HP 6177B series zener diodes ??

[coppercone2]

I am replacing the diode on this thing because alot of them are like chipped. I got all the replacements but on the PCB there is a deviation from the manual. VR13 has two diodes soldered together in series.

The manual says
6.19V 5% 1N753.

But on the PCB there is two diodes.

one is a "light blue (almost greyish), yellow ----> brown" (brown I believe indicates the diode orientation, on black glass body).

The second diode looks like a much physically smaller diode.
its bands are

"brown, deep sky blue, yellow".

So it looks like this

"light blue, yellow, brown" -> yellow, sky blue, brown


WTF is this?? they twisted the leads together and bend it over with alot of uncut lead left over


But when you probe it, it measures like a clamp. That is, both diodes are facing each other. so positive lead in middle makes both ends beep.


The smaller diode drop is 0.5, the bigger one is 0.7

[coppercone2]

did they decide to drop the voltage by 0.5 volts? as far as I can tell thats all it does

[coppercone2]

Oh Its in change notes but not documented in the diagram. its hp 1901-0461

[andrewtaylor]

did they decide to drop the voltage by 0.5 volts? as far as I can tell thats all it does

the simple but significant reason: temperature compensation for the voltage in the overall circuit..

[coppercone2]

is this still recommended with modern diodes?

the device did not improve by enough to negate this benefit did it?

[coppercone2]

also is it possible this component was matched for drift? maybe I should leave it alone, even if its chipped. it reads fine on the meter

[ArdWar]

That's a signal voltage clipper/limiter zener, no one got time to put temperature compensation for that.

More likely they just don't want it to forward conduct when some user inevitably do a fuck up and force the guard to higher voltage.