Hi guys,
I have an EIZO F77 21" monitor which does not power up (not even the LED indicator light).
At first I thought that the famous faulty flyback is overloading the low voltage (B+) power supply causing it to shut down, but this is not the case.
The flywheel tests fine with a very high Q.
This monitor has a fairly complicated power supply.
The line voltage gets into the main board where it is rectified and sent to a power supply board (this board is mounted vertically, behind the CRT neck, slightly to the left).
The power supply board contains two stages. The first stage is a pre-regulator based on the UC3854 chip that produces a stable DC voltage (about 250 volts) that is insensitive of load and line voltage variations. This stage works fine. The second stage is a switching power supply based on the ubiquitous UC3842. This is where the problem is.
This supply produces a multitude of voltages (7V, 120V, 13V, -13V and 80V, to name a few). For some odd reason all the voltages are about half, the 80V output gives 45V, the 120Voutput gives 67V, etc.
As always with the UC3842 some of the output voltage is fed back (through two opto-isolators in this supply!) to the chip and is used to regulate its pulse width so as to maintain the desired output level regardless of load variations etc.
I conducted most of the usual tests like connecting a known good capacitor across all the electrolytic capacitors, (including the one in the UC3842 VCC supply), checking the outputs for shorts and overload conditions, replacing the opto-isolator (There are two such things in this supply, and I do not have a replacement for one of them, so this test is not perfect), checking for bad diodes in the outputs etc.
Just to make things clear, this behavior of the power supply does not change if I load it, i.e. if I connect it to the other boards in the monitor, or not.
During these tests I found a shorted SMD zenner diode ZD901 (Though maybe I killed it with my soldering iron by mistake) but replacing it with a variety of other zenners (I tried quite a few since I do not know its exact value) did not change anything.
This zenner is part of a circuit biasing a MOSFET that has its source connected to the 80V output, and its drain connected to the 7V output. When this MOSFET conducts (for example, when ZD901 is removed) it effectively shorts the 80V output to the 7V output bringing it down to 7V, confusing the UC3842 feedback and causing it to shut down. If under these conditions I disconnect the 7V output (by unsoldering the appropriate transformer pin) the 80V output comes up to it’s full 80V value (but of course without the 7V supply the monitor does not work). Very strange!
The whole UC3842 feedback arrangement in this supply is complicated and my suspicion is that this is where the problem lies.
I can not go any further without a schematic (or very good advice).
I will be very happy if somebody can provide me with a schematic/service manual, for free or for a modest fee.
Advice is always welcome.
Thanks in advance,
Benjamin
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