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Old relic Tandy CM-4 CGA Monitor died

P

Powermac

Jan 1, 1970
0
Recently I snagged an old Tandy 1000 desktop for my collection that
came with a CM-4 Tandy CGA monitor. The system worked just fine for
about 20 minutes and then the video screen went blank. The power light
stayed on and there was some kind of buzzing going on inside but no
magic smoke that I could see (slight bit of smell but nothing crazy
could have been dust). I turned it off and let it sit a bit and turned
it on again. The power light is on, no video shows, and I get that
same buzzing noise. Since it doesn't show video after it cooled down I
figured it wasn't heat related (cracked solder). I tested the Tandy
1000 with another CGA monitor and it works just fine.

I removed the back casing and took the main board out of the monitor.
Both fuses are intact and I don't see anything burnt or popped,
visually all the components look fine with no overheated areas visible
to me and no chips with plastic blown off..

Any idea what has died? Would hate to toss a hard to find CGA monitor.
 
H

hr(bob) [email protected]

Jan 1, 1970
0
Recently I snagged an old Tandy 1000 desktop for my collection that
came with a CM-4 Tandy CGA monitor. The system worked just fine for
about 20 minutes and then the video screen went blank. The power light
stayed on and there was some kind of buzzing going on inside but no
magic smoke that I could see (slight bit of smell but nothing crazy
could have been dust). I turned it off and let it sit a bit and turned
it on again. The power light is on, no video shows, and I get that
same buzzing noise. Since it doesn't show video after it cooled down I
figured it wasn't heat related (cracked solder). I tested the Tandy
1000 with another CGA monitor and it works just fine.

I removed the back casing and took the main board out of the monitor.
Both fuses are intact and I don't see anything burnt or popped,
visually all the components look fine with no overheated areas visible
to me and no chips with plastic blown off..

Any idea what has died? Would hate to toss a hard to find CGA monitor.

With everything back together, but with the back off, turn it on and
see if the CRT filament lights up. That's step 1. Step 2 is to check
the high-voltage going to the clip on the side of the CRT. Do you
have any electronics or tv repair experience?
 
P

Powermac

Jan 1, 1970
0
With everything back together, but with the back off, turn it on and
see if the CRT filament lights up.  That's step 1.  Step 2 is to check
the high-voltage going to the clip on the side of the CRT.  Do you
have any electronics or tv repair experience?- Hide quoted text -

- Show quoted text -

What kind of light should I be seeing at the back of the tube when it
is on (specific color or just any light)?

What voltage should be at the clip on the side of the CRT (under the
rubber)? Checking it to ground with a multimeter or to a specific
point on the board?

I don't have much experience with TV repair other then replacing a
part here and there , I do have experience with electronics mostly
fixing old computer equipment I collect. Usually if a CRT monitor
blows up or gets fuzzy from old age I just replace it unless it is
hard to replace (like this one).
 
B

Bob Larter

Jan 1, 1970
0
Powermac said:
Recently I snagged an old Tandy 1000 desktop for my collection that
came with a CM-4 Tandy CGA monitor. The system worked just fine for
about 20 minutes and then the video screen went blank. The power light
stayed on and there was some kind of buzzing going on inside but no
magic smoke that I could see (slight bit of smell but nothing crazy
could have been dust). I turned it off and let it sit a bit and turned
it on again. The power light is on, no video shows, and I get that
same buzzing noise. Since it doesn't show video after it cooled down I
figured it wasn't heat related (cracked solder). I tested the Tandy
1000 with another CGA monitor and it works just fine.

I removed the back casing and took the main board out of the monitor.
Both fuses are intact and I don't see anything burnt or popped,
visually all the components look fine with no overheated areas visible
to me and no chips with plastic blown off..

Any idea what has died? Would hate to toss a hard to find CGA monitor.

Fire it up with the cover off & look for arcing on the flyback
transformer & around the ultor cap.
 
M

mark

Jan 1, 1970
0
Powermac said:
What kind of light should I be seeing at the back of the tube when it
is on (specific color or just any light)?

What voltage should be at the clip on the side of the CRT (under the
rubber)? Checking it to ground with a multimeter or to a specific
point on the board?

I don't have much experience with TV repair other then replacing a
part here and there , I do have experience with electronics mostly
fixing old computer equipment I collect. Usually if a CRT monitor
blows up or gets fuzzy from old age I just replace it unless it is
hard to replace (like this one).
Don't check with a meter under the cap when it's on thats the high
voltage in the thousands of volts maybe 20KV it takes a high voltage
probe to read that.
 
P

Powermac

Jan 1, 1970
0
   Don't check with a meter under the cap when it's on thats the high
    voltage in the thousands of volts maybe 20KV it takes a high voltage
probe to read that.- Hide quoted text -

- Show quoted text -

Just got around to checking the monitor with the back off, there is no
light at all in the back of the tube, just a bunch of noise when I put
power to it like a small jey taking off.

Any ideas?
 
B

Bob Larter

Jan 1, 1970
0
Powermac said:
Just got around to checking the monitor with the back off, there is no
light at all in the back of the tube,

That means you've got no filament power. You should normally see a
red/orange glow.
 
P

Powermac

Jan 1, 1970
0
That means you've got no filament power. You should normally see a
red/orange glow.



--
    W
  . | ,. w ,   "Some people are alive only because
   \|/  \|/     it is illegal to kill them."    Perna condita delenda est
---^----^----------------------------------------------------------------Hide quoted text -

- Show quoted text -

So when that goes what should I look for as far as fixing it?
 
B

Bob Larter

Jan 1, 1970
0
So when that goes what should I look for as far as fixing it?

That's outside my expertise, but I'm sure someone else here can give you
a few ideas.
 
J

JB

Jan 1, 1970
0
Bob Larter said:
That's outside my expertise, but I'm sure someone else here can give you
a few ideas.

The bunch of noise like a jet is probably arcing. Look for the source but
keep your hands away. It could be on the board around the flyback
transformer. could even be a fractured board. It could be around the tube
socket. Good to start with the power off and discharge the HV to ground and
clean the board with a vacuum or compressed air and inspect carefully for
burned spots. Also clean the side terminal and rubber cover. As was stated
the filament for the tube may be out. This can usually be tested with an
ohmmeter, but you will have to look up the base diagram. Dead filament will
allow the HV to go to maximum (because the tube draws no current) and may
cause arcing. It is usually best to avoid testing or measuring the actual
voltage at the side cap because you can get zapped even if you do everything
right.
 
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