I have repaired a few dozen of AS-2518-16, AS-2518-22, SDU-100 boards.
Most of them I fixed with ease.
Now I have two boards which got me puzzled.
The 5V section is no problem, fairly simple the fix.
The solenoid driver section also no problem.

The High Voltage section is a bit harder.
When this section is broken it is very easy to notice the burned R51 (22K) resistor.
I replace the 2N3440 transistors and the expencive 2N3584 gets replaced with a BUX85G transistor.
When all the transistors are off the board, I check the diodes and the resistors for correct value.
Check the potmeter for correct working order and replace when nessesary.

On the two problem boards I can't get the voltage regulated.
It will stay at a low 100V and does not change when adjusted via the potmeter.
It looks like the 140V zener diode is working correctly (240V-140V=100V output).
Can someone send me in the correct direction to search for the broken/incorrect part?
Can someone explain exactly how this high voltage regulating section works?

Peter

Bally_AS-2518-22_High_Voltage_Regulator_schematic (resized).jpg

I looked at some of the pages Google shows, but these are to generic to get to understand this (above) circuit.
Non of them show the 3 transistor type regulator.
Stern SDU-100 even got a 4 transistor regulator, but for now let's stick to this type of regulator.

I've printed the schematic and will take some measurements this weekend.

Which part would be the best replacement for this zener diode?
My local parts shop recommends 1N5382B 5W zener diode.
It's a bit too big for the holes in the board and you would have to drill new ones.
Is the 1N5275A 0.5W zener to light for the board?
Is there a better alternative?

I guess the cheap ass China 2N3440 transistors ($0.60) must be low quality....
ebay.com link: feedback_profile

I'm getting more and more the feeling this could be my problem.
More measure time tomorrow.

This is the result of todays measurements.
The green values are from a working board, set at 171V output.
The red values are from the not correctly working board (97V output).

Which transistor (or other component) is not working correctly?

Bally_AS-2518-22_High_Voltage_Regulator_schematic_measurements (resized).jpg

In this case, yes.

Yes, I'm sure, but will check again.

It was just 0.3 to 0.5V lower than the emitter, not higher.
That might be why the transistor does not turn on?

I doubt it is the zener.
It was already behaving like this with the old zener, before I replaced it with a 1N5382B.

Today I replaced both Q22 and Q23 to be sure.
No change....
With the board out of the game I measured the resistors.
One odd thing...
Some resistor values can't be measured (with a DMM) correctly because of the capacitor in the circuit.
For example R56.
This is a 82K resistor, but when measured in circuit it will start at 1M and slowly drop down (on the good board).
When measured on the bad board to goes to 82K rightaway.
Another sign is the normal resistance over diode CR21.
On the good board it is a steady 1K5.
On the bad board it starts at 134K and goes up in value.
I even disconected the collector from Q21 to rule the transistor out, but no effect in both situations.
When you measure the totall resistance of R54, RT1, R56 theoreticly you should be getting something around 115-120K.
When measured on the good board it starts around 1M and goes down slowly (due to the cap).
When measured on the bad board it goes rightaway to 115-120K.
It does not matter how I use the red or black lead of the DMM.
AS-2518-22_back_bad (resized).jpgAS-2518-22_back_good (resized).jpgAS-2518-22_front_bad (resized).jpgAS-2518-22_front_good (resized).jpg

Don't be fooled by the zener.
On this bad board it is the old one, but I have another bad board with a new zener behaving the same way.

No, the RT1 does nothing to change the voltage.

R51 are new old stock on both boards and the value is good.
Both boards had them burned previous to repairs.

Checked both R35 with one leg disconnected and are both good.

The zeners on these boards are original.
I have another bad board with a new zener and it behaves the same way as this bad board.
Even a new RT1 potmeter.
Replaced all the suspects...

C26 does discharge...
I always wait for the capacitor to discharge before taking it out of the game.
I check with my DMM to see if it does/is discharged.
I hate those shocking experiences.

Replaced the zener with a new Z4KE140A.
Replaced the 1N4004 diode.
Replaced the 2N3440 transistors again.
Checked all the resistors with one leg desoldered.
All checks out fine, but no change....
It does not explain why there is too low voltage.

The only thing I did not replace are the two disk capacitors C27 and C28 (I do not have them).
They look good and the resistance is high.

I've been working on these boards again....
Indeed R51 was getting very hot in a short time.
This led me to the decision to replace the 2N3440 transistors again.
I've received the new 140V zeners, so replaced them too.
This time I had succes!
Both boards are working like they should.

I must say I do not trust those cheap Chinese transistors.
Next time I think I will use the TIP50 transistor.

No other source, just the crapshoot I did with the transistors I have.
I still have a few left to go.
When I'm out I'll order other type of transistors.
The source I got them from is in post #11. Unreliable seller!!! like Ed said in post #14.

This whole problem of the low voltage is thus typical for bad 2N3440 transistors.
Given the short time I keep the test machine switched on, the R51 did not burn right away.
If I would keep it switched on it would burn.
Another lesson learned...

Replacement for which transistor do you mean?
Image please...

If you mean for the high voltage regulator, the voltage for the TIP47G is too low.
My input voltage to start with is 270V.
The TIP47G maximum is 250V.
https://nl.mouser.com/datasheet/2/308/TIP47_D-1814988.pdf

barakandl Would this heat sink work for the TIP50G?

https://nl.mouser.com/ProductDetail/Wakefield-Vette/291-H36AB?qs=VVKQmw408U%252Bz99MOvnriAQ%3D%3D

No screws to get lose, not too heavy.
Giving just a bit more cooling....

Heat_Sink_Wakefield_Vette (resized).jpg

Done!
Thanks for the tip.
He had only 2 lots of 20 left, so I bought both.
40 good transistors on their way...

Nope, I fix boards for a lot of other people.
This will allow me to fix 20 boards.
They'll be gone in a year or so, maybe two.

Just to summerize the possible replacement parts....

Q22 and Q23 are 2N3440 and is getting harder to find.
Replacement can be a TIP50, but be aware of the correct orientation (as with all transistors).
The use of a small heatsink is advised for Q23 (like Wakefield-Vette/291-H36AB).

Q21 is a 2N3284 (very expensive and hard to find).
A good replacement is a BUX85 mounted on the original heatsink (very easy to do, cut the middle leg).

The zener diode VR1 is a 1N5275 and is getting harder to find.
A good replacement is a Z4KE140A zener diode.

The adjustment pot RT1 (25K) is an open type and cause bad adjustment.
This can/should be replaced with a Bourns 3306P-1-203 (20K).

Do you mean the 4 transistor version like this schematic?

Stern_High_Voltage_Regulator_schematic (resized).jpg

I've seen it happen when you put the display connector in reverse on the display board.
It instantly fries the 1K2 resistor on the MPU.
Some cheap ass repairman did not put in a lock pin and the connector got twisted around.
I failed to see the red wire on the wroung side of the connector before it was too late....

That is why I keep repairing these boards.
Most of the time it is not only the HV section that needs to be repaired.
The 5V caps needs to be replaced and the upgrade needs to be done.
Solenoid drivers and CA3180's need to be checked too.