Valiant Bias Potentiometer Mod

[Wb4mak]

I'm restoring a Viking Valiant and could use a bit of elmering on a couple of issues please:

1. The VFO (on 160/80M) measured with a digital scope at pin 9 of the the buffer grid following the VFO will not tune BELOW 2 MHz. I've ordered replacement caps for the 160/80 grid circuit but is there something else I should check? Works fine on 40M. This results in low/no grid drive at the finals. I'm thinking one of the four fixed capacitors is open or has gone very low in value. The variable padding capacitors and VFO main tuning capacitor section seem to be OK.

Note: The dropping resistor on the 0A2 6AU6 VFO screen regulator was bad (very low) and it caused the 0A2 to short and really cook the board immediately around the middle of the 0A2.

2. I've heard of a solid state replacement for the Drive bias pot to reduce heat using a horizontal output transistor. Does anyone know if this mod and have a schematic and parts list?

I've read through all the other suggested mods on this list and Google but for now I plan to just use it mostly stock on my daily AM net.

Thank you in advance for all suggestions.

Vry 73,

Mack de WB4MAK

[Steve - WB3HUZ]

2. I've heard of a solid state replacement for the Drive bias pot to reduce heat using a horizontal output transistor. Does anyone know if this mod and have a schematic and parts list?

Although not specifically for the Valiant, this one should work or get your pretty close.

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

[W3GMS]

Mack,
I don't have the schematic in front of me but looking at the schematic it should be obvious which caps are used in conjunction with the main variable tuning cap.  Those dog-bone style NPO's right under the top of the VFO plate are known to go in time.  The last time I needed to replace them, I ordered them from Surplus Sales of Nebraska.  I usually start with a good cap bridge and measure the caps to see which ones are shot.  I replace all the original block mica's with the newer dipped mica style caps and have had good luck. 
Hope that helps!
Regards,
Joe, W3GMS

[w4bfs]

Hi Mack ... I see you came to the right place .... the schizmatic HUZ pointed out is a good starting place .... if the hortz output  transistor has a low beta you may need to use a lower value resistor than 470k ....I used 100k and had good results .... have a good trip and see you hext week ... a dead crt have have what you need iffin it wasnt the H.O.T. that caused it to die in the first place ...73 ...John

[WQ9E]

Mack,

Before you tear into the VFO capacitors, listen to the spotting signal in a receiver to see if it truly is as indicated on the scope.  The combination of fundamental plus harmonics can sometimes cause incorrect readings on digital counters and digitizing scopes.  Also, how do the calibration controls impact the frequency range?  There is a fair amount of range from these controls so an incorrect adjustment here will cause trouble.  Also make sure the adjustments are actually turning the adjusting components in the VFO enclosure-it is easy to have them not re-engage when you are installing the top VFO cover.

If you haven't already done so you do need to replace the dropping resistor to the VR tube with a higher wattage one (10 watts is a good choice).  Replacing with the same rating as the original will result in another failure.  Putting the replacement in the VFO enclosure (as done originally) will cause the VFO to stabilize more quickly.

If your Valiant is an early one, be sure and check the wiring from the LV transformer to the 866A cathodes.  Early ones used standard hookup wire not suitable for the plate voltage that appears on the leads and they will break down and short to the chassis taking out the LV transformer in the process.

Later Valiants (and those modified for use with the 6N2) will have a pair of pots to set the modulator and final bias.  This is not a required mod but it does make setting the proper bias more convenient.

The modification to use a transistor to control the screen voltage for the multiplier/driver is a good one and gets around searching for expensive and fragile high wattage pots.

[W3GMS]

Mack,
If you run into beta issue's with a bi polar transistor, use an N Channel FET.  They are cheap and work very well.  I use one in my Ranger and never had a problem.  I also used the same scheme but with a zener on the gate to gnd as a series pass transistor for a regulated screen voltage circuit for the modulators.  I can send you a very proven circuit for that as well if your interested.  Lots of ways to solve the problem. 
Regards,
Joe, W3GMS   

[DMOD]

For the screen grid control voltage circuit, I have used a 2N6740 or an NTE379 bipolar trnsistor and placed a 0.1 uFd 500V at the base and 0.01 uFd's 500V at the collector and emitter.

Phil - AC0OB

[Pete, WA2CWA]

For the screen grid control voltage circuit, I have used a 2N6740 or an NTE379 bipolar trnsistor and placed a 0.1 uFd 500V at the base and 0.01 uFd's 500V at the collector and emitter.

Phil - AC0OB

Can we assume you also used a pot (what value) to control the voltage and a base to collector resistor (what value). Any other parts? How did you mount it? Etc.

[W3GMS]

Hi Pete,
Normally no resistor is necessary in the collector or drain of the bipolar or FET unless your trying to reduce the differential voltage across the transistor for power dissipating purposes concerning the transistor.  The circuit looks like either an emitter follower or a source follower if a FET is used.  So the wiper of the pot goes to the base or gate and one end of the pot goes to the original screen voltage source and the other leg of the pot gets grounded.  The nice thing about the FET is the pot can be a very high value since you do not need to supply any current to the gate.  This helps on sizing the pot.  I used a 500K pot on my version which works out to .18 W of dissipation assuming its across the 300V rail.  For the FET I got an 500V insulated package type FET which was mounted directly to the chassis.  The FET I used was an IRFS840B.  No caps were needed other than the normal RF bypass cap from screen to ground.
Regards,
Joe, W3GMS

[Pete, WA2CWA]

Thanks Joe. My query to Phil's post was to enlighten the general reading public on his design if it truly is similar to the initial design link posted and/or similar to what you described. His, "I have used" only documents a transistor and caps. Readers might want to know more and we might want to archive the full design either on Steve's site or this site.

[DMOD]

Can we assume you also used a pot (what value) to control the voltage and a base to collector resistor (what value). Any other parts? How did you mount it? Etc.

Yes, very similar to HUZ's link.

In my case, I was controlling a pair of 6146's so the total screen grid current is 25 mA. I mounted everything around the 500K, 0.5W potentiometer on the front panel. The wiper of the potentiometer controls base current. Bottom side of pot goes to ground.

My screen supply voltage is around 265 Volts; I adjust the screen voltage to about 210 volts, so transistor dissipation is around 1.5 Watts nominal. The transistor I mentioned earlier is a 100 Watt HV (450 Volt) bipolar. If you are going to dissipate more than 5 Watts, I would heat sink the TO-220 transistor to the panel or provide a heat sink to the transistor's tab.

The top of the pot and the collector of the transistor go to a 1.0K 1 Watt resistor which then goes to the 265 volt screen supply potential. If you are going to pull 50mA or more, I would place a 22 uFd@350Volts (or larger value) electrolytic at the junction of the 1.0K and the pot/collector to ground.


Phil - AC0OB

[Wb4mak]

Thank you very much to everyone for the most helpful advice.

I finally have the Valiant on the air successfully. While there were lots of electrolytics replaced, here were the final issues:

1. The 18K dropping resistor on the screen of the 6AU6 had gone to 400 ohms.
2. That in turn took out the OA2 regulator and caused a lot of heat as indicated by the charred area around the circuit board.
3. I "suspect" the heat ruined the 91 pf cap in the 160/80M grid circuit of the VFO. It measured 30 pf. Replaced the cap.

I aligned everything and all appears to be right with the world. I'll get around to the drive pot mod soon so thank you again for all the schematics. I have some HV FETs here that should work fine.

This is an awesome list and now I have a new (old) beauty to add to my vintage collection.

Vry 73,

Mack de WB4MAK