Hor sync problem

[Giulio Maiocco]

Hi guys,

I'm working on the first model of Italian TV, a Geloso from 1954. The set now works fairly well, but I still have a big problem: the Hor sync often goes out and the image is shakey in the horizontal direction. I made the regulations on the synchroguide hor oscillator, but little to no changes. I still changed all the capacitors (except of course the micas in the RF and IF sections and the ceramics models). If you want a copy of the schematic, please write at my address g.maiocco(at)tiscali.it and I send it to you via Email. Please remember that the file size is about 700Kb

I could clearly hear the sync buzz in the loudspeaker, but the contrast control does nothing as to attenuate it, it just blank the whole screen. The video out tube was changed from a 6AH6 to a 6AQ5..

Thank you

Giulio Maiocco

[jpdylon]

I would start troubleshooting the AGC and sync section, you may just have too high an input signal that is overloading the set. Moderrn signals from VCRs, cable, etc are often just too high for these sets to handle.

Unless you are certain that 6AQ5 is the proper substitute, I would put the right tube back in before you start troubleshooting.

Just my two cents..

[Giulio Maiocco]

Hi Jordan,

The image, when the Hor is correctly synchronized, looks very good, so I think the 6AQ5 sub is OK. Usually when the TV is turned on, the hor is fine, then after about 30 mins, it starts to shake and finally the sync goes out. Shortly after I started the thread, I noticed the regulation on the oscillator coil needs to be "all screwed in" to get sync when the hor is shakey and it would need to be in even further in to adjust properly.. Could it be a mica cap in the hor section breaking down after a while?

Thanks again

Giulio Maiocco

[Chad Hauris]

You may need to do a thermal test...either take a heat gun or hair dryer when the set is cold and apply heat, or wait till the trouble occurs...then take some freeze spray and start spraying resistors and caps in the horizontal or sync area to see if it will come back.
It probably would be a good idea to unplug the set while spraying the freeze spray on the component, then plug back in and note results.

[bgadow]

If it were me I'd go along with what Chad suggests. I would target resistors first. You could also try subbing the horiz osc tube.

[Giulio Maiocco]

Hi,

I think I found the problem: now the TV won't synchronize, no matter what you do to it, so I hooked up my old trusty (tube based) Oscope to the test point on the hor oscillator coils and, surprise, there is no trace of the two typical toes, just a plain sawtooth. Now that reminds me that when the TV was working, I saw the toes disappearing for the brief moment the TV lost sync... What does it mean?

Please excuse my poor English!!

Thanks

Giulio

[Giulio Maiocco]

Hi,

A little update:

The toes displayed with the oscilloscope hooked on the hor osc coils test point sometimes reappears, but they won't last for more than a second. I also noticed that the sawtooth waveform varies the amplitude randomly. When I try to adjust the regulations on the hor osc coils, I obtain a raster only on a small portion of the range of regulation and at the same time the 1B3's filament comes on, otherwise it is dark..

Any suggestion?

Thanks

Giulio Maiocco

[sean]

On a couple of sets I worked on the horizontal would go out of sync after working fine for awhile. In both instances it turned out being mica caps that became leaky. In one set it was the cap coupling the sync separator to the hor. phase detector. This put a high voltage on the phase detector (something like 110V when it should have been a couple of volts). In the other case it was a capacitor in the horizontal oscillator circuit. Start checking your voltages in the horizontal section.

[Giulio Maiocco]

Hi,

Sean, you were right, the problem turned out to be a 200pF mica capacitor in the hor oscillator circuit. Now the oscillograms are very stable

I wish you all a happy christmas and a happy new year

Thanks

Giulio Maiocco