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TV has 130V DC on A/V input!

O

Orange

Jan 1, 1970
0
I have some old TV, and it works fine. Only recently I discovered that
voltage between shield of composite input and gnd of device being
connected is about 130 Volts DC!!!!! Its about 80V when voltmeter is
set to AC.

I did notice every time some sparks when connecting device (Amiga1200)
to TV, and then I was slightly "shaken" by current (don't know the
right word), but since the picture quality HAS BEEN OK, it didn't
matter. The picture is still OK, but I'd rather not use TV any more.

Could this possibly be normal for TV, because picture is perfectly OK
(are other TVs like that) ? Should I take it to repair?
Amiga1200 behaved strangely, would freeze if not "left alone" for 30
min after powerup; could this be the reason?
 
A

alpha_uma

Jan 1, 1970
0
Orange said:
I have some old TV, and it works fine. Only recently I discovered that
voltage between shield of composite input and gnd of device being
connected is about 130 Volts DC!!!!! Its about 80V when voltmeter is
set to AC.

I did notice every time some sparks when connecting device (Amiga1200)
to TV, and then I was slightly "shaken" by current (don't know the
right word), but since the picture quality HAS BEEN OK, it didn't
matter. The picture is still OK, but I'd rather not use TV any more.

Could this possibly be normal for TV, because picture is perfectly OK
(are other TVs like that) ? Should I take it to repair?
Amiga1200 behaved strangely, would freeze if not "left alone" for 30
min after powerup; could this be the reason?

No, how can it be "normal" to get an electrical shock? Is the TV power cord
using a two-prong plug or three-prong plug?
Al-U
 
T

Tom MacIntyre

Jan 1, 1970
0
No, how can it be "normal" to get an electrical shock? Is the TV power cord
using a two-prong plug or three-prong plug?
Al-U

TV's typically do not use 3 prong.

Sounds like a blocking capacitor may be shorted, or at least leaking;
that's the sort of thing that happens when the blocking capacitor in a
TV's tuner is bypassed.

Tom
 
O

Orange

Jan 1, 1970
0
I forgot to mention it ,there is no GND wire, only two wires.
Many things aren't normal in my country, but thats offtopic :)
TV's typically do not use 3 prong.

Sounds like a blocking capacitor may be shorted, or at least leaking;
that's the sort of thing that happens when the blocking capacitor in a
TV's tuner is bypassed.

Tom
Thanks for the replies, I'll take it to repair. When I tell the
service man what it could be, he wont rip me off :)

Do you think it could have damaged the Amiga 1200?
The computer is behaving strangely: if it is being used within 15 min
after turning power on, it freezes and crashes and sometimes hard disk
drive (2,5") makes funny noise. But if its "left alone" for that
period, it works well after. Though hdd still might make that noise.
I don't know where to look for help. c.s.amiga.hardware didn't know
what it could be.
 
F

Franc Zabkar

Jan 1, 1970
0
I have some old TV, and it works fine. Only recently I discovered that
voltage between shield of composite input and gnd of device being
connected is about 130 Volts DC!!!!! Its about 80V when voltmeter is
set to AC.

I think this may be normal. I actually measure ~95VAC and ~70VDC
between the tuner case of my Sanyo TV and mains earth. Even so, I do
not get a "bite" from any of the terminals. The TV has a 2-pin plug,
and the chassis is not live. I remove the aerial lead and all other
leads when measuring. I've also measured similar voltages on a Teac
set in the past.

IME TVs often have a high value resistor and capacitor linking the
reference points on both sides of the switchmode transformer. For
example, my Sanyo TV has a 5.6M resistor and two 2200pF caps between
the negative terminal of the main 400V filter cap on the AC side and
the 0V reference on the DC side.

|---- R ----|
|- C --- C -|
_|_ _|_
= ===

Assuming your electricity supply uses the MEN system (neutral tied to
earth at distribution box), then this would mean that the OC voltage
appearing at the AV terminals (or the tuner case) would be a half
sine.

**** ****** **
* * * *
* *

Assuming the supply is 240VAC, an ideal non-RMS DMM would measure this
waveform as 240VAC /2 = 120VAC. However, as a real DMM would have an
input impedance of 10M, then the voltage it would measure would be 120
x 10 / (10 + 5.6) = 77VAC. The DC value of the above waveform would be
240 * sqrt(2) / pi = 108V. The DMM would measure 108 x 10 / (10 + 5.6)
= 69VDC. Of course this assumes that the impact of the two caps is
negligible. In fact the impedance of the two caps at 50Hz is 2.89M, so
the measured voltages should be somewhat higher than those calculated.


- Franc Zabkar
 
A

alpha_uma

Jan 1, 1970
0
TV's typically do not use 3 prong.

Sounds like a blocking capacitor may be shorted, or at least leaking;
that's the sort of thing that happens when the blocking capacitor in a
TV's tuner is bypassed.

[Sorry for the delayed response.]
Would that pose a serious risk of electrocution if the home wiring is not
equipped with CFCIs? Are TVs all "double-insulated" these days, and how
trustworthy are they?
Al-U
 
A

alpha_uma

Jan 1, 1970
0
alpha_uma said:
[Sorry for the delayed response.]
Would that pose a serious risk of electrocution if the home wiring is not
equipped with CFCIs? Are TVs all "double-insulated" these days, and how
trustworthy are they?
Al-U

Sorry for the typo; I meant GFCIs.
Al-U
 
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