[Boatanchors] And Now for Something Competely Different: Zenith

[Glen Zook]

One thing that I have found in restoring receivers from especially the 1930s is that some circuits, especially oscillator circuits, fail to work when a modern capacitor replaces an old paper type.  It seems that some leakage through the capacitor was needed for the circuit to work.  In those cases, a resistor from around 470K to 2.2 meg has to be put in parallel with the capacitor to get the circuit operational.

Most circuits work fine with the new capacitors.  However, one can definitely go bald from "pulling their hair out" when the oscillator fails to oscillate trying to locate the problem.  I found that adding a high resistance in parallel with the capacitor suddenly got the circuit working again just by trial and error having tried about everything else.
 
Glen, K9STH


Website:  http://k9sth.com


________________________________
 From: Joe Connor <joeconnor53 at yahoo.com>
To: Glen Zook <gzook at yahoo.com>; "Charlie , W5COV" <cvest at cox.net>; David Stinson <arc5 at ix.netcom.com>; "boatanchors at mailman.qth.net" <boatanchors at mailman.qth.net> 
Sent: Saturday, March 30, 2013 1:02 PM
Subject: Re: [Boatanchors] And Now for Something Competely Different: Zenith
 

The quality and sound from these radios is pretty amazing. My theory is that after we restore them, they perform better than they have since they left the factory.

Here's why: back in the '30s, capacitors were a high-failure item even when new. A lot failed quickly and a lot more probably suffered leakage even then. A radio with a dozen or so caps that are leaky to one degree or another isn't going to sound as good as it should. It's sort of a cumulative degradation. If there was one particular cap that shorted or caused a noticeable problem, the repairman would replace it but I've never heard of any repairman back in the day replacing all the paper caps. It would probably have cost more than a new set. Instead, the set was written off as "old" and "tired" and relegated to a dusty attic.

Now, we come in and replace all the caps with high-quality modern caps. We replace the out-of-spec resistors and align the IFs with precision (thank you, frequency counter). The end result is that the radio performs the way it was designed to perform, probably for the first time since it left the factory.