On Sat, 04 Feb 2017 04:28:09 -0600, oldschool wrote:

On Fri, 3 Feb 2017 22:05:48 -0000, "Benderthe.evilrobot"
wrote:


True. But the reason to replace them ALL is that if you only replace
the one(s) causing a problem, another will fail later, then another
still later. It is much easier to do them all at once than to have to
repair the same unit over and over as they fail one after another.
Been there, done that.

So have I.


I can totally understand what you are saying. However, I am not so sure
about whether I would want to replace all of them in certain parts of a
radio or tv. Here are my thoughts on this.

Lets take my Hallicrafters SX-99 (which I paid for but dont have yet).
That radio is 62 years old. (made in 1955). The seller said it works
fine, (and I was sent a video of it working. I did detect a very slight
hum. (But the video's audio is not the greatest).

So, as soon as I get it and play around with it, I will replace the
electrolytics in the power supply and any other 'lytics (if there are
others). Even if there is no hum, I'd replace them, just based on age.

However, this is a working radio. I ask myself if I really want to
replace all the other (small) caps. [Then I say to myself.... if it
works, dont fix it].

But, I know there are caps in specific circuits more likely to fail,
than in other circuits. Those would be caps connected to the high plate
voltages, especially at the audio output tubes and in the power supply.
I also know that if those short out, they can damage other parts, such
as tubes, resistors, and more. So, I would likely consider replacing
those. I might even consider replacing ALL the caps in the power supply,
and all audio stages, and feel safe doing that, since those caps are not
real critical as far as affecting the overall performance of the radio,
even if the new caps are a little different in their capacitance.

Where I do NOT feel comfortable changing them, are in all RF and IF
stages. The reason is that I know that a cap/coil circuit plays a big
part in the inductance, which affects the radio alignment. I am not
equipped to align the radio coils, and would prefer to leave them alone,
as long as the radio is getting good reception.

I also know that those RF and IF stages do not operate on as high
voltages as do the audio output stages. So, once again, I ask myself,
"Do I really want to risk throwing this radio out of alignment, when
it's working fine, and knowing those caps are not as likely to fail".

I answer myself "Probably not".... (As long as the radio is working
well, dont screw up a good thing.... Then too, if the caps in those
circuits are .05 or .003, I WANT a .05, not a .047. (And it seems that
.05 is no longer made).

So, if I'm satisfied with the performance of this radio, I will replace
the 'lytics regardless. I may also replace all caps AFTER the volume
control, as well as all caps in the power supply. But I will likely NOT
touch any caps in the RF and IF stages.

That's my thinking on all of this right now. Not just for this radio,
but anything..... Now, if it were simply an audio amplifier, I'd likely
replace all the caps, because audio is not all that critical.

I think that there is a misunderstanding here. The statement to replace
all the capacitors actually means all the electrolytic and paper
capacitors. Usually ceramic and mica capacitors are still good and
nobody replaces those unless they are proven to be bad.

The paper capacitors in the IF and RF stages need to be replaced even if
the radio "works". Leaky capacitors change the voltages on the tubes
causing them to work at less than their best performance. Also, these
capacitors are used as supply bypasses and AVC filter capacitors. They
do not affect the alignment of the set; the ceramic and mica capacitors
may. Their value is not particularly critical; pick the closest modern
value.

Consider the value issue. If the radio has a .05 uF, 20% capacitor in
it, its actual value can be anywhere from 0.04 uF to 0.06 uF. A modern
0.047 uF 10% capacitor can be between .0423 uF and 0.0517 uF. So the
0.047 uF capacitor can be closer to 0.05 uF than the old one marked with
that value.

As for life expectancy, there was a time in the late '50s and early '60s
when both paper and plastic film capacitors were used. The paper
capacitors I have from that period are universally bad while the plastic
film ones are almost always good. Plastic film has passed the test of
time.

As for what type of plastic film to use, polyester (AKA Mylar) is the
cheapest and has the poorest performance. But it is still better than
the paper capacitors of old so it is suitable for use just about anywhere
a paper capacitor was formerly used. Polycarbonate (no longer made) and
polypropylene are better but more expensive. Polystyrene capacitors are
also very good and inexpensive but are usually seen only in small values
and are frequently not seen at all. They also have the problem that they
melt at lower temperatures than other plastic capacitors and solvents
dissolve them. Still, within their limitations, they are excellent.

--
Jim Mueller

To get my real email address, replace wrongname with dadoheadman.
Then replace nospam with fastmail. Lastly, replace com with us.