6 May
2013
6 May
'13
5:40 a.m.
Fun times with a TRS-80 Model 4 (such a nice looking machine!).
http://www.classic-computers.org.nz/blog/2013-04-06-replacing-model4-screen…
Terry (Tez)
6 May
6 May
9:22 p.m.
Fun times with a TRS-80 Model 4 (such a nice looking machine!).
http://www.classic-computers.org.nz/blog/2013-04-06-replacing-model4-screen…
Why did you replace the CRT? I notice that your final (?) photo has a
caption of the form 'enjoying the green screen', but I thought all M4s
had a green phosphor CRT. Well, M3s that have been upgraded don't I guess.
As regards removing the final anode connector ('anode cap'. 'EHT flower'
:-), etc) most of the time the CRT is automatically discahrged either
via a bleeder resistor or via the beam current of the CRT. The only time
I've had a reasonable residual voltage on the final anode was on a CRT
where the cathode (yes, I mean cathode and not heater) had become
disconnected from the base pin, so no beam current.
The residual charge in any case is unlikely to be dangerous iteslf, but
it may make you jump and hit the CRT or similar.
I use my EHT voltmeter (a Heathkit one cosnisting of an 800M resistor and
50uA meter in series). It passes enough current to discharge the CRT and
of course you can see when it's discharged. But not everybody has such
an instrument (although it's a very useful thing to own if you work a lot
on monitors).
-tony
9:50 p.m.
Reminds me of working on a recent acquisition... a Mac Plus. The display
wasn't working and the case was REALLY dirty to boot, so I removed the
CRT/power supply board and all the other guts to R&R and clean the case in
the kitchen sink. I told my wife who was sitting on the couch, "you're
going to hear a loud snap, so don't freak out." I grounded the tube and...
nothing. On most other monitors I've worked on, I've gotten at least a snap
out of them (especially color ones), but not with this one.
On Mon, May 6, 2013 at 3:22 PM, Tony Duell <ard at p850ug1.demon.co.uk> wrote:
Fun times with a TRS-80 Model 4 (such a nice looking machine!).
http://www.classic-computers.org.nz/blog/2013-04-06-replacing-model4-screen…
Why did you replace the CRT? I notice that your final (?) photo has a
caption of the form 'enjoying the green screen', but I thought all M4s
had a green phosphor CRT. Well, M3s that have been upgraded don't I guess.
As regards removing the final anode connector ('anode cap'. 'EHT flower'
:-), etc) most of the time the CRT is automatically discahrged either
via a bleeder resistor or via the beam current of the CRT. The only time
I've had a reasonable residual voltage on the final anode was on a CRT
where the cathode (yes, I mean cathode and not heater) had become
disconnected from the base pin, so no beam current.
The residual charge in any case is unlikely to be dangerous iteslf, but
it may make you jump and hit the CRT or similar.
I use my EHT voltmeter (a Heathkit one cosnisting of an 800M resistor and
50uA meter in series). It passes enough current to discharge the CRT and
of course you can see when it's discharged. But not everybody has such
an instrument (although it's a very useful thing to own if you work a lot
on monitors).
-tony
11:09 p.m.
Why did you replace the CRT? I notice that your final
(?) photo has a
caption of the form 'enjoying the green screen', but I thought all M4s
had a green phosphor CRT. Well, M3s that have been upgraded don't I guess.
No, only the later M4s had green screens. Earlier ones did not. My B/W
one had some screen burn. There was a faint pattern etched into the
screen. In the past life the unit must have been on for a long time
showing just that one screen image.
I've worked on Macs in the past and usually have grounded the tube first.
Someone told me that if the machine (at least this TRS-80) had been off
for 24 hours then there is little risk of getting a shock, so I didn't
bother grounding first. As it was there was no problem.
Terry (Tez)
11:31 p.m.
On Tue, 7 May 2013, Terry Stewart wrote:
>had a green phosphor CRT. Well, M3s that have been
upgraded don't I guess.
When the M4 came out, the college paid $1000 each to upgrade the M3s to
M4s. They could have bought M4s for $600 and paid $100 to have the drives
and RAM tranferred.
screen. In the past life the unit must have been on
for a long time
showing just that one screen image.
VisiCalc?
7 May
7 May
8:06 p.m.
On Tue, 7 May 2013, Terry Stewart wrote:
IRIC< the upgrade was a new CPU board and new keyboard. Were those parts
really more expensive than a complete machine?
>had a green phosphor CRT. Well, M3s that have
been upgraded don't I guess.
When the M4 came out, the college paid $1000 each to upgrade the M3s to M4s. They could have bought M4s for $600 and paid
$100 to have the drives
and RAM tranferred.
Drivee, ues. The floppy drives, PSUs, FDC board and RS232 board are the
same between the M3 and M4.
But not the RAM. M3s use 4116s (16K*1 RAM). An M4 can have 1 bank of
4116s (the very uncommon 16K version), 1 bacnk of 4164s (64K) or 2 banks
of 4164s (128K, you need t ofit a PAL chip in place of a jumper block for
this upgrade). So the 4116s from a M3 are no use in an M4.
-tony
8:02 p.m.
I've worked on Macs in the past and usually have
grounded the tube first.
Someone told me that if the machine (at least this TRS-80) had been off
for 24 hours then there is little risk of getting a shock, so I didn't
bother grounding first. As it was there was no problem.
The charge is stored betwee nthe final anode (aquadag coating on the
inside of the 'flare') and a similar groudned coatign on the outisde.
This forms a capacitor with the glass envelope of the CRT as the
dielectric. This is the smoothing capacitor for the EHT supply, of course.
Now glass is a pprtty good insulator, and it's unlilely the capacitor
would self-disharge due to such leakage in a day oe so. But there are
other discharge paths. Some better monitors, paricualrly colour ones have
a bleeer resistor across the EHT supply. Often this is actually a
potential divvder used to measure the EHT voltage and thus control a
regulator circuit. In any case, it'll discharge the CRT in a few minutes
at most.
The otehr discharge path is via the electron beam. If, at turn-off, the
HET genrato suts fown first, there will be some reisidual beam current
(the cathode will eb hot enough to emit electrons, etc) and this will
obviously discahrge the CRT. I susepct that's what haappens in the M4,
sicne I am not aware of a bleeder resistor in the circuit. This will take
a minute or so at most.
I think leaving it overnight will do no more good than leaving it for 5
minutes.
However, it's always safest to discharge the CRT before working on it.
Beelder resitors can fail. Due to a fault there might be no beam current
at all. I anm not in favour of just shroting the anode conenctor to
chassi groudn, though. If there is a considerable residual charge, the
peak current will be quite high .This can damage the conenction between
the fianl anode and its conenctor inside the CRT. Yo uca nalso end up
with currents flwoing where you least espsct them,zapping semiconductors
all over the machine. No, I prefer to use a resistor or EHT meter for
this.
-tony
4032
days inactive
4033
days old
6 comments
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participants (4)
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ard＠p850ug1.demon.co.uk -
cisin＠xenosoft.com -
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terry＠webweavers.co.nz