In Part 1 I started working on the Agilent 6627A quad power supply with one bad channel. Replacing Q319 did not fix it, however there were changes. The -7 Volt rail was now low. Curiously this rail is called -5.75V on the schematics for the 1 and 3 power output boards, even though they are identical to the 2 and 4 boards, where it is correctly shown as -7. The circuits are the same only the PCBs are sort of mirrored so two of these boards can fit next to each other with the power modules facing the fan. I assume this is a mistake in the schematics, or maybe the remains of an older version. Anyway, I had to find out what pulled the voltage down (well, up, towards 0V).
Q319 was replaced so I traced which signal goes where. The circuit with Q319 has only one output-like signal, the BLEED_SINK line for the downprogrammer circuit. The only obvious fault which could pull the -7V down is a shorted CR355, which is a 1N4850 diode. A quick check proved that it was indeed shorted. The 1N4850 is a bit hard to find, but it should not be too different from the very common 1N4148, other than a lower voltage and a higher current. Actually the higher current requirement was inconclusive, as some datasheets gave the same current for both. Since the 1N4148 is higher voltage it should not matter at least for a trial. So I replaced CR355 with a 1N4148.
This still did not fix the power supply, but there was yet again some change. The output would now not stay at zero, but tried to output some voltage, then drop to 0 and start again. The supply was still alternating in CV, CC and UNREG modes. The -7V rail was still very low and pulsating as the output would try to regulate.
So it was time to look at what else could have caused this. Disconnecting R314 fusible resistor would unload the -7V supply, which could be now verified that it worked without load. I then scrutinized the schematics for all appearances of this power line, looking for possible sources of fault. CR352 and CR353 diodes were OK. The next suspect was the C370 tantalum. Tantalums are always suspect! It was a bit hard to get to it, as it is mostly covered by the heat-sink, but removing the board made access possible. Indeed this capacitor measured 5 ohms in both polarities, which is not good. I did not have replacement, so simply removed it, which should again be fine for testing. Once this capacitor was out, the power board came to life and provided rock solid output. C370 is not a filter cap across the power rail, so until the other problems were fixed it did not affected the -7V. Once the broken trace to Q319 was reconnected, this cap came into play. At the beginning of the troubleshooting the -7V power was in spec.
I ordered a new 1N4150 and a 39uF tantalum, they should be here in the coming days. But I consider the repair successful. I will still need to verify and if required calibrate the supply, but that should be simple.
There were 4 fault is the unit. A shorted transistor, a broken trace, a shorted diode and a shorted tantalum. The first 3 are in the same part of the circuit and their demise could be related, but I don't really see how the trace broke, as the -7V is current limited probably below damage levels. Maybe some external impact? The capacitor most likely became shorted by itself.
The moral of the story is that there could be several fault creating similar symptoms, never assume that replacing one faulty component should fix the problem. Actually fixing something might uncover other hitherto unknown problems. This means one needs to restart the troubleshooting and not think that if a power rail was good at the beginning it would stay so during the repair.
Study the schematics and figure out which bad components could result in the observed fault. Go methodically and test them.
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