Commodore 1084S-P1 No image but HV is present

[decoder_82]

Hi !

I am trying to repair a Commodore 1084S-P1. The monitor has high voltage (noise, static electricity on the screen, spark when discharging) but image remains dark. Heater filaments are not glowing on the tube.

When I measure on CRT neck board, I get not voltage for the heater pins. When I test for continuity, from the heater pin, I have continuity to flyback pin 1, 2, 9 and 10.
I would like to measure if some heater voltage on the flyback itself, but I am not sure where is the heater voltage. The flyback is a HR7533 (https://www.donberg.ie/catalogue/line_output_transformers/hr_diemen_line_output_traformers/hr_7533.html).

The service manual for the monitor can be found here: https://gona.mactar.hu/Commodore/monitor/schematics/Commodore_1084S-P1_Colour_Monitor_Service_Manual_Full_High_Quality.pdf

Any advice would be appreciated.

Many thanks

[BrokenYugo]

The heater circuit looks pretty simple to me, start buzzing things out and find the break, starting by confirming you have continuity through the heater right at the neck connector. Perhaps one of the inductors on the neck board went open?

It is possible your multimeter may not like measuring the filament voltage as it's running at 15KHz.

If the oscillator is running (must be since you have HV), the heater is there, the winding in the flyback is there, and the circuit between is all there, you'll have a hot heater.

[fzabkar]

The filament voltage is generated from FBT pins 1 and 2 in your circuit. If you can measure 23V at capacitor 2547, then the transformer is doing its job.

[floobydust]

I would measure continuity at the CRT heater pins and work backwards to the flyback transformer. Because it's really three filaments RGB in parallel, opens for all 3 are uncommon.
Check for cold/fractured solder joints at the CRT socket, two ferrite chokes, etc. CRT sockets can oxidize and make a bad connection due to the Kovar pins. So pulling off the socket and reattaching it can fix some intermittents.

If the CRT has air in it, the filaments will stay low resistance and will not glow. You can check the CRT datasheet, using a bench PSU with some current-limiting to inject say 6VDC 1A and see if they light up, this is with the flyback (heater) winding disconnected of course. Look if there is Getter present in the CRT.

[shakalnokturn]

Usually if CRT has let air in you'll have other obvious symptoms if the HV supply is still working.

[decoder_82]

Thanks all for the answers !

Some additional info based on the replies:

• There is continuity on CRT neck between the heater pins. It does not seem the filaments are open.
• There is 22.3V at capacitor 2547. Should I rule out the FBT as being the issue, right ?
• There is continuity between CRT heater pins and work backwards to the FBT to pin 1 and 2
• Solder points on the FBT have all been reflowed to make sure there is no issue there

For now I will continue to look on the main board and see if there is any component in the area that is bad. I do not what else could be wrong.

[james_s]

Usually if CRT has let air in you'll have other obvious symptoms if the HV supply is still working.

Yes I once powered on a TV that had been damaged in shipping and there was a spectacular crackling light show in the neck of the tube. If it was up to air you would absolutely know as soon as the HV came up.

[decoder_82]

OK I managed to find the issue. There was a bad contact at the level of the M1 connector on the CRT neck board.
After soldering the wire that had bad contact directly to the board, I got my filaments glowing and the image on the tube.

[james_s]

Nice job, another piece of vintage gear gets a new lease on life.