I like the idea of having a general purpose power supply that can both source and sink current and act as a DC-40kHz power amplifier. So I picked up an HP 6825A bipolar power supply / amplifier for 85 USD about a month ago. I've got a stack of old HP analog power supplies (6177C, 6110A and 6236B) where it will fit in nicely.
After inspecting the power supply, I noticed that when the mode switch is set to "power supply" and the voltmeter selector is set to AC that the analog voltmeter pegs hard to the right. Interestingly, this only happens when the output voltage is positive (meter reads zero when output is negative). The ammeter, when set to AC, does not exhibit this effect at all.
Initially I thought that A1CR20 may have failed, but it tests fine (though it's some unobtainum part that has a very low Vf ~.35V @ 1mA).
Looking through the circuit schematic more closely, I eventually noticed that HP didn't put a DC blocking capacitor in the AC voltmeter circuit but did do so for the AC ammeter circuit. So it makes sense why the AC voltmeter pegs when a positive DC output voltage is present at the HI and LO terminals. (The half-wave rectification of the AC detector makes it easy to go over FS with DC without a blocking cap.)
Is this a design flaw or am I the problem here? I get that it's pointless to set the voltmeter to AC when configured as a power supply (i.e. DC), but it just seems silly not to put a blocking capacitor to prevent the meter from pegging when accidentally switching to the AC mode.
Eitherway, I did find one other potential issue. The 1.6V uncal AC setting pegs well before it should while monitoring the output with my benchtop DMM. Interestingly, the 16V AC mode looks to be spot on. Hopefully this can be massaged out with the adjustment pots, rather than another slog through the schematic chasing gremlins.
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