Thyristor gate driver problem

[artis]

Hey folks. please help me out, I am having trouble finding the problem with a thyristor driver circuit. So this circuit has been working for many years without problems. It works from 2 of the 3 phases, so 400VAC (I am in Europe). The thyristor is in series with a large coil, aka electromagnet located inside a box attached to a sand separator. The box works like a vibrator. It can be powered directly from 2 phases but in order to lower the vibration amplitude the thyristor circuit is installed.
Recently the thyristor circuit started working intermittently. It works for about 5 mins then stops for about 5 mins or so.
I took the whole circuit box home, checked every part, measured the capacitors (with capacitor tester), measured the transistors, checked the circuit board, connections everything, and it was all fine.
The problem still continues. Then today I was measuring the circuit as it was working LIVE. The thyristor gate driver is supplied from a rectified but unfiltered DC supply of 25 volts, this sags to 22.5 roughly as it begins working.
When the gate drive circuit stops working and the thyristor shuts off, I noticed there is a voltage of 25 VDC across the pulse transformer primary (measuring across the 1N4007 flyback diode) The transformer primary has a resistance of some 2 ohms or less. It is a small gate drive transformer. Clearly a voltage drop of 25 volts across the transformer isn't resulting from current flow through it but most likely I'm measuring from - terminal to which one side of the primary is attached through the unijunction transistor across the 220 ohms resistor connected to + side.

Now here is the weird part. When the gate drive circuit has stopped working, as i touch the cathode side pin of the primary flyback diode (1N4007) with a screwdriver or multimeter probe, it immediately starts working.
Clearly as I touch it even though with an isolated probe, there is a small change in capacitance introduced by my body and this is enough to get the drive going again. Then it stops after a little while again, touch it once more it works again.

It seems to me this could be some kind of a problem with a trimpot maybe changing it's originally set resistance (the two yellow ones are on board 3 pin potentiometers ) or maybe one of the capacitors is bad, although I checked all of them. I even changed the 2N2646 unijunction transistor simply because I had one and I wasn't sure whether the original measured correctly.

Any suggestions? Please see attached schematic

[mtwieg]

Then today I was measuring the circuit as it was working LIVE. The thyristor gate driver is supplied from a rectified but unfiltered DC supply of 25 volts, this sags to 22.5 roughly as it begins working.
When the gate drive circuit stops working and the thyristor shuts off, I noticed there is a voltage of 25 VDC across the pulse transformer primary (measuring across the 1N4007 flyback diode)

I think this clearly indicates the transformer primary is open circuit (either from a fault in the transformer itself or the wiring/pcb). But...

Now here is the weird part. When the gate drive circuit has stopped working, as i touch the cathode side pin of the primary flyback diode (1N4007) with a screwdriver or multimeter probe, it immediately starts working.

Ok now I'm confused, you previously said you measured 25V across it when not working, but here you say that touching it with a multimeter it starts working?

[artis]

@mtwieg Well as I said it is enough to just gently touch the cathode side of the flyback diode and the circuit oscillates again. The reason I measured the 25 volts across the primary I think was because I must have put the probes at different places in a different way before as it was not working and also it didn't trigger the circuit. The voltage could have appeared across the primary because one primary side is attached to the negative diode bridge terminal while the other is attached to one of the two bases of the 2N2646 unijunction transistor with the other base through a 220 ohm resistor attached to the + side of the diode bridge. Well that is my guess.

It also seems to me that the real switching during each half period as it passes through the circuit from the unfiltered DC of 100 Hz ripples is switched by the BC107 transistor and this happens through the charging and discharging of the 1000 uF capacitor connected to it's base. Truth be told I'm not sure what is the actual role of the unijunction transistor.

[Kim Christensen]

Clearly as I touch it even though with an isolated probe, there is a small change in capacitance introduced by my body and this is enough to get the drive going again. Then it stops after a little while again, touch it once more it works again.

Did you try touching it with an insulated stick? Something like a old tuning tool. Perhaps it's a intermittent mechanical fault.

[artis]

Clearly as I touch it even though with an isolated probe, there is a small change in capacitance introduced by my body and this is enough to get the drive going again. Then it stops after a little while again, touch it once more it works again.

Did you try touching it with an insulated stick? Something like a old tuning tool. Perhaps it's a intermittent mechanical fault.

If you think, did I try mechanically knocking it, then no. I checked all electrical and mechanical connections when the board was on my table and all was fine, I couldn't find a single problem. It seems to me that the diode touching is somehow just slightly changing the capacitance of the circuit and that does the job

[schmitt trigger]

The unijunction transistor is the actual device which controls the firing angle.
The NPN transistor appears to be what the GE SCR Manual called the ramp and pedestal circuit, to provide higher resolution.

[artis]

The unijunction transistor is the actual device which controls the firing angle.
The NPN transistor appears to be what the GE SCR Manual called the ramp and pedestal circuit, to provide higher resolution.

Yes it seems that way to me too. But the more I look at it , it looks like the BC107 transistor is almost useless as all it does is provide a parallel path for the current to charge the 0.68uF cap to fire the UJT. But why the transistor, why not just one potentiometer as current path to the 0.68uF cap? I fail to see how that increases resolution?
I am planning to bypass the bc107 transistor and the 23k resistor and just leave the 60k potentiometer and see how that solves the problem.
The diode bridge creates 100 Hz half periods so each half period as it starts it starts also charging up the 0.68uF cap and if it was done only through one potentiometer then the pot would determine how fast the cap charges which would determine the place in the half period at which the UJT fires the thyristor, but now the same happens only through 2 parallel routes as it seems to me, firstly through the 23k resistor which is fixed and secondly through the 1000uF cap in series with the 60k pot series with the 0.68uF cap, when the 1000uF cap has opened the bc107 bipolar, the charging happens faster.

Anyway I would appreciate your thoughts and comments, is the bc107 close to useless in this schematic and can be omitted?