Sunday, July 23, 2017

Fixing up an old Heath C-3 Condenser Tester


I brought my C-3 over to test the capacitors we removed from the old Alto power supplies, but the unit failed in transit as the magic eye tube wouldn't glow. As the only output to indicate the results of a test, that meant the unit was unusable. 

I discovered several flaky things when I looked into the unit at home. It uses a transformer generating nominally 500VAC through a 1626 tube rectifier to produce both a high negative voltage for testing capacitor leakage current and the B+ voltage of 130 to 200V for the 1629 magic eye tube. It has a resistor that establishes the ground point and therefore divides the DC into the + and - amounts. The two sides have electrolytic capacitors rated at 8uf 475V.

Unfortunately, the transformer is producing more like 600VAC. Instead of dividing 660VDC into -450 and +200, already close to the capacitor rating, it is producing over 600V negative and my B+ was under 100V. I have one problem that is causing the current through the dividing resistor to be too low, which causes B+ to be too low, but that also exposes the other cap to way way more than its rating.

It is very hard to find capacitors at 8uf or higher with ratings of 675V or higher, thus I decided to put two 16uf 450V caps in series and include some high resistance bleed resistors to ensure voltage balance. The target current will be around 0.5 ma in the higher side, so that I will burn only 180mw with that resistor but ensure that two caps divide the voltage evenly. That means I need two 680K 1/4w resistors. The low side will burn about 60mw.

On recommendations from others who have refurbished these, I am replacing the main voltage divider string of resistors on the 'Scale' switch with 2W units as the existing 1/2W parts are close to rating at the high setting and can be damaged when testing caps with high leakage current.

To wrap things up, I ordered new 1626 and 1629 tubes. I will do some cleanup of the unit while waiting for my parts to arrive.

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