New Caps in Heathkit IM-13 VTVM

[AustinTxBob]

I picked up a used VTVM and started looking at replacing the caps since they all seem to be original (old at least).  A couple of the caps appear to be out of spec however.  I am looking at what the schematic claims are two .005uf but inside I see .05uf.  Also I see a line on the caps but they are not electrolytic on the schematic.  I should be able to replace these with film caps, right?  .05uf or .005uf? 
 

[cvanc]

I can't speak to the discrepancy in component value - hopefully someone else can comment on that.

The line on older 'paper' style caps denotes the outer foil.  If you think about how a capacitor like this is made, it's basically two long skinny pieces of metal foil, with some kind of insulator (dielectric) between them.  This 3-layer sandwich is rolled up tight into a cylinder, so naturally one of the foils ends up as the outside layer of the finished product.

Back in the day, it was argued that in some circuits it made a difference to hook the outer foil to the 'input' or 'output' node.  For instance if your cap is being used as a HF shunt to ground then maybe connecting the outer foil to the grounded node of the circuit helped it to act like a shield?

I'm not really sure this actually made much difference, however.  Can't say I've ever run across anyone who swore this was A Big Deal.  (your mileage may vary LOL)

Good luck with your rebuild.

[TimFox]

I was taught that the outside foil (bar marking) should go to ground or to the lower DC potential as a safety matter.  The older capacitors had just paper and wax outside of the outer foil, so it was not wise to connect +300 V to that end.  Since then, there have been silly postings in audio literature about the direction of signal flow, etc.  Of course, there is capacitance from the outer foil to ground (if neither end is grounded), but is much smaller than the actual capacitance of the capacitor so the effect on shunt capacitance from the signal node to ground is negligible.
However, it is definitely a good idea to replace non-hermetic paper capacitors of that vintage, since they tend to absorb moisture over the years.  Following such advice, I replaced all the similar plastic-cased capacitors in a Tektronix 130 L,C meter with modern polypropylene foil units.  Measuring each original capacitor after removal showed bad Q values on each.
For a VTVM, I think the discrepancy between 0.005 and 0.05 uF is not important.  You may find it easier to buy 0.047 uF.

[cvanc]

For a VTVM, I think the discrepancy between 0.005 and 0.05 uF is not important.

This is quite likely true.  The meter probably will work fine with either value, it isn't very critical.  And being a kit that was made for many years there might have been different values used in different production runs.

Let us know when you get it working, OK?

[Marc M.]

I pulled the cover off my Heath/Schlumberger SM-5228 I got way back in the late 70's which is pretty much the same thing except it's the bench version and factory built (hence the SM prefix rather than the IM which is the kit version).  The only difference is I installed the solid state conversion kit where one of the resistors changed.  I confirmed that both caps are indeed .05 uF @ 400 wvdc. Notice how bad the construction is on the factory unit .  Whomever built your kit took a hell of a lot more pride in their work than the person who assembled mine at the factory  .

I still use this on the bench when working on tube gear (it has a 1500vdc range), higher voltages (I also have the high voltage probe), and when doing alignments.  Not the most accurate thing in todays world of sub-percentage accuracy, but near enough for the types of equipment and / or purpose I use it for (I actually have 2 of them).  I made my bench shelving out of 8020 extrusions and using a piece of aluminium channel I can pull them off the shelf and hang them off the front rails when I need them.

I was taught that the outside foil (bar marking) should go to ground or to the lower DC potential as a safety matter....

I was taught the same thing in my high school electronics class, but I remember it being more for reducing hum in audio circuits and stray RF in RF circuits because it provides some shielding.

[Johnny10]

What I have found helpful for changing and replacing old caps.



and



Always great videos from Mr Carlson

[AustinTxBob]

Thanks everyone.  All of the caps replaced and it powers up and zeros out.  I'll assemble a set of probes tomorrow for it and go through the alignment process.

[TimFox]

One big advantage of the traditional VTVM is that the 11 megohm input resistance includes 1 megohm inside the probe handle just after the tip.  This reduces capacitive loading on nodes in high-frequency circuits.  Normal DMMs now often have 10 megohm input resistance on DC, but there is no isolation from the probe tip to the meter.  My -hp- 410A VTVM goes further:  10 megohm in the probe and 100 megohm in the meter.