Any TV engineers here?

TV-Engineer said:

I'm not sure what to try next on it. Do you know if the EHT can be too high even when the HT line is spot-on?

Click to expand...

I wouldn't have thought so? - also the x-ray protection should shut it down if the EHT was too high.

High EHT usually gives a reduced size picture.
It can go high independently of the HT if the LOPT tuning cap goes O/C

could you please describe the fault again
put it plainly and short

Okay, here goes...

With no signal supplied via RF or Scart:

LOPT pulses every few seconds, causing screen disturbance. Sticker near EHT lead shows potential change with each pulse by flicking toward lead and back. Potential present on chassis reading 100-120VAC on meter to mains earth. Gives nasty shock if any metal parts of chassis touched, like RF socket. Metal parts can be connected to mains earth, but LOPT still pulses.

At switch-off, crackling all around tube and the case around it, along with 15 - 20 seconds of arcing at 4-pin connector on mains-in board, between pins 2 and 3:
Pin 2 - Live from the chassis to switch.
Pin 3 - Live from mains board (hence from mains plug) to switch.

Mains earth lead held near metal parts of chassis will draw arcing to the lead instead and arcing at connector reduces, stopping almost completely if earth lead touches chassis.

EHT at anode cap dissipates very quickly after switch-off. Once arcing is over, no potential remains at anode cap.

With signal from RF or Scart:

Same symptoms, but LOPT does not pulse. Excellent picture. Geometry perfect. Just don't touch the RF socket at back or phono sockets at front, unless you like juice. No good with kids around, so will not put up with fault. Hefty shock even on carpet with rubber-soled shoes on, and it will jump, but only at switch-off.

Any more concise than that and important details will be left out.

It's been literally decades since I've done TV's, but my gut tells me there's a leak in that tube somewhere...

ok first you need to determine whether the reading you get of 100-120ac
is coming from a leak of the ps primary or a fault in the eht,ht stage
To check for a primary ps mains leak disconnect the ht supply rail to line output stage this will shut down all high
voltage generation then measure again to see if youre getting the
100-120ac reading from chassis to earth ground
remember servicing tvs IS DANGEROUS work always put your safety
first and dont be careless at it
come back and tell us what reading you got
and ill get back to you

Mastertech, Hi. I've been so busy, I've only just had chance to try what you suggested.

I removed the LOPT transistor and put a 60W bulb across the collector and emitter, as my father taught me to do - really just to show current is present on the HT line - but I wanted a load to be present so the TV would start up, which it is doing after tripping a few times beforehand.

So, the line output stage is now dead. There is still 118VAC reading from chassis to earth ground. No arcing or anything, but that does seem to be EHT leaking from the anode cap, since the charge there disappears as the arcing carries on.

So, potential still on the chassis.

...I'm always careful with TVs, back when I first started on them, I got a nasty whack or ten from the smoothing block, live heatsinks discharging juice from capacitors around about, and so on. So, I learned to be far more careful. Good advice nonetheless. So far (touch wood) I've been free from any EHT going through me. I wanna keep it that way.

Thanks.

The isolating components between chopper primary and secondary consist of a capacitor and two 8.2MOhm resistors in series.

Interestingly, on the primary side resistor, there is a reading of 118VAC. On the secondary resistor, there is 115VAC. At the junction between the two series resistors, there is a reading of 80VAC.