Andrew Back writes:

It still dropped out with the current set at 7mA. After a while a
buzzing sound develops which becomes increasingly low frequency
until the plasma is lost, and sometimes it will then strike back up
again. Not sure why it's dropped out unless some component is
heating up and causing failure -- perhaps related to the voltage
being down also.

If it's low even when the discharge is stable, then likely tube contamination.
I doubt a He soak will help. Unfortunately, it's not uncommon for SP-120s to
have tubes that won't stay lit even at 8 mA though the color looks good.

One test to confirm it's the tube and not the power supply is to put a fan
on the tube. If it's the tube, that should enable it to stay lit at least
for a longer time.

(This assumes that the 3.7 kV value in the Laser FAQ is correct. It's from
the spec sheet but I have not measured a healthy one.)

Possibly time to install a modern laser tube or head.

--
sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.