Power Xformer heat query
 

I've got a particularly nice NC-183 in the collection. Although nothing
untoward has happened after a number of continuous hours of operation, I
do notice that the power xformer get noticeably hot.. as in it can't be
touched but for a few seconds. Voltage drop considerations aside (they
can be addressed, after the fact), would using a solid-state rectifier,
in place of the 5U4, provide a noteworthy temp decrease? 15 watts is 15
watts, of course, but I'm curious if anyone's actually made note?
Thanks de K3HVG

Hi,

It isn't too unusual for a power transformer to be run uncomfortably
hot. The temperature rise is due mostly to i2r losses in the primary
and secondary windings, and partly due to losses in the core. The
transformer manufacturers very often run the transformer with a magnetic
flux that is approaching the saturation point of the core. This results
in the cheapest/lightest transformer possible... But it comes at the
cost of cool operation.

If you remove the 15W filament load due to the 5U4, it will cool the
transformer down a bit, but probably not as much as you would like.

Here is a way to tell: Pull the 5U4, and leave the radio on for
1/2 hour, or so (without HV). Check how hot the transformer becomes.
This t rise is better than the best you can do by replacing the 5U4
with a SS full wave rect. If it is substantially cooler, then you will
get an improvement with the SS rect. Otherwise, you should probably
just live with things the way they are.

-Chuck Harris

k3hvg wrote:
I've got a particularly nice NC-183 in the collection. Although nothing
untoward has happened after a number of continuous hours of operation, I
do notice that the power xformer get noticeably hot.. as in it can't be
touched but for a few seconds. Voltage drop considerations aside (they
can be addressed, after the fact), would using a solid-state rectifier,
in place of the 5U4, provide a noteworthy temp decrease? 15 watts is 15
watts, of course, but I'm curious if anyone's actually made note?
Thanks de K3HVG

"k3hvg" wrote in message
...
I've got a particularly nice NC-183 in the collection. Although nothing
untoward has happened after a number of continuous hours of operation, I
do notice that the power xformer get noticeably hot.. as in it can't be
touched but for a few seconds. Voltage drop considerations aside (they
can be addressed, after the fact), would using a solid-state rectifier,
in place of the 5U4, provide a noteworthy temp decrease? 15 watts is 15
watts, of course, but I'm curious if anyone's actually made note?
Thanks de K3HVG


First, have you replaced the filter caps, and the original wax capacitors?
Leaky audio coupling caps will greatly increase the current draw.

What is your line voltage?

If its over 120, I'd suggest using a small 12-volt @ 2 Amp filament
transformer wired in series-buck (or autotransformer) configuration
to drop the AC line voltage at the set to 110 volts. That will help a lot
with the transformer heating!

regards

Peter, k1zjh

In article , k3hvg writes:

I've got a particularly nice NC-183 in the collection. Although nothing
untoward has happened after a number of continuous hours of operation, I
do notice that the power xformer get noticeably hot.. as in it can't be
touched but for a few seconds. Voltage drop considerations aside (they
can be addressed, after the fact), would using a solid-state rectifier,
in place of the 5U4, provide a noteworthy temp decrease? 15 watts is 15
watts, of course, but I'm curious if anyone's actually made note?

I've always used the rule-of-thumb that if you can't keep your hand on a power
transformer for more than a few seconds, it's running too hot. But in the case
of old gear, the transformer may be near the end of its useful life, rather
than overloaded. It's also possible that there is some sort of overload
elsewhere in the receiver, causing excessive B+ current draw that will destroy
components. For example, a cathode bypass capacitor in the output stage that is
shorting out will reduce the bias on the stage, causing excessive plate
current. A leaky coupling capacitor will do the same thing by putting positive
bias on the tube grid(s) (and it doesn't take a lot of leakge!). Filter caps
that are leaking will do the same thing. National tended to use power supplies
that were just adequate for the receiver, so any overload can cause excessive
heat.

Try this: Pull out the 5U4 and run the receiver without it for an hour or so.
The transformer load will then be just the filaments. See how hot the
transformer gets under those conditions. Also measure the B+ voltage and
current under normal operation and see what's going on.

73 de Jim, N2EY

I had a Collins KWM-380 where the xfmr got to hot to touch even after I shed
most of the load. Luckily I found a replacment, and the problem was solved.
I monkeyed with the bad one, poltted temp vs time and worte it up. I came
to the conclusion that it ahd a shorted turn. Any time you can hold your
had on a component except for a tube or power resistor it's running too hot
and something is wrong. As mentined before, shed some load by removing the
5U4 and testing and then a few tubes at a time to see if that helps. If
that doesn't help look for obvious leaks on the heater feed lines, If none
are found my guess is you have an xfmr that has a shorted turn/ turns.
hank wd5jfr
"k3hvg" wrote in message
...
I've got a particularly nice NC-183 in the collection. Although nothing
untoward has happened after a number of continuous hours of operation, I
do notice that the power xformer get noticeably hot.. as in it can't be
touched but for a few seconds. Voltage drop considerations aside (they
can be addressed, after the fact), would using a solid-state rectifier,
in place of the 5U4, provide a noteworthy temp decrease? 15 watts is 15
watts, of course, but I'm curious if anyone's actually made note?
Thanks de K3HVG

I appreciate all the replies, ladies and gentlemen. This receiver has been
re-capped, fully and the B+ draw was nominal. But as many remarked, primary
power could be playing a big part. I therefore installed a 20A variac in
the power distribution setup for all the B/A's and set it for 110v. No more
heat problems. Thanks to all!!

k3hvg wrote:

I've got a particularly nice NC-183 in the collection. Although nothing
untoward has happened after a number of continuous hours of operation, I
do notice that the power xformer get noticeably hot.. as in it can't be
touched but for a few seconds. Voltage drop considerations aside (they
can be addressed, after the fact), would using a solid-state rectifier,
in place of the 5U4, provide a noteworthy temp decrease? 15 watts is 15
watts, of course, but I'm curious if anyone's actually made note?
Thanks de K3HVG