On Tue, 21 Jul 2015, tony duell wrote:
On Mon, 20 Jul 2015, Tothwolf wrote:
Yes, the grid cap would /usually/ be a non-polarized wax paper type,
which tend to be very unreliable. I've yet to find a wax paper type
which will pass a leak test and those are also on my replace on sight
list.
Of course you wouldn't want to replace mica, ceramic, or plastic film
parts without good reason, but if a set is going to be more than just a
shelf queen, aluminum electrolytics and wax paper capacitors are a
It depends a lot on the circuit. If replacing the capacitor is going to
involve major realignment and the original is probably OK and leakage is
not going to do further damage (likely in the case of a tuning
component) then I will leave it and only replace if it fails.
IMO an alignment is simply part of the restoration process. When I service
a set, I do so expecting that it is going to be used and thus needs to
have an accurate dial vs just sitting on a shelf. Simply installing
replacement aluminum electrolytics and wax-paper capacitors is not likely
to affect alignment. It is extremely common however to find sets where
someone else has previously mucked up the original alignment in an attempt
to work around electrically leaky wax-paper capacitors which have caused
the band the drift.
must-replace item. Carbon film resistors in this sort of equipment
should also be tested, however I only replace those which are either
bad or out of tolerance (some brands held up better than others).
This is inconistent. A capacitor which is failing (starting to leak,
say) may get worse. A resistor which is drifting may get worse. Either
can do more damage when it fails. Why replace the cap and not the
resistor?
Why is that inconsistent? If I test a carbon comp resistor and it measures
within spec, there isn't much reason to replace it. Unlike an aluminum
electrolytic capacitor, a carbon comp resistor is very stable chemically.
Carbon comp resistors tend to drift due to absorption of moisture, and
while it is possible to dry one out in a toaster oven at a controlled
temperature, the resistor will again drift out again over time, so if one
is out of spec, replacement is the best option.
I probably would replace certain safety-related capacitors in live
chassis sets, like ones that isolate external sockets, using class Y
replacements. But that;s about it.
That's a good idea, however something to keep in mind is that class Y
safety rated capacitors are not designed not to short (and not put say a
I thought that was the difference between class X (will fail in a safe
way, but may short) and class Y (will not short). The latter are to be
used where 'failure of the capacitor may expose a person to electric
shock' according to the data sheets I've read.
In general class X go across the mains, class Y from mains to ground.
Except that the chassis in modern equipment is /expected/ to be connected
to ground, unlike a floating or hot chassis in a vintage radio. Both class
X and class Y can fail short. A class Y tends to have a thicker dielectric
and/or a lower voltage rating, which means it is less likely to fail
short, not that is cannot fail short.
I consider replacing aluminum electrolytics to be preventive
maintenance. One wouldn't drive a 20-50 year old car with original
hoses, belts, and tires, and IMO it is just common sense to replace
electronic components such as aluminum electrolytic capacitors which
have extremely well documented life expectancies and failure rates.
I do wonder if this data is based on the cheaper components used in
consumer electronics (paticularly things like AA5s) and that the
capacitors used in computers were of a much higher quality and longer
life.
Possibly. Radio repair shops of the AA5 era also had a vested interest in
turning a set around as quickly (and as cheaply) as possible, and a set
back in again in the same year for another repair was also good for their
business. Back then, consumers expected their radios to need "routine"
service, so people were less likely to even question it. I've come to this
conclusion based on the types and quality of radio shop repairs I've seen
in these old sets. I have a radio in my to-do queue right now (an AA5)
which was owned by my grandparents, where a shop needlessly hacked the
leads off a Centralab hybrid module and replaced about half of its
functionality with some really cheap wax-paper capacitors and a handful of
resistors (after searching for a number of years, I actually managed to
find a NOS module for it, so that part of the circuit will be restored to
its original condition when I eventually get to that project).
As far as shotgun-repairs go, one of my own pet peeves are those out
there selling "cap kits" (usually really low quality [sometimes
counterfeit]
Oh don't get me started....
Cap kits or counterfeits? ;)
Best way to avoid counterfeits...do not buy modern name brand aluminum
electrolytic capacitors on eBay. When I've looked, it tended to be about 9
out of 10 capacitors were counterfeits when I searched for Nichicon or
Rubycon parts.
I wouldn't even really mind high quality cap kits for some projects, but
those selling them usually make them from the absolute cheapest parts they
can source, including the fake junk found on eBay, which includes
counterfeit semiconductors (transistors and voltage regulators).
capacitors, too) to newbies which also include a bunch of
semiconductors (diodes, voltage regulators, and transistors) on the
theory that those parts fail because they run hot. I've gotten to the
point where I will not even attempt to service a board which has been
botched up by a fat fingered newbie who has attempted to install one of
those kits.
There is however one component besides certain capacitors which I
absolutely will replace on sight, no exceptions, period, and those are
selenium rectifiers. There is nothing good that can be said of selenium
rectifiers, and it is absolutely trivial to solder in a silicon diode
as a modern replacement.
In some cases you need a series resistor to compensate for the forward
resistance of the selenium rectifier or the output voltage goes too
high. Particularly in those mains/battery valve radios with 1.5V
directly heated valves that have filament burn-out if you look at them
wrongly...
But yes, selenium rectifiers rarely work now (although there are
exceptions) and when they fail they can take out the mains transformer.
And they smell horrible (think of school dinner cabbage!)
They fail in the worst possible way, a rotten garlic smell that seems to
completely permeate a room, which can take days, and sometimes weeks for
the smell to completely clear. Not to mention how poisonous selenium
itself is, which is best not released into an indoor environment. Since
the selenium rectifier has basically zero reuse value, unless it is burned
up, I usually just leave the old rectifier stack attached to the chassis
and disconnect it.
