On Nov 19, 9:57 am, "N Cook" wrote:
In a valve Dansette Bermuda of early 60s
Been in a loft for years.
The mains wiring is perished and other wiring looks dodgy so testing some
things cold first.
Using 9V battery and 300 ohm dropper shows near enough 9V over the
rectifier, how high V do you have to go with a good one to check it ?
Are there safety grounds anyway for changing to a 1N4007 or so ?
Single element rectifier
Siemens lazy S logo
Made in Germany
2250c50
Kc 0.6e 11/16

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list onhttp://home.graffiti.net/diverse:graffiti.net/

Mpffff... let's try to answer the question, at least a little bit.

a) The OEM selenium diode is prone to failure. When it fails it does
so in a spectacular manner such that you will offend any and all
parties within smell-range and at the same time release fumes that are
toxic (but by no means as toxic as they smell). So: Get rid of it.
Better yet, leave it in place for appearance but hide the new silicon
diode behind it.

b) There is a significant voltage-drop across the diode. In battery
sets (typically with 1.4V tubes, not accounting for this drop can
either greatly shorten tube life *or* blow filaments altogether.

c) As selenium diodes age, they increase in resistance. Many battery
sets (with specific reference to the post-war, miniature-tube Zenith
TransOceanic series) will fail due to too-low filament voltage from
this cause. On the other hand, with some of their mains-only radios,
with specific reference to the post-war AM/FM units, this is not a
major issue.

So: Yes, replace the diode. Add a resistor in series to get the proper
filament voltage if this is a concern. For the T/O series, that can
work out somewhere between 30 and 80 ohms @ 2 watts (use 5 if you can)
or better. Such a resistor is harder to conceal than the diode, but it
does make the radio functional with reasonable safety. But if the down-
line components from the rectifier are not voltage-sensitive, you may
not have to add the dropping resistor.

Now, keep in mind that if you are replacing a single selenium diode
(i.e.: NOT a stack), the forward voltage drop is insignificant such
that no additional resistor is required. Or, you can put a zener diode
across the filament string (if so equipped), to protect the tubes.

Peter Wieck
Wyncote, PA