Viking II drive control

[K9PNP]

The drive pot on my VII rather spectacularly shorted out and tried to burn.  A real mess inside the pot's case.  Luckily it only took the LV rectifier tube with it before the fuse went.  I found that a 25K 25W WW control will fit in its location and not interfere with the 6AQ5 stage tuning, at least on 160 thru 20 M so far.  Part is an Ohmite # RHS25KE, available thru Mouser as p/n 588-RHS25KE.  About $35 but it will probably outlast me.  For the purists, there is an issue of the shaft and bushing sticking out of the front panel a little too far.  I'm looking at modifying a control knob with a little countersinking to make it look more original.  My VII is definitely not an example of a mint rig in any case.

[K6IC]

Hi Mitch,

Sorry to hear about the drive pot demise.  This is not at all uncommon.

You may also want to consider this mod from the The AM Window pages of this site:

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

73  GL  Vic

[K9PNP]

Hi Mitch,
Sorry to hear about the drive pot demise.  This is not at all uncommon.
You may also want to consider this mod from the The AM Window pages of this site:

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

73  GL  Vic

I had that article printed out for use.  When I went through my HV-capable transistors, found the 2 I needed were both bad.  Murphy loves me anyway, so should not have been suprised.  So, being a genuine Old Timer, I dropped back to the 'go big or go home' theory of power handling.  Thanks.

[K6IC]

OK Mitch,

Sorry,  I cannot help on the HV XISTOR.  Eventho the pot you mentioned is a bit expensive,  it is good that someone is still making them.  At 25 Watts,  is should last about forever.  73   Vic

[W1AEX]

Hi Vic,

My Viking One drive pot failed about 25 years ago. I used a circuit that Pete W1VZR described to me and it has worked without any problems since I installed it. It's nice and simple and uses inexpensive "old school" components. The drive pot valued between 500k - 100k can be a cheap 1 or 2 watt linear taper type.

Good luck,

Rob W1AEX

[WD5JKO]

My Viking One drive pot failed about 25 years ago. I used a circuit that Pete W1VZR described to me and it has worked without any problems since I installed it. It's nice and simple and uses inexpensive "old school" components. The drive pot valued between 500k - 100k can be a cheap 1 or 2 watt linear taper type.

Rob, I wonder where you mounted that 12AX7? Also that 100 ohm resistor in that schematic sure looks out of place. Maybe it goes from the 300v supply to the 12AX7 plates instead?

Jim
WD5JKO

[W1AEX]

Hi Jim,

It's been a long time since I looked at that. The 100 ohm resistor is actually R-26, which is present in the Viking One at the base of the driver's tuned circuit, but is not present in the Viking Two. The .000025uf cap refers to C-25 in the original circuit. I'm not sure why I drew them in like that (on a 3 x 5 card back in 1985) except that I probably hooked the circuit in at the LV supply side of R-26 and was too lazy to draw the tuned circuit on the other side of R-26. It certainly makes it more confusing, but that's a good catch by you! I removed them from the circuit diagram to make it less confusing.

I mounted the 12AX7A over on the speech amp side of the chassis. There's a little open territory in front of one of the 807 sockets.

Thanks for catching that!

Rob W1AEX

[WA1GFZ]

A FQA11N90 FET will work just as well as a tube and easier to mount.

[sndtubes]

I use an NTE 164  (ECG 164).  They work just fine for this application.  They are a little expensive.....about 10 bucks.

[w5omr]

I use an NTE 164  (ECG 164).  They work just fine for this application.  They are a little expensive.....about 10 bucks.

I use a pair of ECG 164's on the home rig, lashed up in an emitter follower configuration to directly drive the grids of the 250TH modulator.

http://www.qsl.net/wa5bxo/driver1.html