W6MSC AM Transmitter -- 13-Dec-2004 C. Howard


Update – September 26, 2005

Not much happened with the W6MSC transmitter over the summer.  Over the past weekend I cleared out a bunch of stuff in the garage/radio-shack and finally started thinking about it again.  Yesterday I put my SWR/Power meter between the output of the exciter deck and the input of the final PA.  It showed nothing… completely dead.  So I have some work to do on the exciter deck.  The first task will be to replace the selenium rectifier installed on the back of the deck.  Then I need to try to follow the signal path through the exciter deck and see if I’m getting anything from the crystals on forward.  It will be a challenge to see if I can get the exciter to work outside of the rack… I will need to examine the interconnections and see if that can be done.  While I’m on it, I need to create a true as-built schematic of the exciter.

Update – May 12, 2005

I have the 250-TH tubes installed and all tube filaments light up nicely.  But I am still having trouble getting the thing to go.  When I power up the filaments come on, then there is a timed delay.  After the delay the green light comes on.  I push the ‘Start’ button and I get activity in the rectifier tubes and I get some readings on the meters for plate voltage.  But red light also comes on.  When I release the ‘Start’ button, it goes back to just having the filaments on.

So I suspect there is some kind of relay or interlock that is not working right.

One mystery is the meaning of the term ‘NEMO’ which appears on some of the documents whose images are below.

Here are PDF file scans of some of the few documents I received with this thing.   The documents are old and yellow, so they didn’t scan very well.  If you have a specific question, let me know and I will examine the originals.

On these power control schematics, I don’t know how close they are to the actual construction.  Everything checks ok so far, but I have not done a complete trace of any of the power control decks.

main power control deck  |  main power deck - front panel

aux power control

unknown power control

The first exciter page shows a schematic somewhat different than the final construction.   There is no 807 nor  an 814.  Instead there is a 832A  followed by what looks like a 4E27A (the label is completely gone).  But the 6AG7 and 6V5 stages look similar to what was constructed.

exciter page 1 | exciter page 2  |  exciter page 3




Update - March 12, 2005

I was able to locate the modulation transformer from the original source. But no modulation or PA tubes were found. Most of the interconnecting wires were labeled, as were the connections on the rear of each deck. So I now have the wires connected and the connections documented in a notebook. I also got the bias supply from the original source, and some oil-filled capacitors and a large inductor ( 1 henry ) for the plate supply filter. So as far as I can tell all of the hanging wires now have a home and everything connects to something!

I spent quite a bit of time getting (temporary) 220V house power installed and a large power switchbox.

Current Condition

Right now I can fire up the filaments and everything populated glows nicely. I am waiting on shipment of some 250TH PA tubes. That should fill all the sockets. I was already able to gather up 4 working 813s for the modulator.. even though my advisors are telling me that 2 would do the job I'd like to start out with all the sockets filled. I also acquired the 866 tubes for the auxillary plate rectifier circuit. The main plate rectifier once had 872s but came to me modified with a large diode array.
I am hoping I can apply plate power after those 250th tubes are installed.

To Be Done

I need to rebuild a coil for the PA output swinging link for use on 80 mtrs. I have half of the coil in good shape but the other half is mangled. Coils for 40, 20 and 15(?) look ok.

Also need an external VFO to augment the crystals that came with.

Audio system needs are unknown at this time. I believe the modulator 813s need a relatively small amount of drive so I'm going to start small and move up as needed.

Dec 13, 2004

Here are some pictures of the W6MSC transmitter with all of the decks mounted in the rack.

Here are a couple of shots of the front

Transmitter Front #1

Transmitter Front #2

Transmitter Front #3

And a shot of the back

Transmitter Back

I have some closer shots of the front:

Front - upper left -- meters and exciter        

Front - upper right -- meters and power amplifier deck

Front - middle left -- rectifier deck on top, power on bottom

Front - middle right -- modulation deck over power level controls

Front - lower -- power control

And closer shots of the back:

Back - upper left - power amplifier                                Back - upper right -- meters and exciter deck

Back - middle left -- modulator                      Back - middle right -- rectifier deck

Back - lower left -- power control                 Back - lower right -- medium power deck

Back - bottom left -- mod xformer deck       Back - bottom right -- plate xformer and space

You can see a couple of problems in these photos.

One is that it looks like the modulation transformer is missing.
Another is that a bunch of labeled wires below the plate transformer has no corresponding deck to connect to.