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Taxan Supervision 620 repairs

3

3pcedev

Veteran Member
Joined
Jun 8, 2014
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735
Location
Australia
Hi all,

Thought I would tackle one of my 'rainy day' repair projects this weekend. Just thought I would post up what I have discovered; if anyone has any input it is quite welcome.

I've got a Taxan Supervision 620 (CGA) monitor which failed a few years ago. I recall that when it failed it was working fine, then turned itself off and smelled like burning electronics. I pulled it apart and discovered a blown fuse; as well as smelt/looked around for what had burnt but found absolutely nothing that looked amiss. I replaced the fuse which immediately blew again; frustrated I put the thing on the shelf and there it has sat since.

The other day while browsing I discovered a copy of the Taxan service manual for the 630 series on minuszerodegrees (thanks modem7!). Since mine is a 620 I figured it was about the same, so I had a bit of a read. To my surprise I discovered that this monitor might support the Olivetti M24 colour adaptor. Since my M24 only has a monochrome screen I figured it would be fun to have a colour display; so I got it off the shelf again...

First thing I did was re-verify my findings. I replaced the fuse again, checked everything 3 times, then turned it on. This time the fuse blew instantly, but a puff of smoke came out from the 'main' power supply. Eagerly I pulled it apart (again) and looked for the source of smoke. Nothing! The only thing that was hot was a 15 ohm resistor after the main rectifier. I removed the resistor and measured it's resistance - still spot on 15 ohms. I then thought the rectifier had smoked; so I again removed/tested it and it passed just fine.

I then went back to basics. The DC side of the rectifier fed into a STK7308 - a very common TV switchmode controller which has been used in just about everything. Doing a quick resistance check between the + and - side of the STK7308 gave 3 ohms. 3 ohms at 300VDC is a problem... it's something like 30kW dissipated power! No wonder it blew fuses. I then figured it must be a fault with either the output transformer, or the STK7308 has a shorted power transistor.

Turns out it was the latter. I removed the STK7308 and the resistance went up to >1Mohm. Consulting the data sheet (which is all in Japanese) I somewhat deduced the pins corresponding to the collector/emitter of the output transistor. Checked the resistance between the pins and sure enough, 3 ohms.....

I've ordered a new one which I hope will fix my problem. My only concern is that it may have damaged the switching transformer; but I guess I will have to wait and see.....

Does anyone have any other suggestions as to why this part may have failed? Unfortunately the service manual does not have a circuit diagram; so I am unable to easily check if anything else is amiss except using the 'remove everything and test' method.
 
I have a 635 and Gold Card. I have only plugged it into a Leading Edge model M thus far. Got all these friggin beeps. Maybe I have to set up the mobo properly, but then again maybe they just don't like each other. I plugged my AT in, I think for the first time the other day, and got nothing. I have to crack that open this week and see what's ailing it. I figure that's my best chance of seeing the T635/GC combo working. I do like my M though. It's quite the sexed up box.
 
An area where I once worked used to maintain a quantity of Taxan monitors. Some notes of mine on the 640A and 650 follow. Given the commonality in the Taxan 6xx monitors, the notes may assist you later.

* COMMON: High voltage zener diode (ZD941) in power supply goes short.

* The fuse on the main board has a tendency to blow. Not easy to see; located next to the connector for the deflection coils. Fuse is sometimes found on the solder side of the board. When this fuse blows, the power supply is seriously unloaded.

* Units that 'squeal' have faulty power supplies. Replacement of the switchmode IC (IC901 - STK7308/STK7310) usually cures the problem. Sometimes a faulty electrolytic in the power supply can cause this problem.

* Units that have ripple running through the characters have faulty power supplies. Replacement of the switchmode IC (IC901 - STK7308/STK7310) usually cures the problem.
 
Wow I couldn't see the wood for the trees with that one.... I saw the 'service manual' and then stopped looking. Two items down and there are the schematics.....

I've done a quick check on ZD941. I *think* it's bad; although my diode tester isn't great. It certainly could have caused the failure of the power supply.

For $2.50 I will just replace it.....

Thanks for looking up your notes; these little tips make all the difference!
 
Well the replacement parts came in yesterday.

I replaced the switchmode IC, bridge rectifier and the overvoltage diode and then retested. Now I get the following:

- Fuse doesn't blow (great!)
- Voltage after the rectifier is ~300VDC which is as it should be.
- NO voltage on the output of the supply (should be ~118V).
- Replaced the output diode of the switching transformer; but still get no output.

Modem7's references were a big help, but unfortunately they do not contain the schematics of the PSU itself (just the main monitor board). This gives me component values; however I can't really do much diagnosis this way.

Any suggestions on how to proceed? I have very limited information on even the switchmode controller IC (only datasheet I can find is more of a sales brochure than a true datasheet).....
 
Success! Took me a few hours of testing but I finally got it sorted out.

Here was the repair process:

  • The STK7308 switchmode controller had failed. This manifested as the emitter/collector of the internal power transistor shorting together. This caused an almost dead short through the switchmode transformer between the +300V and internal ground. As such it blew fuses (instantly) and caused the 15ohm safety resistor (just after the rectifier) to get very hot and smell. Since the fuse blew quickly the resistor was undamaged; however it was worth testing before reassembly.
  • Once the ST7308 was replaced the fuse held in; however the 118V output of the power supply was at 0V. By removing and testing discrete components I found that D921 (3.6V, 1W Zener Diode) had failed into a short circuit condition. This zener is used as part of the feedback circuit to the switchmode controller.
  • Replacing D921 resulted in the power supply functioning normally.
  • Note I also replaced ZD941 as per Modem7's notes, as well as the bridge rectifier and output diode from the switchmode transformer. This was a belt and braces approach as I figured any of these could have either caused the initial failure, or been excessively stressed as a result of the STK7308 failure.

I now have a raster pattern; but have not tested the monitor on a computer. Once I have tested it I will post back again.
 
Damn it; spoke to soon.

Monitor was working for about 15 minutes (was in the process of adjusting, had a working but distorted image) then lost power.

Quick diagnosis with the multi meter shows that D921 has blown again!!

I'm trying to figure out exactly what D921 does; however I know the anode is connected to the output of the switchmode psu (+118V) via a very high value (>20MOhm) resistor. The cathode is connected to the cathode of another diode; and I *think* this feeds into a transistor (not sure which pin).

Looking at the STK7308 datasheet it makes no sense; the 'typical' application uses no feedback from the output side (datasheet here: https://www.promelec.ru/pdf/STK7309.pdf). Note the two 3.6V zeners near the centre of the 'typical application' circuit.....

The only other thing besides (another) fault is that I used a simple 1W 3.6V zener as a replacement (found it in my toolbox). Perhaps the part was not rated correctly (maybe I need a 5W part?). The parts list for the power supply didn't specify wattage, just "GZB-3.6B/C" as the part reference....
 
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The 3.6V zener is just to provide a startup at power on. The diode it is connected to is for isolation after the supply starts up, the anode swings negative when the supply is running. If the zener shorts again, I would suspect the diode it is connected to is leaky (circled in red). Pin 10 probably swings a little negative during operation and if the diode was leaky, it would forward bias the zener, killing it.Capture.JPG
 
Good news - I replaced the zener and its reverse protection diode (along with the STK7308 again). PSU started up normally and I've run the monitor for at least 3+ hrs with no issues.

I also replaced the electrolytics in the PSU while I was at it. They tested OK but were starting to smell like they were leaking. There are only 2 of them so it was cheap insurance for the future.

Thanks everyone for the advice. It's nice to have this screen up and running again.
 
2 weeks later and the monitor is still going!

It does have one other peculiarity though... When I turn it on it takes ~5-10 minutes for the brightness to stabilise. It's very dim to start with (even with brightness on max) for about 30 seconds, then I need to slowly wind the brightness back over 5-10mins as it gets way too bright. I swear the focus drifts a bit during this time too; not badly enough you cant read things on the screen, but enough you notice it. The focus does readjust using the trimpot; but like the brightness it's almost like it 'warms up' and requires little adjustments over time.

Any usual suspects for this behaviour? I can live with it; but it would be nice to see if it can be solved.
 
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