1995 Toshiba TIMM MM20E45 Loss of sync?

[NJRoadfan]

Trying to diagnose this one. A video is worth a thousand words right?

https://www.youtube.com/watch?v=KqdH2nx1DX8

I'm trying to determine what the problem exactly is. Troubleshooting guides point to things like bad horizontal sync oscillator and/or problems with vertical sync. This TV is a bit unique that it is a multisync monitor that can take both TV and VGA (480p) signals. Not shown in the video is occasionally the OSD oscillates in a sinewave pattern from not moving like in the video to very quickly and back.

Looking in the service docs (yes I have the service manual and schematics), the synchronization and H/V drive is handled by a Sanyo LA7860. I strongly suspect that chip has gone bad, or one of the electrolytic caps in the circuit is failing. No visible signs of leaking caps or blown components on the PCB that I can see. How the TV handles the OSD is quite complex (since its generated by the jungle chip, but the set uses an external sync driver), but its odd that its mostly working while video input is totally fubared.

This set was sitting for about a year in climate controlled storage and was working fine when last used. I turned it on one day and well, this happened. Normally I wouldn't bother, but this set is low hours and quite rare. It handles a wide variety of video game consoles and vintage computers with its RGB input.

[old_tv_nut]

The onscreen display is probably triggered by the h sweep, so it works (mostly) no matter what the frequency is. But the H frequency is far off. Since sync is multifrequency and apparently handled by an IC, this will be hard to diagnose. You would need at least a schematic showing the correct waveforms at the IC pins and an oscilloscope. My guess is this set was probably meant to be "repaired" by replacing a whole board. You could try blindly replacing lytics, without any way of knowing if its one of them or the IC.

[Yamamaya42]

That TV/monitor uses Phase-locked loop for sync, and will be very hard to fix, it could VERY WELL BE any electrolytic capacitor in the entire PLL chain, and unlike filter caps, values DO matter here, before pointing fingers at the LA7860 , EVERY cap and resistor in the feedback loop has to be checked.

This won't be easy.

[NJRoadfan]

I have the schematics, which show what the waveforms should be in TV mode and in VGA mode at various points around the sync circuit. What I don't have is an oscilloscope, which I'm going to have to rectify.

I can't imagine too many techs knew what to do with these sets when they came in for repair back in the day. Unless they were already working on computer monitors, they wouldn't be familiar with multisync circuits. Toshiba's technical training manual goes into great detail how the sync circuit works as a result. They designed the thing as a standard TV chassis and grafted on the RGB port and multisync drive. At times they note similarities to other Toshiba TV chassis.

Almost forgot. The OSD isn't stable even when the set is in "free run" with no signal input.

[zeno]

Hoz freq is off. If this set has 1 or 2 small sub boards pull &
resolder them. the pins get cold joints. If not dig into the
hoz osc. Sync & H. AFC usually cause a floating pix with one bar.

73 Zeno
LFOD !