Hello
I have just found this thread on IBMU repairs which is interesting. Having repaired many of these units, most of the comments given above are factual and valid.
The main problem with these units is, as noted by Iain, the Tantalum Capacitors failing Short Circuit. Most problems lie in the Power Supply Card although the Analogue modules frequently cause problems. Faulty Tantalum Caps often appear burnt out and these are easy spotted. Otherwise I would advise the use of a short circuit locator, such as the Polar "Tone Ohm" which will save a lot of time.
The white PSU modules on the PSU card are fairly resilient but will fail if they see a short circuit for long periods. Spares are difficult to obtain but other commercial PSUs can be substituted by drilling new mounting holes in the PCB.
I suggest running the main PSU card from a 24V bench power supply and checking the various voltage outputs.
I am currently preparing a document that details basic repairs on the IBMU which will be available shortly. I also have a number of PSU cards that have been serviced and fully tested should this be of interest. My location is in Eastbourne.
Barry G8DXU