The AM Forum

I recently bought a dx-100. After some work, I had it up and running very well. Then at some point, it lost output and sparks occured in the high voltage rectifiers (5R4's). Now if I operate the plate switch, it draws a lot of current and dims the light in my shack, so I switch it off immediatly before I blow one of the fuses in the power cord. I suspect the 125 ufd 450 volt caps, but I am not sure as this is one of my weak areas of expertise. Any thoughts will be helpful to me.
Thanks, Scott N0GXM

That is a very common problem with DX-100s Scott.  Those 5R4 sockets tend to arc over and develop carbon tracks.  You may have to clean them up or replace them.  Also the filter caps will most-likely need to be replaced.  Check them for shorts-high leakage.  There are several on-line sources for new caps.

73,  Jack, W9GT

yeah, what jack said.  8)

Ya, there may be carbon tracks from the top side of the socket, where the HV AC goes to the plates of the 5R4, to the edge of the hole cut in the chassis. Mine had this, removed the sockets and cleaned up the mess, and solid stated the HV rectifiers after that.

Have heard of 5R4 socket arcs in DX-100s a number of times.   In my DX-100s this has NOT been the problem.  Altho,  all of the 100s that Ive had have used large ceramic sockets for the 5R4s

AND,  Scott did mention that the arc was INSIDE the 5R4s.  This is the arc that I've had with my DX-100 -- arcs INSIDE the 5R4s  My solution was a pair of paralleled Negetive Temp Coccicient (NTC) Thermistors (Keystone  KCC CL-60 in my case) on the primary of the HV Plate Transformer.  This solved my sparkie problem.

73  Good Luck,    Vic

After making some resistance readings, I believe the 125ufd 450volt filter caps are shorted. I have ordered new ones and hope this fixes the problem. Thanks to everyone who answered my posting. I will be back here to update you on my progress.
Thanks, Scott  N0GXM

Tom NØJMY at Hayseed Hamfest,   http://www.hayseedhamfest.com/
has new ones in stock.  He does not list them on his site, but he does have them.
http://www.hayseedhamfest.com/capkit.htm

Thanks for all the responses to my DX-100 problem.  Well, I now have a very nice working DX-100 and made my first contact this morning on 3.875 Mhz. I was given an S-9 + 30 Db with excellent audio quality. Replacing the 125 ufd caps made all the difference, it runs real cool and with all the rest of the work that took over a month, I now have a nice transmitter. I hopr to work some of you on the air.
73, Scott N0GXM

This problem will continue to exist and will surface again and again because the DX-100 was designed (in the early '50's) to operate from 110 to 115VAC mains power. Your AC power could very well be in excess of 125VAC as mine is (126 VAC). I suggest the rig be run on a variac or multitap auto/isolation transformer with no more than 115VAC presented to the transmitter. Lower the voltage - raise the reliability! Carl WA2UJX

Regarding 5R4 socket arcing. The socket is mounted  with a fiber washer between the  socket
and chassis. Over the years the fiber washer gets compressed and the spacing is reduced.
Then it becomes a very intermittent arc source. I replaced mine with nylon washers from the local
hardware store.