[ARC5] Off topic - OS-8

[David Rossetti]

Got a Hickok 380A O-scope off of eBay very inexpensively, probably because
no one recognized it as a Hickok direct sale of the military OS-8 (Hickok
did that with some of their tube testers as well: e.g. the KS-15750 and the
RD 1575, the KS-15874-L2 and the nearly identical USM-118). Don't remember
exactly which version of the OS-8 it crossed to.

Have digital copies of the B and C OS-8 manuals and schematics harvested off
of the web. Contact me off-line if interested and I will send them to you.

Dave Rossetti
410-279-0226 (mobile)


-----Original Message-----
From: ARC5 [mailto:arc5-bounces at mailman.qth.net] On Behalf Of Brian
Sent: Sunday, July 19, 2015 1:07 AM
To: kgordon2006 at frontier.com; ARC5 at mailman.qth.net
Subject: Re: [ARC5] Off topic - OS-8

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 

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