Hopefully someone will have come across this before.

I am working on a Sony VPH1271 3 tube projector, which gives a superb
picture apart from one (almost invisible) problem.
On low average brightness scenes, all is well. As soon as there is a
reasonable proportion of peak white in the scene, I am seeing horizontal hum
bars of fairly high frequency, and not locked to the video. Turning the
contrast up and down (and hence varying the mean beam current) changes the
frequency and visibility of the "hum", sometimes upwards, sometimes
downwards. All three tubes are affected. My guess is that one of the
supplies to the tubes either has induced hum (judging by the frequency
probably from the chopper), or one of the supplies is running out of steam
due to dried up caps etc. My guess is that this must also be on the
secondary side of the smps due to the high frequency shash.

I'm hoping this isn't EHT related.

Any thoughts or solutions before I dive in with the scope?

Assuming you're a technician, or at least reasonably competent to work
around potentially lethal voltages, a 'scope is exactly what you need. Look
at the various supply lines as the problem occurs. You should see the noise
on the affected capacitor(s). Or you could just replace all the caps in the
secondary circuit, and any smaller ones in the primary, like 1uF @ 250V etc.

Mark Z.
"Trevor Brown" wrote in message
...
Hopefully someone will have come across this before.

I am working on a Sony VPH1271 3 tube projector, which gives a superb
picture apart from one (almost invisible) problem.
On low average brightness scenes, all is well. As soon as there is a
reasonable proportion of peak white in the scene, I am seeing horizontal
hum
bars of fairly high frequency, and not locked to the video. Turning the
contrast up and down (and hence varying the mean beam current) changes the
frequency and visibility of the "hum", sometimes upwards, sometimes
downwards. All three tubes are affected. My guess is that one of the
supplies to the tubes either has induced hum (judging by the frequency
probably from the chopper), or one of the supplies is running out of
steam
due to dried up caps etc. My guess is that this must also be on the
secondary side of the smps due to the high frequency shash.

I'm hoping this isn't EHT related.

Any thoughts or solutions before I dive in with the scope?

Yeah that's what I figured. Since this thing is on the roof and 20 feet up
(and weighing a couple of hundred pounds), what I need really is advice like
replace c6 c10 etc...

I guess I'm just going to have to temove the smps with the rest of it still
up there, and fire it up on a dummy load on the bench.

Sorry I'm being a bit lazy, but this thing is H U G E !

I used to fix broadcast electronics and this isn't really my thing....

"Mark D. Zacharias" wrote in message
...
Assuming you're a technician, or at least reasonably competent to work
around potentially lethal voltages, a 'scope is exactly what you need.
Look
at the various supply lines as the problem occurs. You should see the
noise
on the affected capacitor(s). Or you could just replace all the caps in
the
secondary circuit, and any smaller ones in the primary, like 1uF @ 250V
etc.

Mark Z.
"Trevor Brown" wrote in message
...
Hopefully someone will have come across this before.

I am working on a Sony VPH1271 3 tube projector, which gives a superb
picture apart from one (almost invisible) problem.
On low average brightness scenes, all is well. As soon as there is a
reasonable proportion of peak white in the scene, I am seeing horizontal
hum
bars of fairly high frequency, and not locked to the video. Turning the
contrast up and down (and hence varying the mean beam current) changes
the
frequency and visibility of the "hum", sometimes upwards, sometimes
downwards. All three tubes are affected. My guess is that one of the
supplies to the tubes either has induced hum (judging by the frequency
probably from the chopper), or one of the supplies is running out of
steam
due to dried up caps etc. My guess is that this must also be on the
secondary side of the smps due to the high frequency shash.

I'm hoping this isn't EHT related.

Any thoughts or solutions before I dive in with the scope?