Along the sme line - while adjusting an older (1988) RPTV, I noticed that
the focus would change (go blurry) when vhite screen appeared; immediately
return in focus on a dark scren; put a meter on the B+ and noted it stayed
steady. Any ideas?

Emilio


"Yukio YANO" wrote in message
news:f7%sf.209738$ki.45771@pd7tw2no...
wrote:
Sadly, it's out of warranty.

That being the case, would it be worth it for me to have someone look
at it? Is there a common cause that an ordinary electrical engineer can
repair?

-Ed

Thermal/Mechanical Intermittant to the "Aquadag coating" on the CRT .
(could be internal to the CRT)
The Odd Focusing problem is due to a fluctuating Focus Voltage occuring
at the Horizontal Rate. The Focus Voltage is a rectified Voltage
derived from the Horizontal Sweep/Flyback Transformer. The Focus Voltage
is taken off a Voltage divider across the HV supply that is filtered by
a capacitor formed by the Internal Coating on the CRT and the Black
"Aguadag" coating on the Outside. If this Outside Coating is poorly
Grounded (usually a length of coiled spring wire) the HV will fluctuate
slightly at the Horizontal Rate but will not be readily apparent
because the Picture Tube Phosphours glow at the Frame Rate ~ 16
milli-second.
The Focus Voltage fluctuation are Synced to the Scan Rate and would
appear somewhere near the same distance from the edge of the screen on
every horizontal scan. This Voltage is Not supposed to fluctuate !! The
only Filter Capacitor in this part of the circuit is the Coatings On and
In the CRT. A small sheet of Aluminum Foil under Spring should improve
the contact with the Aquadag Coating.

Is it practical to pay someone to TRY to repair an INTERMITTANT !!!
A lesson in frustration for ALL parties involved.
There is no Joy in repairing an intermittant, only Relief. (could still
be internal to the CRT)

Yukio YANO