The AM Forum

Could anyone tell me what pot to buy for the Viking II
Drive? R25 in manual shows a 25K pot but does not
say what wattage. Mine let the smoke out last nite.

                                              Joe KF0XV

Joe,

It is a 4 watt rated wirewound pot which is rather difficult to find new.  There are a number of manufacturers who list 4 watt pots but if you look closely at the temperature de-rating they are only good for four watts in the Arctic region and with the higher ambient temperatures inside your transmitter the rating is down to about 1 watt and they will NOT work.  Unlike the original pot, they are not wire wound and will not last under the type of service in your Viking 2.

You may be able to find one out of a parts unit but the better long term solution is to replace the 25K pot with a standard pot and use a horizontal output type transistor as the current carrying element.  You can find the basic circuit idea through this AM website at:  http://www.amwindow.org/tech/htm/drivepot.htm

Rodger WQ9E

YUP Joe,
The mod for the solid state is worth the effort and it will last a long time

Fred

Yup, did the pass transistor modification to my Viking One 20 years ago when the big drive pot opened up and it has worked fine ever since. Highly recommended!

Rob W1AEX

If you do find a replacement wirewound pot, I would check to make sure something else in the circuit isn't malfunctioning to make it smoke, before applying power.  WW pots do sometimes spontaneously open up with age, due to oxidation of the wire inside, or gradual deterioration due to long-time heating effects, but if smoke came out of it just before it crapped out, that sounds like it was drawing excessive current.  There could be a short circuit elsewhere, such as inside a capacitor, etc.

This should work for you.

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

Thank You all for the info. I ordered 7 of them from
Digi Key.

                   Joe/KF0XV

Greetings:

I need to make this repair to my Ranger - the original drive pot is working to a degree but acting flaky and probably on its last legs.

Does anyone have some nomenclature  or part number from Digikey or Mouser for a suitable horizontal output transistor?

Thanks,
Rob KA8JBY

I found this circuit as a good starting point for my JVII rebuild.  The driver is a 6AQ5 rather than a 5763.  I could not get sufficient screen voltage without reducing the 470k resistor to 100k.  This is still only about 3 mA base current max and may be due to low gain in my emitter follower.  Since it worked fine, I pursued this no further.   73   John

Thanks, John.

As it turned out, I had to halve the resistance to 235 in order to get sufficient screen voltage.
Seems to be working well now.

73,
Rob

 Hello All , what is the best transistor to use for this ? I think I have some  2SC1308 I think that would be OK for this just don't remember there specs. and have not took time to look them up.....Thanks ...Tim...WD4TC.
   I've got 3 bad pot to replace.!   

Those Horizontal bipolars have crap for gain you are better off with a high voltage FET