Greetings all from New Zealand. I think this post will probably have a few of you shaking your heads but I still thought it was worth asking for advice...
I imported a matching monitor for my IIgs via EBay from the US, obviously using it with a step-down transformer as we're on 220V over here. It was sold "parts only". I was pleasantly surprised when it arrived to find it actually worked, it just needed the focus adjusted. I duly took the monitor apart and adjusted the focus giving me a pretty much perfect picture.
This is where things took a turn. Since I had the monitor apart I thought I may as well do a re-cap and a retro-brite. I had a capacitor kit from Console 5. After discharging the CRT I carefully replaced all of the electrolytic capacitors. On reassembling and powering up I now have a non-functional display.
I definitely have high-voltage - there's plenty of static on the screen and my iPhone app is reporting 15kHz. The power LED does not light however and I get no display. I triple checked the polarity of the new capacitors and reflowed the solder to the flyback and the connector on the neck board but no joy.
I know it was probably foolish to replace all the caps in a functioning monitor but I've recapped a few motherboards and apple II PSUs never with any problems, one cap at least showed signs of leakage when I removed it.
Any suggestions on where to start troubleshooting? I do not have an oscilloscope, but I do have a mulimeter that can check capacitors.
I figure I could check and replace the original capacitors.
If I can't repair it the case is likely headed for an LCD conversion (the case now looks essentially new).
Any suggestions for help welcome!
Start by getting the schematic and tracing the video signals from the input through the signal chain. If you have static charge on the display, then EHT is good and the horizontal drive is good. The power LED not working is quite odd and could be a major hint for troubleshooting.
One thought is that much of a CRT display runs off the so-called B+ power supply, which is around 200 VDC. If you have EHT and H drive, B+ must exist. However, semiconductors run off a much lower secondary power supply, Vcc at 5 VDC. The power LED, as well as the video signal chain needs 5 V. Looking at the schematic, see where 5 V is regulated, and check that pin's voltage with the DMM. Then follow it to the power LED to locate where along the power rail is broken. There may be a shorted component, even a capacitor, that is bringing Vcc down. If that's the case, you may have no other option than to remove components until the short goes away.
You're far from the first to find a non-functioning device after replacing capacitors, if it's any consolation. I'm also nervous about "kits", since the suitability of any replacement depends on understanding its function in the circuit. I just don't feel comfortable outsourcing such a judgement to an operation of console modders.
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I feel like such a rookie though... turns out there are two identical connectors on the neck board (one to the led) and I'd reversed them! They are plugged in correctly now and all is well.
I'm glad it was as simple as that!
Interesting that they ran the LED from the neck board. One possibility is that they derive the LED current from the [cathode] heaters in some way, which would indicate a heater failure by turning off the LED.