We had an old tiny 1972 B+W 200mm/8" Magnavox TV in our kitchen for
years. When it finally died a year ago, I replaced it with a small
color set, but always wanted to get the set going again. The set
could also be 12V battery operated.

Started out tonight checking the 12V power supply by substituting a
14V bench supply. Same symptoms, very blurry picture. The output of
the regulator was right at 12.2 volts, and the B+ Boost was at 305
volts, almost right on the 311V shown on the Photofact 1295. Dragged
out my High Voltage probe, it seemed to be only 4.5 KV on the CRT, at
any brightness setting. The other CRT voltages were right on.

I figured the HV transformer had died, but decided to substitute the
solid-state HV rectifier with an old solid-state unit from a 25 kV
rectifier from a color set. I figured even with the higher forward
drop of the substitute diode since there were many more individual
diodes in the 25 KV rectifier, I might get lucky since the current
drain would be much smaller. The diode was about 4 inches long, 3/4
inch diameter, with bare ends like the caps on an old horizontal
output tube (for those of you old enough to remember what they looked
like).

After cutting out the old diode, and putting the new diode in the
circuit with clip leads and some judicious use of plastic spacers, I
turned the set on. 9.2 KV high voltage right on the spot with the
Photofact 9.2 - 9.7 KV. Now I had to figure out how to mount the new
diode permanently, since it was twice the length of the old diode and
would not fit in the original plastic clips.

I finally connected the new diode to the old leads with alligator clip
lead insulators covering the ends of the diode. Additional electrical
tape provided further insulation, and I used still more tape to put
the diode in the middle of an open space away from anything. Done- so
I thought ......... wrong !!!!.

Now the sound was garbled, and it had been fine earlier. I noticed
that whenever I put a grounded screwdriver between the diode and the
circuit board, the sound improved a little. I tried a small piece of
grounded aluminum foil between the diode and the board and the sound
was perfect. Apparently there was sufficient radiation from the
plastic body of the diode in its substitute location to interfere with
the sound circuits which were the closest on the circuit board to
where I mounted the diode. A grounded piece of foil covered with
electrical tape to keep it from shorting to anything proved a complete
cure, and it ended up in a position that should not impair air
circulation.

Now my wife can have her favorite set back if she wants. I'll bet
though that she will prefer the newer color set.

H. R.(Bob) Hofmann

"hr(bob) " wrote in message
...
We had an old tiny 1972 B+W 200mm/8" Magnavox TV in our kitchen for
years. When it finally died a year ago, I replaced it with a small
color set, but always wanted to get the set going again. The set
could also be 12V battery operated.

Started out tonight checking the 12V power supply by substituting a
14V bench supply. Same symptoms, very blurry picture. The output of
the regulator was right at 12.2 volts, and the B+ Boost was at 305
volts, almost right on the 311V shown on the Photofact 1295. Dragged
out my High Voltage probe, it seemed to be only 4.5 KV on the CRT, at
any brightness setting. The other CRT voltages were right on.

I figured the HV transformer had died, but decided to substitute the
solid-state HV rectifier with an old solid-state unit from a 25 kV
rectifier from a color set. I figured even with the higher forward
drop of the substitute diode since there were many more individual
diodes in the 25 KV rectifier, I might get lucky since the current
drain would be much smaller. The diode was about 4 inches long, 3/4
inch diameter, with bare ends like the caps on an old horizontal
output tube (for those of you old enough to remember what they looked
like).

After cutting out the old diode, and putting the new diode in the
circuit with clip leads and some judicious use of plastic spacers, I
turned the set on. 9.2 KV high voltage right on the spot with the
Photofact 9.2 - 9.7 KV. Now I had to figure out how to mount the new
diode permanently, since it was twice the length of the old diode and
would not fit in the original plastic clips.

I finally connected the new diode to the old leads with alligator clip
lead insulators covering the ends of the diode. Additional electrical
tape provided further insulation, and I used still more tape to put
the diode in the middle of an open space away from anything. Done- so
I thought ......... wrong !!!!.

Now the sound was garbled, and it had been fine earlier. I noticed
that whenever I put a grounded screwdriver between the diode and the
circuit board, the sound improved a little. I tried a small piece of
grounded aluminum foil between the diode and the board and the sound
was perfect. Apparently there was sufficient radiation from the
plastic body of the diode in its substitute location to interfere with
the sound circuits which were the closest on the circuit board to
where I mounted the diode. A grounded piece of foil covered with
electrical tape to keep it from shorting to anything proved a complete
cure, and it ended up in a position that should not impair air
circulation.

Now my wife can have her favorite set back if she wants. I'll bet
though that she will prefer the newer color set.

H. R.(Bob) Hofmann


I've used diodes meant as replacements for commercial microwave ovens to
repair ancient B&W monitors. They're quite small, only about an inch long,
and will handle 13KV at 500mA. They're cheap too.

On Dec 31, 12:55*pm, "James Sweet" wrote:
"hr(bob) " wrote in message

...





We had an old tiny 1972 B+W 200mm/8" Magnavox TV in our kitchen for
years. *When it finally died a year ago, I replaced it with a small
color set, but always wanted to get the set going again. *The set
could also be 12V battery operated.

Started out tonight checking the 12V power supply by substituting a
14V bench supply. *Same symptoms, very blurry picture. The output of
the regulator was right at 12.2 volts, and the B+ Boost was at 305
volts, almost right on the 311V shown on the Photofact 1295. *Dragged
out my High Voltage probe, it seemed to be only 4.5 KV on the CRT, at
any brightness setting. *The other CRT voltages were right on.

I figured the HV transformer had died, but decided to substitute the
solid-state HV rectifier with an old solid-state unit from a 25 kV
rectifier from a color set. I figured even with the higher forward
drop of the substitute diode since there were many more individual
diodes in the 25 KV rectifier, I might get lucky since the current
drain would be much smaller. *The diode was about 4 inches long, 3/4
inch diameter, with bare ends like the caps on an old horizontal
output tube (for those of you old enough to remember what they looked
like).

After cutting out the old diode, and putting the new diode in the
circuit with clip leads and some judicious use of plastic spacers, I
turned the set on. 9.2 KV high voltage right on the spot with the
Photofact 9.2 - 9.7 KV. *Now I had to figure out how to mount the new
diode permanently, since it was twice the length of the old diode and
would not fit in the original plastic clips.

I finally connected the new diode to the old leads with alligator clip
lead insulators covering the ends of the diode. *Additional electrical
tape provided further insulation, and I used still more tape to put
the diode in the middle of an open space away from anything. Done- so
I thought ......... wrong !!!!.

Now the sound was garbled, and it had been fine earlier. *I noticed
that whenever I put a grounded screwdriver between the diode and the
circuit board, the sound improved a little. *I tried a small piece of
grounded aluminum foil between the diode and the board and the sound
was perfect. *Apparently there was sufficient radiation from the
plastic body of the diode in its substitute location to interfere with
the sound circuits which were the closest on the circuit board to
where I mounted the diode. *A grounded piece of foil covered with
electrical tape to keep it from shorting to anything proved a complete
cure, and it ended up in a position that should not impair air
circulation.

Now my wife can have her favorite set back if she wants. I'll bet
though that she will prefer the newer color set.

H. R.(Bob) Hofmann

I've used diodes meant as replacements for commercial microwave ovens to
repair ancient B&W monitors. They're quite small, only about an inch long,
and will handle 13KV at 500mA. They're cheap too.- Hide quoted text -

- Show quoted text -

Hi Jim:

Thanx for the suggestion of the uwave diodes. I did try two different
uwave diodes, but only got about 8 KV from either one, not sure why,
as they should have easily handled the CRT current, but the forward
drop seemed to be the problem, so I took the easy way out and used the
old color tv diode.

Bob H

Thanx for the suggestion of the uwave diodes. I did try two different
uwave diodes, but only got about 8 KV from either one, not sure why,
as they should have easily handled the CRT current, but the forward
drop seemed to be the problem, so I took the easy way out and used the
old color tv diode.

Bob H


Interesting, I wonder if they were not fast enough? Domestic diodes may not
be as good as commercial/industrial diodes too, not sure.

The ones I was using were configured as a voltage doubler, so that may have
got around issues with them.

At any rate you got it working and that's the important part.