I was using my Cornell Dubilier BF-50 (vintage) capacitor analyzer with all of the right things showing on the “Magic Eye” 6E5 (Philips JAN (Mil-Spec)) electron tube’s eye for leakage, short, and open tests. By the way, the BF-50 uses a 12A7 (Tungsol) electron tube for both rectification of the AC input and amplification. Both of the tubes are fairly new purchases, both tested at or above NOS ratings on respectable tube testers. When I went to switch the BF-50 to discharge mode I noticed something, nothing was in the test leads, meaning it has just tested an open circuit! This immediately brought the unit’s validity into question for me. This unit is partially made to test low value capacitors that we now know, isn’t the easiest thing to do; (None of your run of the mill ESR meters on Ebay or Amazon for under $50 can test a capacitor below 1uF) Silver Mica, Ceramic and “Paper Caps” are examples of such capacitors. This unit is able to test down to insanely low capacitance levels, with help from two electron tubes and a dedicated ground to ensure accurate voltage (no moving or dirty ground). It also tests electrolytics up to a limit of I think 500uF or so, but that doesn’t matter nearly as much to me. I trust the newer technology for testing higher value capacitors, very few of them can truly test a low value cap now.
Update: After drilling out these old school rivets I was able to get to the very bad looking electrolytic capacitor at the bottom of the device. The .5 MFD 600 Vdc with what looks a cork about to blow.
Finally, after I removed this suspect looking 0.5 uF capacitor, it actually reads out on my Extech 380193 LCR meter as okay for capacitance, ESR and quality rating! I am still going to replace it, as I don’t trust it.
To get us started, here is the Cornell Dubilier BF-50 Owners Manual (hosted here at Hallman Labs).
Cornell Dubilier Official BF-50 Schematic w/ Part Values
The question with repair comes to this, do I need another unit like mine in order to verify which capacitors are the issue? Or is it possible the unit did take an open as satisfactory due to the low capacitance range it was in? I don’t think so, but it’s worth asking. Once I know which capacitor/s and/or resistors are the issue, the decision turns to how nice of a capacitor do you replace it with? Sprague Atoms? Sprague Bumblebees? Cornell Dubilier’s own capacitors of the same type (I have them)? I have provided a schematic as well as a link to where you can find the owner’s manual on my website. I appreciate any input on the subject. I know to replace the caps with ones of the same type and stock rating, but just wondering which parts are the most important for this design to function accurately.
If I need another unit to repair/check, should I be looking for a place with a working Sprague Tel-Ohmike on hand? Or could I get by using my ($200) Extech 380193 LCR meter? Extech claims the device can read down into the sub-pico range for capacitors. This device can give you quality, dissipation and ESR alongside the capacitance measurement. It can do either 1kHz or 120Hz. The less things change from the frequency, I assume that is what we want. What it really specializes in is measuring inductance, but it’s not in the scope of this discussion. The quality and dissipation always change due to frequency. I also have a SDS1202X-E 200Mhz 2 Ch digital SPO oscilloscope along with a DDS Function Generator (programmable and random waves available, with TTL output).
Note: this is what a Sprague Tel-Ohmike “TO-6” looks like in comparison. It is a little fancier and usually runs triple what I paid for the BF-50.
Thanks for reading and please let me know if you have any ideas or thoughts on part selection for a unit like this.