Re: 7854 fixed, now for some newb questions


Chuck Harris <cfharris@...>
 

The USA market for USA made test equipment back then was essentially
unlimited. The European market was just a little extra greed. So,
the bean counters wanted it, but the engineers doing the designs didn't
spend much time thinking about it.

As a result, things like using 120V safety caps that though rated to
handle 240V were used... even though the safety margin was much reduced
of what you would like to see in 240V land. And the biggest offense
was treating 50Hz as if it the 16% reduction if frequency wouldn't matter
to 60Hz power transformers and fan motors.

Not a conspiracy, just greed and indifference of USA engineers.

Solid state scopes from the discrete component era usually have a
some method to affect a slow increase in intensity on power up. My
7854 doesn't start bright, but rather the intensity ramps up to just
about right.

As far as I know, all 7854 keyboard units have sticky switches. It is a
simple clearance issue. Tektronix made the holes in the aluminum front
panel too close tolerance to the plastic buttons. Further, the buttons
are tapered so that they are narrow at the front, and wide behind the
panel... Only, the holes in the panel are punched square.. not tapered.

If you file the squareness into a taper by relieving the hole on the back
of the aluminum panel, you can improve the situation without making a
noticeable change to the panel. It must be disassembled, which isn't
hard at all.

Lubrication won't do the job.

There is a lot of very fast digital circuitry, packed very tightly, into
the 7854. It needs the air. Do not put a reduced capacity fan in this
scope.

Also, it is easy to have the fan rotating the wrong way. It has a plug
with no key, and it is a DC fan. The fan should suck the air out of the
case, not blow air into the case.

The RAM in the 7854 is not low power stuff. It will suck a big battery
down pretty quickly, which is why the older scopes had the ability to use
a large external battery. The last version of the scope had more modern
memory, and tektronix decided to use an internal battery... but I would
bet that it would be shot lived too, as they put a switch to disable the
battery on the back of the scope.

One of the nicest features of the scope is the standard GPIB interface
on the back. It works very well. But, by now, the address selection
DIP switch will be dirty, and the address might not be what the switch
says it is. You should replace it, or clean it, and verify its function.

The 7B87 has an ability to trigger the scope's waveform memory at any
point in time after the 7B87 is triggered. That way you can record
with great fineness an event that occurs later. You don't really need
it, but it is nice to have.

Single shot events are not the 7854's forte. It is only capable of
50K samples per second in that mode. The 7854 is really a sampling
scope with storage, that doubles as a 400MHz 7000 mainframe.

-Chuck Harris



Nenad Filipovic ilmuerte@gmail.com [TekScopes] wrote:

Dear All,

A few days ago I've become a proud owner of a mint condition 7854 with the
calculator keyboard (still in its original plastic wrap). What was wrong
with it - exploded capacitor in the integral EMI filter of the power
connector. It splattered a load of brown goo on the outside, the filter
itself bulged so hard that I couldn't pull it out at first. I was already
thinking about a conspiracy of fancy US gear against European line voltage,
but then noticed that the filter itself is "Made in Switzerland", splendid.

Cute mainframe, especially the programming mode that constantly reminds me
of the T-800's red first person view from Terminator 2. And that postfix
notation gets me talking like Yoda after a long programming session. It's
my first 4-bay mainframe from the 7000 series, so I have a lot of questions:

1. Excessive brightness on power up. My 465 does this as well, when powered
up from cold state the image comes up quite bright, somewhat defocused, and
then settles to normal after ~10min. Not a major showstopper but it's
annoying to have to turn it down on every power up, and then back up once
it warms up. On the 7854 it seems more pronounced on scope trace
brightness, and less pronounced on readout brightness. Any suggestions?

2. My unit is a late model, it does not have the usual memory backup power
connectors at the back, but instead a memory backup switch. What is the
life of that backup battery inside, should I replace it? I read that long
thread about 7854 RAM/ROM board upgrade, ROM rot was reported as a common
issue on 7854s, are the late models free from this problem?

3. Sticky buttons. I remember there was a thread discussing this, but could
not find it in the group archive. There were talks about plastic swelling
over time and getting stuck in those tight tolerance holes, so filing away
hole edges was proposed as a solution. I wonder if this issue could be
fixed without taking everything apart? Keywords to find the original sticky
buttons thread would also help (I've read "7854 build in keyboard sticky"
but it's not that one).

4. Noisy fan. It's not worn out, it's the sheer speed that creates the
noise. I'm not sure if it's original, so I was wondering if it's normal to
be so noisy? Of course it keeps the scope quite cool (even with the
monstrous 7L18), but November in Europe is chilly so most tech stuff like
it. I'm wondering if it would be safe to turn the RPM of that fan down?

5. I don't have the 7B87, so am I missing much? To be honest, at the moment
I do not have any proper time bases to experiment with (while waiting for
7B10 and 7B15 to arrive I'm temporarily using the 7L18's sweep to mock time
base functionality). I would like to use my 7854 to record single shot
events, I hope a delaying time base + it's internal trigger would be
enough. Not sure what I'd really gain with the external sampling clock
input of the 7B87?

Best regards and thanks,
Nenad Filipovic






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Posted by: Nenad Filipovic <ilmuerte@gmail.com>
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