[Amps] FL-2100B input swr problem

[K2CB]

Good news. I found the causes of the tuned input issues. 

One, I misinterpreted the indexing of the input band switch wafer for the tuned input coils. I had the switch's input and output sides reversed.  When I initially ohm'ed it out, I neglected to realize that both common wipers (tube input and antenna relay sides) obviously will show continuity to the five positions on both sides at the same time due to the low resistance path of the coils. 

That's what I get for taking all of the old components out first. Once I went back and looked at some photos of the original component layout, I realized I had the mics component values and coil sides reversed.  Once I properly repositioned the mics caps to the proper sides of the coils, things began to tune properly.  

Well, sort of. That solved the issue for 80, 40, and 20m. 

On 15 and 10m, while better (no longer an infinite SWR), the input SWR was still between 2-3:1. 

What I found was that how and where the 10 and 15m caps are grounded makes a big difference in the overall tuned circuit. The lower bands were not critical, but the upper two bands needed the mica caps tied to different ground points individually, with the 10 and 15m ground leads spaced apart. The 80 and 40m mica cap leads didn't seem to matter. 20m exhibited some interaction, but with only a minor overall effect on input SWR.  Can you say inter-electrode capacitance?

My advice to the next person tackling this repair job is to tie the replacement mica cap ground leads to the exact same points as the OEM caps, even if you think you can arrange them in a neater and easier fashion. In the end I wound up using solder lug star washers at the original mounting hole locations of the doorknob caps.  The initial method of soldering the mics cap leads to the two or three terminal strip ground points obviously didn't fare well!

Now to finish the rest of the amp's issues tomorrow. 

You learn something new every day!

Eric
K2CB




> On Jun 30, 2017, at 7:58 AM, Jim W7RY <jimw7ry at gmail.com> wrote:
> 
> Yes and yes to the band switch being in sync.
> 
> 
> "SWR thru amp when in standby is fine".  This has nothing to do with the SWR though the amplifier only when the amplifier is operating.
> 
> 73
> Jim W7RY
> 
> 
> -----Original Message----- From: K2CB
> Sent: Friday, June 30, 2017 6:55 AM
> To: Jim W7RY
> Cc: amps at contesting.com
> Subject: Re: [Amps] FL-2100B input swr problem
> 
> Yes and yes to the band switch being in sync.  SWR thru amp when in standby is fine.
> 
> The only thought I came up with overnight is that possibly the coax from the relay to the band switch may be compromised. I am going to bypass it with a replacement piece this morning and give that a try.
> 
> Eric
> K2CB
> 
> 
>> On Jun 30, 2017, at 7:21 AM, Jim W7RY <jimw7ry at gmail.com> wrote:
>> 
>> Is the input and output band switch still synced? Is the bandswitch knob in sync with the front panel?
>> 
>> 73
>> Jim  W7RY
>> 
>> 
>> 
>> -----Original Message----- From: Bruce Blain K1BG
>> Sent: Friday, June 30, 2017 5:59 AM
>> To: 'Eric Dobrowansky' ; amps at contesting.com
>> Subject: Re: [Amps] FL-2100B input swr problem
>> 
>> Inspect EVERYTHING between the output of your exciter and the first
>> components in the amplifier closely.  Test the coax to a dummy load.  Look
>> at the SO-239 connector on the amplifier chassis.  Use an external SWR meter
>> (ie, don't trust the one in your transceiver).  Eliminate EVERYTHING as a
>> possibility.
>> 
>> Hope it works out.
>> 
>> Bruce, K1BG
>> 
>> -----Original Message-----
>> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Eric
>> Dobrowansky
>> Sent: Friday, June 30, 2017 1:56 AM
>> To: amps at contesting.com
>> Subject: [Amps] FL-2100B input swr problem
>> 
>> I am working on an FL-2100B amplifier which is exhibiting high input SWR on
>> all bands except 40m.
>> 
>> I went ahead and replaced all of the capacitors on the tuned input circuit
>> with new micas, including the 1000pf from the band switch to the tube
>> socket. I also replaced the two 200pf caps in the grid circuit of each tube.
>> 
>> SWR is still high on all the bands except 40m.20m will work somewhat, at
>> about a 3:1 SWR, but all the other bands are exhibiting a very high input,
>> to the point the exciter folds back hard.
>> 
>> I've even tried a spare set of tubes just for the heck of it. Every other
>> component I've checked on the input circuit is checking good at this point.
>> 
>> Is is possible that the 1000pf plate blocking/coupling doorknob cap could
>> have shifted value enough to cause such issues on the input side?That's
>> about the only thing left....all of the band switch contacts, both input and
>> output, check good.Unfortunately I don't have a 3kv cap on hand to try at
>> the moment.
>> 
>> Searching the archives I see CO8LY posted the exact same symptoms a few
>> years ago. But I do not see any solution to his original post.
>> 
>> Any suggestions greatly appreciated!
>> 
>> Thanks.
>> 
>> Eric
>> K2CB
>> 
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