Andrew Back writes:

Hello,

I recently picked up an old Spectra Physics 120 HeNe laser head plus
256 Exciter, and I'm not getting any laser output. However, the tube
glow looks to be around the right sort of colour and I don't think
there is air leakage.

I've yet to try cleaning the optics, but I have checked the tube
voltage and current and I'm not sure if I have a problem here, as
it's 2.7kV and 6mA, whereas Sam's Laser FAQ suggests it should be
3.7kV and 7mA. Also, after being powered up for a while the power
supply seems to drop out -- I get a buzzing sound and the plasma
goes intermittent, before stopping altogether. When it's
intermittent the tube voltage spikes up to around 4-5kV (at least
that's what I see it jump to on my DMM). So, perhaps a PSU component
is on its way out?

I'd be grateful of any suggestions that folks may have. I'm hoping it's
simply a case of resistor or cap that has not aged well.

Hi:

The tube probably IS at least one of the problems assuming no one has
messed with the mirror alignment.

There's a range of current that would be OK. I'd suggest increasing it
to 7 mA. That should stop it from dropping out. I don't think there
is anything wrong electrically but the tube has leaked. If you can run
it for a few hours or days, it may recover at least partially.

--
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