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Successful Goldstar Green-screen mono Monitor Repair (with pictures!)

BradN

Member
Joined
Aug 27, 2006
Messages
32
Hi all, I just wanted to report a successful repair of a GoldStar MBM-2233 monitor, mfg. date Dec 1984.

Click here for the photo album: http://imgur.com/a/27Dws

Early symptoms: Randomly the horizontal picture width would twitch a little bit, this went on for some months.

Total failure symptom: Eventually something went and no picture would show, and the power light would only come on dimly.

Troubleshooting:

So, here I am with my prized monitor no more useful than a doorstop, but with an oscilloscope, multimeter, and some electronics understanding.

I start by checking all the diodes and capacitors and transistors for shorts. Nothing seems shorted out. Nothing looks burnt.

I find that there is a power board that provides ~12V to the main board, and that is the only power supply rail in the monitor. When the thing is turned on, the 12V rail drops low and a large power resistor on the power supply board heats up a lot. It seems obvious this was designed to limit the impact of a short in the 12V section, and it does perform this task well, but wouldn't be good to leave the monitor turned on with something shorted.

Next I start looking up the datasheets for the two ICs used on the main board. These are generating the timings for the horizontal and vertical outputs. It turns out from some more testing that if I force the horizontal output to stop, the short disappears and the supply rail comes up.

Well, this horizontal output goes to a transistor that switches power into the flyback transformer. Because the transistor tested out okay (as well as everything else I looked at), I assume that something has gone wrong in the flyback transformer itself or something powered from an output of it. I unsolder the flyback transformer and pull it out of the board, and now with the horizontal signal running, there is still no short circuit.

The recommended way to test a flyback for shorts within a single coil is doing a ring test. Attach a scope to one of the coils (find them first with a multimeter), charge up a small capacitor, and briefly strike the capacitor across the coil. The inductance and parasitic capacitance should make the coil voltage oscillate repeatedly. I did this test (and it took very many tries to get decent looking results due to how long the capacitor should be in contact), and it oscillated a few times before decaying away. Having never performed this before, I wasn't 100% sure this was a failed test but it didn't look like a very good oscillation at any rate from any of the trials.

Unfortunately at the time (early 2012 I believe) I couldn't find any suitable replacement flyback transformers ANYWHERE on the internet. So, it gets packed away in a mostly disassembled state.

Now, a few days ago, just for the heck of it I try looking on ebay for the various cross compatible flyback models and find a seller on ebay, and at a reasonable price too ($16)! The original flyback is 154-067C, a possible replacement I could find in cross references on the web is HR42002, which is also a replacement for 154-067D, and the -D is the one available. So I really didn't have anything saying that a -D would do everything a -C does, only that HR42002 covers them both. But, I take a risk and order it anyway. (It turns out the -D is a physically smaller part than -C, probably a cost reduction)

I receive the new transformer, solder it into the board, nervously remove the old flyback cable from the tube (this is the part that could possibly kill you if the tube isn't discharged and your finger slips...), manage to get the new flyback cable attached, and power it on.

Success! A green glow heralds victory.

I suspect what was happening during the early failure symptoms was a partial shorting out in one of the flyback coils that eventually overheated and led to a more complete short.
 

Shadow Lord

Veteran Member
Joined
Jun 16, 2010
Messages
3,163
Location
California
As someone with a some understanding of electronics I must say you sir have anything but some understanding of electronics. My hats off to you for your technical prowess and for getting your monitor back!
 

BradN

Member
Joined
Aug 27, 2006
Messages
32
Thanks, I guess it's not easy to feel accomplished in electronics because every hill you climb you just see a taller one from there. But in the grand scheme of things I still don't consider myself really an expert, just someone who's learned enough tricks to get by!
 
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