Fuse Blows
 

I repaired a circa 1970's tube Panasonic portable tv. There was a
blown power line 'slo blow' fuse. This set also had the infamous
"insta-on" feature that kept the tube filaments lit while the set was
off. I disabled this feature by snipping a diode across the on-off
switch. The set played fine for about four days then the fuse blew.
I replaced with another that promptly blew as the set warmed up.

Any ideas on what could be suddenly drawing too much current/

Mike

Ish wrote:
I repaired a circa 1970's tube Panasonic portable tv. There was a
blown power line 'slo blow' fuse. This set also had the infamous
"insta-on" feature that kept the tube filaments lit while the set was
off. I disabled this feature by snipping a diode across the on-off
switch. The set played fine for about four days then the fuse blew.
I replaced with another that promptly blew as the set warmed up.

Any ideas on what could be suddenly drawing too much current/

Mike


There are lots of possibilities. A shorted capacitor. An internal
short in a tube. Shorted windings in the flyback or power transformers.

Start by pulling all the tubes (making sure you know where they go)
and see if the fuse still blows. Then replace the rectifier tube
(if there is one -- it might have silicon rectifiers). Then add the
horizontal output tube (typically the biggest tube) and associated
drive tubes. At some point the fuse will blow and you'll have a
better idea what section is malfunctioning.

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