Question about HOT voltages - Videokarma.org TV - Video

TinCanAlley · #1 11-09-2021, 06:00 PM

Okay, I'm trying to fix an IBM Monochrome Monitor from '82. I have the Sams Photofacts service manual and have been following the diagnostic steps. I got only so far before I found something odd and no explanation for it in the SM.

I tested the power supply section and all readings looked good. So I moved to the horizontal output section and that's where I ran into a problem. The SM said to check for 24.7V at the emitter of the TR24. I got a reading of 27. Then it said to check the collector of TR24 for 14.9V. I got 27V again. Both the E and C have 27V. The SM only notes what to check if the voltage is missing, not if it is wrong or, in my case, the same. Now TR24 is what I always considered a HOT based on the package TO-3, but in this SM it is a transistor and the HOT is package TO-220.

Yes, my knowledge is limited, but I'm trying to learn. That's why I always defer to the expertise here. If you need more info, as I'm sure I've left out something needed, just let me know.

I wanted to attach the SM, but it is too large.

Yamamaya42 · #2 11-09-2021, 08:23 PM

Quote:

Originally Posted by TinCanAlley

Okay, I'm trying to fix an IBM Monochrome Monitor from '82. I have the Sams Photofacts service manual and have been following the diagnostic steps. I got only so far before I found something odd and no explanation for it in the SM.

I tested the power supply section and all readings looked good. So I moved to the horizontal output section and that's where I ran into a problem. The SM said to check for 24.7V at the emitter of the TR24. I got a reading of 27. Then it said to check the collector of TR24 for 14.9V. I got 27V again. Both the E and C have 27V. The SM only notes what to check if the voltage is missing, not if it is wrong or, in my case, the same. Now TR24 is what I always considered a HOT based on the package TO-3, but in this SM it is a transistor and the HOT is package TO-220.

Yes, my knowledge is limited, but I'm trying to learn. That's why I always defer to the expertise here. If you need more info, as I'm sure I've left out something needed, just let me know.

I wanted to attach the SM, but it is too large.

it would really help if you can link the Schematic on someplace like imgur or other such site, I have not seen one of those monitors since my very young teen days, and do not have quick access to docs, what I DID find, is CLOSE, but most likely not correct, there is NO way for the V at that point to be that high if the regulator is working correctly, the schematic I found shows 24.7 going into the regulator and 14.30 out, this goes to the HOT, but as mentioned, it's most likely CLOSE but not exactly what you have.

https://i.imgur.com/RLFCEKB.png

TinCanAlley · #3 11-09-2021, 09:20 PM

Quote:

Originally Posted by Yamamaya42

it would really help if you can link the Schematic on someplace like imgur or other such site, I have not seen one of those monitors since my very young teen days, and do not have quick access to docs, what I DID find, is CLOSE, but most likely not correct, there is NO way for the V at that point to be that high if the regulator is working correctly, the schematic I found shows 24.7 going into the regulator and 14.30 out, this goes to the HOT, but as mentioned, it's most likely CLOSE but not exactly what you have.

https://i.imgur.com/RLFCEKB.png

That is the schematic I have. What I just discovered is this monitor needs a horizontal signal to produce a raster, so I'm assuming this means there is no way to do an accurate test without it being connected to a source for the HSynch?

TinCanAlley · #4 11-09-2021, 09:29 PM

Quote:

Originally Posted by Yamamaya42

it would really help if you can link the Schematic on someplace like imgur or other such site, I have not seen one of those monitors since my very young teen days, and do not have quick access to docs, what I DID find, is CLOSE, but most likely not correct, there is NO way for the V at that point to be that high if the regulator is working correctly, the schematic I found shows 24.7 going into the regulator and 14.30 out, this goes to the HOT, but as mentioned, it's most likely CLOSE but not exactly what you have.

https://i.imgur.com/RLFCEKB.png

Oh, I have another question. The SM says to look for 22V AC between the cathodes of D602 and D603. So do I just put the DMM in AC and connect on lead to the cathode of one and the other lead to the cathode of the other? When I do this, I get nothing. I must be doing something wrong. I figure if one of these were bad, there would be no voltage to the T24 REG. Am I right?

Also, should I break out the scope and start checking the points noted on the schematics? And is an isolation transformer a necessity?

Yamamaya42 · #5 11-10-2021, 05:45 AM

correct, there is NO horizontal oscillator, it's direct driven off the monochrome card, and since this is meant for use on a PC, it's already very well isolated. i would think.

Yamamaya42 · #6 11-10-2021, 06:04 AM

do you HAVE a IBM pc with monochrome output to drive this thing?

TinCanAlley · #7 11-10-2021, 12:11 PM

Quote:

Originally Posted by Yamamaya42

do you HAVE a IBM pc with monochrome output to drive this thing?

I have an IBM PC (5150), but it has a CGA that will damage the monitor as it has a much different H frequency. My MDA is dead, so I order a replacement. Until that arrives, I can't go beyond testing before the drive circuits.