IBM 5153 monitor not working!!!

[CollinB1]

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]

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 an IBM monitor.htm

 

[CollinB1]

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]

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]

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.

 

[CollinB1]

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]

Also, what do you mean by the HOT to be running?

HOT = Horizontal output transformer, A.K.A. flyback transformer.

 

[paul]

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]

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]

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]

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]

...
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]

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]

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]

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]

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?

 

[jmetal88]

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.



View attachment 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]

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]

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]

OK thanks i'll try that!