In my previous post, I mentioned that the previous owner of the C3 Capacitance Checker had indicated that it had been rebuilt, and that it works.  I decided to take a look inside to see what had been done before powering it up on the bench for evaluation.

Bottom View

Top View

I’m glad I took a look, as it appears that all of the original capacitors are still in place in the unit.  I measured all of the capacitors in place.  None appeared to be shorted, and all except the 2 uf were actually pretty close to the marked values…

0.01 uf = 0.010 uf
0.02 uf = 0.024 uf
0.05 uf = 0.057 uf
0.25 uf = 0.307 uf
2.00 uf = 0.660 uf, 2.46 ohms ESR
8.00 uf = 12.84 uf, 1.09 ohms ESR
8.00 uf = 12.97 uf, 0.94 ohms ESR

The only real suspicious ones were the two 8 uf electrolytic caps, which were reading about 12 uf, and the 2 uf, which was reading 0.6 uf.  I’ll still replace most of them (especially the electrolytics and wax paper caps), but they had held up far better than I expected.  Of course, I have no way of testing leakage yet, as that is the goal of this project.

Next, I measured all of the resistors in the tester.  Again, most were in good shape, with just a few outside of a 5% tolerance.   I had to pull one leg of the 90K ohm precision resistor to get a good reading on it, as other circuitry was causing it to read as nearly a short. I haven’t traced out the circuit to see why that is so.

Nominal Actual
1000 ohm 974 ohm
2000 ohm precision 2.02k
10k ohm 10.37k
22k ohm 21.04k
22k ohm 22.19k
22k ohm 20.43k
22k ohm 21.43k
22k ohm 23.23k
47k ohm 46.4k
90k ohm precision (2x180k) 90.2k
200k ohm precision 202.1k
220k ohm 207.7k
470k ohm 421k
1M ohm 1.007M
10M ohm 9.87M

The good news is that the precision resistors were very close to nominal. This is good, because those values are hard to come by now.  I will replace the resistors that are out of tolerance with new 1-watt metal oxide resistors.

Since there were no obvious failures to this point, I decided to slowly  power up the tester on my variac.  First I wanted to check the transformer, as if it was bad, the tester would not be worth the trouble of refurbishment.  I pulled the 1626 triode and the 1629 eye tube to minimize effect on the transformer.  Then I measured the AC voltage on the three windings.

With 120vac input to the unit, I read the following…

Yellow – Yellow : 14.4 vac
Green – Green: 64.6 vac
Red – Red/Yellow: 566 vac

So it looks like the transformer is good!

After turning the unit off and inserting the two tube, I brought the voltage up again.  It was now time to check the DC voltages of the power supply.

With 115 vac applied, I read the following voltages:

Pin 8 of the 1626 tube: +226.6 vdc (a little high, should be about +150 vdc)
Pin 1 of the 1626 tube: -367.5 vdc (a little low, should be -410 vdc)

Pin 3 of the 1629 tube: +159.6 vdc ( high, should be +80 vdc)

These voltages were not so far out of line that I thought it dangerous to operate, so I continued with testing.

I tested some capacitors and found that the calibration for the value is off by quite a bit on both small and large caps.

Nominal Actual
0.10 uf 0.05 uf
22 uf 10 uf
47 uf 34 uf
100 uf 72 uf
470 uf 220 uf

Next, I tested the leakage test voltage at the bannana jacks on the bottom of the front panel.  These voltage were off quite a bit from the expected value, as well.

Setting   Measured
25 v 39.36 v
150 v 129.3 v
250 v 212.4 v
350 v 291.4 v
450 v 376.8 v

I also tested the unit in Resistance mode with a 47 K resistor.  It read about half of the correct value.

So there are a few things to wring out with the unit.  I’ve placed an order with justradios.com for the capacitors and some resistors. I’ve also ordered a safety capacitor to replace the 0.5 uf going from the line to ground.  I’d hate to have that short out and put the chassis at line potential.

Finally, two of the knobs cannot be tightened enough, as they are cracked.  I’ll have to find a way to repair them, as well.

Broken Knob

I’ll post again when I make the parts replacements and do further testing.