-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Monday, 21 December 1998 11:12
Subject: Re: Electrical knowledge, was Another ~1960 computer kit
>> A screwdriver (insulated handle!) blade touched to the top cap of the
>> 1S2 would produce a healthy arc when held somewhere near chassis
>
>1S2 = DY86, I believe. OK, I understand now - that's the EHT rectifier,
>right...
Never heard of a DY86, but yes, either that or the damper diode. Can't
remember.
Creeping senility.
>Normally just holding the screwdriver up to the cap would draw a spark -
>you didn't need to have it anywhere near the chassis. And if you didn't
>get a spark, you removed the top cap connector (anode) and tried again -
>to the connector. A common fault was that the diode valve would
>short-circuit, thus effectively short-circuiting the AC EHT via the
>capacitance of the CRT.
>
>> if the line/eht stage was running. This practice did not survive the
>> transition
>> to solid state devices that disliked the spikes etc it could cause!
>
>No, indeed. Producing sparks in transistorised TVs/monitors is a good way
>to blow semiconductors all over the chassis. When I was working on a
>Barco monitor (actually off a classic-computer graphics display system),
>I had an EHT flashover to my fingers (ouch!!) which took out a couple of
>transistors in power supply unit.
Sounds about right. Murphy.
>Some valve output transformers could match pretty low loads. The famous
>Williamson transformer has 8 secondary windings which can be wired in
>series/parallel to match down to <1 Ohm, I think.
I've seen 3 ohm secondaries on valve gear, so that sounds possible.
The 1930's radio it was in had a field coil loudspeaker, first one I ever
saw.
Cheers
Geoff Roberts
Computer Room Internet Cafe
Port Pirie
South Australia.
netcafe_at_pirie.mtx.net.au
Received on Sun Dec 20 1998 - 18:26:30 GMT