B&O TV AVANT 100Hz
in [Repair]
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From: Brasto on 13 Dec 2008 15:20 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
From: Franc Zabkar on 13 Dec 2008 17:40 On Sat, 13 Dec 2008 12:20:32 -0800 (PST), Brasto <bram.stolk(a)gmail.com> put finger to keyboard and composed: >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 -- Please remove one 'i' from my address when replying by email.
From: Brasto on 14 Dec 2008 03:24 On 13 dec, 23:40, Franc Zabkar <fzab...(a)iinternode.on.net> wrote: > On Sat, 13 Dec 2008 12:20:32 -0800 (PST), Brasto > <bram.st...(a)gmail.com> put finger to keyboard and composed: > > > > > > >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-4985... > > Would it be possible to lift the CI pin and short it to ground? > > - Franc Zabkar > -- > Please remove one 'i' from my address when replying by email.- Tekst uit oorspronkelijk bericht niet weergeven - > > - Tekst uit oorspronkelijk bericht weergeven - 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
From: Brasto on 23 Dec 2008 17:36 On 14 dec, 09:24, Brasto <bram.st...(a)gmail.com> wrote: > On 13 dec, 23:40, Franc Zabkar <fzab...(a)iinternode.on.net> wrote: > > > > > On Sat, 13 Dec 2008 12:20:32 -0800 (PST), Brasto > > <bram.st...(a)gmail.com> put finger to keyboard and composed: > > > >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-4985... > > > Would it be possible to lift the CI pin and short it to ground? > > > - Franc Zabkar > > -- > > Please remove one 'i' from my address when replying by email.- Tekst uit oorspronkelijk bericht niet weergeven - > > > - Tekst uit oorspronkelijk bericht weergeven - > > 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
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