Adventures with the Nixdorf 8870 Mini-Computer
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Completed spare-part testing

5/6/2014

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Major spare parts testing was completed last night.

I'm suspicious of the 1830 hardware arithmetic unit but that could be because I'm not 100% sure how to configure it in NIROS. I know you have to deactivate $DEC14H then activate a $DEC18H driver in it's place, but I've got a niggling feeling there is something else that needs to be done. It could be one or more DISCUBS but I just can't remember.

Whilst I have an 8" FD and a box of new discs as it happens, I think that either I've got the wrong controller (apparently it's for a 5" FD not 8"), or the controller / FD is faulty as the system becomes unstable when it's installed, but I need to experiment more with this.

I've not tested the PLC2 I have because to be honest I don't need it and it's certainly not a requirement for a minimum running system. Also, if memory serves you need to have the BCU (Batch Communication Utilities) software (or the MAFI), and I don't.

The only failed parts I have that are required; excluding the 2 x SMC drives but see a previous blog about those, is a 3109-02 PSU. I've got two working ones but the 3rd is playing up.

I suspect it's a leaky capacitor. Electrolytic capacitors don't age well and really don't like high temperatures, and a high proportion of failures in any SMPS (switch mode power supply) can usually be attributed to leaky or failed electrolytic capacitors. There are 16 high-value capacitors in this PSU and a job on the list is to get replacements then swamp them all out. It will either work or it won't... it's one of those types of repairs.

The 8870 modules are unfortunately packed with tantalum capacitors which, IMHO are the spawn of the devil as when they fail they nearly always go short circuit usually with pyrotechnic results, along with a smell you wouldn't believe. This PSU contains several so I'll probably swap them out for safety, if replacing the main electrolytic capacitors fixes the problem; pointless otherwise.

So, all in all and except for DAP4 replacement parts, I'm in pretty good shape for now.



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    I'm a software developer, an engineer and I love vintage computers, but the 8870 has a very special place in my heart.

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