That's one of the early Zirffa clocks, with the large Supertex (now Microchip) driver chips. Your symptoms seems to say that the HV driver is no longer working well.
First thing I would do is get some alcohol and clean up the gunk around the pins of the tubes. These look like flux residue and flux has a habit of causing being or becoming conductive. It's a long shot, but you may find that the situation improves.
Check if anything (especially the large three legged device on the right gets hot. After that you might want to check the voltage across the large capacitor (10uF 400V). You will probably find that it is not reaching 170V but instead is getting stuck at between 120 to 150V or so.
Ian, thanks for your quick replay.
- I have cleaned around the pins - Didn't fix the problem.
- Nothing get hot.
- I have checked the voltage between the big capacitor and it starts from 160 V and slowly drop down to lower voltage 140, 130,120 and after minutes or so even to 90 V. Does it mean that the big capacitor is the cause of the issue ?
Well, the clean up was worth a try, but it really was a long shot.
It's a bit hard to put the blame on a single component at this stage. I would have expected something to get a bit warm. If it is the large cap which is leaking, you're putting a couple of Watts into it and something should get warm. If nothing at all is getting warm, then it may well be that the HV is not being generated in the first place.
If I recall correctly, this is a totally traditional MC34063 boost circuit, and the thing that would cause the boost to drop slowly like this is that the frequency is not stable, or the power is not arriving to the circuit properly.
If you have a scope I can help you look at what is going on. Otherwise we'll have to swap parts out and hope for the best.
The first place to start is pin3 on the 34063 and check that the frequency there is stable. My suspicion is that the frequency is drifting and we're no longer in the boost area.