Harris FM K5 Freq Change

FM does it with frequency!
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RGORJANCE
Posts: 1406
Joined: Mon Dec 31, 2007 7:48 am
Location: RACINE, WI

So what was the snake-oil that he found?

Post by RGORJANCE » Thu Jan 11, 2018 11:51 am

As I recall the detail, his FM-20G had taken some kind of hit----possibily lightning. It totally messed up the IPA. Story goes that he fiddled around with the coil and possibly moved things around. He put things back together, fired the brute back up and tuned for max smoke. He got his TPO, but it showed a whole lot less plate current. He got really confused and called Quincy for help.

As a result of all this, I heard that the original IPA design was causing under drive to the PA tube, and the modification allowed a whole lot more drive to make it to the final. I suspect that much better impedance matching was also part of the mod.

I never got a real technical explanation and I probably wouldn't have understood it anyway becuz I was only a salesman. :lol:

A bit more history if ya'll don't object.....

FM-G series-M-6095 tube exciter, M-6146 tube stereo generator
FM-H series-solid state TE-1 exciter
FM-H3 series-TE-3 exciter with PHASE LOCK LOOP AFC
FM-K Early series MS-15 solid state exciter and color change from the beige grey to "black and blue"
FM-K later series change to MX-15 exciter

Fossil
(also black and blue) :twisted:

TvRadioGuy
Posts: 20
Joined: Mon Sep 12, 2016 10:40 am

Re: Harris FM K5 Freq Change

Post by TvRadioGuy » Fri Jan 12, 2018 12:53 am

Thanks for the interesting and education on How to get an HT Tuned with 80 Percent efficiency

I have not looked in the Transmitter yet But Next week I will Get under the hood.

(Forum)
There are a couple items in the ipa/pa section that will require change....

1) coil in ipa section = The instructions It said Contract Or Expand Coil It also Mentions to change coils if needed
2) the metal fixtures that connects the anode of the tub to the pa tuning tube.- Instruction said It Needs 1 strap 88.1-94.9_ 2 straps 95.1 107.9
Not sure Its onn 88.1 Now We may have 1 strap
Q Where I can hunt down another Or what Materials can be used as substitute to make One.

Q what is - the driver matched to the grid on the PA. On the 10 and 20 there is some( * "bend tuning" involved. EXPLAIN *)

Question- where would I be able to get parts.

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RGORJANCE
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Joined: Mon Dec 31, 2007 7:48 am
Location: RACINE, WI

Re: Harris FM K5 Freq Change

Post by RGORJANCE » Fri Jan 12, 2018 6:06 am

The metal band connections from the tube anode to the PA tank tubing are aluminum. There may be some dimensional info in the data. Use a very long stainless hose clamp,or two if you can't find real long ones.
I think those were 1/2 inch width.

I seriously doubt that you will be able to get the PA efficiency any where near 80% on the FM-5K. Consider yourself lucky if it gets into the mid 70's.

The HT series is totally different construction in the pa/pa compared to the FM=K series.

Fossil

Lee_Wheeler
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Joined: Sun Jul 24, 2016 1:53 pm
Location: Kansas City

Re: Harris FM K5 Freq Change

Post by Lee_Wheeler » Fri Jan 12, 2018 2:05 pm

TvRadioGuy wrote:
Fri Jan 12, 2018 12:53 am

Q what is - the driver matched to the grid on the PA. On the 10 and 20 there is some( * "bend tuning" involved. EXPLAIN *)
There is an inductor that is physically in the form of a thin strap that is shaped somewhat like a "Z" on the bottom of the PA tube socket on the 20s. It is likely about the same on the 5.

The inductor is adjusted by bending the z shape to be either taller or shorter. Since you can't do it while the transmitter is on you really can't ever get it perfect. You just do it in small steps to see if the PA grid current gets better or worse. Fortunately for you I don't think the 5 ever had the same arcing problem that the 20 has. On the 20, as you are tuning it, it will go nuts and sound like a shotgun going off next to your ear when it arcs over. Even now when I work on a 20K it scares the crap out of me when it arcs and I know it is coming.

...Lee

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