[ARC5] Off topic - OS-8

[Jim Wiley]

Just a note of reminder: Anything with a "KS" number is almost certainly 
something built for the Bell System, not the military.  These tub 
testers are Bell System items.  Doesn't really affect the price of tea 
in China, but just sayin'.

- Jim, KL7CC



On 7/23/2015 4:44 PM, David Rossetti wrote:
> 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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