On Wed, 9 Mar 2016 17:48:50 -0000 (UTC), Cursitor Doom
wrote:

On Wed, 09 Mar 2016 00:20:35 +0100, Dimitrij Klingbeil wrote:

There's no need to unsolder, neither to actively avoid unsoldering. That
resistor is connected through a diode on the board. Just apply the
proper polarity signal, and the diode will take care of the isolation.


Hi Dimitrij,
Actually I did unsolder it in the the end as I wanted to inspect the
underside for any clues as its specifications. Sadly there were none
visible all around the device. Clearly all was not well with it and I'm
inclined to believe that it's not an original part; just another poor
replacement.
When I removed it from circuit I was happy to give it 2W worth of power
and see what happened. It just got hotter and hotter until I had to pull
the plug at 120'C because my probe won't go any higher. In contrast, a
known good 20ohm 2W resistor fed with the same 6.3V rose to 70'C and
stopped at that. I have now replaced the old brown 'thing' with a 6W
ceramic power resistor which after half an hour of running hooked up to
the scope and under load only gets up to 40'C which I'm entirely happy
with. The transformer is making a nice gentle, barely audible hiss; not
that rasping sound it used to before your diode was replaced; all the DC
Vouts are fine and there's no acrid scorching smell coming from the board
any more; just a pleasant warm 'smell' of electronics behaving
themselves.
I now feel confident enough to re-insert the board and connect up the VHT
and EHT and heater supplies - then we'll see if there's any life in the
tube! But that will have to wait until tomorrow or I'm looking at divorce
again, apparently.
Later....

From previous traffic on this issue:

"If these are the maroon-colored parts, they are Philips flame-proof
parts designed to run with body surface temperatures in excess of
175C.

"The long preformed leads are thin dia steel, with poor thermal
conductivity, in order to reduce thermal conduction to the printed
wiring.

"Your real concern should be the temperature of film caps and
insulators in the immediate viscinity, which have a lower tolerance to
overtemperatures. They should not touch.

"Book hot spot limits for Philips PR01, PR02 and PR03 is between 220
and 250C, depending on the series. This is typical for later metal
glaze films. Book derating for normal use is linear, to zero watts at
150C ambient."

Larger bodied resistors with the same power ratings may run at a
lower body surface temperature, but this will be of little value
if their bulk results in reduced air flow or physical contact
with nearby components.

This position should not be filled with a wire-wound component.
Metal glaze, at certain values and body sizes, offers a reduced
self inductance, allowing them to function in higher frequency
snubber circuitry.

RL