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We are fixing up an old Atari game cabinet, and we keep blowing the Zener diode D802. This is a 51V zener diode, but we don't understand what it's doing and so we can't figure out what is causing it to be overloaded.

The concept of operations for the supply is here:

enter image description here

Below is the 110-120VAC to 123VDC power supply schematic (relevant part shaded in white).

The AC goes through a half-wave rectifier at D801, and is smoothed by the bulk 800uF capacitor, C803.

Our local electric grid supplies 120VAC, so the rectified voltage is 170VDC. Somehow, this 170V winds up flowing backwards through the Zener diode with enough energy to fry it.

With an IR camera, we watched a brand new Zener die during a 1 second power blip.

We have pulled the fuse and replaced the load with a 65W light bulb, which seems to be a good fit as a dummy load (about 25% of the fuse's 1.5A rating.)

If we desolder D803, leaving an open circuit, then the system stabilizes at about 74V. Coincidentally, 74V + 51V = 125V, which is that the user's manual says we should see on the base leg for Q801.

enter image description here

  1. What is D802 Zener's purpose in the voltage regulator?
  2. Knowing that we have tested the D803 diode, the R804, R805, R807 and R808, and that Q802 has the appropriate 12.8V on the emitter (this value is defined off screen), what's a guess for where to look for the current sink which is destroying the Zener?

We'd love some insight into how to make this old cathode ray tube Millipede game live again.

JRE
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Kenn Sebesta
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  • @jsotola Looking up the part number, it does appear to be an avalanche diode, though: https://pdf1.alldatasheet.com/datasheet-pdf/view/38186/SANKEN/RM25.html. It doesn't appear to be used in breakdown here, though, so it's probably just being used as a rectifier. – Hearth May 02 '21 at 05:39
  • Mmmm one more guess, fuse resistor R803 – carloc May 02 '21 at 13:12
  • If you haven't already done it, check or replace all the electrolytic capacitors in that section of the power supply. When they fail, they can leak (electrically,) allowing current to flow in directions it wasn't intended to. – JRE May 02 '21 at 15:18
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    Possibly a really dumb question, but are you sure the text "51V Zener Diode" corresponds to D802 (where you drew the red arrow)? RM25 appears to be a 40V avalanche diode. D804 actually uses the zener diode symbol, so perhaps "51V Zener Diode" refers to D804 not D802? – abligh May 02 '21 at 15:53
  • Further to @abligh's question, the first thing I noticed was that the symbol for D802 is a regular diode, whereas the only Zener in the diagram is D804... – Eight-Bit Guru May 02 '21 at 16:53
  • We had wondered about that ourselves, because the manual draws a zener diode correctly elsewhere in the schematic, however the parts manual we have is specific that D802 is a 51V zener diode. (According to the manual, D804 is an 11V Zener diode). It is possible the parts manual is in error. – Kenn Sebesta May 03 '21 at 18:05
  • This may be a really dumb thing to suggest but did you get the polarity of the zener right? – mvds May 03 '21 at 20:00
  • Not a dumb thing at all! We had a similar question. We concluded we did it right because we followed the circuit board silkscreen, checked the silkscreen was correct, and referenced a photo of the failed part as installed on the circuitboard. – Kenn Sebesta May 03 '21 at 21:35

4 Answers4

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I think it's for soft start. Q801 conducts as soon C803 reaches those 51V. Before, only the 20W R802 feeds into node B2.

Janka
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  • Makes sense. One strange thing is that we've noticed when we pull the diode, the regulator stabilizes at ~73V. It might be a coincidence that 73+51 = 124, which is the desired base leg voltage, or it might be by design. – Kenn Sebesta May 02 '21 at 12:21
  • That's because the regulator is trimmed that way to diminish the effect of that diode during normal operation. – Janka May 02 '21 at 13:45
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I'd rather double check the series pass transistor and if okay go ahead chasing for some overload downstream.

i.e. disconnected the HV flyback driver. Most likely you have a faulty HV flyback transformer.

Those circuits are usually rather critical in the sense that they work fine as long as all their parts are okay but otherwise unexpected interactions arise.

In this case could be that enough drive for the "normal" load comes from R804 and R805 while exceeding one could call D802 and R809 into play .

carloc
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D802 and R809 make C804 charge up quickly until getting close to the regulated voltage (when R804 takes over to provide ripple filtering).

If Q801's Base-Emitter junction is short circuit or its Collector is open-circuit, or R803 or connection B8 is open-circuit then the load will draw high current through the Base (and D802) because the Collector isn't supplying any.

Other possibilities include:-

  • Q801 has low gain, causing increased Base current.
  • C804 is leaky, dragging the Base voltage down below 120 V and causing D802 to continue conducting.
Bruce Abbott
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  • Thanks, that was just what I was looking for. I simulated the circuit with falstad, https://tinyurl.com/yj8jgwgu. You can absolutely see the rapid voltage stabilization by playing with the simulation, in particular by cutting the diode out of the circuit. It takes much, much longer to bring the voltage up to spec. – Kenn Sebesta May 03 '21 at 21:29
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Any kind of a short on the 123V supply will cause the diode to conduct through the BE junction of Q801 and R809 (only 15 ohms). So probably something not shown. I would look at the deflection transistors (especially horizontal deflection).

Spehro Pefhany
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  • Right now we have the load removed and replaced by a lightbulb so the only thing which isn't shown is that Q802's output is used as the base leg input to a 12V regulated supply. We've pulled that 12V regulation transistor, so in theory there should be nothing left. We'll keep looking. Thanks for the advice to focus on the deflection transistors, though, since something on the main board obviously made the power supply fail. – Kenn Sebesta May 02 '21 at 12:19