The AM Forum

I'm just getting the A.M. station back on the air and have been digging out some of my stuff.
The Viking II that was gotten on Ebay needed alot of work but I knew that.
It has also given me a chance to remember some things I had forgotten about radio and that's a good thing.
This project is very exciting trying to get this transmitter back on the air.
I just completed the recapping and replacing resistors on the Viking II.
I had no grid current starting out.
I now have over 100 watts on 75 meters into a dummy load.
Nice C.W. sound on the receiver.
However, no modulation at this time.
I am using an EV 611 microphone that I used on a Heathkit DX-100.
I'll look at the modulation section tomorrow.
Any modulation trouble shooting tips appreciated.
Thanks.

P.S. Thanks for the tips on the rusty knobs..worked real well.

I've seen a couple with leaky caps between the gain pot and second stage causing the second stage to be saturated.

Check for a PTT mod relay.

Carl
KM1H

One of the most common failures in the modulator section is the big tapped power resistor under the chassis. The tap is moved to adjust the idling current by changing the screen voltage on the 807 modulators and quite often the winding on the resistor is damaged, causing the resistor to become open. This of course can result in no screen voltage being supplied to the modulator tubes. More dangerously, it results in no bleeder across the B+ supply, which of course will allow HV to linger for quite some time after the rig is shut down. If you see no idling current on the modulators when transmitting in the AM mode, that would be a good place to start.

Good luck!

Rob W1AEX

http://www.amwindow.org/tech/htm/viking2.htm

I have done this in 4 Vikings. (1 V1 and 3 V2s). Works great. Plenty of gain.
A .1uf to the top of the pot makes a nice line level inpoot too!

Yup, Agree with Rob. Both I did were replaced with Zener regulators on a 1X1 L bracket. 

Rob has a good point about the resistor often being open.  If your resistor is still good keep in mind the fine wire is very fragile so be careful to properly loosen the adjustable tap before moving it.

If you haven't replaced the cathode bypass caps in the audio section they are a common issue although with full gain and shouting there is generally some weak modulation.  The audio driver transformer fails pretty commonly in all of the Johnson line.  Suitable replacements are available from Antique Electronic (and others) if you want to keep it stock.

Rodger WQ9E

Update:

Well, I had an open resistor and a bad 807.
I went ahead and changed all the resistors in the rig to specs (1 and 2 watt) and new 807's.
I now have modulation.

The case as said before was really ragged out.
All was left of the little Viking guy was one horn and his nose.. :P
The paint at the top and sides was down to the copper in large areas.
The 'brown' paint and the lettering was untouched.
The brown paint was either very dark, almost matte black or the oxidation had taken it's toll.
At any rate, my xyl taped off the lettering very precisely and I primed then I shot the case in black krinkle paint found at the car shop for valve covers after a good sanding down.
When it drys it gives it a nice matte finish.
Takes about 2 hours for full krinkling..krinkle?..whatever..
It krinkles good.
The echelons for the meter and tank were removed and sprayed with Krylon plastic black spray paint (matte).
It turned out really well.

A few more tweaks and she'll be on the air..
I'll try the PTT mod when I know everything is working as it should.

Thanks to all for your suggestions.

how about a picture? I'd like to see it.  :D

Ok..I'll post one tomorrow less knobs.
I had a couple of the knobs knarly and cracked.
Fear not.
I have some coming in.

Have to weld up a rack for it and the receiver.
Start on that tomorrow.

And in the truest tradition of amatuer radio of yesteryear, the rack is going to be made from angle iron from old bed rails, black of course! ;D

Come to think of it.
My microphone base is black.
It's a matching set. :P

Where did you get replacement knobs for the V2?  The one I am working on had apparently been wet at one time and some of the set screws were rusted.  Had to use an extractor until it broke and I ended up drilling out the set screws on 2 knobs.

I have original knobs. What mite you need?
Regards,
Gary