Modding Graphic EQ unit

[HoodTube]

Hi folks. This is not about a pedal, but I've got an old Akai EAA22 EQ unit with a snapped power supply board. Instead of trying to sand and solder this back together, could I just replace the wires that connect to that from the main EQ board with a socket for a universal power adapter? Also (if that were possible), how would I figure out which polarity/voltage etc to run it at? I don't have the service manual and can't find a free one online. Any advice would be great, cheers.

[Sooner Boomer]

I'm not familiar with the device in question, but that's never stopped me from making up an answer.  Look on the circuit board and see if there are and clearly identifiable integrated circuits.  Look up the data sheet for these ICs.  Locate the power and ground pins.  Use your continuity tester or voltmeter to trace the circuit paths.  Hint - power and ground should be common to all the ICs - verify this.

[vigilante397]

Many commercial PCBs use ground pours, which make it pretty easy to spot what connects to ground. Do you have pictures of the board? I'm sure we could figure it out.

[PRR]

Manual for $5

However you can see a lot of this one in the preview images, which may be saved and opened in an image-peep program.

> snapped power supply board.

Snapped where??

The main DC supply is on the main board with all the audio. This takes 21VAC from another board which is the power transformer and fuses and switch-leads. The DC supply is limited at 60mA, so a new power transformer can be 21V 0.12A or 2.5VA (teeny!). If you truly must convert to an external DC supply, welll.... it's difficult because this has a turn-on/off thump protection which needs to see the raw AC to anticipate power-down. 

https://i.postimg.cc/2z9M8zdZ/EAA22-schematic.gif

[Rob Strand]

The AC says 21.1V but the DC says 25.4V.  My guess is it is an 18V transformer/AC supply.  When unloaded the voltage will rise.   The regulator stage is 15V, which I'm assuming is on the cracked board. A directly connected external PSU would lose the current limit present on the unit's PSU ckt.

If the de-thump circuit is on the same board that would mean you would lose that circuit altogether.   So ideally you would have to re-create it or use that part of the broken PCB.  It is possible to do a similar de-thump from a DC supply.  Simple circuits use a zener diode.  Perhaps checkout the other commercial audio schematics for ideas,  eg CD players.

Get the manual from here, open the images individually on large scale,
https://www.manualscenter.com/manuals/akai/eaa22-service-manual.html#.XbEITTgeZpi

Click to enlarge: