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Thread: Taxan Supervision 620 repairs

  1. #1

    Default Taxan Supervision 620 repairs

    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.
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

  2. #2

    Default

    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.

  3. #3
    Join Date
    May 2006
    Location
    Melbourne, Australia
    Posts
    5,710

    Default

    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.

  4. #4
    Join Date
    May 2006
    Location
    Melbourne, Australia
    Posts
    5,710

    Default

    Quote Originally Posted by 3pcedev View Post
    The other day while browsing I discovered a copy of the Taxan service manual for the 630 series ...
    Quote Originally Posted by 3pcedev View Post
    Unfortunately the service manual does not have a circuit diagram;
    See http://minuszerodegrees.net/manuals.htm#Taxan

  5. #5

    Default

    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!
    System 80 Expansion Interface located! Thanks to all who helped out and the good people in the NZ vintage computer forums!

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