Nuisance 36" Zenith

[Chris F.]

Got this set in the other day, don't have the model number handy but it
was made in Feb. '95. It had been looked at by another shop, they had given
the owner an estimate and then the owner died, and the guy who "inherited"
this set brought it to me. I knew the tech at the other shop, and phoned him
up to see what his diagnosis had been. He said it needed the whole power
supply / horiz. output module replaced, at a cost of $300 Cdn. The guy is
competent but I decided to have a crack at it anyway.
The set had sound but no HV, and the HOT (and some fuses) were blown. When
I tested the flyback on a homemade tester (built from the plans on the
repairfaq site) it was arcing badly between the pins on the flyback.
Something told me this was not right, so with the customers permission I
ordered a new FBT. Sure enough, the old flyback was indeed defective. Once
the new one was installed, the set powered up - for about a minute. It seems
to have a decent picture and everything, but if the G2 control is turned up
anywhere past minimum, or if a bright scene comes on, the horizontal drive
is shut down. The high voltage stays within 25kV so I don't think this is
the cause. Turning the set off, then on again, cures the problem until the
next bright scene.....
Considering that the set was "worked on" by at least two other shops, I
fear there may be some unknown damage done by a technicians carelessness. I
already repaired some cold solder around some connectors.
Any ideas? I'm so close, I don't want to just give up on this thing. Plus
I'd like to get the it done and out of there - this monster takes up half my
shop!
Thanks.

 

[David]

More likely damaged components along the ABL+protect line due to the flyback
arcing and failure. You will need the schematic to trace down all the
failures from the arcing flyback.
David

 

[John Del]

Subject: Nuisance 36" Zenith

From: "Chris F." [email protected]
Date: 1/15/04 4:34 PM Pacific Standard Time
Message-id:

.. Once

the new one was installed, the set powered up - for about a minute. It seems
to have a decent picture and everything, but if the G2 control is turned up
anywhere past minimum, or if a bright scene comes on, the horizontal drive
is shut down.

The problem is likely the flyback you put in, which I'd bet is probably not the
original but a generic. If Gold Star still offers the original, get one from
them. If not, you need to modify the prov line because the new flyback has a
higher pulse on one of the secondaries that the prov circuit monitors.

John Del
Wolcott, CT

"I'm just trying to get into heaven, I'm not running for Jesus!"
Homer Simpson

(remove S for email reply)

 

[Chris F.]

I got this replacement from Global Electronics. I really don't feel like
ordering another one from somewhere else; is there any way I can just modify
the set without a schematic?
It's worth noting that I when I powered the set, after the heat had been
turned down in the shop for a while, the set seemed to work normally. Also I
noticed the picture shrunk in a bit on the sides, as the CRT was warming up.
Maybe some bad caps?

 

[john]

Yikes ESR the Caps ..

kip

 

[John Del]

Subject: Re: Nuisance 36" Zenith

From: "Chris F." [email protected]
Date: 1/16/04 7:53 AM Pacific Standard Time
Message-id: <[email protected]>

I got this replacement from Global Electronics. I really don't feel like
ordering another one from somewhere else; is there any way I can just modify
the set without a schematic?

What is the sweep board number?

John Del
Wolcott, CT

"I'm just trying to get into heaven, I'm not running for Jesus!"
Homer Simpson

(remove S for email reply)

 

[Chris F.]

The sweep board number is 9-1270.

 

[Mike]

same board is mine!

yours might not happen to be a SR3587DT?

mine has the same problem. no HV. it blows the HOT instantly upon powerup.

anyway, there is a cap that increases in value, and the driver transistor
has a tendency to go bad.

 

[John Del]

Subject: Re: Nuisance 36" Zenith

From: "Chris F." [email protected]
Date: 1/17/04 7:32 AM Pacific Standard Time
Message-id: <[email protected]>

The sweep board number is 9-1270.

I'll get you the mod Monday. If I don't post it here, email me and remind me!
John Del
Wolcott, CT

"I'm just trying to get into heaven, I'm not running for Jesus!"
Homer Simpson

(remove S for email reply)

 

[Chris F.]

I will thanks.
So this is a common problem, huh?

 

[Chris F.]

Something like that, although I don't think it has a DT on the end of the
model #. When I got this thing, the HV would try to start up for about a
fraction of a second, then shut down. There were two shorted HOTs enclosed
in a package, inside the TV, obviously from the previous shop. The HOT I put
in didn't blow, but I only made one or two attempts to power the set in this
condition.
It's also worth noting that I replaced the NTE 2331 HOT with an NTE 2302,
as I didn't have any of the former in stock. According to the books this is
a superior replacement, so I don't suspect that this is the cause of the
problem.
Build the flyback tester described on www.repairfaq.org, and try it on
your flyback (try a known good one first, just to make sure the tester
works). If you can't get much/any HV on the output side, or if you see
arcing between the pins, the FBT is bad. Thanks to Sam for that handy idea,
much cheaper than a multi-thousand dollar ring tester.......

 

[John Del]

Subject: Re: Nuisance 36" Zenith

From: "Chris F." [email protected]
Date: 1/17/04 4:06 PM Pacific Standard Time
Message-id: <[email protected]>

I will thanks.
So this is a common problem, huh?

It is on that series of boards when using non original flybacks. Since Gold
Star is rapidly dropping support of Zenith built product, then generic subs
will be the only recourse, and creative redesign (fudging) will help out when
the subs don't work right.

BTW, it's not just Gold Star (LG) that's doing this, but others. I just did a
1999 Sony projo that crapped it's flyback. NLA from Sony. I got a sub from
CRC that had a regular CRT anode cap, plus G2 and Focus controls on it!
(remember this is a projo). It looked like it came out of a 27" TV. It also
had the control pin at the bottom, while the board wasn't drilled for one. I
removed the anode and attached the little metal thing from the original fly so
I could plug the anode into the splitter, bent over the control pin and
soldered a ground wire to it, and installed it. Strangely, the sweep and HV
are perfect, and the new sub fly runs cool. Problem is that the focus controls
at the divider must be rotated all the way to the end to achieve some level of
sharpness. Rotating the G2 and Focus controls on the sub flyback didn't help
much. Not perfect, but usable. Customer was happy to get it back, so I never
tried fudging the focus (which I will do on the next one when I have more
time).


John Del
Wolcott, CT

"I'm just trying to get into heaven, I'm not running for Jesus!"
Homer Simpson

(remove S for email reply)