[sdiy] Zener diode wattage tolerances

Harry Bissell harrybissell at wowway.com
Mon Jul 9 20:09:58 CEST 2012

At least its a uA723 regulator based design. Try looking at app notes for that chip. The schematic is probably
right off the data sheet.

BTW if the zener is a SHUNT regulator (no series pass transistor) the very act of leaving it unloaded might cause the zener
to burn off the board.

It would be a fight between the expected load and the series resistor, remove the load and the zener must eat ALL
the current.

That would be a sh!tty design imho but if they were sure what the load would be, it could be a cheap solution...

H^) harry

----- Original Message -----
From: Bob K <farfisa5 at gmail.com>
To: ASSI <Stromeko at nexgo.de>
Cc: synth-diy at dropmix.xs4all.nl
Sent: Mon, 09 Jul 2012 14:01:18 -0400 (EDT)
Subject: Re: [sdiy] Zener diode wattage tolerances

I was just doing some research in regards to the photos I took.  It
appears that this pcb is actually taken from a Gottlieb pinball
machine from the early 1980s!  That would definitely explain the large
voltages.  The person must have hacked it into the synth for some

Weird weird stuff.

Well, it looks like I have something to tinker with now.  :)

I'll definitely be careful and do some more searching.

Thanks again everyone!


On Mon, Jul 9, 2012 at 10:48 AM, ASSI <Stromeko at nexgo.de> wrote:
> On Monday 09 July 2012, 09:53:07, Bob K wrote:
>> I picked up a homebrew synth at a yard sale over the weekend.
> A homebrew synth with a professionally made PCB and a serial number?  There
> are a lot of unusual voltages on that PSU board (60V, 42V, 12V, 8V, 5V),
> what is this?  The connectors seem to indicate some Telco stuff?
>> Documentation and some crude schematics came with it and, after
>> opening it up, I saw this burnt out component:
>> The documentation tells that it's a zener diode (8.2V, 2W, 10%).
>> Looking online, I'm finding 8.2V zener diodes but only with a 1.3W and
>> lower or higher than 3W.  Nothing in the 2W range.
>> I'm guessing a 1.3W diode should the job, right?
> That diode burnt out for some reason.  If I'm judging the size of it
> correctly from the surroundings, there is no way that it would be able to
> dissipate 2W over a longer period of time.  So we'll have to assume that the
> 2W specifictaion really means something around 250mA maximum reverse current
> and quite a bit lower (like 75mA) on average.  If anything, I'd replace it
> with something having a little bit higher rating, but not lower -- unless
> the schematic clearly tells you that the reverse current is in fact limited
> to some much lower peak and average value (150mA in the case of a 1.3W
> diode).  But in that case you will need to carefully check why that current
> limiting did quite apparently not work.
>> This is my first time trying to diagnose and repair something.
> Whenever you see something that has failed this badly you should assume that
> the actual fault was (or still is!) someplace else and proceed with caution.
> Careful with those 60V and 42V, my assumption is the synth probably doesn't
> use those, but any short to these supplies will produce another round of
> fireworks.  You should get hold of a bench supply that can current limit
> (backfolding characteristics preferred) and check that anything past that
> PSU is fault free.  Then get the PSU back into operating order and check
> that it works seperately, connecting it to a dummy load.
> Regards,
> Achim.
> --
> +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
> SD adaptation for Waldorf rackAttack V1.04R1:
> http://Synth.Stromeko.net/Downloads.html#WaldorfSDada
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
Synth-diy mailing list
Synth-diy at dropmix.xs4all.nl

Harry Bissell & Nora Abdullah 4eva

More information about the Synth-diy mailing list