there's no sign of overheating I can
> find anywhere and I haven't been beating
> on the amp, I'm assuming the drift is from
> the 160M padder caps breaking down over
> the years and they should be replaced.
>
> Alpha could have made them easier to
> access, they're right underneath the band
> switch, screwed to the front RF panel and
> the front panel of the RF deck is not
> removable (this would have made access a
> cinch). The only access to the front area
> of the deck is from above and from the
> left side.
>
> I did not remove the side panel at this
> time, I'll do that when I order & get the
> padders. Here's the view of the padders
> from above.
>
> 100pF band position A
> http://doctorgary.net/100pF.jpg
>
> 1000pF band position B
> http://doctorgary.net/1000pF.jpg
>
> I understand the early models had one 500
> pF padder and later models like this one
> have two 1000 pF padders in parallel.
>
> The two paralleled 1000pF caps for
> band-switch position B are easily
> accessible from the side, the two
> paralleled 100 padders for Band-switch A
> are located deep where I can't get a
> screwdriver in to undo them.
>
> For anyone who has replaced these on one
> of these amps, have you any suggestions of
> how to best get to the 100 pF caps and
> remove/replace them with a minimum of
> effort?
>
> Also, I'll probably get the HT58 padders
> from RF Parts, any other worthy places I
> might also check into?
>
> Thanks & 73,
>
> Gary
> KA1J
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps