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.
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.