The Fine Things are Always Hand Made
I finally have gotten up the courage to start working on my 30 tube Scott Philharmonic BOL radio that I got last year. I asked some initial questions last year but now I'm finally starting to work on the amp and chassis.
I'm starting with the amp/power supply first. It looks relatively untouched but I found discrepencies with schematics that I have. My amp uses dual 5Z3 rectifier tubes. Schematic shows input cap should be 30uF but the can capacitor mounted on top of the amp is a 4uF, 600V. I believe it to be a replacement because it is different than the other three but I'm puzzled about the low value. Should I go back to a 30uF, 500V capacitor?
I also started removing tubes from the radio chassis. I ran into one puzzle (so far). The 6B8G tube in the record scratch supressor circuit has been replaced with a 6SN7 and the grid cap wire that would have gone to the top of the 6B8G is gone. Any thoughts on this? The 6SN7 is a dual triode while the 6B8 is a pentode/dual diode tube. I'm no electronics expert but they don't seem to be the same??
Norm - what instrument do you use to measure the cap values?
I have a choice of an EICO 950-B I recapped ... and the big Hickok 209 A. For capacitance, neither is real accurate, so when I care, I compare to new stock. The big revelation was using the EICO 950-B to check for leakage under real voltage instead of just using my VTVM. The old Sprague black beauties seem to be particularly bad now.
For capacitance I use a late model digital capacitance meter that I inherited. I believe it is an early Tenma digital instrument. Generally I am not a fan of digital instruments but based on use I have developed great confidence in this instrument. Before inheriting the digital capacitance meter I relied on a General Radio 1650A bridge for accurate capacitance measurement. For leakage and for reforming electrolytic capacitors I use a Sprague Teleohmike TO-6A.
Regarding Sprague "Black Beauty" capacitors, it is my understanding that only the black cased capacitors with red printing were called Black Beauty capacitors by Sprague. These are Mylar and paper "Difilm" capacitors and are very reliable. The Mylar layer is responsible for very low leakage and high reliability. The black cased Sprague capacitors with yellow printing are wax-paper and with silver printing are paper in oil. Both of these types are very leaky and unreliable today, the paper in oil capacitors due to cracking and evaporation of the oil. The black cased Sprague capacitors with colored stripes (AKA "bumble bee") are OEM parts almost all of which are wax-paper but some of which are difilm and paper in oil. They must be tested individually. The only place I have found difilm "bumble bee" capacitors are in the high voltage circuits of a late McIntosh MC-275.
Progress Report: I'm starting to work on the top of the chassis. I've removed the coil and tuning cap covers. I see some original and some replacement caps. I plan to replace all of the paper caps and check the resistors. One cap , in particular, on the rearmost IF coil (IF #1?) seems to have the wax melted out of the bottom of the paper cap. I hope that didn't do any damage. I've attached a couple of photos. Still, progress is being made!