2019 June 16       Electronics      How-To

Introduction


In Part 1 we looked at initial repair steps for an RCA vacuum-tube volt meter, the model WV-87A.

Now let's see what else it needs.

WARNING:  Line-powered electronics may contain dangerous voltages.  Do not work on electronics if you lack the proper skills and knowledge.  Disclaimer.


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In This Article


The Mica Capacitors

The Other Caps, Re-Visited

The D-Battery Holder

Precision Resistor


Conclusion





The Mica Capacitors


Capacitors C-4 and C-7 are supposed to be 3300 pF, 500-volt mica. 

Mica dielectric has good RF stability;  that may be important if you're measuring AC voltages up into the megahertz range. 

The ones I found in there were not original to the unit, possibly not even real mica caps.  (One of them was from 1941.)  Leakage tests showed they were good only to about 100 volts.  So I soldered in some 3300pF, 500-volt mica caps.


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The Other Caps, Re-Visited


In Part 1, we looked at the electrolytic and paper capacitors. 

This time I checked the big paper capacitor (C-1) for leakage.  It was definitely bad.  Supposed to be a 1600-volt cap, it started to leak at about 100 volts.  So I replaced it with a brand-new 1600V, 0.1uF film capacitor.  It helps to use radio pliers or something on the lead you're soldering, between the component and the heat.  This will act as a heatsink so you don't melt the dielectric. 

The other two paper caps were replaced.  The one was shorted;  the other showed leakage above 80 volts.  (These were, at one time, 400-volt capacitors.)  I replaced both of these with modern 600-volt 0.047 uF caps.


C-3 and C-6, replaced.


The electrolytic (C-5) was actually a pair of 16 uF's in parallel, making 32 uF.  This is the filter capacitor for vacuum tube diode V2, which is half of the 6AL5 connected to the HV secondary of the transformer. 

Transistorized gear uses high capacitance for the filter caps, which decreases ripple.  32 uF would be low for solid-state, but it's on the upper end for vacuum tubes.  Much more than that, and the inrush current will shorten the life of the tubes. 

The manual calls for 10 uF;  16 uF would be just dandy.  Put two 33uF 250V caps in series, and you get a 500-volt capacitor of about 16 uF.  Balancing resistors (220K) might not be necessary, but they will discharge the filter caps when you unplug the unit.  So for that reason, I would add them.


         


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The Battery Holder


Soldered-in batteries tend to get left in the equipment for too long...


This one hasn't been changed in a while.


This is one reason why we use battery holders.  An old-style metal holder would look better, but plastic is what's available now: 



The original mounting screw is #10-24, so drill an 11/64" hole in the battery holder.  There's not much room to keep the holder far from the transformer, but try to leave some. 


Get your WV-87A here

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Precision Resistor


Most of these types of test gear rely on hard-to-find resistors.  The tolerances are much more precise than typical resistors.  If all you can find is the right value but too wide a tolerance, you can get around this.  Just measure a few with a multimeter and pick out one that's closest to the exact value you need.  Will it drift?  Maybe, but it's better than nothing.



The resistor that failed here was R-42, the 900 ohm.  Looks like it ran too much current, probably from someone testing high voltage while in the RX100 setting.  A 12-volt battery would not have done this to a 900 ohm 1/2 watt resistor.  110 volts may well have.  A 200- or 300-volt B+ voltage definitely would. 

Figuring out where current will go on a schematic when it's not even supposed to be there is harder than figuring out the designed-for currents.  But when you see a scorch mark and a burned-through component, you can be pretty certain where the current was.  900 ohm resistor gone;  90-ohm and 9K ohm resistor intact.  But the 9K resistor took some external heat damage, and the multimeter says it's out of spec.  So, might as well replace it.


Get your WV-87A here

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Power Up Yet??


So far it's had a complete re-capping, a burned-through resistor replaced, and the transformer checked OK.  Shouldn't this be ready to power up?

Almost.  After these repairs, when it gets a new power cord, it's about ready to power up with the variable autotransformer.  But before that, one more thing:  it's going to need every tube tested or replaced with known-good tubes.  There is no point in all this work if you're going to power it up with a shorted tube or something. 

So that's probably where we'll pick up in Part III.


Get your WV-87A here

Table of Contents



Conclusion


So there you go:  Part II of the WV-87A functional restoration. 

If you buy yours on-line, make sure to ask the seller some detailed questions before purchasing.  Some of these units can need quite a bit of work before they're usable. 

    

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