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(Topic ID: 228367)

Force II, Gottlieb System 80 Power Supply. High 60/42v after rebuild

By tman67234

1 year ago

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  • 10 posts
  • 3 Pinsiders participating
  • Latest reply 1 year ago by G-P-E
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a1 solder side (resized).jpg
a1 (resized).jpg

#1 1 year ago

I'm currently working on getting my power supply board (A2) in order and ran into something a bit suspicious and I'm not quite sure if its normal.

First things first, I checked my transformers and rectifiers and everything seemed good. I ran them by G-P-E (Thanks Ed!) and he agreed.

I just finished installing the following parts to my power supply board (A2):

- TIP31C Transistor (Q1)
- 2N5550 Transistor (Q2)
- 2N6058 Transistor (Q3)
- 1N4004 Diode (CR1-CR4)
- 470uF 100V Axial Electrolytic Capacitor (C1)
- 47uF 100V Axial Electrolytic Capacitor (C2)
- 470uF 10V Radial Electrolytic Capacitor (C4)
- 1N4734A Zener Diode (CR8)
- 1N4738A Zener Diode (CR7)
- 1N4746A Zener Diode (CR6)
- 1N4759A Zener Diode (CR5)
- T10681 (SCR1)
- 500 Ohm Trimmer Pot (POT 1)
- 12K Ohm, 1/2 Watt Resistor (R3)
- 680 Ohm, 1/2 Watt Resistor (R10)

I also made a new fuse panel and replaced the orange cap with a new 6800uf 25V cap.
My 5v stuff appears to rock solid. The 12v coming into power supply board (A2) is a little high, but with some adjustment of the pot, it holds 5v great!

My 42/60v stuff is not so good. Both are about 9v too high! 60v (TP1) is reading 69v and 42v (TP2) is reading 51v. The 60v AC coming in is 64.6v.
I compared some values to the following diagram:

Here were my readings:
E1 76.4v. Mine = 87v
E2 63.6v. Mine = 70v
B of Q2 58.2v. Mine = 64.2v
E of Q1 65.1v. Mine 69v

Note: All of these measurements are under no load.

I seem to gain a ton of voltage after the 4 1N4004 rectifier.

I found this:
Which claims "Vdc = Vac * 1.41" during rectification with no load. While the calculation isn't perfectly correct in my situation, it sort of explains why that voltage is higher after rectification. I'm still worried that something isnt quite right.

Does something seem incorrect? Or am I losing my mind and everything is nominal? Hahaha.

Thanks for the help in advance!

#2 1 year ago

First - don't worry about voltages surrounding Q2 at this time. Q2 is your over-current shut down, it is supposed to shut down Q1 in the event of a shorted output. When everything is up and running - you *could* test the over-current shut down function by purposely shorting TP1 to TP3. But this causes R4 & Q1 to heat up and Gottlieb boards of this age are rather fragile... not sure you really want to do this.

Now as to why output is so high:
E2 voltage is what sets the output voltage from Q1. In theory, the output voltage is normally Vb minus Vbe.
Vb (base voltage) = E2 = 70V on your board.
Vbe is typically in the 0.7V range.
Using your base voltage - output voltage from Q1 (emitter of Q1) should be 70 minus 0.7 or approx 69V. This matches what you measured so this says the Q1 transistor is working fine.

The problem here seems to be that E2 is too high. This would be caused by the 62V zener diode CR5.
CR5 has a normal tolerance of + or - 5%. 62V plus 5% = 65.1... it appears CR5 is out of tolerance.
This can also be caused by a bad solder joint or trace at CR5.

(BTW - Gottlieb's normal emitter voltage from Q1 is 63.1 and not 65.1)

#3 1 year ago

Awesome. Really appreciate the explanation! I see what you are saying and have ordered some new 1N4759As! I'll report back with my results! Thanks!

#4 1 year ago

So I replaced CR5 with a new 1N4759A and same result (if anything it was worse, higher voltage). Reading ~72 volts at the cathode of CR5 (E2). I currently have Q3 and the plate off, so I can access the solder side. The CR5 traces and soldering joints look fine to me.

Here are some pics of the board:
a1 (resized).jpg
a1 solder side (resized).jpg

I'm a bit stumped. Could my new 1N4759A be junk?

#5 1 year ago

Bump. Still looking for advice. I went ahead and ordered a 5W 62v zener (1N5372B) for CR5, just as something to try.

#6 1 year ago

If you come to wits end I will make you an excellent deal on this: link An even better deal if you offer a trade in. I'd tell ya' to buy a Rottendog but I don't like the system 80 version, while the system 1 RD version is or was "the cat's ass", the system 80 falls short. Tim

#7 1 year ago

If the TIP31C is in so-so, questionable condition, it can also wreak havoc on those voltages.

If I could ever find time to make more System 80 PS boards.... mine are quite reliable and heard of anybody having an issue with them.

#8 1 year ago

Hi Ed, NOOOOOOOOOOOOOOOOO make this first: Part Number 120-103. I'll buy 2 and put 1 in my pcb hall of fame.

#9 1 year ago

I ended up hooking it all up, and with everything under load the display voltages were only 2v over spec and are working great (power wise that is. I have some connector bugs to chase down still). So I'm gonna leave it as is!

Thanks for all the help everyone!

#10 1 year ago

No doubt, lots of hands slapping foreheads on this one (or at least for me).
Usually need a load to get a decent measurement, I should have caught that one.

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