Monday, April 4, 2011

Drake R-4b Calibrator power supply

Top C190 Bottom C192 originals 

The calibrator in the R4b was sounding very raspy until it had been on for a few min.  (maybe 10 min) and then it worked normally.  I decided to order some more cap with the recent DigiKey order and today I replaced C190 with a 25V 220uf Digikey # 4035PHCT-ND and C192 with a 16V 1000uf DigiKey # P13119-ND and the Calibrator worked right away and sounded good.

I tested both of the caps on the Heath C-3 tester and the 1000uf was definitely leaky and would not provide a good null on the bridge.  The 200uf 6V original provided a good null at about 150uf but the min leakage test on the C3 is 25V and it definitely fails that which is probably normal for a 6V rated part.

The 1000uf is under the power transformer and it was not too difficult to get to and is the probable real fix.  The other cap is on the Cal board and I removed the 2 screws holding the board to the chassis and then the cap was readily accessible .

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