[Boatanchors] Repair Advice Sought

Mon Jan 7 14:06:35 EST 2013

Scott,

I have a couple VariArm VFOs along with the later 90711 model.  There really isn't anything that complex about these units but do not undertake wholesale replacement of components, particularly in the oscillator compartment.  

First make sure the power supply is operational and since you aren't going to find a "tapped" line cord anywhere you will have to install a separate resistor in place of the 160 ohm line cord resistance that feeds the filaments and pilot light.  With higher voltages being the norm a 180 ohm power resistor is more appropriate as a replacement and a 25 watt rating is reasonable.  I didn't want to make any changes/additions to my Variarm units nor add additional heat so I put this resistor (and a fuse since none was included in the original) in a small box inline with a power cord to the VFO unit.  I did replace all filter caps in the power supply.  

Note that Millen utilized a "safe" transformerless setup in that B- is not connected directly to chassis ground but is RF grounded through capacitors.  Because of this C15, 16, 23, 24, and 25 are good candidates for replacement with new parts but observe proper precautions in any case to make sure any wiring replaced or components changed corresponds to the schematic. The Rice VFO from which this VFO was derived did not isolate the line from the chassis.

With the power supply operational and "safety" caps replaced proceed to operational testing and if there is no oscillation (best determined with a scope or external receiver rather than checking drive to the transmitter at this point) check the tubes.  If you have oscillation but no transmitter drive it could be an issue in the V2 or output coupling circuit.

Good luck getting it going.  This is an interesting early VFO and makes a great companion to early transmitters.  One of mine drives a Utah junior transmitter and the other is used with a Stancor 20P while the later 90711 is hooked up with a Millen exciter and power amp.

Rodger WQ9E

-------- Original Message --------
> From: whitebear1122 at comcast.net
> Sent: Monday, January 07, 2013 9:00 AM
> To: boatanchors at mailman.qth.net
> Subject: [Boatanchors] Repair Advice Sought
> 
> First off, I really appreciate the advice that is given on this list, thanks!  

I am repairing a Millen 90711 VFO and would appreciate some general troubleshooting advice specifically with close loop systems like an oscillator.  Some history:  t his VFO is brand spanking new, came from the original box, and the coupler was still wrapped in the original wrapping and tied to the cabinet with string.  Point being that it has not been powered up or messed with since it came from the factory maybe in the 50's or 60's.  

I've never seen a power cord so rotted before. When I moved the VFO from the desk to the operating bench, it left behind a pile of cord insulation droppings and then left a trail of insulation across the floor.  :)    S o far I've found an open 15K Ohmite power resistor, the current limit resistor driving the 150v voltage regulator tube.  I never expected a power resistor to just open up on its own . 

Back to the ques tion.  The question is how to best troubleshoot an oscillator that doesn't work, and has parts that are 50-6 0  years old.    An oscillator is a close loop system where any single failure may result in it not working at all, so  all components and all connections are suspect.  In order to measure the value and determine the condition of the  resistors and capacitors , most  have to be isolated from the rest of the circuit, removing any other LC connections that would impact the measurement .  Usually one has to lift a component lead for this.  The typical example is a capacitor with a resistor or inductor soldered to or across it.  

To make that measurement, I have to l ift the component leg of all those components that need to be measured.  I've not  had good luck unsoldering component leads, often making a mess of the connection or damaging the solder lug or joint, so I am more inclinded to cut and splice the connection back together with a little wire "coil sprig"over the two leads .   So in this closed loop system, do you just whack your way through all the components until all are measured?  Otherwise, how would you test every part in there?   

I've thought of just shotgun replacing all the caps in the circuit (I don't have much confidence in old Micamold caps), and when the caps are removed, then meaure up the resistors and replace as necessary. I don't have the schematic in front of me but there are maybe 7  Micamold caps in the oscillator section.   If that 15K Ohmmite power resistor opened up on its own, I don't have any confidence in the other small resistors either.   

So I have the power supply running.  I checked the oscillator tube and put a little DeOxit on the pins.   I loosened and tightened all the screws.   I measured filament and plate voltage at the tube socket.  When I put the scope probe on the plate in order to look for oscillations all I see is 60 hz.   Tonight I'll do some continuity testing, to see if connections are there.  Things like connections to capacitor rotors, corrosion, or bad solder joints I suppose.    

I am  curious of the  technique   or strategy that you use to resolve an oscillator issue like this where it is not oscillating at all and you have no confidence in any of the oscillator components. 

73, Scott WA9WFA 

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