Update time!
During the holidays I was tinkering with the DRO and wanted to try the VGA converter. But before I could check that there were other hurdles to be taken.
Spoiler alert, the cheap converter does not work (yet). I can access the menu and it shows on a screen with HDMI input but cannot get it to work with the DRO.
Plot twist.
When I switched the unit on, nothing displayed. That was the moment I knew why the previous owner had dismantled the DRO from the machine. A few months before it worked fine for the few minutes I tested it..
Having learned a little from people who are much smarter with electronics than I am I remembered that its often best to disconnect the power supply and check if it is OK. Removed the whole back plane from the DRO and switched it on. All but one pin had voltage present (all from shared ground). Because the PSU does not look to be super smart and interfaced/digital/etc I expected that that pin should have power too.
Switched the unit off, unplugged it (some filter components are connected before the double pole on/off switch..).
With a strong light source I managed to trace the PCB traces to a separately mounted 78T12CT linear voltage regulator. First suspected this regulator but remembered that these more often show a failure mechanism that makes them pass through the voltage without regulating.. Then I found a Thyristor next to it which looked to be part of a
crowbar circuit (click) to clamp the output to ground if a regulator fails. But when I checked resistance, the output rail of the PSU (at the connector) was showing zero Ohm, which is not how the crowbar circuit protects, as it is only activated when the overvoltage is present..
Tracing the PCB with a strong led, as the board is difficult to remove from the back plane/heat sink. Top row connects the toroid transformer and regulators/components on heat sink to the PCB
Decided to break the problem in two: disconnected the output pin of the linear regulator and check voltage there (regulator is connected via long leads to the PCB). To my surprise this regulator provided clean 12v! Checking resistance again, the circuit on the PCB was still shorted out.
(auto-rotation of the message board isn't messed up, the multi meter is showing the year when the measurement was taken)
Found one 25v 100uF electrolytic capacitor that was in the circuit. Removed it and checked the circuit again, no short anymore! Had a fresh 63v 100uF capacitor left over that I didn't need when I did another repair.
Soldering the new capacitor to the board was slightly tricky, the PCB cannot be removed from the back plane as the voltage regulators and a few other components are attached to the heat sink without access for tools to safely remove them.. This also makes it difficult to renew the thermal conductive paste. Anyways, if it were easy everyone would do it.
Connected the PSU again and now the screen came back to life!
For anyone troubleshooting the PSU, here is the pinout, checked with a DMM (left to right, looking at the component side of the PCB, right side up):
(left to right on picture as well, note new
red capacitors)
- Ground (connected with pin 2&3)
- Ground
- Ground
- +5v (connected with pin 5&6)
- +5v
- +5v
- +12v (output I fixed on my unit, apparently supply for monitor)
- -12v
- +5v
- +5v (appears to be the same voltage rail as the pin 4-5-6)
Because the other electrolytic capacitors of the PSU are of the same age I replaced those too:
- 2x 6800uF 35v
- 2x 100uF 63v
- 1x 2200uF 40v FRAKO EFR capacitor (will be replaced by a high end capacitor, apparently something special was nescessary there)
The ones from Wurth Electronics had the same pitch so chose those.
There are various Tantalum capacitors on the PSU board as well which I will replace too. Will post a comprehensive list when I have done that and verified they work. The bridge rectifier, consisting from four rather toasty rectifier diodes will be getting a few fresh Schottky diodes as well (Schottky diodes are overkill but 1: less heat is better and 2: overkill is consistently more fun).
Total component cost is ~15 Euro, maybe 20 with shipping.
If the whole PSU is completely broken I would expect that a Meanwell triple output (+5v/+12v/-12v) can be a suitable replacement as well.
Rest of the unit appears to have a Lithium
Sonnenschein (click) battery, which still sup-plied 3.6v after all these years:
And for reference, if anyone is looking for the power connector, it is a STAK2 connector made by Hirschmann.
Disclaimer: THE POWER SUPPLY UNIT RUNS ON MAINS VOLTAGE (AND THE CRT SCREEN ON MUCH HIGHER VOLTAGES EVEN).. be careful!