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CollinB1
July 9th, 2017, 09:47 AM
Hello,
So I have this IBM 5153 color monitor and the first time I tried to turn it on,
It didn't work. Not even the power light turned on.
There was a pop in the power supply so I replaced the power transistor and
a large concrete resistor that measured no resistance value(even though it was like 10ohm
or something).
The monitor still didn't turn on.
If I had a schematic, maybe I could trace the LED back until I find the problem.
I dont have a schematic though, could somebody please help me out here?
Some of the diodes looked a little corroded but measure fine while in the board.
Maybe I need to take them out and then test them?
Any help would be much appreciated!

paul
July 9th, 2017, 12:54 PM
It's in the 5153 section on http://minuszerodegrees.net/

The PS supplies 115 VDC to the chassis at about 450 mA, you can test it against a 40W light bulb. However, the PS working alone will not light the LED on the monitor.

If the PS works and the monitor still doesn't function check the items listed in "repair database."

Also, Tezza found an issue here similar to what you describe: http://www.classic-computers.org.nz/blog/2008-8-23-repairing%20an%20IBM%20monitor.htm

CollinB1
July 9th, 2017, 01:04 PM
So it doesn't matter that the LED and power switch are hardwired into the power supply.
The only wires exiting the power supply besides power LED and switch is the DC output.
It seems like even with a broken main board that the LED should turn on.
Maybe I'm wrong but I'll definitely check that site.

CollinB1
July 10th, 2017, 03:59 PM
Yep, I was wrong.
Turns out my memory has failed me.

The Power LED is hardwired into the main board.

I tested the power supply output and its 120V which is good.

Any ideas on why Its not turning on?
I don't think it would have anything to do withe the crt because if it were broken,
the power LED would at least light up.

paul
July 10th, 2017, 05:17 PM
The Power LED is hardwired into the main board. I don't think it would have anything to do with the crt because if it were broken the power LED would at least light up.
CRT broken? Perhaps check that the filament is on.
Look where the LED is wired, the secondary of the flyback, and note that it requires the HOT to be running.

Is the PS output stable despite the monitor not working? If yes, have you checked through the troubleshooting information provided so far?

If the PS output shuts off, start by changing out this 47uF cap in the PS per the attached image.

39660

CollinB1
July 10th, 2017, 05:42 PM
Yes I measured the power supply while in the circuit and with power on. The power supply
did not shut off, just constant 120V.
The CRT filament does not light up, But I don't think that it is broken.
It makes sense to me that something is keeping the output of the power supply from reaching the
chips and flyback, causing both the LED to stay off and also the CRT to not light up.
I feel like they both come from the same problem.
I'll check the schematic to see if I can find something like that.

Also, what do you mean by the HOT to be running?
Does that just mean power is running?

I guess it couldn't hurt to try replacing that capacitor.
I'll try that.

Thanks!

modem7
July 10th, 2017, 08:24 PM
Also, what do you mean by the HOT to be running?
HOT = Horizontal output transformer, A.K.A. flyback transformer.

paul
July 10th, 2017, 08:41 PM
Horizontal Output Transistor, but close enough! Q403 on schematic. The LED is smack in the middle of the first page schematic in the SAMs manual, which is why it's so hard to locate.

If the tube neck is warm after a half hour the filament is almost certainly working.

modem7
July 10th, 2017, 08:50 PM
Horizontal Output Transistor, but close enough!
Doh !

Has a thorough visual inspection been done? That can sometimes save a lot of effort.

From my reading of the circuit diagram in the SAMS document, there are two sources of 12V, and the LED is powered from the second.
1. 12V derived from 115V (on the diagram, the "12.76V SOURCE" square with 5 in it)
2. 12V derived from a coil in the flyback transformer (on the diagram, the "12.76V SOURCE" square with 9 in it)

The voltage for the CRT filament also derives from a coil in the flyback transformer.

CollinB1
July 11th, 2017, 10:03 AM
So the HOT has 2 pins and an outer shell.
The outer shell measures 115v
while neither of the pins measure anything.

Would this be the problem?

And yes it has been thoroughly inspected.\

Also the tube does not light up or turn on.
I think the filament is the thing that light up
in the tube neck...But that doesn't light up
either.

retrogear
July 11th, 2017, 11:44 AM
BTW don't measure the collector of the HOT with your meter (metal outer shell you describe) because it has like a 800 volt pulse which could fry your meter but since you have no voltage on the base or emitter (the 2 pins) you
have no horizontal drive so only 115VDC on the collector. Check voltages on Q401. Lead labeled C should have 54V, B .32V, E 0V

Larry G

paul
July 11th, 2017, 12:59 PM
...
I think the filament is the thing that light up
in the tube neck...Clearly you're not as old as I am :)

Sounds like your HOT could be fried. You might want to check it out of circuit with an ohmmeter rather than risk your health with the voltages present.

CollinB1
July 11th, 2017, 05:25 PM
Even if the HOT were fried, I would still get a voltage on the 2 pins, correct?

Well I measured Q401 and got 0v on all three pins!

I guess we must trace it back from there now

CollinB1
July 11th, 2017, 05:47 PM
So I measured my power supply again and it measures 127 now for some reason.
Does this mean something has broken, consuming less power?

Also, I followed the Collector of Q401 to T401.

One end of the coil is connected to the 127v
while the other end is 0v.

This is a faulty transformer, correct?
But does this explain the 0v at the base?

Also, would this affect the power LED?

retrogear
July 11th, 2017, 07:17 PM
If the horizontal output section is not running, there is essentially an open load on the power supply so I wouldn't be concerned the 115V is high. T401 primary should be 91 ohms. Either it is open and / or Q401 is shorted.
Yes, with no horizontal drive from Q401, the B of Q403 HOT would be 0. I am using SAMS CMT4-2 which is available online.

PS - the power led is driven from the horizontal output transformer which is dead at this point

Larry G

paul
July 11th, 2017, 07:20 PM
First, the HOT is Q403. The SAMs schematic has the voltages at each terminal of that and Q401.
The power supply should be stable at 115 +/- 1 VDC. You can adjust it with an insulated trimpot screwdriver, but it must be under load, which it probably isn't right now.
It's hard to diagnose this logically from your responses. I would suggest doing preliminary checking with the power off. 2k ohms range for the two transistors.



Even if the HOT were fried, I would still get a voltage on the 2 pins, correct?
Well I measured Q401 and got 0v on all three pins!
I guess we must trace it back from there now

So I measured my power supply again and it measures 127 now for some reason.
Does this mean something has broken, consuming less power?
Also, I followed the Collector of Q401 to T401.
One end of the coil is connected to the 127v
while the other end is 0v.
This is a faulty transformer, correct?
But does this explain the 0v at the base?
Also, would this affect the power LED?

39692

jmetal88
July 12th, 2017, 04:30 AM
First, the HOT is Q403. The SAMs schematic has the voltages at each terminal of that and Q401.
The power supply should be stable at 115 +/- 1 VDC. You can adjust it with an insulated trimpot screwdriver, but it must be under load, which it probably isn't right now.
It's hard to diagnose this logically from your responses. I would suggest doing preliminary checking with the power off. 2k ohms range for the two transistors.



39692

I think you're misunderstanding him -- he said he followed the circuit back from the HOT and found no voltage on Q401, so he's not mistaking Q401 for the HOT, he just thinks the problem is further back in the circuit.

CollinB1
July 12th, 2017, 11:53 AM
Yes I get 0v on the 2 pins of the HOT and all three pins of the Q401.
Sorry for not being clear. I'll try better

I think the coil of T401 that connects to Q401 is disconnected because I measure no
connectivity between the 2 pins of T401.

Also, would a broken horizontal drive also cause the filament not to light?

Thanks!!

retrogear
July 12th, 2017, 12:32 PM
yes the filament is fed by the flyback transformer, so no horizontal drive, no filament. Unsolder T401 and see if there is corrosion on the leads. The coil wires will be soldered to the leads that goes thru the board. Maybe you'll get lucky and find the break.
I doubt you'll be able to source a new transformer.

PS - or even apply heat/solder to the leads when it's off the board. It might heal the connection. Don't heat too long, however.

Larry G

CollinB1
July 12th, 2017, 06:10 PM
OK thanks i'll try that!

CollinB1
July 13th, 2017, 03:15 PM
Well, I took out the transformer,
Re-wrapped the entire thing,
and Now the power LED will turn on
and I can hear the hissing noise that
a monitor makes.
The filament does not light up.
I checked voltages at Q401 and C was 36.6 instead of 53.8
and B was .3 which is right.

Also the power supply now measures 119 volts.

I think I may have just put T401 back together poorly.
Could that affect the filament turning on?

retrogear
July 13th, 2017, 05:34 PM
Progress !! Turn the lights off and look at the filament to see if it's lit. You might not see it in room light. Did you hear the "crunch" of the high voltage when you turn it on? Also when you turn it off you should hear a slight "crackle"
as the HV decays. Run the set maybe 30 seconds then turn off and feel the temperature of the horizontal output transistor. Then increase to one or two minutes. Insufficient drive from your homebrew transformer
can cause the HOT to run hot. It should be warm but not too hot to keep your finger on it.

PS - also watch the face of the tube in the dark as you power on/off. See if you see a flash of the raster.

Larry G

retrogear
July 13th, 2017, 05:40 PM
There may be DC voltage on the HOT after powered off but usually it discharges fast. I don't think it will bite you.

Larry G

retrogear
July 13th, 2017, 05:46 PM
Check the voltage on the B and E of Q403 (the two leads). Should be about 1 volt if you are getting drive. Remember DO NOT measure the metal can (C) of that transistor, it will fry your meter !!!

Of course, only put your finger on the HOT when powered off. It is deadly when powered on !!!

Larry G

CollinB1
July 13th, 2017, 05:58 PM
I am not familiar with this "crunch" of high voltage, but I do hear the expected continuous buzz
of whatever makes that noise. I do not hear the HV decay. By the way, What is the HV?
I know what noise you're talking about but don't know what it is.
After a few minutes of running, The HOT feels warm but not too hot at all.
I have one concern though: the transformer may have the wrong polarity.
I forgot to remember which pin goes on which side of T401 :( oops!

Could I measure a certain pin on the crt board to see if the filament is getting power and the tube is broken,
or if it isn't getting power at all?

Sorry I am not too educated on this type of stuff.
Monitors and crt's are unexplored territories to me.

EDIT:
So I measured both pins of HOT and they are both 0.25v.
They are connected directly to the pins on T401.
They should be 0.94v and 1.11v
Does this mean T401 is not working correctly?

Thanks!

retrogear
July 13th, 2017, 06:17 PM
HV stands for High Voltage. There is about 20,000 volts to the red rubber cap on the side of the tube. The buzz you hear is probably what's called vertical drive and not the horizontal drive.
Vertical drive is about 60Hz. Is the power LED on the front lit? It is powered by the same drive that lights your filament. If your horizontal drive is working, you wlll get 12V across C428 and 5V on C212.
Check those voltages.

Larry G

retrogear
July 13th, 2017, 06:29 PM
Is there any voltage on C428? I assume not. Yes, .25V on the HOT means T401 is not working correctly. It takes at least .6V for the HOT to turn on at all.

retrogear
July 13th, 2017, 06:41 PM
As Paul said, the HOT Q403 could also be bad. If you remove T401 again, measure resistance between the two leads of Q403 to see if there is a low ohms short, like 200 ohms or less.
You can unsolder a lead to make that measurement as well.

PS - bedtime for me

Larry G

CollinB1
July 18th, 2017, 07:03 AM
So I measured the resistance of the leads of HOT after desoldering the wire off of one of the leads.
It measured 50 ohms. This means that I have a bad HOT right?

That would explain why .25v was on both leads of Q403.

Should I replace this and try it again?

Also, I can't believe I forgot to add that the power LED did turn on!

retrogear
July 18th, 2017, 09:22 AM
Just to clarify, you unsoldered a lead of Q403? Then 50 ohms across the two leads?
Yes, then replace Q403 but make sure T401 is hooked up correctly before powering on.
That would also explain why the C of Q401 was 36.6 instead of 53.8 since it's driving a short.
If you want, measure Q401 C while Q403 lead unsoldered to see if it came back up to 54 volts.
That might also tell you if T401 wired correctly.

Larry G

paul
July 20th, 2017, 01:45 PM
Every time CollinB1 disappears for a while I worry about this ...!

I am not familiar with this "crunch" of high voltage ... By the way, What is the HV?

retrogear
July 20th, 2017, 02:40 PM
He seems like a young guy so his heart can take it. I got zapped many times in the TV repair business and am still talking to you ...

On the other hand, microwave oven power supplies do kill. I had a coworker killed servicing one years ago.

evildragon
July 20th, 2017, 03:39 PM
CRT's are high voltage, but not high current. It's the current that kills..

Just keep one hand in pocket, and you should be fine.

Stone
July 20th, 2017, 03:49 PM
It's the current that kills...That's not quite accurate.

It's the actual combination of amps and volts that's actually lethal, not just one or the other.

evildragon
July 22nd, 2017, 06:37 PM
Well, I debate that, and here's my reasoning. The human body is a constant resistant, and yes, while you need a high voltage to get something through your body, you need 100mA to stop a heart. If it was a high voltage with little current, it'll shock you, but not stop you (permanently).

But at the same time, you can't have high current without high voltage, but a car battery can certainly do some damage, and it's not high voltage, just high current. I've heard a story about a navy officer who died from a 9v battery, however it involved open cut wounds. As to if it's true I never checked.

But it seems to stand without reason, that so far in my observations, it certainly is the current that kills, weather the higher amperage requires higher voltage, or something with lower voltage, but enough current.