Audio Developments AD062 Mixer -- Blowing Fuses

[rs_]

Hi, I am working on a new (to me) Audio Developments AD062 mixer. It's a very, very nice portable mixer; it cost something like USD $15,000 when new, around the year 1984:
http://www.trewaudio.com/i/consign/7910.jpg

Here is a link to its Service Manual, which I refer to below:
http://saravanbuskirk.com/rspics/AD0...20Part%201.pdf

It looks like it has been well-maintained (recapped and so on) over the years, and the (reputable) seller said it was working fine when they tested it last. It is not working now. It is blowing power supply fuses within a half-second or so of turning on.

Thanks to the "backplaned" modular design, I have been able to isolate the issue to the Master module (which is the one with the VU meters on it in the picture). I have removed every module from the "mainframe" except the Master Module and fuses are still blowing.

What I know so far is that you turn on the power switch (the unit uses a *tested* external +15VDC power supply on a 4-pin XLR) and the power goes to the Power Board, which can be seen on Page 79 of the Service Manual.

The power goes directly from Page 79 to a "Low Voltage Test Circuit" on the Master Module. This can be seen on Page 35 of the Service Manual. The circuit in question is at the top-left of Page 35. The Low Voltage light is illuminated briefly, and then the main fuse blows and everything shuts off.

The only way I can prevent blown fuses is to remove the socketed IC1 at the top-left of Page 35. It is a dual-opamp that, as nearly as I can tell, is determining whether or not the Low Voltage light should illuminate. If I remove it, the Low Voltage light stays on and fuses do not blow (but the mixer does not pass audio regardless.)

I have checked all components in the "Low Voltage Detector" circuit on the top-left of page 35. None of the resistors, thermistors, or capacitors are open. The diode is working properly.

Thanks for any further suggestions on how to troubleshoot this.
rs

[Steve Conner]

If IC1 blows the fuses, it's probably shorted, replace it. Maybe you'll get lucky and the output of it also drives a turn-off mute circuit.

[rs_]

If IC1 blows the fuses, it's probably shorted, replace it. Maybe you'll get lucky and the output of it also drives a turn-off mute circuit.

Thanks for the prompt reply. I forgot to mention that I tried that. Same situation.

rs

[Steve Conner]

So, you replaced IC1 with a fresh known-good one and it still blows the fuse?

IC1 isn't part of the audio signal path anyway, so you have some other fault. Check the + and -12V supply rails and the +48V too while you're at it. They come from the DC-DC converter on page 79, but they feed everything else.

If any of the rails is missing or low, break into the wire as it leaves the DC-DC converter board and measure the current draw on that rail.

[rs_]

The presence of IC1 blows the main fuse even with a new, known good replacement, that's correct.

I will take a step back and get the "full scoop" on the Power Board voltages before blowing anymore fuses...

Thanks, rs

[rs_]

Getting closer to the heart of the matter...

Service Manual:
http://saravanbuskirk.com/rspics/AD0...20Part%201.pdf

Pic of Power Board:
http://saravanbuskirk.com/rspics/AD062PowerBoard.jpg

The power board is receiving the correct voltages through the external XLR connector on the right side (+15VDC, +15VDC, 0V) but not putting the correct ones out. The voltages being put out of PL1 (top left-ish) are very low, all around 0.1 volts.

IC2 is registering 0VDC or close to that as well (when measured at pins 14 and 11 per Page 79 of the Service Manual). The voltages at these pins should be +12VDC.

What is a good place to start on debugging the Power Board? Do the measurements I already have imply that some component in particular might be at fault? Trying really hard not to brick this thing...

Thanks again,
rs

[g1]

In your pic, R11 beside the 7812 regulator looks like it got pretty hot.

[Steve Conner]

Are the power board output voltages low even when it's disconnected from the rest of the mixer? If so, no need to worry, the power board is bricked already and you can work on it without fear of damaging the rest of the mixer. But do make sure it is putting out the right voltages before connecting it back up.

The first suspects would be the secondary rectifiers and the power transistor that drives the primary. The secondary rectifiers are the three diodes over on the left hand side by the yellow inductors. If any one of them is shorted, it shorts out the whole power supply, so test them.

The driver transistor is the heatsinked device near the transformer. It operates under considerable stress, especially since the designer of this power supply forgot to include any snubbing for it. You can pull it from the circuit and test it. I would replace it and see if the power supply bursts back into life. If you have a shorted secondary rectifier, the replacement might just blow again, hence why I suggested checking them first.

If all these parts check out OK, we are probably looking at a fault in the control circuit that wedges the driver transistor permanently on.

[oc disorder]

My 2 pence worth.. it's not just something stupid perhaps ? like a short in one of the molex connectors or on the distribution
rails? eg like a nut has fallen off somewhere and resting across tracks ? I say this as you mention it came from a reputable seller?