Getting erratic voltage reading near and around IC901 (over voltage
detection IC). Voltage goes from 0v - 4v back and forth at the base of
Q902 and generally around that area. On the schematic the voltage at
that point should be 0.4v. I have checked all capacitors and can't seem
to find any bad ones, infact I've checked for bad caps or transistors
on most parts of the set but can't seem to find what is causing the
problem. I thoght that I should mention the erratic voltage though as
that is about the most promising thing that I have found. At first I
though it may be an over voltage problem on the horizontal output
circuit which was tripping out the over voltage protection cicuit but
now I'm beggining to wonder if the problem may actually be the over
voltage cicuit or the chopper circuit which are both connected and the
same cicuit really.

I was also wondering if it may be something breaking down under load
due to the fact that the set will not trip out when the HOT is removed.
So it seems that the problem only happens when the set is under load.

Would really appriciate some help with this as I've been trying to get
this set working for a long time now and it's starting to do my head in.

As you seem to know these sets are plagued with problematic caps.
Although one can change loads and still not find the culprit!.
I would scope each supply rail to check its smooth as this is probably the
most likely route to find whats throwing the prot cct. Is the 8v rail ok- I
seem to remember probs with its reg. Plus its always worth changing the cap
in the base feed to the chopper Tr.
If you have definitely eliminated psu rails as the cause - and the primary
supply is ok (runs a bulb ok when loptx isn't connected) then ok you have a
problem around frame/line, presumably drawing a lot of current.
I have also had eeprom corruption that causes a dead set (ie stuck on stby)
but yours doesn't sound like that. its not worth doing the initialise data
unless you're certain of that though as all eeprom values need setting up.
Hope that helps
Andrew

"kel-fbs" wrote in message
ps.com...
Getting erratic voltage reading near and around IC901 (over voltage
detection IC). Voltage goes from 0v - 4v back and forth at the base of
Q902 and generally around that area. On the schematic the voltage at
that point should be 0.4v. I have checked all capacitors and can't seem
to find any bad ones, infact I've checked for bad caps or transistors
on most parts of the set but can't seem to find what is causing the
problem. I thoght that I should mention the erratic voltage though as
that is about the most promising thing that I have found. At first I
though it may be an over voltage problem on the horizontal output
circuit which was tripping out the over voltage protection cicuit but
now I'm beggining to wonder if the problem may actually be the over
voltage cicuit or the chopper circuit which are both connected and the
same cicuit really.

I was also wondering if it may be something breaking down under load
due to the fact that the set will not trip out when the HOT is removed.
So it seems that the problem only happens when the set is under load.

Would really appriciate some help with this as I've been trying to get
this set working for a long time now and it's starting to do my head in.

Your advice does help alot as it makes me realise that I need to learn
a lot more about TV repairs before I will be able to fix this set. You
see I'm an ex electrician and electrical tester (domestic comercial
industrial ect.) that has now been out of work for a while bringing up
a child on my own. So in my spare time I decided to learn about TV
repairs, not sure why but it has always interested me to know how TV's
work. Anyway back to the subject, up until now I have managed to fix a
few TV's with easily found faults, mainly bad chopeer transistors or
HOT's and caps. As they have been easily found I have got away with
just using my multimeter. I'm not in it for a quick fix or to even use
or sell the TV's that I manage to fix, it's more to do with learning a
bit more from each set as I go along. I think that it is now maybe time
to go onto the next step like you mentioned and buy and learn how to
use a scope or learn how to use a variac or series light bulb to help
with fault finding. I have also read about using isolation transformers
for saftey puposes when working on the primary side of power supplies
but am a bit confused about this one as the info was a bit vauge. If
there is any advice or tips you can give me from your experience about
using this kind of equipment then it would be appriciated. Thanks

OH and I forgot to mention in my first post that there were some
syptoms before the set died but as it happened so long ago I nearly
completely forgot. first syptom was that the fuse in the 13A plug
popped so I replaced it and the set came back on. Once it was back on
the sound was fine but there was no picture until gradually over a
minute or two the picture started to go from nothing to a very very
dark image, infact it was so dark that I could only make out some very
dark blurs and silhouettes. After this process of the picture gradually
becoming visible over a minute or so the fuse in the 13A plug would
then pop again. Being as though at the time I knew absolutely nil about
TV's I very stupidly kept replacing the fuse somehow thinking that it
would sort it'self out and after about 2 or 3 replacement fuses the set
turned to the state that it is in now and trips out into protection
mode. It probably would have been a bit easier to find the fault if
only I had not kept replacing those fuses

Anyway hopfully this info may make some sense to someone more
experienced than me.

"kel-fbs" wrote in message
oups.com...

I have also read about using isolation transformers
for saftey puposes when working on the primary side of power supplies
but am a bit confused about this one as the info was a bit vauge. If
there is any advice or tips you can give me from your experience about
using this kind of equipment then it would be appriciated. Thanks

The switch mode power supplies in use now are generally isolated ok unless
you have to fix the SMPS itself, in which case isolation is a good thing.