I happen to have a 3562A for repair, which had a defective power supply, and after fixing this, it still doesn’t work – traced the error to the ROM board. It is really a coincidence that I own a more or less identical unit, which is working. Therefore, after checking all the supply rails to make sure nothing is going to damage it, I swapped out the ROM board with the working unit, and, there we go, it does the trick.

Now, how to find the defect? – first, a quick check of all the address logic, to no avail.

So the defect must be located in one of the ROMs. Have a look, the board has 2×18 pcs, Intel P27256, 32 kByte each:

These chips are representing 589814 words of data (16 bit bus): a massive 1.179648 Megabytes, holding the program for the 3562a.

To get it powered up, first surprise, the thing draws nearly 2 Amps, about 55 mA each. Checked with the datasheet – and in fact that’s what these little heaters need.

Checking with the schematic, seems that this is an earlier version of the board, Revision B. That’s a pitty, because Rev C is much more common, and ROM images would be available off the web. Judging from the datecodes, manufactured in 1986. Still, a great instrument, low noise, build in source, and easy to use.

The circit of the board is really nothing special, a few decoders, and the memory bank, with some bus drivers.

The plan is as follows:

(1) Read out the bad board (reading data from this first, just in case I accidentially damage something).
(2) Read the working board.
(3) Compare the EPROM images from both boards.
(4) Replace any defective EPROM(s). The ones installed are single time programmable, plastic case, but not an issue at all to replace them with regular UV-erasable EPROMs, if needed.

Desoldering all the 36 EPROMs – absolutely no option with the tools I have around here. With a few wires, and a ATmega32L board (JY-MCU AVR minium board V 1.4, always handy to have a few of them around), it was just a matter on an hour to get everything set up, not the fastest way, but the data will be clocked out byte by byte…

Now it is just a matter of time, for the defective chip to show up.