21 May
2012
21 May
'12
2:20 p.m.
I have a working Pet 2001-8 that has one annoying issue. When I power up the unit I
typically don't see any characters displayed unless I apply a slight amount of
pressure on the back of the brightness control in the back of the monitor. I suspected
bad solder joints but couldn't see anything wrong. Turning the control brightens the
screen but somehow this control is causing the character display to turn on/off. Any
ideas what might be going on here?
Thanks,
--Jim
21 May
21 May
2:43 p.m.
On Mon, May 21, 2012 at 5:20 PM, Jim McCusker <jim_mccusker at hotmail.com> wrote:
I have a working Pet 2001-8 that has one annoying issue. ? When I power up the unit I
typically don't see any characters displayed unless I apply a slight amount of
pressure on the back of the brightness control in the back of the monitor. ? I suspected
bad solder joints but couldn't see anything wrong. ?Turning the control brightens the
screen but somehow this control is causing the character display to turn on/off. ? Any
ideas what might be going on here?
Bad solder joints or cracked/lifted traces should be easy to visually
identify and easy to correct. If you are careful not poke around live
circuits with conductors (i.e., screwdrivers), you could remove the
back of your monitor, power up the PET, and tap at the body of the
brightness control with something non-conductive, like a chopstick.
The idea is to see if the problem follows the entire pot or just the
handle of the pot - it's possible you have bad/broken traces, in which
case moving the body would also produce symptoms, or it's possible the
pot itself is mechanically defective, in which case, the body is
stable but agitating the knob causes symptoms.
I've done some minor work on those boards (defective 7812 regulator),
and they aren't that hard to work on, but if you haven't worked around
CRTs, do look up and follow safe handling procedures.
-ethan
3:22 p.m.
New subject: Tandy Model 4 with typical problem
I have a model 4 that developed a problem with the video / floppy
drives. When both floppies are plugged in, no video.
With one floppy drive installed, the system works OK. I remember the
flex cable was usually the culprit.
I've inspected the cable and it looks ok, should I be looking at
something else?
Thanks,
Jim
22 May
22 May
12:06 p.m.
New subject: Tandy Model 4 with typical problem
I have a model 4 that developed a problem with the video / floppy
drives. When both floppies are plugged in, no video.
With one floppy drive installed, the system works OK. I remember the
flex cable was usually the culprit.
I've inspected the cable and it looks ok, should I be looking at
something else?
THe flex cable (the one from the CPU board to the floppy controller
board) is a common cause of problems with the M3 and M4, but I don;t see
how it could cause the machine to work with 1 drive and fail with 2. It
carries part of the CPU bus, the floppy controlelr board works out how to
select drives.
I would suspec that for some odd reason the PSU (the one for the drives,
on the side of the drive tower) is shutting down when you add the second
drive. Either that drive is defective (and drawing too much current --
does it matte which derive is conencted if you only have one of them)? or
that the PSU is defecitve nad can't supply enough current to powe the
controlelr and 2 drives.
-tony
11:57 a.m.
Bad solder joints or cracked/lifted traces should be
easy to visually
identify and easy to correct. If you are careful not poke around live
circuits with conductors (i.e., screwdrivers), you could remove the
back of your monitor, power up the PET, and tap at the body of the
brightness control with something non-conductive, like a chopstick.
A 6" length of 1/4" nylon rod (sold for extending control spindles, or
available from some materials suppliers) is good for this.
The idea is to see if the problem follows the entire
pot or just the
handle of the pot - it's possible you have bad/broken traces, in which
case moving the body would also produce symptoms, or it's possible the
pot itself is mechanically defective, in which case, the body is
stable but agitating the knob causes symptoms.
My expeirence, alas, is that bad connections are anything but helpful in
this repssoect and that tapping the body of the pot wil lcause the wiper
to break contact if it's defective.
I've done some minor work on those boards (defective 7812 regulator),
and they aren't that hard to work on, but if you haven't worked around
CRTs, do look up and follow safe handling procedures.
THe high voltages are certainly unpleasant, but they are at pretty
limited currents and are unlikely to be fatal. The UK 240V (or 230V,
depending on which you beelive) mains is a lot more dangerous.
That is not to say you don't follow safe practices, of course you do. In
particular work one-handed (keep your left hand firmly in your pocket),
assume everythign is live unless _you_ have proved otherwise, etc. But I
find there are a lot of unnecessary fears about working on small
monochrome monitors.
-tony
21 May
21 May
2:56 p.m.
On 21/05/12 5:20 PM, Jim McCusker wrote:
I have a working Pet 2001-8 that has one annoying issue. When I power up the unit I
typically don't see any characters displayed unless I apply a slight amount of
pressure on the back of the brightness control in the back of the monitor. I suspected
bad solder joints but couldn't see anything wrong. Turning the control brightens the
screen but somehow this control is causing the character display to turn on/off. Any
ideas what might be going on here?
Thanks,
--Jim
Wild guess from an electronics n00b here; perhaps the wiper of the
potentiometer has bad contact with the resistive strip?
--Toby
3:06 p.m.
On May 21, 2012, at 5:20 PM, Jim McCusker wrote:
I have a working Pet 2001-8 that has one annoying issue. When I power up the unit I
typically don't see any characters displayed unless I apply a slight amount of
pressure on the back of the brightness control in the back of the monitor. I suspected
bad solder joints but couldn't see anything wrong. Turning the control brightens the
screen but somehow this control is causing the character display to turn on/off. Any
ideas what might be going on here?
Could easily be dirty contacts on the wipe for the pot. Try isopropyl
alcohol, or any number of commercially-available contact cleaners for
stereos, etc.
Or, given your description, the slight flex you're applying might be
wiggling other bad solder joints elsewhere on the board to the point
where they connect. I've had that happen (it was a nasty bugger to
find, too).
- Dave
4:34 p.m.
On May 21, 2012, at 5:20 PM, Jim McCusker wrote:
Thanks Dave, I'll give the isopropyl idea a try, followed by closer inspection of the
circuit board while the back is off. I have the circuit schematics but not enough
experience with monitors to feel confident touching very much inside while the circuit is
energized. I'll take some photo's if it's still not working and see if
anyone familiar with the circuit sees anything. Once I get this working I'll be
working on a few bad chicklet keys and then I'll have a complete working system.
--Jim
I have a working Pet 2001-8 that has one annoying issue. When I power up the unit I
typically don't see any characters displayed unless I apply a slight amount of
pressure on the back of the brightness control in the back of the monitor. I suspected
bad solder joints but couldn't see anything wrong. Turning the control brightens the
screen but somehow this control is causing the character display to turn on/off. Any
ideas what might be going on here?
Could easily be dirty contacts on the wipe for the pot. Try isopropyl
alcohol, or any number of commercially-available contact cleaners for
stereos, etc.
Or, given your description, the slight flex you're applying might be
wiggling other bad solder joints elsewhere on the board to the point
where they connect. I've had that happen (it was a nasty bugger to
find, too).
22 May
22 May
6:32 p.m.
On May 21, 2012, at 7:34 PM, Jim McCusker wrote:
Thanks Dave, I'll give the isopropyl idea a try,
followed by closer inspection of the circuit board while the back is off. I have the
circuit schematics but not enough experience with monitors to feel confident touching very
much inside while the circuit is energized. I'll take some photo's if it's
still not working and see if anyone familiar with the circuit sees anything. Once I get
this working I'll be working on a few bad chicklet keys and then I'll have a
complete working system.
Yes, be careful inside monitors, especially if you're not confident of what
you're touching. The tube can have several to tens of kilovolts on it.
Ethan's suggestion of probing around with a wooden or plastic chopstick is
a very good one.
- Dave
23 May
23 May
11:50 a.m.
Yes, be careful inside monitors, especially if
you're not confident of what
you're touching. The tube can have several to tens of kilovolts on it.
Ethan's suggestion of probing around with a wooden or plastic chopstick is
a very good one.
It is _very_ difficult to come into contact with the EHT (fianl anode
voltage) accidnetally unless the insulation is breakign down. It's pretty
much confined to the flyback transformer, possibly an external rectifier
diode, and the CRT anode connecotr. OK, some colour monitors have a high
vlaue sense resisotr/potional divider hung off this supply too, but it's
all encapsulated and insulated so you won't accidentally touch it.
But you will find high voltages ont he CRT base pins and thus the monitor
PCB tracks. Expect a 80-110V supply for the video output stage (CRT
cathode), and up to 800V on the earlier anodes in the electron gun.
Although they are not likely to be able to supply enough current to be
fatal, they are certainly painful!
It's not uncommon to find a few hundred voltas on the brighness control
wires, so be careful there.
(Most monitors have a brightness control that varies the control grid
voltage and this can be a hundred volts or so _negative_ wrt ground.
However at least oen monitor (the one used in the HP120 and HP150?) has a
brightness cotnrol that varies the first anode votlage, and has 400V
positive on it. No I didn't find that the hard way!)
-tony
22 May
22 May
11:47 a.m.
I have a working Pet 2001-8 that has one annoying issue. When I power up =
the unit I typically don't see any characters displayed unless I apply a sl=
ight amount of pressure on the back of the brightness control in the back o=
f the monitor. I suspected bad solder joints but couldn't see anything wr=
ong. Turning the control brightens the screen but somehow this control is =
causing the character display to turn on/off. Any ideas what might be goi=
ng on here?
Well, it could be a dry joint, but the other obvious poissiblilty is that
the brightness control itself is worn and that the wiper is not making
good contact wit hthe carbon track.
Have you tried giving it a short of contact cleaner (propan-2-ol, for
example)?
-tony
1:16 p.m.
From: ard at p850ug1.demon.co.uk
Subject: Re: Commodore Pet 2001-8 w/flaky brightness adjustment
To: cctalk at classiccmp.org
Date: Tue, 22 May 2012 19:47:55 +0100
I'll be giving this a try tonight and will post an update. Hopefully it'll do the
trick.
--Jim
I have a working Pet 2001-8 that has one annoying issue. When I power up =
the unit I typically don't see any characters displayed unless I apply a sl=
ight amount of pressure on the back of the brightness control in the back o=
f the monitor. I suspected bad solder joints but couldn't see anything wr=
ong. Turning the control brightens the screen but somehow this control is =
causing the character display to turn on/off. Any ideas what might be goi=
ng on here?
Well, it could be a dry joint, but the other obvious poissiblilty is that
the brightness control itself is worn and that the wiper is not making
good contact wit hthe carbon track.
Have you tried giving it a short of contact cleaner (propan-2-ol, for
example)?
4596
days inactive
4598
days old
11 comments
6 participants
participants (6)
-
ard＠p850ug1.demon.co.uk -
ethan.dicks＠gmail.com -
fraveydank＠gmail.com -
jim_mccusker＠hotmail.com -
jimpdavis＠gorge.net -
toby＠telegraphics.com.au