Test your zener diodes

[oski_98]

I recently had an interesting experience refreshing the power circuit of an Atari Lynx II and learned some things that might help others. I had just replaced Q12 (MOSFET), Q8, Q7, and D13 (zener diode) of the power circuit. I had done four of these before and inspected the work closely so I just put the unit back together and powered it on.

 

Immediately I noticed that the screen was very bright, much brighter than I had ever seen it before - so bright that even at the lowest brightness setting I could barely see the game splash screen. I immediately powered it down and suspected that too much voltage/power was issue. I turned it back on one more time and measured the voltage across C41 at 8.5V, much higher than the 4.8-4.9V I would expect. I thought about turning it back on and troubleshooting, but decided instead to just go to bed and come back to it tomorrow with a fresh mind - I didn't want to risk turning it on more.

 

So the next day I best guessed that probably the issue was the zener diode, since that is the component responsible for stepping down the voltage and so I decided to remove it and test it. So I fired up the hot-air rework station and as soon as I saw the solder start to melt I gently tried to move it with my tweezers and it came apart. It just shattered into two pieces in my tweezers. First time that's ever happened to me. I'm positive I was handling it gently.

 

So I grab a new zener diode and decide to test this one with my multimeter before installing it. Forward bias reads .71V and reverse is 0L (open circuit) as expected. Then I install it and remember that I had taken forward and reverse bias measurements of the diode when installed from another unit: forward biased .29V and reverse bias .94V. I have no idea why those are the measurements when installed, I just know that they are for another Lynx II that works fine (if anyone can explain it I would love to know). The newly installed diode is close enough to these measurements (.29V forward biased .82V reverse biased) so I fire it up and finally get 4.8V across C41

 

Moral of the story: test your zener before you install it. And if you want, test it after you install it using the measurements provided in this post. Who knows it might even save a Lynx or two.

 

 

 

[+Stephen Moss]

2 hours ago, oski_98 said:

Forward bias reads .71V and reverse is 0L (open circuit) as expected. Then I install it and remember that I had taken forward and reverse bias measurements of the diode when installed from another unit: forward biased .29V and reverse bias .94V. I have no idea why those are the measurements when installed, I just know that they are for another Lynx II that works fine (if anyone can explain it I would love to know).

When measuring components in circuit it is not always the component alone you are measuring as the voltage supplied by the meter when measuring resistance/continuity/diodes will flow through other components connected to it, parallel components in particular will affect the result so having different measurements when in circuit compared to out is not unusual when measuring resistance or checking diodes.

 

Is it possible that you inserted the Zenner forward biased instead of reverse bias?

Zenner's conduct in both directions, when forward biased (Anode to higher potential) it will act like a regular diode (forward voltage drop of around 0.5-0.7V), only when connected in reverse biased (cathode to higher potential) will it act like a Zenner dropping 4.7V.

[oski_98]

Thanks for the reply. I was looking for a more specific explanation of how the circuit works that would explain the difference.

 

I understand that the surrounding components would affect the measurements I take with my multimeter when the component is installed vs. uninstalled. The reason I provided the measurements in my previous posts was precisely to quantify what the effect is when I take measurements with my DMM (Fluke 117). Though I guess I could be making some bad assumptions about the consistency of these measurements across different boards and different DMMs.

 

I'm 100% confident the diode was installed in the correct direction: with the black stripe towards the RF shield. Here's a WIP picture I took while re-capping the board.

 

 

I think more likely explanations are that the diode was defective or that I damaged it during installation.

[ChildOfCv]

The best way to test reverse bias of a zener is to put it in a test circuit with a resistor and a DC supply.  At least 10K would probably be safe for sane input voltages, or you could gradually increase the supply voltage until the diode voltage tops out.

 

If you do this a lot, you could possibly build a board with the resistor permanently mounted and inputs for the power supply, and contact pads for the diode.

[Igor]

Anyone looking for details of how to replace the MOSFET and zener here's a guide I put together -

 

https://atarigamer.com/pages/replacing-the-mosfet-and-zener-diodes-in-an-atari-lynx

[oski_98]

@ChildOfCv the test circuit approach also turned up during my research on the topic. It seems like that's the best way to test it. I confess I was simply too lazy to build a test circuit.

 

I wonder if one of these ATmega328-based component testers sold on Amazon for $15 would do the job just as well.

[ChildOfCv]

I seem to recall them reporting on zener voltage.  But cheaper designs will have low limits on their capabilities, like either 5V or possibly 9V.

 

I see one that's made specifically to test zeners, a Zen50 tester.  Sells for a bit over $50 on DigiKey.  Claims it can test them as high as 50V.  At least it's more convenient than attaching meter probes and a battery.