Saving Circuit Board w/o Tube

[Outland]

I have a 1994 Panasonic TV with a bad tube but good circuit board. I want to save the circuit board. From watching videos online, separating the circuit board is simply a matter of pulling the board off the gun, and that's it? It's been switched off for more than a week, so I think there is little residual charge left.

Similarly, I have the same model TV with a good tube and a (occasionally) bad circuit board. I can simply swap in the circuit board as I removed it from the old one?

[MadMan]

The board hanging off the ass end of the CRT? Should just be friction fit plugged on, yeah. If so, it really should not require any effort to pull it off. If it resists more than a moderate tug, then maybe I'm wrong. Also, there shouldn't be any residual charge on the board, unless it has some electrolytics on it, and even then, not after a few days.

[dishdude]

The board pulls right off the CRT. Discharging the anode is what you need to be concerned with.

[Outland]

When will it be safe to pull the board off?

[Popester]

If the tv is unplugged and you have discharged the second anode connection to ground you should be good to go.

[Outland]

I’ve never discharged an anode. The TV has been off for a few weeks, and it was made in 1994. Is there an instructional video?

[MadMan]

You know that big cable that plugs straight onto the side of the CRT? And it has a rubber boot over the connection? Get yourself a screwdriver, wrap the bare end of a wire around the screwdriver, and connect the other end of that wire to a big chunk of metal on the tv chassis (ground). Stick the screwdriver under the rubber boot and fish around until you've touched metal. Careful not to touch the metal of the screwdriver. If it had a charge you'll hear a snap.

But as it's been several days, it's probably naturally discharged by now.

Also, don't let anyone scare you, the high voltage charge on a crt anode is not particularly dangerous. It won't even injure you. It does hurt like a mother, but it's no worse than being zapped by a car's ignition coil.

[Outland]

Thanks, I’ll give it a shot!

[Electronic M]

Originally Posted by MadMan

You know that big cable that plugs straight onto the side of the CRT? And it has a rubber boot over the connection? Get yourself a screwdriver, wrap the bare end of a wire around the screwdriver, and connect the other end of that wire to a big chunk of metal on the tv chassis (ground). Stick the screwdriver under the rubber boot and fish around until you've touched metal. Careful not to touch the metal of the screwdriver. If it had a charge you'll hear a snap.

But as it's been several days, it's probably naturally discharged by now.

Also, don't let anyone scare you, the high voltage charge on a crt anode is not particularly dangerous. It won't even injure you. It does hurt like a mother, but it's no worse than being zapped by a car's ignition coil.

To add to this a bit if you start to do regular service work on TVs where you need to turn the set off and pull the chassis or main board quickly after power off then you want to put a 1M resistor in series with your screw driver to ground lead. If there was a lot of charge stored in the CRTs integral capacitor (which is the case imediately after power off) shorting to ground without a series resistor can cause the HV charge on the CRT to bounce back in a few moments.

Early on in my TV resto work I didn't know about that and kept getting HV bites from my Silvertone Roundy...Cussed like a pirate over it a bunch then learned about bounce back and made sure to only resistively discharge after that.(though I'll still short discharge if I'm in a situation where I don't have what I need to resistively discharge on hand)

[old_tv_nut]

Originally Posted by Electronic M

To add to this a bit if you start to do regular service work on TVs where you need to turn the set off and pull the chassis or main board quickly after power off then you want to put a 1M resistor in series with your screw driver to ground lead. If there was a lot of charge stored in the CRTs integral capacitor (which is the case imediately after power off) shorting to ground without a series resistor can cause the HV charge on the CRT to bounce back in a few moments.

Early on in my TV resto work I didn't know about that and kept getting HV bites from my Silvertone Roundy...Cussed like a pirate over it a bunch then learned about bounce back and made sure to only resistively discharge after that.(though I'll still short discharge if I'm in a situation where I don't have what I need to resistively discharge on hand)

I didn't know that using a resistive discharge would prevent the bounceback (dielectric relaxation) - interesting. I wonder if bounceback would be eliminated with a direct discharge if you just held the contact for a few seconds more?

[Electronic M]

Originally Posted by old_tv_nut

I didn't know that using a resistive discharge would prevent the bounceback (dielectric relaxation) - interesting. I wonder if bounceback would be eliminated with a direct discharge if you just held the contact for a few seconds more?

I've experienced bounce back minutes after disconnecting from IIRC over 10sec long discharge... I've also done multiple discharges and gotten multiple bounce backs on some CRTs. I either resistively discharge, leave the CRT HV connector shorted to ground, discharge immediately before contact, or regret my lapse... assuming the CRT hasn't previously been discharged beyond bounce back.

It is one of life's little annoyances.

I've always liked the SS and hybrid Zeniths that use a resistor voltage divider to obtain focus voltage from the HV since they self discharge fairly quickly.

[Outland]

What would happen if I just pulled off the board without discharging?

[dishdude]

Originally Posted by Outland

What would happen if I just pulled off the board without discharging?

It might bite you. Just follow Madman's instructions above, super simple and takes a second. I usually use the screwdriver to push in the metal prong on the connector to easily remove it from the CRT.

[Outland]

Just wondering, kind of scared.

[zeno]

Almost any solid state set will bleed off in minutes through the
focus divider that is usually built into the tripler or FBT.To be sure
hook a screw driver to a clip lead. Attach the other end to the
CRT ground. Usually there is a bare wire across the CRT where it looks
painted black. That is the best ground. slide driver under the boot &
touch arround the middle of it. To unhook slide the boot to one
side at a 90 deg angle to the wire. lift then slide back.
Only trouble with chassis swap is you may have to make a few adjustments.
No big deal.

BTW DO NOT do this with an old Sony. They have a coax lead & must
be handled with care !

73 Zeno
LFOD !