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

[Jim]

Here is the link for the Titan 425 loading padder replacement:


http://s900.photobucket.com/user/w7ry/library/Ten%20Tec%20Titan%20425%20Replacement%20Padders


Thanks
73
Jim W7RY

On 3/17/2019 9:12 PM, Jim wrote:
> 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
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