What is generally indicated when a power transformer is very hot, and
the 7w power transistor in its vicinity is also very hot? The radio
works but will often "go quiet" after 10 minutes or so of operation.
Any suggestions would be greatly appreciated.
Pete

On Mar 9, 5:00 pm, wrote:
What is generally indicated when a power transformer is very hot, and
the 7w power transistor in its vicinity is also very hot? The radio
works but will often "go quiet" after 10 minutes or so of operation.
Any suggestions would be greatly appreciated.
Pete

I meant resistor, not transistor

I'd bet on shorted caps.
If you keep running it like this you'll
soon have a burnt up power transformer too.
New caps will be a heck of a lot easier to find
than a replacement power transformer.

Ron

wrote:
On Mar 9, 5:00 pm, wrote:
What is generally indicated when a power transformer is very hot, and
the 7w power transistor in its vicinity is also very hot? The radio
works but will often "go quiet" after 10 minutes or so of operation.
Any suggestions would be greatly appreciated.
Pete

I meant resistor, not transistor

"Ron in Radio Heaven" wrote in message
...
I'd bet on shorted caps.
If you keep running it like this you'll
soon have a burnt up power transformer too.
New caps will be a heck of a lot easier to find
than a replacement power transformer.

Ron


Good possibility Ron. Not only that but these old rigs were designed to run
on lower line voltage and with the higher mains voltage we see today only
pushes these old rigs harder. Beefing up the components in the power supply
wouldn't hurt but because the old iron in these was designed for the lower
voltage, it doesn't hurt to run our old rigs on a variac or power supply
set up 110v etc..

BH

I'll second that. Both my NC-183,183D, and various other vintage rigs
ran very hot at my nominal line voltage, here, of about 120-125 volts.
I now use a Variac in power distribution line for the older radios and
set it at 110v. The drop in temp of the transformers was dramatic....
There may, of course, be other problems like leaky caps, etc. so a
complete check may be necessary. de K3HVG

Thanks. I went and replaced the electrolytics. Replaced the 330 ohm 7w
with a 500ohm 25 watt. The 7 watt was swinging beyond 1K when hot. All
the other caps are mylars. The 12BE6 socket had a 6BE6 there. I dont
have a 12BE6, so I sent the 6V filament rail to that socket and will
run it with a good 6be6. Not the best remedy, but you know how it
is! Powered up and got some reception on the two lower bands, nothing
on the top three. My changes may have knocked the alignment way off,
so I'll realign and see if I can "wake it up", powering thru a variac
at 110. Any comments are appreciated.
Pete

wrote:

On Mar 9, 5:00 pm, wrote:

What is generally indicated when a power transformer is very hot, and
the 7w power transistor in its vicinity is also very hot? The radio
works but will often "go quiet" after 10 minutes or so of operation.
Any suggestions would be greatly appreciated.
Pete


I meant resistor, not transistor


There are two likely situations as to why a transformer runs 'hot'. The remedies
have been briefly discussed. Let me add some background.

First, the design of the transformer uses a magnetic material that is
non-linear. As the magnetizing force is increased to higher levels the B-H curve
goes sub linear and the cores starts to saturate [the self inductance decreases]
and more primary current is drawn into the transformer [more heat].

The transformer equation for 60 Hz sine waves is E = 4.44*f*N*B*A. Where f
equals line frequency, N equals the number of turns on the primary, B equals
flux density, and A equals cross sectional area of the core primary. Since f, N
and A are fixed, an increase of voltage requires an increase in flux density
[b]. B equals the permeability [u] time the magnetizing force [H]. The value u
is very non linear and decreases in transformer material as H is increased to
higher values above the 'linear' region.

Conclusion, transformer get hot. Solution: reduce the primary voltage as
suggested in other responses.

Second, older designed electrolytic capacitors used a 'wet' dielectric. Simply
because capacitors get old [don't we all] the degree of 'wetness' dries out and
the dielectric dissipation factor increases. The capacitors, in common language,
become lossy. The isolation resistance can change from millions of ohms to a
value of a few tens of ohms.

Conclusion, I^2*R losses increase in the lower [lossier] resistance. Some
capacitors can be reformed. If there is large design margin, spec value divided
by stress value [eg; 450 VDC rating/200 VDC operating condition], then it is
possible to 'reform' the capacitor by gradually increasing the operating voltage.

Preferably, the older electrolytic capacitors should be replaced.

Either or both situations should be checked to assure that the 'hot' transformer
is not over stressed.

Note: transformers typically run at 95 percent efficient. So, some heating is
normal.

K3HVG wrote:
I'll second that. Both my NC-183,183D, and various other vintage rigs
ran very hot at my nominal line voltage, here, of about 120-125 volts. I
now use a Variac in power distribution line for the older radios and set
it at 110v. The drop in temp of the transformers was dramatic....
There may, of course, be other problems like leaky caps, etc. so a
complete check may be necessary. de K3HVG


One trick you can use to achieve 110V from a 120V nominal power line is
to take a 12.6V filament transformer with a secondary current rating that is
greater than the current drawn by the load, and hook it up as a series
bucking auto transformer... The primary goes across the 120V power line,
and the secondary is in series between the power line, and the radio to
be powered. Pick the 12.6V secondary's polarity so that the voltage drops,
rather than raises. If 12.6V is too much of a drop for your situation, you
can always use a 6.3V transformer to get 115 to 117V.

Much cheaper than a variac.

-Chuck

The changes you have made should not knock the alignment off. I generally
like to fix the problems before alignment. Alignment is generally "gilding
the lilly".

It is not uncommon to acquire a radio that is off because of alignment.
But, even if that is the case, it often means that there was another
unsolved problem which precipitated alignment.

You can usually tell when an alignment has been attempted by someone not
qualified as they leave their calling cards on the adjustment points.

Once the basics are corrected, you can usually use a signal applied at
various stages to check stage amplification. If the radio is working on the
lower bands, that is a good start. Look for switch contacts, or high
frequency oscillator failure (I have had that with flat 6C4 tubes) or even
the mixer tube. Then, carefully check the alignment - being sure to follow
the alignment manual and always makr your starting point before any
adjustment.

The previous comments have been good.

Colin K7FM

"Chuck Harris" wrote in message
...
K3HVG wrote:
I'll second that. Both my NC-183,183D, and various other vintage rigs
ran very hot at my nominal line voltage, here, of about 120-125 volts. I
now use a Variac in power distribution line for the older radios and set
it at 110v. The drop in temp of the transformers was dramatic.... There
may, of course, be other problems like leaky caps, etc. so a complete
check may be necessary. de K3HVG


One trick you can use to achieve 110V from a 120V nominal power line is
to take a 12.6V filament transformer with a secondary current rating that
is
greater than the current drawn by the load, and hook it up as a series
bucking auto transformer... The primary goes across the 120V power line,
and the secondary is in series between the power line, and the radio to
be powered. Pick the 12.6V secondary's polarity so that the voltage
drops,
rather than raises. If 12.6V is too much of a drop for your situation,
you
can always use a 6.3V transformer to get 115 to 117V.

Much cheaper than a variac.

-Chuck

Another trick that works well on smaller transformer-powered
receivers is adding wire wound power resistor in series with
the transformer primary winding. I use Dale chassis-mount
30-ohm resistors (the style with tabbed aluminum shells)
for a lot of early Zenith restorations.. Often the filament
voltages are pushing 7 volts, and the transformers running so
hot you can't touch them--and that's after replacing all caps.

Pete k1zjh