Main pc board failed repairers can't find replacement

Even with a multimeter you will be at a disadvantage without the other thousand dollars of test equipment. The best bet is to get a repair kit.

https://www.fixya.com/support/r29790007-repairing_own_television

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Back - light problem. Check and replace damaged component/s at its back-light inverter board, or replace this board as card basis. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts. It will be best to replace the inverter board as card basis. http://electro-medical.blogspot.com/
To some model sets; especially LED back-lighted types; the back-light inverter circuit will be part of the main power supply board [SMPS]. In such cases, the power supply board might need replacement. To some models sets, the back-light inverter and SMPS section circuit will be integrated at its main board itself. If so; the main board should be replaced. To some models of sets, all these section circuit will be integrated at its main board. If this is the case, you have to replace the main board.

There are several fuses on the power supply board.

Usually the one that fails is not the main AC line fuse (which is close to the AC line input), but the PFC fuse located, you guessed it, next to the PFC regulator IC on the board.

When the PFC fuse is blown (measuring open) you also need to replace the PFC IC and possibly other components on the board. Most of the time the PFC is sufficient though.

Here's a link for repair service for the board:
http://www.coppelltvrepair.com/p/751/toshiba-46rv535u-power-supply-board-repair-service-for-dead-refusing-to-power-tv-or-faint-display-problem

Also search there for the TV model, we may have repair kits or exchange services available!

Hope this helps!

F1 error code is for main board failure, replace main board

Good day,
the board in this machine is notorious for failing. Testing by jumping L1 to test terminal may tell you, but other components in the board may give you a false response.

The main heater rarely fails and you can test it with an ohm meter to verify that.

If the main heater checks good for continuity, replace the defrost thermostat, and board. Best long term repair.

A U-6 means the memory on the main board failed.
If the machine is old, the battery on the board is old and not holding the values in the machine.
Replace the main board will correct the problem.
Since it is in the middle deep in the machine you might need to have Xerox repair it for you if they still provide service to this model.
Good luck.

In the absence of any warnings of impending failure, the main power supply has probably failed.
This is likely to be a separate assembly (board) that you could replace on your own. If you remove the back lower panel, follow the AC cord to locate the supply, get the part number from the assembly (there may be two different numbers). The bare circuit board may have a number that was printed in the same manner as the component identifiers and then another number on a label which might contain a 'revision' number too. This board is probably not terribly tought to repair with the necessary instruments but you would be better off buying a new board and then looking for someone to repair the other since it will probably be needed when the replacement fails.   

With a 657, most likely the main board has failed. It is not uncommon at all, That is a general problem with that model.
What you can try is, looking from the front of the machine, the main board is located on the back right side. removing 2 screws and releasing the levers will allow you to slide the processor board out of the machine. You can try just reseating,,,, removing and shoving the board back into the machine, and seeing if that helps.
Most likely, either the board has failed, or the memory battery had exceeded its life and the memory is not working.
Wish I had a more positive answer for you, but the 657 is getting older, and those lithium batteries are failing. I have to add though, that those processor boards, have failed pretty frequently over the years.
What you can do is go onto Google and look up "the parts drop" ,,, call them and ask for Britt, and see what he has as far as that producut is concerned. That machine is actually made byn Toshiba, you may be be able to find either a Xerox, or Toshiba repair dealer in your area that has that board and will fix it reasonably. If you find a Toshiba dealer other than a Xerox dealer. Tell them it is the same machine as a Toshiba E-120, they are exatly the same.

I i knew the model, But what is happening is the machine is not reading the machine ram. One of the memory sticks is print memory and one contains the machine firmware.
In order of likely hood here is what has happened.
1. The memory stick simply needs to be removed and re seated in the slot.
2. The memory itself has failed. Needs to be replaced and the machine firmware reloaded.
3. The machine firmware has corrupted and needs reloaded.
4.You need to to an NVRAM reset.
5. The battery maintaining the memory has failed.
6. The main board in the machine has failed.
7. Corruption, which a reset will remove. Can't tell you how to or if you can do a NVRAM reset without knowing the model number.

The Main Board contains the DM circuitry. And you are right, this board does not fail in the way of its predecessors. What is the unit doing?