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The topmost board is the RAM board. The slot above it is free and empty. The board has number 09825-66523 rev B, supposedly a 16KB RAM card.:

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After removal of the RAM board, by moving it to a 90% angle and then sliding it out of its hinges by moving it sideways, we see the cassette board:

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A detailed picture of it after removal:

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Power supply check

Checking the voltages on the PSU:

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