[ARC5] Off topic - OS-8

[Brian]

Hello Ken,

The OS-8 series is a great little servicemen's portable CRO ... if you only 
need to see up to 50 kHz. I have had several, and still have one (OS-8G/U), 
though I fire up the TEK more frequently, now.

For me in Australia, the major limitation is the mains input Voltage. I got 
around that by selecting a suitable capacitor to put in series with the 
primary to get 240 Vac down to 110 Vac. I learned this trick when I was 
teaching at Warwick University in the UK - we had all sorts of test 
equipment from the US, and rather than increase the pressure on the floor 
through extra iron, I built up a box with a switchable range of mains-rated 
capacitors - I still have it ... somewhere. I could get the correct 
capacitor value in seconds.

When you replace the selenium rectifiers with silicon, rather than double - 
or even treble - your refurb work, just put one, carefully selected 
capacitor in series with the primary.

The only other problem I had with this CRO was two, moderately large 
resistors - 11 k as I recall, that drifted and took the picture off the face 
of the Earth. If I had the manual, I could tell you where they were. An 
electronic copy of the manual would be helpful ... pretty please?

73 de Brian, VK2GCE.

On Sunday, July 19, 2015 6:34 AM , you said:

In my stash of "stuff" I found two OS-8s: an OS-8 and an OS-8B.

I have manuals for both units.

Both "work" after a fashion, but the trace is skewed and not a 
straight-line.

I suspect the main problem is in the power supply section.

Looking at the schematics, I see that there are three diodes, all selenium
rectifier types. It has been my experience that selenium-diodes become,
after a long while, not so much a diode, but a dual-resistor which shows a 
bit
less conductivity in one direction than another.

Therefore, I plan to replace them with modern diodes and a series resistor 
to
keep the voltage closer to the original voltage.

What is the group's experience with OS-8(X)?

Ken W7EKB