Lightning strike blew out tuner pre-filter

[Joe]

A while back a lightning strike blew out the pre-filter of my TV tuner.
I was watching the TV when this occurred, so I know it was the cause.
I say 'pre-filter' as it is a little sealed box that has the cable TV coax
connector protruding thru the back cover, for which to connect the cable TV
cable to.

This pre-filter has just 2 components in it, a capacitor and what looks like
a resistor, so I would assume this just performs some type of filtering.
I checked with the local electronics stores to see if I could find a
replacement for the unit, or even just the 2 contained components ... no
luck.

Thru trial & error, using a box of old parts for testing, I was able to get
a decent (but not perfect) picture by removing both old components from the
pre-filter, and replacing them with a single 5pF capacitor.
But, sometimes after a few hours operation, the picture becomes unwatchable
(fuzzy), and I have to simply disconnect & reconnect the cable to instantly
clear the picture.

I would like to get the correct components for this pre-filter to fix this
correctly.
Is this 'pre-filter' a typical component of a TV?
Does anyone know what these 2 contained components typically are?

Here's the TV:
Sanyo AVM-2508G
Chassis G5C-2508G0
Serial B8260330606047

Thanks for any help.
Joe

 

[Mark D. Zacharias]

It's an isoation block. Safety-critical part. Isolates the hot chassis of
your TV from a cable system ground or antenna ground.

Mark Z.

 

[Charles Schuler]

A while back a lightning strike blew out the pre-filter of my TV tuner.
I was watching the TV when this occurred, so I know it was the cause.
I say 'pre-filter' as it is a little sealed box that has the cable TV coax
connector protruding thru the back cover, for which to connect the cable TV
cable to.

This pre-filter has just 2 components in it, a capacitor and what looks like
a resistor, so I would assume this just performs some type of filtering.
I checked with the local electronics stores to see if I could find a
replacement for the unit, or even just the 2 contained components ... no
luck.

Thru trial & error, using a box of old parts for testing, I was able to get
a decent (but not perfect) picture by removing both old components from the
pre-filter, and replacing them with a single 5pF capacitor.
But, sometimes after a few hours operation, the picture becomes unwatchable
(fuzzy), and I have to simply disconnect & reconnect the cable to instantly
clear the picture.

A 5 pF capacitor is too small (it has a reactance of almost 600 ohms at
channel 2). I'd advise a larger capacitor (470 pF or so). The other
component might have been voltage clamp ... MOV...zener...spark gap...diodes
(1N914s perhaps). The TV will work without it but will have no transient
voltage protection.

 

[David]

That is a critical safety device to isolate the HOT chassis of the tv set
from the coax connector.
Do not bother to attempt to fix it, it is not worth the time or effort when
replacement isolation blocks are well under $20.00. MCM electronics carries
a few. Take a look there first.

David

 

[Joe]

Thanks much for all the replies.
Can I assume that any isolation block will work?
I'm not concerned about mounting.

 

[David]

Any isolation block that fits the mounting and does not come into contact
with the hot part of the chassis will work. Was that isolation block simply
mounted to a hole in the cabinet or soldered into the circuit board or
sticking out of the end of the tuner?

If you are not worried about cosmetics, you can mount the isolation block
with the 9" leads into a new hole up on the plastic cabinet with no issues
at all, except knowing where to hook up the cable.

David

 

[Asimov]

"Joe" bravely wrote to "All" (05 Jun 04 16:40:13)
--- on the heady topic of "Lightning strike blew out tuner pre-filter"

The "filter" is an antenna isolation network. It does a couple of
things. The first is to isolate the TV ground from the antenna ground.
The other is to discharge any minor static build-up safely for example
walking across a carpet. As you experienced lightning is just a little
too much. The small disk is often a combination capacitor with a large
value resistor. Often 0.001uF (1KV or more) and about 150K ohms or
more. The lone resistor may be around 470K or more but must be a
higher voltage spec (600 volts & more). Take a box from a junked tv.


Jo> From: "Joe" <[email protected]>

Jo> A while back a lightning strike blew out the pre-filter of my TV
Jo> tuner. I was watching the TV when this occurred, so I know it was the
Jo> cause. I say 'pre-filter' as it is a little sealed box that has the
Jo> cable TV coax connector protruding thru the back cover, for which to
Jo> connect the cable TV cable to.

Jo> This pre-filter has just 2 components in it, a capacitor and what
Jo> looks like a resistor, so I would assume this just performs some type
Jo> of filtering. I checked with the local electronics stores to see if I
Jo> could find a replacement for the unit, or even just the 2 contained
Jo> components ... no luck.

Jo> Thru trial & error, using a box of old parts for testing, I was able
Jo> to get a decent (but not perfect) picture by removing both old
Jo> components from the pre-filter, and replacing them with a single 5pF
Jo> capacitor. But, sometimes after a few hours operation, the picture
Jo> becomes unwatchable (fuzzy), and I have to simply disconnect &
Jo> reconnect the cable to instantly clear the picture.

Jo> I would like to get the correct components for this pre-filter to fix
Jo> this correctly.
Jo> Is this 'pre-filter' a typical component of a TV?
Jo> Does anyone know what these 2 contained components typically are?

Jo> Here's the TV:
Jo> Sanyo AVM-2508G
Jo> Chassis G5C-2508G0
Jo> Serial B8260330606047

Jo> Thanks for any help.
Jo> Joe

.... You mean 15" sparks are SUPPOSED to come out of this thing?!?