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Author Topic: 75A4  (Read 2900 times)
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WA4WAX
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« on: April 25, 2024, 05:38:20 PM »

Question for the board:

Does anyone service the 75A4 in this day and age? Mills is out of it.

Also:

Look at these capacitors.  Would they be a good substitute for silver mica caps?  We use caps like these in MRI coils.

Thanks!

https://datasheets.kyocera-avx.com/100B.pdf
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pwfallon
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« Reply #1 on: April 26, 2024, 07:20:55 AM »

Those caps have great electrical specs for RF use.  That being said these may be hard to use in recapping as the parts are not leaded but rather intended for surface mount PCB use.

I am in the process of replacing all the silver mica caps in the A4 with 500V dipped SM caps.  Obtained the needed parts from Tube Depot and Digi-Key


I just posted on another topic about the input transformer T2 in the 75A4 I am working on.  For reasons unknown it looks like the input (antenna side) winding had been removed.  Trying to get a good picture of what a "stock " one looks like to try and restore it.

Joe
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K8DI
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« Reply #2 on: April 26, 2024, 07:30:41 AM »

The AVX type MLCC (multi layer chip capacitor) are favoured by the class E folks here as well as RF designers all over.  MRI, plasma generators, etc.  Other than the form factor, they’re a better than original replacement for silver mica.  Although uncommon/more expensive, they can be ordered with tabs or strips as well, which would make using them in place of leaded components easier.

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
W3SLK
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« Reply #3 on: April 26, 2024, 10:34:49 AM »

Talking with Howard, W3HM last year about some issues with my 32V2 revealed some easy repairs. For by-pass caps he uses the polystyrene jobbers that you can get about 50 for $10. Where micas were used he replaced with ceramic discs. I have a couple micas that were showing almost a dead short. Don't be afraid to use what your common sense tells you. Unless you want to maintain 'factory mint' with everything as built you will probably have a difficult time finding those components and with their age they will probably have a high failure rate when you do find them.
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Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
WA4WAX
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« Reply #4 on: April 26, 2024, 07:14:05 PM »

A word about those capacitors: They make them with both axial leads and ribbon leads.  Most of the MRI work we do sees ribbon leads.  You do NOT need the nonmagnetic version.

I have a like new 75A4.  Serial number 4973.  The original owner was a certain KH6CON.  He took possession in January of 1958.  It has lots of noise.  I suspect caps.  The main dial rotates smoothly.

BTW, there is an article about working on the PTO in the December 1974 edition of Ham Radio Magazine.  This may be found on World Radio History.
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WA4WAX
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« Reply #5 on: April 26, 2024, 07:16:06 PM »

I can take a picture of the RF input transformer on mine.  A poster indicated need.
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pwfallon
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« Reply #6 on: April 27, 2024, 05:31:53 PM »

That would be very helpful if you can take a picture of T2.

Tnxs,

Joe
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WA4WAX
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« Reply #7 on: April 28, 2024, 06:19:08 PM »

I will get some pictures within a week.
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WA4WAX
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« Reply #8 on: May 08, 2024, 05:48:25 PM »

I hope I am in the right location.  Please advise.

I do not have a camera with a macro lens.  These are phone shots.


* A41.jpg (3803.9 KB, 4032x3024 - viewed 156 times.)

* A42.jpg (114.18 KB, 1080x810 - viewed 116 times.)
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WA4WAX
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« Reply #9 on: May 10, 2024, 09:18:31 PM »

Joe:

I hope I captured the detail you need.
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pwfallon
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« Reply #10 on: May 11, 2024, 09:07:11 AM »

Hi Matt,

Thanks for taking these pictures!!

The primary (white twisted pair) on  mine was removed and modified.

Is it possible to see how many turns there are of the twisted pair on the transformer? 

Tnxs,

Joe
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RolandSWL
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« Reply #11 on: May 11, 2024, 09:37:59 AM »

I found this information concerning a problem with the Collins R-388 receivers as used by the military. I don't know if this applies to 75A4.

 Cross-Modulation with R-388, 51J-3 and 51J-4 - The first receivers, the 51J-1 and 51J-2, used RF transformers in the Ant/RF input section of the receiver but this resulted in a fixed antenna input impedance of 300 ohms. The Army found this terribly inconvenient since most of their antennae were whip verticals or dipoles with impedances a lot less than 100 ohms, let alone 300 ohms! Collins modified the RF/Ant input section when implementing the changes needed for the R-388 receiver. The change removed the primary winding on the coils and just used a parallel LC for tuning the Ant/RF along with adding an Antenna Trim to allow matching the antenna low impedances better.
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WA4WAX
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« Reply #12 on: May 17, 2024, 06:50:42 PM »

Joe:

I have one of those 1K fiber optic widgets with screen and capture capability.

I will try to fire it up and get you a close up view.  No promises.
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Tom W2ILA
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« Reply #13 on: May 27, 2024, 01:03:06 PM »

Question for the board:

Does anyone service the 75A4 in this day and age? Mills is out of it.


Where did you hear “Mills is out”?  He just serviced a PTO for me and sent it to Dayton last week  with his buddy for me to pick up.
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WA4WAX
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« Reply #14 on: May 27, 2024, 06:04:15 PM »

I may have got a bum steer.

Do you have contact info for him?

I have a PTO.

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Tom W2ILA
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« Reply #15 on: June 02, 2024, 04:43:52 PM »

sent via pm
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