Alesis Multimix 16 power supply

[tired eyes]

I have a stubborn issue here that I'm hoping someone else has encountered.

This Alesis Multimix 16 USB 2.0 mixer came in a box with an aftermarket power supply that the customer had bought. It's the same one selling here: Alesis Multimix 16 12 FW Power Adapter | eBay advertised as a replacement for the mixer. It's a dual 18VAC adapter wihch feeds the full wave rectifier of the power supply. I've attached a JPG of the schematic, full link is here: http://diagramas.diagramasde.com/aud...ServManual.pdf

I found two bad caps (shorted positive filter cap, PC9 2200uF and PC5 100uF was leaking electrolyte). The 7815 and both LM2575 regulators were also shot. I replaced all those parts, along with the negative side filter cap for good measure. When I power up with this adapter, I get negative voltage but no positive (only about 2.1V at the input of the 7815). Investigating further, both legs of the AC coming from this adapter go negative, i.e. 18Vp-p 60hz sine wave centered at -9V. Disconnected, it reads 18VAC, 0VDC like it should. This results in -41V on the negative side of the rectifier and +2 on the positive side. So I get a nice clean -15V from the 7915 but of course no positive voltages.

If I bypass the rectifier and connect +/-18V from my bench power supplies, everything on the power supply works: I get +/-15, +5, +12, +3.3 and +48. Nothing appears to be drawing any current that would overload the adapter and cause that DC offset. My bench supply on the positive side only shows 10mA. So I immediately suspected this adapter (he did have it connected to a shorted filter cap for a while) and set out to try another one. I borrowed a similar unit from a music store I'm friendly with and got the same results. This one was different but also advertised for use with this mixer.

So, is it a ripoff power adapter(s), or what the hell else on the board could be causing this? I've tried lifting just about everything else in the circuit and I can only get +10 out of the regulator. It goes down to 2 with any of the positive regulators in curcuit. I replaced all the rectifier diodes just to be sure.

Any insight would be appreciated.

[g1]

I'm not sure how you can get the +48 supply when you are feeding it +18VDC ?
But I would suggest disconnecting the 2 caps that feed the phantom supply and try the AC supply again.

[52 Bill]

Are you sure that all of the power supply diodes are okay?

What ac voltage do you get directly from the adapter, 18vac ct or 36vac ct?

[mozwell]

PC3, PC4 are like a voltage doubler circuit to the bridge PD1 - PD4, the out of this adds to the volts on PC9 cap, to get the higher voltage, which is then regulated to 48V with PD9, PIC5
Here is the full sch
the power adaptor should be 18 - 0 - 18V, there should be 18VAC between connector PTE4 pin 1 & 2, and also between pin 2 & 3, 36VAC between pin 1 & 3......

[Jazz P Bass]

Check the pins as per Mozwell's suggestion.
It's clearly stated on the power pack & the schematic that it is an 18Vac x2 supply.
http://thumbs3.ebaystatic.com/d/l225...De6ILKX6xA.jpg
It may be something simple like a bad power switch not letting the voltage through.

[km6xz]

It sounds like a reference problem, you have 18 volts between the AC lines but either is not referenced to ground as expected, with DC offset. Check from pin 2 of the power jack to pin 1 and 2 of PTE3 for both 1 and 2 having 18v ac to pin 2 of the power connector PTE4.
If those are not right, the switch or ground connection through the pc board is at fault. If those are correct, with 18vac from 2 to 1 and 2 PTE3, than check for any difference between pin 2 of the power connector and the common ground point at the common junction of the two large filter caps. I suspect one of those is not at the same potential. If that is OK, check the input of either 15volt regulator from pin 2 of the power connector. Verify that the Audio ground is intact between the each of those point(pin 2 of the power connector, ground terminal of the regulators, etc should all be very close to each other in voltage.

You mentioned changing all the rectifiers, if one was backwards or open it would explain the 9volt offset. It is probably not a component but a pc trace open. Even it it looks like it is ok, measure the traces of the ground on both sides of the pc board if any of the ground to voltage test points mentioned are wrong.

[tired eyes]

Thank you for all the responses. I'll try to address all the suggestions here in order.

I have lifted the caps for the doubler circuit and diodes PD2 and PD3 with no change. I also have the inputs of all regulators lifted to attempt to isolate the rectifier.

Checked AC voltages at the power connector and PIC3 and PIC4 as suggested, all are correct, and checked the power switch. Checked all grounds on the board for continuity and for DC voltage when powered. Can't find any issues with the grounds.

Seems like either a rectifier problem or a bad / incorrect adapter. I had replaced all the rectifier diodes but checked them again for open/short, and verified the direction.

Going back to the power adapter, I noticed that when it is not connected, I get 36V between pins 1 and 3, but measuring from 2 (ground), I get around 14V on pin 1 and 15V on pin 2, and these voltages are floating. This suggests to me that pin 2 is not internally connected to the centre tap of the transformer, thus it is not a ground reference. What are the odds I got 2 bad transformers? I'll try pulling a piece of gear with a known good ~18V CT transformer and wiring it up.

Thanks again for the quick replies,
Jon

[172pilot]

Old thread.. but, what was the solution?

Thank you for all the responses. I'll try to address all the suggestions here in order.

I have lifted the caps for the doubler circuit and diodes PD2 and PD3 with no change. I also have the inputs of all regulators lifted to attempt to isolate the rectifier.

Checked AC voltages at the power connector and PIC3 and PIC4 as suggested, all are correct, and checked the power switch. Checked all grounds on the board for continuity and for DC voltage when powered. Can't find any issues with the grounds.

Seems like either a rectifier problem or a bad / incorrect adapter. I had replaced all the rectifier diodes but checked them again for open/short, and verified the direction.

Going back to the power adapter, I noticed that when it is not connected, I get 36V between pins 1 and 3, but measuring from 2 (ground), I get around 14V on pin 1 and 15V on pin 2, and these voltages are floating. This suggests to me that pin 2 is not internally connected to the centre tap of the transformer, thus it is not a ground reference. What are the odds I got 2 bad transformers? I'll try pulling a piece of gear with a known good ~18V CT transformer and wiring it up.

Thanks again for the quick replies,
Jon

I know it's been a while, but I just went through a house fire, and somehow managed to save my Alesis mixer, but not the adapter, so I was wondering if I should try my luck with what you bought, or if I should go to radio shack and just buy a 36v ct transformer and hack one together myself.. It sounded to me as if "18v x2" meant an 18v ct transformer to that company, and really it was 18v from side to side, with no ground reference, but not having one to put my hands on, I wouldn't know for sure.. Did you end up having any luck?

Thanks in advance, and thanks for the link to the schematic - that was quite helpful!!
Steve

[Jazz P Bass]

Yes, it is an 18V x 18V adapter.

The trick is to find the connector.

I would just purchase the right one.
Alesis Multimix 16 & 12 FW Power Adapter | eBay

[172pilot]

Yes, it is an 18V x 18V adapter.

The trick is to find the connector.

I would just purchase the right one.
Alesis Multimix 16 & 12 FW Power Adapter | eBay

I actually think I found the connector - Just search for "3 pin aviation connector" on ebay: and you will find stuff like this but by the time I find the right transformer, get the connector, and put it in a box, I'm not saving much.. Kills me to pay $50 for a glorified wall wart that should cost $20, but I ordered one tonight from your link..

The description of that adapter certainly looks to me as if it's just 18 volts, meaning that rather than centertapped 36v, maybe both 18v connectors are from the same secondary and actually the same rail.. I didn't look at the schematic enough to know if that'd be a problem, but since they say it's for the specific device, I guess it should work..

THanks for the confirmation!
-Steve

[g1]

The schematic in post #1 shows it. 36V center-tapped. Or as some would say, 18-0-18.

[172pilot]

The schematic in post #1 shows it. 36V center-tapped. Or as some would say, 18-0-18.

I agree with that.. I understand the ORIGINAL one was 36v ct, but the picture of the ebay one simply says 18v AC, with no indication of a CT or two windings to be able to put in series.. I'll definitely test and report back when I get it.. Hopefully Monday!
-Steve

[g1]

Ok, I see what you mean. I think probably something got lost in the translation there .