16 Jun
2006
16 Jun
'06
1:23 a.m.
For many years I used an analogue VOM only. To be
honest, it's rare that
you need to make an accurate meashuremnt when repairing a classic
computer (you want to know if the 5V line is pressent and correct, but if
it's acutally 4.95V, things will still worl fine). Then I picked up a
cheap Fluke 85. I use that almost exclusively now, but the analogue meter
stays on my bench, it's better for osme things.
On this (semi) subject, I vividly remember for a while seeing a "solid state
VTVM"
that had the extremely high input impedance of a VTVM/DMM + the good low-range
response with an analog output. Now, all I see are the "classic" passive VOMs.
What happened to the VTVMS? were they "not necessary"? I'd possibly be
worried about
meter loading messing up some computer circuits with a passive VOM (10-20k vs 10M
meter resistance).
Scott
16 Jun
16 Jun
1:37 a.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer checked)
On Fri, 16 Jun 2006, Scott Quinn wrote:
On this (semi) subject, I vividly remember for a while
seeing a
"solid state VTVM" that had the extremely high input impedance
of a VTVM/DMM +
sounds nice
but what is a "solid state vacuum tube" volt meter ???
1:52 a.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer checked)
On 6/15/2006 at 10:37 PM Fred Cisin wrote:
sounds nice
but what is a "solid state vacuum tube" volt
meter ???
I think it works like this--you take a 12AX7, and, using a diamond file,
carefully cut the tip off. Fill it with a good polyester resin and glue
the tip back on. Absolutely solid! Put that in your Eico!
Anyone remember the April Fool's joke article in (IIRC) 73 many years ago
that advocated filling vacuum tubes with brine to improve conductivity?
Cheers,
Chuck
2:05 a.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer checked)
>but what is a "solid state vacuum tube"
volt meter ???
On Thu, 15 Jun 2006, Chuck Guzis wrote:
I think it works like this--you take a 12AX7, and,
using a diamond file,
carefully cut the tip off. Fill it with a good polyester resin and glue
the tip back on. Absolutely solid! Put that in your Eico!
Do you have to pay a "hazardous waste disposal fee" when you let the
vacuum out?
28 Jun
28 Jun
10:29 a.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer
At 05:11 PM 6/16/2006, Tony Duell wrote:
It depends on how the vacuum was produced, no? Making a low
vacuum with mercury might result in a fair amount of
highly regulated mercury vapor in the "vacuum."
- John
Do you have to
pay a "hazardous waste disposal fee" when you let the
vacuum out?
No. In fact as a small amount of polution goes into the device in place
of the vacuum, they should be paying you. 
10:48 a.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer
From: John Foust <jfoust at threedee.com>
At 05:11 PM 6/16/2006, Tony Duell wrote:
It depends on how the vacuum was produced, no? Making a low
vacuum with mercury might result in a fair amount of
highly regulated mercury vapor in the "vacuum."
Hi
The amount of mercury in a vacuum is no more than the same volume of
air with exposed mercury in it. The partial pressure is the same. If it is
greater,
the mercury will condense out. Of course if you heat the mercury, you can
increase the partial pressure.
Dwight
Do you
have to pay a "hazardous waste disposal fee" when you let the
vacuum out?
No. In fact as a small amount of polution goes into the device in place
of the vacuum, they should be paying you. 
16 Jun
16 Jun
9:05 a.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer checked)
On Thu, 15 Jun 2006, Chuck Guzis wrote:
On 6/15/2006 at 10:37 PM Fred Cisin wrote:
sounds nice
Something I have on my desk.
A Heathkit VTVM with Teledyne a FET module
replacing the 12AX7...
Peter Wallace
but what is a "solid state vacuum tube"
volt meter ???
9:58 a.m.
From: Fred Cisin <cisin at xenosoft.com>
On Fri, 16 Jun 2006, Scott Quinn wrote:
Hi Fred
When they first came out with the FET volt meters, they called them
solid state VTVM's. Of course this doesn't make sense but they were
only trying to indicate that these had the higher input resistance similar
to a vacuum tube input.
It was just marketing.
Dwight
On this (semi) subject, I vividly remember for a
while seeing a
"solid state VTVM" that had the extremely high input impedance
of a VTVM/DMM +
sounds nice
but what is a "solid state vacuum tube" volt meter ???
4:18 p.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer checked)
>but what is a "solid state vacuum tube"
volt meter ???
On Fri, 16 Jun 2006, dwight elvey wrote:
When they first came out with the FET volt meters,
they called them
solid state VTVM's. Of course this doesn't make sense but they were
only trying to indicate that these had the higher input resistance similar
to a vacuum tube input.
It was just marketing.
AH! The marketing people decided that an FET was a
"solid state vacuum tube"
I shouldn't have taken it so literally.
Thanks,
--
Grumpy Ol' Fred cisin at xenosoft.com
4:37 p.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer checked)
On Friday 16 June 2006 04:18 pm, Fred Cisin wrote:
Well, shoot! Ain't a vacuum tube really also a "hollow-state FET"?
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
>but what
is a "solid state vacuum tube" volt meter ???
On Fri, 16 Jun 2006, dwight elvey wrote:
When they first came out with the FET volt
meters, they called them
solid state VTVM's. Of course this doesn't make sense but they were
only trying to indicate that these had the higher input resistance
similar to a vacuum tube input.
It was just marketing.
AH! The marketing people decided that an FET was a
"solid state vacuum tube"
I shouldn't have taken it so literally.
Thanks,
--
Grumpy Ol' Fred cisin at xenosoft.com
10:47 a.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer checked)
On Friday 16 June 2006 01:37 am, Fred Cisin wrote:
On Fri, 16 Jun 2006, Scott Quinn wrote:
Uses a transistor setup of some kind, or more likely a FET, rather than a
tube...
I wouldn't mind having one.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
On this (semi) subject, I vividly remember for a
while seeing a
"solid state VTVM" that had the extremely high input impedance
of a VTVM/DMM +
sounds nice
but what is a "solid state vacuum tube" volt meter ??? 
6:09 p.m.
New subject: VOMs (WAS Re: SBC6120 (a build-it-yourself PDP-8 clone) grammer
response with an analog output. Now, all I see are the
"classic" passive VOMs.
What happened to the VTVMS? were they "not necessary"? I'd possibly be
worried about
I think they got replaced by DMMs...
meter loading messing up some computer circuits with a
passive VOM (10-20k vs 10M
meter resistance).
Most good classic VOMs are 20000 Ohms/Volt, which is a cryptic way of
saying they take 50UA to get the needle all the way across the scale :-).
In other words, on the 5V range (if you have one), the resistance is
100k, on the 10V range it's 200k, and so on.
I wouldn't worry about that sort of extra load on the PSU lines or buses
of a classic computer (at least one not decigned to be very low power).
Probing around a crystal oscillator circuit might be another story, but
stray capacitance would cause problems there anyhow.
Note that most RMMs (and VTVMs) are 10M ohms input impedance, no matter
what voltage range you set them to. Which means, actually, a VOM on the
1000V range probably takes less current than a DMM.
-tony
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