For some reason none of my many power adaptors fit my Prokeys 88....is this adaptor specific to this model?

sure will but none of the connectors will line up and neither will the coolant lines, but relax they do make adaptors , just google adaptors for model number and your see

Ac adapters for PCs can be bought from most of the electronic spare part selling shops. http://electronicshelponline.blogspot.com/

e.g.

http://www.ebay.com.au/itm/Global-AC-Adapter-4-M-Audio-Prokeys-88-Stage-Piano-Charger-Power-Supply-Cord-PSU-/260991184879

Reset the 88 by switching off --> hold the - and + buttons while turning on. Your device will reset and usually everything should be solved.

try to remove the battery first and plug in the ac power adaptor and see if it works. if yes, then your battery is faulty.

if it doesn't work i suspected that there's a problem with the power socket from your machine. you can check it with the nearest service center near you.

You might check with these two eBay stores which import tons of stuff, among their offerings are charger/adaptors and battery packs for a wide range of equipment. I have purchased from both and was happy with price, service and quality.
Optimum Solutions
Itrimming   I didn't find any for specific charger for your model but searching on the adaptor number might be more effective. 

As I search the internet I found out no replacement for this kind of the adaptor model number 090-02180-13, there are some people looking for this kind of adaptor is a faulty a pervasive problem with the TV. There are literally hundreds of complaints on the internet. Most deal with a faulty AC Adaptor and the inability of the company to provide a replacement part.
I have a solution for this adaptor problem we better improvised the parts inside. The faulty parts of this adaptor arise in the transformer. This transformer is easily to burnt because of lower amperage. So we make it higher amperage so it can resist the load heat and cannot burnt easily.
Maybe you can do this for yourself, open the adaptor case, when open already look for the transformer, the visual picture of transformer is a bulky metal with many windings. Take this out and bring it to electronic shop that know how to rewind a transformer. Tell them to reworks or rewind the transformer, only the output voltage will be change input is the same. This is the

NEW SPECIFICATION:

• INPUT 100-240 V
• OUTPUT + 24VOLT DC AT 12 AMP
• 12VOLT AT 6 AMP.

This is the result of new specification when the reworks or rewind is finish ,put it back to the adapter box,and then you can use it.This is higher amperage I mean it to you before,it can resist overheating.If ever happens that you can't find in your place know how to do reworks of the transformer here in philippines i do the rewind or reworks of the transformer ,but the problem is.very far to your place it needs shipment to carry this out .I price you 120 dollars including shipment and i assure it to you that adaptor will last long because of higher amperage .
When i search i found a site that can make an ac adaptor only you give the new specification and give where you are using the adaptor give the brand name PLTV-32 and they will make it..try this option also ..LCDPAYLESS.COM

Hi

If the AC power light is blinking, try the following steps:

1 Cut off power to the computer by disconnecting the AC adaptor and removing the battery. The error condition will be interrupted, and the AC power light will stop flashing.

2 Put the battery back into the computer. Do not connect the AC adaptor. Try turning the computer on again. If the computer starts normally, the AC adaptor may be
defective and will need to be replaced. If the AC power light starts flashing, remove the battery, and continue with the steps below.

3 Connect the AC adaptor to the computer. Leave the battery out of the computer. Try turning the computer on again. If the computer starts normally, the battery may need charging, may be depleted, or may be defective. Turn the computer on, insert the battery, and then leave the computer running for several hours, which will deliver a slow, steady “trickle-charge” to the battery. Once the battery has been trickle-charged, it may begin working correctly again. If the trickle-charging does not prove effective, consider replacing the battery with a new one.

4 Connect the AC adaptor to a different power outlet, preferably in a different room. If the computer starts normally, there may be a problem with the AC outlet itself, or the voltage level available from it.

5 Verify that the AC adaptor is the correct unit for your computer model. The computer may not be able to start from an AC adaptor that is rated for less current (amperage) than the computer requires, even if the rated voltage is correct, and the plug fits correctly in the DC-IN socket. The labels on the bottom of the computer and the AC adaptor show the specifications for voltage ("V") and current ("A") for each device. The voltage level must match exactly. The amperage rating of the AC adaptor must be equal to or greater than that required by the computer

Regards
Mike

Is the AC adaptor model AC Power Adapter: AC-3VX? If so it should fit, if not, have it replaced to where you bought it from...

Below is the URL for compaq's Service Manual for that model. This shows you the details of every single part & how to disassemble and reassemble every screw. Surely it has to be in this document, somewhere. --> ftp://ftp.compaq.com/pub/supportinformation/forum/portables/7400msg.pdf