I have a Gottlieb Volcano that is working, but with some minor booting issues. I'm going to post another thread specific to the power-up issues.

I have the manual and schematics.

I don't know if this is related to boot-up, but I have ~0 Volts DC at Test Pin #5 on the A2 Power Supply board. It should be +8 VDC. I'm fairly confident that's because the Zener diode (CR7) is shorted out.

All other Test Pins on the A2 are showing correct voltages.

Tracing this 8 VDC line (wire #133) back to the transformer, it's labeled as "+8VDC Offset" and is connected between the two sides of a 5 VAC output.

Measurements at the transformer show 2.5 Volts AC at connector #12 to ground and also 2.5 Volts AC at connector #10 to ground.

According to schematics, the 5 VAC lines (#122 & #144) simply goes to the Lightbox where they're labeled "5V AC" and "5V AC Return". I'm assuming this is just used for lighting but I don't know for sure.

I'm not understanding the following...

1. What is the purpose of the 8 VDC offset applied to the 5 VAC output of the transformer? Once the machine is booted, everything appears to function normally... why even bother to apply a DC offset?

2. What specifically is the 5 VAC used for in the Lightbox? My schematics do not show.

Thank-you for your help.

BTW: On the same transformer, there is a 3V AC output with a 5V DC offset. Again, each side measures 1.5 Volts AC to ground. The offset measures 5 Volts DC to ground.

So I have two different AC outputs, each with its own DC offset... and yet the one with the missing/broken DC offset behaves similarly to the one with the working DC offset. ???

I hope it's not inappropriate for me to bump this yet. If so, I apologize.

After Googling, I just can't seem to find a logical explanation about why this 5 VAC source is offset with 8 VDC.

Thanks Chris. That makes sense and I understand the general principal. It's the specific circuit that I was unclear about...

I just couldn't see where this 5 VAC went. It just dead-ends on the "Lightbox Wiring" diagram. However, looking at it again (when it wasn't 110 degrees in my shop), I can now see that wires 122 and 144 also go into each display board at pins 15 & 16.

I'm missing my 8 volt DC offset and maybe this explains the slight/slow pulsation I'm seeing in the displays. It's very slight. Otherwise, the displays appear to be functioning normally.

Anyway, I have a new 8.2 volt zener diode on the way.

Thank-you again.

Ugh! I should have checked the R10 resistor... but I carelessly assumed it was the zener diode and only ordered one of those. (The R10 was totally open, infinite resistance, and why I didn't just put my meter across it I can't explain.)

The original zener and resistor were both rated 2 Watts, but all the pinball places selling replacements for the 8.2 Volt zener are selling 1 Watt units.

Finding a 30 Ohm 2 Watt resistor late on a Saturday was quite the challenge as this is not stocked at your local Radio Shack. However, I figured there's no longer a need for a resistor rated 2 Watts since the zener in the same branch is only rated 1 Watt now.

Anyway, I got lucky and a local friend gave me a 30 Ohm 5 Watt ceramic resistor. It's certainly overkill but I like it a lot better than three 10 Ohm resistors in series.

Ok, so new zener and new resistor and the test pin reads 9 Volts... a little high but at least it's not zero any longer, and it's high enough to achieve the stated goal of bringing the entire sine wave above zero.

Yes, bottom line, I see probably an 80-90% reduction in the pulsation of each digit's segment. This is kinda what I expected, since you could previously see a huge difference between the stability of the Credit Display as opposed to the Score Displays.

The original schematic and parts list specified a 30 Ohm resistor with a 2 Watt rating for R10 as well as a 2 Watt rating on the 8.2 Volt zener.

It may work perfectly fine, but how could I have possibly known that simply substituting with a 680 Ohm, 1/2 Watt resistor was another option?

What part is mathematically incorrect? My original resistor was 30 Ohms (orange, black, black).

Maybe the 2 Watt size was overkill.

That's fine, I understand the principal. Thank-you for the link.

The R10 value apparently is not too critical, but at 680 Ohms, it has better current limiting protection for the zener. With less current, you can then use a zener with a smaller wattage rating.

Since I now have the smaller 1 Watt zener, I'll bump up my resistor value to something closer to 680 Ohms.

680 Ohm 1/2 Watt resistors are stocked at Radio Shack stores.

Not surprisingly the 1 Watt zener was running very hot until I swapped the 30 Ohm resistor for the 680 Ohm. Also, the voltage at the test pin went from 9 to 8.5 Volts... still a little too high but nothing to worry about, considering what it's supposed to be doing, and since it's greater than 8 Volts.

All is good.