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Old relic Tandy CM-4 CGA Monitor died

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.

Reply to
Powermac
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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?

Reply to
hrhofmann

k

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).

Reply to
Powermac

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

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

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.

Reply to
mark

heck

age

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?

Reply to
Powermac

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
---^----^---------------------------------------------------------------
Reply to
Bob Larter

check

igh

oltage

ta delenda est

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

Reply to
Powermac

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

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

and

check

it

voltage

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.

Reply to
JB

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