I just finished my first lap around my newly acquired DDA DMR12 console, all channels pass signal but 19 of the 48 inputs are having a common issue with Phantom power (the rest seem to be functional). I tested first with a dynamic then with an SDC and the symptoms go like this: Engage Phantom push-button, peak and signlal LEDS lights light up, then go out (pop if gain and fader up), no signal passes, disengage phantom push-button, signal rises and fades, like it's really only working momentarily as the caps drain or something.

Got out the multi-meter to measure voltages. PSU puts out a steady +50VDC (and +/- 18VDC). Good channels measure +48~+50VDC on XLR Pins 2 & 3. Bad channels measure about +30~+32VDC on one or both of XLR pins 2 & 3. Dug up the schematic (down below) and measured a bit more

With the SDC mic connected and the Phantom power engaged, good channels measure a voltage drop of about 7.4VDC across R1 and R2, Bad channels measure a voltage drop of about 18VDC across R1 and R2.

Is C81 leaking maybe? or C1 & C2?

I did some more measuring and testing and now it seems maybe dirty pots and switches are to blame. Frustrating since I've gone through and thought I cleaned them all pretty thoroughly by now. Taking a break and will come back to this.

What do you see that makes you suspect the pots and/or switches? The e-caps C1 and C2 should block the +48 from going any further down the signal path. That said, if they were leaking and causing DC to appear on a pot or switch it could cause a scratchy sound when manipulated. Has the board had any recapping done?

If you measure the voltage on a good channel with no mic connected and engage the phantom switch on a bad channel, does it cause a change on the good channel?

A strong signal from the mic, while turning the gain knob will break through, not always with much clarity, but it will come through a bit, but maybe it's the scenario you describe. Will measure to see if DC is getting through C1 and C2 next chance I get to work on it.

The board hasn't been recapped, but at ~30years of age might be time,

ok, getting back to it with a clear head, I should've been able to identify the issue, but it was an off day I guess.

Measuring for DC voltage across C! and C2, it's clear they are the problem. Good channels show no DC on the - side of C1 and C2, bad channels measure the same on both sides of C1 and C2. So looks like I'm recapping those to start with.

Measuring phantom on a good channel with no mic connected, and engaging phantom on a bad channel yields no change in DC voltage on the good channel.

C1 and C2 Stock values are 33uf 63V. any reason to change that up? I might go with a higher voltage tolerance since phantom is measuring 50ishV.

Happy to report that replacing C1 and C2 with new 33uf 100v Panasonic FC caps resolved the phantom power issue. I went ahead and did every channel, except 6 which will get the Neutrik NTM1 input transformers instead once they arrive.

Now the real question: Does the failure of C1 and C2 suggest the need for a full recap or would the phantom caps be expected to go sooner by the nature of their function? And can anyone suggest a good test for whether the caps are problematic besides pulling and measuring them? I get sound through each channel, but haven't had a chance to do any serious listening so I can't say if anything sounds wildly off.

Decided to go ahead with a full recap. Managed to get everything ordered from Mouser, with only 1 value cap backordered (till Oct supposedly) Should keep me busy for a while.