[Premium-Rx] RF-590 display **Found the problem**

[Gary Mitchelson]

It was a .5W board PCB mounted Zener, I'm replacing it with a 1W.
Everthing else in the circuit seems to measure OK.

-----Original Message-----
From: Barry Hauser [mailto:barry at hausernet.com] 
Sent: Sunday, November 24, 2002 22:27
To: Gary Mitchelson; premium-rx at ml.skirrow.org
Subject: Re: [Premium-Rx] RF-590 display **Found the problem**


Gary wrote:


> Well I found the problem on why the unused segments had a slight glow,

> there was no 6.2 V Bias to the Display Converter (the sealed box). VR1

> (provides the 6.2 VDC from the 35VDC supply) was toasted causing a 
> short (really about 20 ohms) to ground. Luckily Radio Shack has 6.2 V 
> Zeners in stock so I'll pick one up tomorrow. Everything else in the 
> converter ohms out OK.

Hi Gary & list:

Yup, that's one of the things I mentioned when you first posted.  Yet
another case where it wasn't the display itself, but a supply or ground
problem.  The vacuum luminescent displays don't seem to fail at the same
rate that the LCD's do which largely replaced them.

However, I had suspected it to be more likely a drifted resistor or
failed cap.  Question is now -- what caused VR1 to fail?  It's possible
that it failed on its own, but could be due another component in that
circuit.  Was this heat-sunk to anything?

I've noticed heat-sink junctions failing on some of the aging solid
state equipment -- on power supply transistors and tab-mount VR's.  The
heat sink grease has turned powdery and/or from heating and cooling over
the years, the device has become loose.  The solution is to remove the
semiconductor or IC, clean the surfaces including both sides of any
insulators, and apply fresh heat sink compound.

It's probably a good idea to check the current draw on the new zener
when you install it.  It would also be good if the replacement is of
next higher wattage than the original.

Barry