75A4

[WA4WAX]

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

[pwfallon]

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
WA1IWQ 

[K8DI]

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

[W3SLK]

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.

[WA4WAX]

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.

[WA4WAX]

I can take a picture of the RF input transformer on mine.  A poster indicated need.

[pwfallon]

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

Tnxs,

Joe

[WA4WAX]

I will get some pictures within a week.