Unhappy NEC MultiSync XP21 CRT vs. my limited skills - Page 1
Forum section: electronics / repair

author: moothefish
author_rank: Contributor
timestamp: August 16, 2025, 01:24:12 pm
content: I picked up a pair of big ol' 21" CRT monitors recently, both NECs but slightly different models. One more or less works fine, but the other has been doing my head inThe core issue is that when plugged in to power, the power LED turns orange as expected, the monitor degausses, then starts clicking a relay on the power supply on and off, about once every second. On each click, the power led briefly turns green, then back to orange. When power is cut, the relay stays latched for a few seconds, a relay on the deflection board engages and the monitor seems like it would power up normally until the remaining juice in the caps drains out. My first thought was that there was a short or overload condition somewhere, causing the power supply to trip and restart in a loop, but I have yet to find anything of the sort. So far I have checked the following:- tested horizontal output transistor and related components for shorts, found none- tested all power supply connections at the deflection board for shorts, found none- tested all caps in the power supply, a couple of them tested a little iffy so i replaced a few, but most were within tolerance both in terms of capacitance and ESR- checked all diodes in the power supply- checked the transistors in the power supply to the best of my ability. I may well have overlooked something here thoughI don't have proper schematics to work from. I managed to track down some for the other monitor i picked up, an XE21. Externally the only difference is the lack of RGB BNCs on the XE model, but internally they are quite different. The deflection board on the XP is more complex, flyback is different, signal board is different and much more complex, horizontal output transistor is on a separate board, etc. The power supplies are also different, pinout is the same but the PSU for the XP is higher wattage.I've included the schematic for the XE21 power supply. Although different in many ways, the relay that is clicking (RL6C1) is the same, which seems to be responsible for firing the degauss coil? Disconnecting the coil makes no difference to the fault. 12v rail sits at 11.6 and the 6.3v rail is at 6 volts, seeing a 1-2v pulse on the degauss pin of the main PSU connector.I have reached the limit of my troubleshooting, I may well have overlooked something simple here so any advice is much appreciated!

author: m k
author_rank: Super Contributor
timestamp: August 16, 2025, 03:11:55 pm
content: What you're feeding in?Back then there were multisyncs and then there were multisyncs.Newer ones start from VGA and originals can stand EGA.

author: shakalnokturn
author_rank: Super Contributor
timestamp: August 17, 2025, 12:23:17 am
content: It has been very long since I worked on any CRT monitor.In addition to previous reply: The earlier models (pre OSD or lacking integrated LCD status messages) can shut-down, act wierd or even fail when fed wit unsupported sync frequencies.If you're feeding video be sure the mode is supported.Have you monitored supply voltages during the cycle?Does the one supplying the high voltage transformer hold up ?

author: moothefish
author_rank: Contributor
timestamp: August 17, 2025, 03:30:03 am
content: So this behavior is happening without any video signal being provided, just mains power. If i do plug in a video signal to it, I get no relay clicks at all and the power LED flickers orange.I did some more poking around and it seems that the power supply is being told to fire the degauss relay constantly from the main control chip. The 175, 80 and 15v rails do nothing while the relay is clicking, but when mains power is disconnected and the power supply briefly starts normally, these rails rise to the right voltage until the rest of the monitor circuitry drains the remaining capacitor charge and it cuts out. Starting to wonder if this might be a logic issue somewhere as the PSU seems to be doing everything it should at this point

author: m k
author_rank: Super Contributor
timestamp: August 17, 2025, 02:30:04 pm
content: So after all just a regular overload, it seems.Check all voltages from main transformer secondaries before and after rectifier diodes.After a diode the bad one wont go up like others.The manual I checked has feedbacks at 8V and 15V.It also has -15V to 80V from one winding, so maybe one higher voltage is pulling down.You can also test it with signal connected.That way the power seems to be totally overloaded, so can have clearer results.