When I power on my 3179 terminal, the monitor is exceedingly dim. As it sits, without a keyboard attached, I will only get a blue line across the bottom and a set of characters below that which indicate a defective or missing keyboard. Still, I should be able to get a decent brightness from those elements, but I don't.
The pictures of the terminal on eBay appeared nice and bright, but I realized that they were taken with long exposure in a darkened room. In fact, it must be night for me to see the characters below the line, even with the brightness dial rotated to full intensity.
The good news with this monitor is that there are no burned in images on the screen at all. If it were normal brightness it would be excellent. Alas, it is not.
POSSIBLE CAUSES
There are a number of possible causes for the very low brightness of the monitor, the most obvious of which are:
- Low voltage at the anode (screen end)
- Low voltage at various control grids
- Exhausted cathode unable to emit sufficient electrons
- Defect in the brightness control circuit
I will set up a voltmeter and probe the various voltages to rule out many of the causes. Most of those can be corrected by finding defective components on the PCBs and replacing them. However, if the cathode is exhausted then I have to attempt a CRT Rejuvenation which applies high voltage to punch through oxide layers that are on the cathode. This process is risky, not guaranteed to work, and often yields a relatively short lifetime at decent brightness before the CRT fades away.
FINDING THE CRT NUMBER ON THE TUBE
A long term fix would be the installation of a replacement CRT in the monitor. It will cost more than the terminal did, but it has two big benefits. The brightness will be fully restored and the monitor will last in that condition for many years. Swapping the CRT involves making many adjustments to get the tube focused, in alignment and with the image properly oriented on the face, but it is achievable.
First, I would need to get a part number for the CRT, other than some opaque internal IBM part number, if I were to have a chance at locating a replacement. Even with that, there is no guarantee that an unused CRT can be bought.
Luckily, there is a clear label on the side of the tube identifying its maker as Matsushita (Panasonic) and its part number as M34JDJ70B/G thus I have that to use for a search. The IBM monitor number is 5954150 but it shows up only listed as a 3179 terminal monitor. I had hoped it might be a shared monitor used with other products such as PCs which might mean that some properly working units can be found.
I did find an online shop that purports to be able to order the CRT for me at a cost of $441.12 not counting tax or freight charges from Illinois to me. That is a bit pricey since I could buy a couple more used 3179 terminals for less than this price, hoping that one of them would be adequately bright.
Carl,
ReplyDeleteWe have a *dim* 7139 too in our c-c-g.de museum.
It's connected to a VM370 Hercules via nice
https://www.tindie.com/products/approachware/3270-usb-interface-ibm/
First I'd like to try is to play with internal RGB brightness potentiometers, but - ehrm - I did not even managed to get the plastic case opened so far. Any hint?
kind regards,
Joerg
The seam between parts of the case has small tabs just inside that interlock to hold the sides together. Once you find the tabs, a small thin screwdriver can be used to bend one side down to release the tab.
ReplyDeleteI will try to find those spots on my 3179 when I go to the shop today, assuming I remember. I can then send pictures of the spots where it opens.
If I haven't responded in a week, it means I forgot. Please reply again to this blog entry because I do want to help.
I have pictures to help you get the 3179 monitor open - reply with your email address. I will not let that reply get published thus nobody else will be able to access your email address but me. I will email the pictures to you.
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