It's fun until you start working on a
live-chassis set and you suddenly
apply mains across your video interface circuit and yourself....
Yep. Nasty. Even with isolation still can get that ZAP!.
True, but the mains is (IMHO) a lot more nasty than the EHT to the final
Seriously, many large-screen (non-portable)
B&W TVs have a live chassis
connected directly to one side of the mains (or worse to one side of a
bridge rectifier connected to the mains, so the chassis sits at half
mains voltage on average).
Every one you described for non-isolated sets are like this. Every
one, I'm referring to cheap color sets old or new. The - side of
In the UK, just about all colour TVs (even small ones, 14", etc) have
what's called a SCART socket. This is a 21 pin connector carrying 2
channels of audio in and out, composite video in and out, RGB in (which
uses the composite input for sync), blanking, etc. It turned out to be
simpler and cheaper to isolate the PSU rather than all those signals
Still watch out if you're repairing the set. Some sets put things like
the horizontal output stage on the mains side of the isolation barrier
(which means the yoke has the horizontal windings 'hot' and the vertical
windings isolated!). But at least the signal circuitry is isolated.
Just about all small B&W TVs over here (for the last 20 years) have a 12V
B+ line. They will run off a car battery or an internal (isolated) mains
PSU. Again, modern sets (even small ones, I saw a 5" one the other day)
have 'AV inputs' (audio and composite video).
rectifier bridge is "hot ground" in those.
Very rare few I seen had
one diode and that was in old citizen sets usually 13" to 19". This
is really downright cheap. In those with non-isolated sets that
does have inputs, always have small signal isolation transformers and
Other than an old, partly valved B&O colour TV, which had a transformer
isolated audio output (and no other I/O), I have never seen isolating
transformers for the signals on a UK TV.
or optical isolation. The others that is both
hot/cold chassis be it
via flyback transformer like this one hitachi and about 7 yr old.
Other kind via SMPS is very common. Using mains isolation
transformer is *extremely rare*. In so many sets in 1
Most UK TVs have an isoleted SMPSU these days
from 1991 onwards. ( chassis TX8xx series) But what
nice about this
13" RCA has no B+ regulator and the pix is very stable because of
direct control of horizontal pulses to regulate power in flyback
Are US TVs really so far behind European ones? We've had fully regulated
supply lines, including EHT, for many years... Over here we'd not accept
a TV where the picture changed in size with the average beam current!
diagram of the set is _essential_. I am not sure where you are
>From original makers is asolute best please. Especially from RCA
much better info and also have writings, and RCA also
training manuals for particular chassis.
JVC doesn't do much on this just an schematic, troubleshooting charts
and raft of adjustment instructions in slim floppy books. Same thing
with Samsung, marginally better but their TV isn't my taste since it
tinkered too much with circuit that it is hard to troubleshoot.
Samsung liked to use philips ICs which is lousy and using AKB for
The advantage of Philips ICs is that you can generally get full data
sheets on them. And there's nothing wrong with them if correctly used!
only way u can probe around. Their philips schematics
is locked and
I can rememebr when Philips would send you a service manual free of
charge if you asked nicely.... Oh well :-(