Crawls Backward (When Alarmed)

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Testing Mica Capacitors on the Hickok 288X and Properly Soldering Connections

I fired up the signal generator after replacing the paper and electrolytic capacitors.  It works...well, after a fashion.  When I connect it to my frequency counters, I get output on AM modulated signals only up to about 1 kc.  I should be able to get AM signals on higher frequencies for sure.

So I took some voltage readings and some of them were pretty far off.  I decided to start testing the mica caps to see if any of them were bad.  Usually, they are very reliable - even after 60 years - but you never know.

Here's the tester I have to test capacitor leakage.   When we say "leakage," we mean the capacitor lets DC pass when it should be blocked.  Hence the term "leakage."

I have this Heathkit IM-11 Capacitor Checker.  It will measure the value of a cap, as well as test for leakage at voltages up to 600 volts DC.  In the leakage setting, you flip a switch to charge the capacitor.  There's an eye tube at the top, and if the cap is good, the eye stays open, or closes and then opens quickly.  If the cap is bad, the eye closes and stays closed.   Just what we need to test these micas in the Hickok 288X.

One end of the capacitor being tested has to be disconnected from the circuit.  Caps can't be tested while they're in the circuit.

On the left is one of the caps under test.  A lot of these old micas look like dominoes.  The dots are a color code indicating their value, voltage rating and tolerance (in this case 100pf, 500v, 10%).

The red and black alligator clips are from the IM-11.  The resistor up in the air on the hemostat was also disconnected - it went to the same point in the circuit as one end of the capacitor.  I like to use hemostats as a heat sink on old resistors - they are sensitive to heat and I don't like to expose them to the full heat from soldering if I can avoid it.

Ve fleep zee sweetch and....the eye tube stays closed.  This cap is bad and will need to be replaced.

So far I've tested six micas and two are bad - a high percentage for these.

(You can count on 100% of paper caps being bad).

A couple of notes on this picture.

The red arrow is the new mica in place.  I reused the old 'spaghetti' insulation on one end, and put new shrink insulation on the other (see the black 'hose' looking insulation).

The green arrow shows the connector - here it's a tube pin - that the resistor and cap connect to.   And this brings me to one of my pet old radio, amp, electronics gear peeves.

A lot of "restorers" will simply snip old leads from components and then use a piece of the old lead to "quig" or "clip" a new component lead to.  For example, in the picture, see the old connection at the yellow arrow.  Some folks might cut that lead and connect a new component to it, or just wrap a new lead around the old connection and solder away.  That, to me, is sloppy, lazy, careless work.  Not to mention it's a much less reliable connection.

What I do is use solder wick to remove all the old solder, take the old leads off, and then attach the new (and old, in some cases such as this one) leads to the nice, cleaned up connector. See how the new cap and old resistor leads go to the connector, have a proper bend, and are ready to solder.  All cleaned up and all new solder.  It's much easier to work with the connector also when it's clean - easy to insert leads.

I read this debate a lot on old radio forums, and to me, not cleaning up the old connection is simply nonsense.  Folks have said "it's slow, and I have customers."  Seriously?  If I were a customer, I'd rather know it was done right rather than done sloppily.  It takes just a few minutes to do it right.  And in a way, it's also a homage to the people who wired this old gear in the first place.  They put it together with care - we should service it with the same care.

Ok, rant over.  Back to testing capacitors!

 
 
 
 

Post a Comment 1 comments:

  • Sven Nyström said...
    March 6, 2012 at 8:37 AM
    I read this post without understanding 90%. Still I enjoyed it. What's wrong with me?

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