Hold on tight....
Hi,
Although potentially lethal, 25 ~ 30KV @1mA, is in fact usually just a profoundly shocking experience.
What is more dangerous (in my opinion) is the fact that most TV's use switch-mode power supplies, which run straight from the mains. This is produces about 350 volts DC which often appears on the main PCB, and is often not well marked.
In addition to this hazzard, there is the question of fire.
An incorrectly repared or inappropriately modified TV is a hazzard. If it has (and most have) safety approval, the board should be self extinguishing, the components that get hot will have to have been approved, the spacing between hot components and other parts (especially wiring) is important.
Another hazzard is X-radiation. The CRT fitted will have a maximum permitted EHT (High voltage). Exceeding this will cause the emission of soft X-radiation. Although there are tell-tale signs of abnormally high EHT that a professional would notice at once, it is likely that non-professionals might not be aware.
My advice is:
Unless you know exactly what your doing, leave well alone.
Hi,
Although potentially lethal, 25 ~ 30KV @1mA, is in fact usually just a profoundly shocking experience.
What is more dangerous (in my opinion) is the fact that most TV's use switch-mode power supplies, which run straight from the mains. This is produces about 350 volts DC which often appears on the main PCB, and is often not well marked.
In addition to this hazzard, there is the question of fire.
An incorrectly repared or inappropriately modified TV is a hazzard. If it has (and most have) safety approval, the board should be self extinguishing, the components that get hot will have to have been approved, the spacing between hot components and other parts (especially wiring) is important.
Another hazzard is X-radiation. The CRT fitted will have a maximum permitted EHT (High voltage). Exceeding this will cause the emission of soft X-radiation. Although there are tell-tale signs of abnormally high EHT that a professional would notice at once, it is likely that non-professionals might not be aware.
My advice is:
Unless you know exactly what your doing, leave well alone.
...Only Too True...
Hi John,
In may early days repairing TV monitors at Thames TV, I once misdiagnosed a fault on a 26" Philips EHT supply. This resulted in me grabing hold of the top cap of a fully charged EHT smoothing capacitor...the shock threw me right across the room and I hit the wall hard enough to knock myself out for a couple of minutes! I was lucky - but you are right EHT shocks rarely kill directly but they often cause effects that kill or maim.
My collegues made me repair all the EHT faults for the next six months 1) To be sure I knew what I was doing 2) as I now had a healthy respect for EHT supplies...
On the other hand your example of 350Volts dc in the switchmode psu is absolutely lethal!!! That's right in the sweet spot for death by electrocution...
Maybe sweet spot is the wrong word...
Of course this means that valve amp B+ supplies are deadly too I do mean deadly I lost a collegue to electrocution from a KT88 B+ supply (guitar amp that had been 'bastardised' - if I ever find the person who did that...
ciao
James
Hi John,
In may early days repairing TV monitors at Thames TV, I once misdiagnosed a fault on a 26" Philips EHT supply. This resulted in me grabing hold of the top cap of a fully charged EHT smoothing capacitor...the shock threw me right across the room and I hit the wall hard enough to knock myself out for a couple of minutes! I was lucky - but you are right EHT shocks rarely kill directly but they often cause effects that kill or maim.
My collegues made me repair all the EHT faults for the next six months 1) To be sure I knew what I was doing
2) as I now had a healthy respect for EHT supplies...On the other hand your example of 350Volts dc in the switchmode psu is absolutely lethal!!! That's right in the sweet spot for death by electrocution...
Maybe sweet spot is the wrong word... Of course this means that valve amp B+ supplies are deadly too
I do mean deadly I lost a collegue to electrocution from a KT88 B+ supply (guitar amp that had been 'bastardised' - if I ever find the person who did that...ciao
James
Some old TV's even has the whole innards connected to the negative on a bridge off the mains. That mean's the ground inside the TV will bounce around between 0 and -300VDC compared to real ground. The antenna signal is usually connected through capacitors. But this isn't used in new TV sets, because it isn't as easy isolating A/V inputs.
Be Very Careful.
Typical Television voltages are as follows:
SMPS 240V Primary Side -
Main cap - 330V DC direct coupled to mains - VERY DANGEROUS !!!.
Internal multiple power rails and may include:
25KV EHT supply to tube ultor cap - DANGEROUS !!!.
105V upto 155V DC supply to horizontal stage - DANGEROUS !!!.
30V DC supply to vertical stage - caution.
12V DC supply to line level circuits - caution.
5V DC supply to logic circuits - caution.
For safe servicing of televisions a 240V-240V isolation transformer is required - NEVER place yourself across chassis to earth ground or antenna plug.
Use insulated shoes and one hand when measuring any running voltages.
Eric.
Typical Television voltages are as follows:
SMPS 240V Primary Side -
Main cap - 330V DC direct coupled to mains - VERY DANGEROUS !!!.
Internal multiple power rails and may include:
25KV EHT supply to tube ultor cap - DANGEROUS !!!.
105V upto 155V DC supply to horizontal stage - DANGEROUS !!!.
30V DC supply to vertical stage - caution.
12V DC supply to line level circuits - caution.
5V DC supply to logic circuits - caution.
For safe servicing of televisions a 240V-240V isolation transformer is required - NEVER place yourself across chassis to earth ground or antenna plug.
Use insulated shoes and one hand when measuring any running voltages.
Eric.
I'm going to close this thread because it is really in a murky area with regard to forum policy.
My final statement is this. TV mods etc are not to be discussed at diyAudio. All repairs etc on TV sets should be made by qualified professionals. TV's have a number of highly dangerous aspects to them once you remove the cover and start poking around. Please, please leave TV servicing / repair to a professional.
Regards,
AudioFreak
diyAudio Moderator.
My final statement is this. TV mods etc are not to be discussed at diyAudio. All repairs etc on TV sets should be made by qualified professionals. TV's have a number of highly dangerous aspects to them once you remove the cover and start poking around. Please, please leave TV servicing / repair to a professional.
Regards,
AudioFreak
diyAudio Moderator.
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