Thursday, September 24, 2020

SPARCstation ELC repair

The SPARCstation ELC is a SPARC workstation built into a monochrome CRT monitor. It is fanless, and gets quite hot, which is bad for electrolytic capacitors. I suspect that also some particular capacitor models used are bad.

Accessing the computer motherboard (under the cover at the back of the top) is very easy. Unfortunately, disassembling the rest isn't very convenient. Maybe the design tried to make it convenient at first, and then changes defeated that.

Before disassembling further, consider the shock and electrocution hazards. Obviously the line voltage side of the power supply is dangerous. The secondary side is dangerous too, because besides supplying safe voltages to the computer, it also supplies higher voltages to the monitor. The main monitor board uses those to make even higher voltages, and sends some of these to the CRT socket board. Due to capacitors, voltages can persist even when the device is turned off and unplugged.

The first problem was the power supply, the board to the right when viewed from the rear. It seems like it could be possible to lift out, but the speaker bracket screwed into its bottom prevents that. So the whole side needs to be freed and moved outwards.

The power supply was pulsing. This can happen when an SMPS detects a fault, shuts down and restarts, but that's not what was happening here. The -12 V rail had excessive ripple, and I replaced its 100 µF 16 V filter capacitor (the missing C528 in the photo) to fix that, but it wasn't causing the problem either.

Just guessing I saw the 100 µF 35 V capacitor near what seems to be the SMPS controller IC. The board in that area was browned due to heat around various holes, so it makes sense that the capacitor may have been ruined by heat. Also, a failure in power supply to that IC could cause the symptoms. After testing the theory by temporarily placing a capacitor in parallel, I replaced the capacitor and fixed the problem.

Now the picture was barely visible. The firmware screen has black text on a white background, but the text was just a bit darker than the background. Replacing the two visibly leaking pairs of back to back 100 µF 10 V capacitors at the bottom of the CRT socket board took care of that.It turned out those transistors, labelled ITT 895 115C, near the corner of the board were bad as well. According to NTE's cross reference, 2N4401 was a good substitute. I don't know why they failed. I thought maybe I damaged them by accidentally solder bridging surface mount tubular C417S to the neighbouring resistor on the underside while replacing the rightmost capacitor, but that solder bridge apparently needs to be there. I also replaced some other capacitors but am not sure any of that was necessary. This board is probably the worst for capacitors because it is in its own RFI shield box inside the monitor.

Note that the CRT socket board requires the RFI shield to make ground connections. The main ground area visible at the CRT socket and the three tabs to the left of it need to be connected by wires if you remove that shield and want to power on the monitor.

Finally there was some vertical foldover at the top. It was interesting to note that its start stayed in place as vertical size and position was adjusted. Multiple guides about this problem with CRTs in general say that the prime suspect is the pump-up capacitor. The vertical deflection circuit needs a pulse of higher voltage to overcome inductance and quickly move the beam back to the top. This is accomplished via a charge pump, which charges a capacitor in parallel with the supply, and then connects it in series, to provide that higher voltage. That was another one of those 100 µF 35 V light blue capacitors, like in the power supply (the large cap in this photo). Here are the definitely bad components:

If you want to see more photos, take a look at the album.

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