Last month, we looked at the problems paper capacitors can
cause in vintage radios, often because they have become electrically leaky.
Paper capacitors are troublesome and require replacement more often than other
components, although perhaps not as often as many people believe.
We also looked at the Healing R401E, a vintage radio receiver
that can operate successfully with quite leaky capacitors. This month, we take a
look at the Healing 505E, which isn’t quite so forgiving.
The Healing 505E
As shown in Fig.2, this set is quite different to its older
brother. We’ll start by considering capacitors C5 and C12. These are screen
bypasses and the leakage across C12 should not be less than 20 x R4 (ie, 20 x
100kΩ) which is equivalent to 2MΩ. If C12’s resistance is much less than this,
the voltage on the 6BA6’s screen will be noticeably less than intended and the
performance of the set will suffer.
By contrast, C5’s leakage can be somewhat greater (less
resistance), as R2 is only 22kΩ.
A high-voltage tester is necessary for testing capacitor leakage resistance.
C4 (the AGC bypass) is supplied with AGC voltage via R7 (1MΩ)
and both the 6BE6 and 6BA6 valves receive back bias via a combination of R7 and
R8. If C4 were to become leaky to any extent, the bias on the valves would be
reduced. As a result, they would work harder and the set could become unstable
Basically, if C4 is leaky, the voltage across R7 increases. If
the leakage is bad enough, little AGC bias will be applied to the two valves and
this will cause distortion and other problems.
In fact, I have always considered the AGC bypass capacitor to
be a very important. In this case, it should have a minimum leakage resistance
of 20 x
(1 + 1)MΩ, or 40MΩ (R7 and R8 are both 1MΩ resistors).
My practice is to replace the AGC capacitor without even
testing it and I like it to have a leakage resistance of at least 100MΩ. In
fact, I usually replace AGC bypasses with 50V disc ceramic capacitors. They are
reliable and easily hidden under other components.