B&O TV AVANT 100Hz

[Brasto]

This Avant 100 Hz Chassis 83/00 is giving me a headache. It is hardly
to service due to the complexity of wiring interconnecting all the
different PCB's.
The problem is very simple:
Set switches on flawlessly
There is Sound, and programs can be selected, but:
There is no video picture
The CRT shows a blank raster with pronounced retrace lines which
cannot be removed completely by readjusting the CRT G2 voltage.
Probing the video processor IC TDA 4780 learned that there is video
INPUT available but NO RGB output can be seen on the oscilloscope. So
as a first approach the IC was replaced by a new one.
Results after replacement identical. Checking the pin-out voltages
learned that ALL showed logical results, except for RGB feedback at
pin? reading low (2VDC ilo 5 VDC)
I hope one of the guys out here can shed some light on my problem box
which is fortunately fully covered with its service documentation....
TY
Brasto

 

[Franc Zabkar]

This Avant 100 Hz Chassis 83/00 is giving me a headache. It is hardly
to service due to the complexity of wiring interconnecting all the
different PCB's.
The problem is very simple:
Set switches on flawlessly
There is Sound, and programs can be selected, but:
There is no video picture
The CRT shows a blank raster with pronounced retrace lines which
cannot be removed completely by readjusting the CRT G2 voltage.
Probing the video processor IC TDA 4780 learned that there is video
INPUT available but NO RGB output can be seen on the oscilloscope. So
as a first approach the IC was replaced by a new one.
Results after replacement identical. Checking the pin-out voltages
learned that ALL showed logical results, except for RGB feedback at
pin? reading low (2VDC ilo 5 VDC)
I hope one of the guys out here can shed some light on my problem box
which is fortunately fully covered with its service documentation....
TY
Brasto

I notice that the TDA4780 has automatic cut-off control. Perhaps the
cathode current feedback voltage is lower than normal because the IC's
outputs have been turned off. If so, then maybe there is a fault in
the RGB amps, or maybe there is something wrong with the CRT, eg
heater-to-cathode short? Having said that, shouldn't the CI feedback
voltage be zero when the RGB outputs are off? Does the voltage
fall/rise as you reduce/increase the G2 voltage?

Do you have a URL for the circuit diagrams?

I noticed the following statement in the TDA4780 datasheet:

"After switch on all signals are blanked and a warm up test
pulse is fed to the outputs during the cut-off measurement
lines. If the voltage at the cut-off measurement input
exceeds an internal level the cut-off control is enabled but
the signal remains still blanked."

http://www.datasheetarchive.com/pdf-datasheets/Datasheets-25/DSA-498568.pdf

Would it be possible to lift the CI pin and short it to ground?

- Franc Zabkar

 

[Brasto]

I notice that the TDA4780 has automatic cut-off control. Perhaps the
cathode current feedback voltage is lower than normal because the IC's
outputs have been turned off. If so, then maybe there is a fault in
the RGB amps, or maybe there is something wrong with the CRT, eg
heater-to-cathode short? Having said that, shouldn't the CI feedback
voltage be zero when the RGB outputs are off? Does the voltage
fall/rise as you reduce/increase the G2 voltage?

Do you have a URL for the circuit diagrams?

I noticed the following statement in the TDA4780 datasheet:

"After switch on all signals are blanked and a warm up test
pulse is fed to the outputs during the cut-off measurement
lines. If the voltage at the cut-off measurement input
exceeds an internal level the cut-off control is enabled but
the signal remains still blanked."

http://www.datasheetarchive.com/pdf-datasheets/Datasheets-25/DSA-4985...

Would it be possible to lift the CI pin and short it to ground?

- Franc Zabkar

Franc that's good info, when I replaced the IC I have mounted it on a
socket , so indeed I'll be able to lift the IC pin and ground it. I'll
work on that next week and keep you posted.
I have no URL for the manual, I have the genuine document

 

[Brasto]

Franc that's good info, when I replaced the IC I have mounted it on a
socket , so indeed I'll be able to lift the IC pin and ground it. I'll
work on that next week and keep you posted.
I have no URL for the manual, I have the genuine document

Franc,
The machine is back to normal life. Doing a check on the RGB driver
IC's mounted on the CRT PCB I noted that they were NOT receiving the
12VSA at the supply pins of ALL 3 drivers. Poking around learned that
it is very unlikely that all 3 little resistors would have opened up.
now using the DMM rather than the Scope I also noted that all 3
Cathodes of the CRT were reading abnormally low voltages....
Where is that 12 VSA coming from? It took a while but then EUREKA,
this TV set was serviced before by the Handy Nephew who had managed to
reconnect the ribbon cable from the CRT PCB twisted over 180 degrees.
(Granted B&O engineering had not bothered to provide a polarization
key to the connector).
Well to keep the story short, after reinserting the connector and
keeping fingers crossed for newly developped problems, the CRT now
showed a picture which only required a readjustment of G2 and some
Focussing.
Another fine machine saved from the junkyard.....

Brasto