October 2014: "1963 Zenith MK2670 Stereo Repair"

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Dean Huster
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Joined: Wed Dec 05, 2001 1:01 am
Location: Harviell, MO (Poplar Bluff area)

October 2014: "1963 Zenith MK2670 Stereo Repair"

Post by Dean Huster » Wed Oct 08, 2014 6:10 pm

Mention is made of replacing paper-dielectric capacitors because leakage (electrical, not chemical) causes heat and heat makes them explode. No mention is made for the major reason to replace these nasty paper-dielectric caps: the same leakage that allows the passage of DC in coupling/blocking cap applications. If the coupling cap from the plate of a preamp to the grid of an output amp (single-ended or push-pull) has leakage, this causes the output tube(s) to turn on .... HARD .... drawing maximum plate current through the primary of the output transformer. A very common problem (I'm facing at least four as I write this) is an open primary on an output transformer caused by a leaky paper coupling cap, both of them push-pull. I don't have these transformers laying around collecting dust like I did as a kid back in the 1960s. They're expensive to find, buy and replace.

Note should be made that you need to leave all of the mica and ceramic caps alone, especially in the front end of a radio receiver. These are often frequency-determining components, rarely (if ever) fail or have leakage, and often tightly placed in crowded circuitry where component movement can mis-align circuits. Only replace paper caps and do all of them.

In the olden days, the unit of measure used for small-value caps were "micromicroFarads", abbreviated µµF,µµf, uuF,uuf, mmF, mmf or MMF since "µ" wasn't a commonly available character and the "µ", "m" or "M" was interchanged at will since everyone knew we were talking about "micro" and not "milli" or "mega" since we were dealing with capacitors. A "micromicrofarad" is the same as a "picofarad". These days, we DO deal with millifarads, especially in Europe and with our fancy little digital capacitance meters. These combined prefixes were not unusual prior to 1970 as millimicroseconds were used rather than nanoseconds; kilomegacycles were used instead of gigaHertz.

Values of paper caps are rarely required to be exact values. Old radios had non-standardized values such as 0.2µF, 500µµF, 0.025µF or 0.005µF. 0.05µF was probably the most common value used. Values can be sloppy, as most are used in "bulk capacitance" applications such as with coupling/blocking capacitors and decoupling capacitors. The industry has since standardized all but custom component values. When replacing the old caps, you're working in the parts catalogs with our modern standard values of capacitors with the significant figures for each decade usually being confined to 10, 15, 22, 33, 47 and 68 (the 20% standard resistor values) or with tighter-tolerance units, 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82 (the 10% standard resistor values). Choose the nearest standard value in most cases, the next highest if it's for power supply filtering or decoupling uses. If you need to replace a 0.05µF cap, use a 0.047µF version; 0.22 for a 0.2; 0.0015µF or 0.0022µF for a 0.002µF. It's not that critical in most cases. In hi-fi amplifier tone control circuits, you might want to use tighter-tolerance components (the 10% resistor standard values) so you can get closer to maintain the filter characteristics. Just understand that values adhering to the 10%, and especially the 5% resistor standard values, are going to be a lot more difficult to find and cost you a lot more. Choose wisely.

A fabulous forum for antique radio and antique test equipment repair buffs is at http://www.antiqueradios.com The folks there are most helpful and it's a BIG forum with lots of posting. A search of topics will likely turn up the exact model you're working on.
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).


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