Help Understanding VFD +100v, -100v Topology

[kolbep]

Hi All
I am working on repairing a Secret Service Pinball Machine by Data East, that somebody had their hands in before.
What I am enjoying about this one is that I have the full schematics/service manual, so it is very easy to work on, unlike every other undocumented board nowadays.

I have already replaced the 2 failed electrolytics on the CPU Board (470uF Power Input Reserviour, and the 4.7uF power on reset capacitor).
The machine is now starting up consistently, and not locking up.

I am now working on the Powersupply board that has been worked on by somebody else previously.



Before powering on, I removed a solder short between the Emitter and Collector on TR3. I don't know if he did it accidently, or if they did it to put the unregulated voltage to the VFD's as a test.
From what I see, I am only missing +100v. the -100v output is fine, and all other voltages are stable.
The Transformer gives out 90v to 100v, which then feeds this section of the Powersupply circuit.
I am just trying to understand the topology, so that I can bench repair the powersupply before putting it back in the cabinet to test.

I measure at various points, and I am getting approx +137v and -137v at various points, but it seems that I am not getting any output from TR1, which causes TR3 to not conduct.

I would like to understand more how this circuit works (how it generates both plus and minus voltages of both 100v from a single 100vac input, and where I can test further.

If somebody has a reference or link to a description of this type of circuit, I would love to read further to see how it operates.

Thanks
Peter

[pcprogrammer]

I'm no expert on these things but think it works like stated below.

The plus and minus from the are made in respect to ground with the diodes D5 and D7. For what I understand from it TR3 and TR4 are setup as emitter followers, so the voltage on the base is almost the same as the voltage on the output. This means that TR1 and TR2 are there to set this voltage in collaboration with the zener diodes D9 and D10. R12 and R13 are there to limit the current through these zener diodes. TR1 and TR2 in their turn are controlled with the zener diodes D6 and D8 and resistors R8 and R9.

At the bases of TR3 and TR4 you should measure about plus and minus 100V in respect to ground.

Not getting +100V from TR3 means that either TR1 is dead or the zener diode D6 is shorted.

Hope this helps.

[floobydust]

So this is how Data East got those VFD displays super bright. They did put Williams to shame.

TR1, D6, R8, R12 make a constant-current source to supply the ref zener D9 around 4.7mA (edit: had wrong zener it's 1N5228 not 1N522B)
Measure voltage across R8, D6, R12 to see where current is not flowing.
The same constant-current source is on the other (-100V) rail as well, using TR2, D8, R9, R13 supplying D10.

[perieanuo]

hi, it's just some mono-alternance rectified standard topology
the voltage regulation after rectufier is also standard zener-driven topo, so easy to debug
check voltage on 1N4764 (100V, the voltage-regulation reference) and 1N5228 (3V9, this one forces TR1 in constant current mode), you get the idea