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

[Barry Hauser]

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