viking 2 clamper adjustment

[kc2etm]

hi my viking 2 seemed to be working ok i changed the two large double electrolytics
in the rear with 4 sprague 16uf 450v after changing those i noticed my modulation current was up to about 200 or so so i adjust r13 for proper current as per the manual by sliding the tap at proper current i get a voltage reading of around 120 off
the center tap of the resistor after this i went through the full clamper adjustment per the manuals instructions but when i do this the radio seems to work ok but i can
not get more then about 50 watts carrier and show no forward modulation on my peak watt meter  now the thing is going through the steps in the manual when i turn r30 full clockwise and engage high voltage they say move it clockwise until the needle hits the first line after 0 well when i turn on the hv the needle is already at 5 on the bottom scale without touching the adjustment unless im reading the top and bottom scales incorrectly so anyway i can increase the wattage to about 85 watts  by adjusting r30 counter clockwise i dont know if the meter in the radio is reading something incorrect or if im just forcing this by adjusting r30 if i place r13 adjustable resistor back to where it was before all this happend i get plenty of forward modulation  i actually just found .47 orange drop 600 volt is that close enough


thanks
fred
kc2etm

[WQ9E]

Fred,

Did you replace the bias filter caps yet?  If you have not, do so because your bias voltage is likely down.

I adjust the clamper control to the point that the plate current just starts to rise from its clamped resting point and this method has worked well for me in all of my "clamped" rigs.

[kc2etm]

i have replace most of the caps in the unit do you happen to know which cap numbers they are is it c13 that was one of the double caps
ive chacked r15 r16 r17 all are a tad high in value but probably ok
thanks
fred

[kc2etm]

when you say from its resting point you mean where it is when before you turn r30 counterclockwise  do you have a reading on the meter like i do in that step the book makes it sound like the meter shouldnt be reading anything then i just bring it up to the first line of the meter scale am i interperting this correct

[WQ9E]

i have replace most of the caps in the unit do you happen to know which cap numbers they are is it c13 that was one of the double caps
ive chacked r15 r16 r17 all are a tad high in value but probably ok
thanks
fred

The bias caps are C12 and C13; it sounds like you already replaced them.

[WQ9E]

when you say from its resting point you mean where it is when before you turn r30 counterclockwise  do you have a reading on the meter like i do in that step the book makes it sound like the meter shouldnt be reading anything then i just bring it up to the first line of the meter scale am i interperting this correct

There is usually a slight amount of current shown and as you turn the clamper adjust it will rise suddenly; set the control just shy of the point where it starts to suddenly rise and this is generally a few mils higher than the fully CW position of the control.  If you have the clamp control set for too much control the output will be greatly reduced and it sounds like this is what you have going on. 

All the clamper has to do is protect the final from excessive dissipation if excitation fails and as long as your transmitter otherwise operates OK with the final not showing more than 15-20 mils in the clamper setup position it is fine.

[kc2etm]

i put the factory bias cap back and it works perfect now i checked the two sprague atom caps i just put in their values are supposed to be 16uf 450 volt when i check them with my meter i get 30uf way out of spec could this be the problem ive replace the other double cap with these also but they seem to be ok i guess they are not critical
thanks
fred

[WQ9E]

Fred,

You would like the input filter cap (the one before the filter choke, in this case C13) to be pretty close to the original spec because increasing the size of the input cap will increase the supply output voltage and also the peak current exposure for the transformer and rectifier.  However, a common tolerance rating for electrolytic caps "back in the day" was it would be within +100%/-40% of marked value which is anything but precise.  I imagine modern caps are much better but if your meter is correct you may have gotten a mislabeled part.

When you replaced the "new" bias caps with the originals how did this affect your modulator resting current?  If the replacement caps were higher in capacity than the originals then the resting current should have gone up after you put the originals back in place.  

If it were me, I would check the voltage at the C10/L2 connection point and if it is between 300-350 I wouldn't worry about the two replacements in the B+ circuit.  While you are checking, see what you find at the junction of C12/L3 and it should be around -75 volts are a bit higher.

ON edit:  Quick thought:  any chance you installed either C12 or C13 backwards?   The bias supply is negative with respect to ground so for C12 and C13 the Positive lead goes to ground.

[kc2etm]

that would be the problem i installed as normal ground to chassis and postive clamper input ill have to try it again tomorrow the other way
thanks
fred

[WQ9E]

Fred,

A common mistake!  If they had been voltage rated closer to the supply and/or if installed in a high current capable supply you would have probably heard a couple of fire cracker like pops.

I would not reuse the ones that were installed in reverse since they have been compromised.  If you don't have any more of that value you can safely use the modern standard value of 22 uf and for safety a 160 volt or greater rating is fine for the bias supply filter caps.

[kc2etm]

Hi rodger i installed two new 16uf 475v spragues (the correct way) and all is well working better then when this started now i have another cap question i believe it is
c9 .5 600v under the oil filled cap its the last old cap in the radio but all i seem to be able to locate in that value are electrolytics could i use one of these in place as long as ground is to the tab
again thanks for all your help with this problem
fred
kc2etm

[WQ9E]

Fred,

I believe the cap you are referring to is the screen bypass cap for the modulator tubes.  Check and see if it is connected to the screen terminals of the 807 modulators.  If this is the one then replace it with a modern .47 @ 630 volt cap available from Mouser or other suppliers.  You don't want to use an electrolytic here.

The big oil cap should not need replacing, they are very reliable, long-life components.