Black horizontal line on CRT monitor

[decoder_82]

Hi !

I am trying to fix a TLH Kontrollempfänger 303 (https://www.radiomuseum.org/r/heucke_kontrollempfaenger_303.html)
Unfortunately, no service manual or diagram available.

The symptoms are the following: a black horizontal line remains on screen. The rest of the picture is correctly displayed. See pictures.

Visual inspection show shows nothing broken or burnt.

Any clue where I should start looking ? My first guess would be to check deflection area or blanking circuits.

[AndyBeez]

Does the black line scroll or remain static?

Not knowing the oscilloscope timings for your input signal, at a glance I might suspect a loss of vertical sync or vertical picture misalignment?

Have you tried with a different video source?

[decoder_82]

The black bar remains static on the screen.
Even without input signal the back bar is there.

[wasedadoc]

Something vey odd with those two arcs in the first picture.  The ends are rounded.  Not a CRT focus thing because the colour subcarrier is clearly resolved in the second picture.

If you increase the brightness, does that bar become grey or does it have no illumination at all?

[AndyBeez]

Something vey odd with those two arcs in the first picture.  The ends are rounded.  Not a CRT focus thing because the colour subcarrier is clearly resolved in the second picture.

Agreed. The black bar is inside the rounded edges?

Otherwise the geometry of the screen looks perfect.

@decoder_82 - Not sure where they are on that TV, but have you turned the vertical position and vertical hold pots?

To me, the width of the black bar looks suspiciously like the back porch time gap.

Your test pattern generator is set to which PAL standard?

Thinks: Maybe the V sync is out of phase or delayed?

[decoder_82]

Yes I tried to play with the potentiometers for v-hold and it doesn't change this issue.
With more brightness the dark bar remains black.

I am using a PAL signal generator. The issue is also present when set on RF tuner. The screen shows snow (without any signal) but the bar is still there.

[wasedadoc]

Maybe something inside the CRT has moved and is blocking the electron beam.  Some ways to prove or disprove that:

1.  Adjust the vertical amplitude control.  Does the bar stay in the same place relative to the face of the CRT or does it stay in the same place relative to the resized picture content?

2.  If the deflection yoke is clamped to the tube neck (ie not glued to it) loosen the clamping mechanism and rotate the yoke about 10 degrees.  Does the black bar also rotate or does it stay horizontal?  (Switch off and unplug the unit while rotating the yoke!!)

[inse]

Are you familiar with German?
I vaguely remembered some special functions with this kind of control receivers.
Radiomuseum says the black bar is a signal level indicator.
Maybe there is a switch to turn that off?

[decoder_82]

Thanks I couldn't find a lot of information about the device and I don't speak German. I think I already checked all the switches but I will dig deeper to see if this can be deactivated.

[AndyBeez]

Maybe something inside the CRT has moved and is blocking the electron beam.  [...]

At first I thought a hair wire from the heater had moved into the beam path, but the edges of the bar are just too sharp. No fuzzy edges - whole lines missing. Maybe it really is a frequency or volume indicator? The TV is certainly more specd than other TVs from this time.

I've seen similar tuning and volume bars, but in white on Soviet made mini TV and radio sets - that I used to teardown as a kid. Makes perfect sense for a black bar as, a solid white bar would burn the screen if left stationary for ten minutes.

Is there a push/pull switch hidden in one of the dials? German's are clever like this.

[inse]

Radiomuseum also says: normal TV operation possible (B/W)
Whether this means that the indicator can be switched off…?

[decoder_82]

There is a switch with several positions among them "bild" and "Peilen". I would expect not to see the bar in "bild" position.
I will check the connections on that switch maybe there is something disconnected.

[picburner]

If you need the service manual this seller has it.

[decoder_82]

Ok problem solve actually there was a button that was stuck (see picture). When it is in pushed position it makes the bar appear.
Thanks all for the help 🙂