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+--- Thread: Microbee twin disk drive unit (/thread-628.html)
Microbee twin disk drive unit - ChickenMan - 11-03-2023
I had a quick look at a twin drive unit for a 56k MicroBee today, it was given to me back in Dec last year. Turned it on, nothing. Check voltages, nothing, check fuse, blown. Replaced fuse (small 1A) and it fired up with A: spinning and LED on. Plugged it into my 56k bee and A: drive boots a disk, fantastic. A couple min later smoke appeared from the power supply, gee whats new ! Turn it off and removed the power supply PCB. It was those 3 large 4700 uF caps ! no RIFA's in this transformer based power supply. Replaced the caps with new ones I bought from Jacar way back, they are a bit taller but still fit ok. So now all good, except B: doesn't work. Cleaned heads, lubed the rails, etc and still no go. So replaced the drive and now all good and working.
These were the caps, 2 turned almost brown and one had its base expanding.
RE: Microbee twin disk drive unit - someone - 18-03-2023
Many of these aged caps lose their capacitance due to their age & it's always best to desolder them and measure them prior to powering them up.
Some caps even gain capacitance!
The yellow microbee filter caps predate the introduction of pressure minimisation creases hence the cracking at the base and it not blowing its top.
These days the estimated longevity of capacitors can be calculated from their thermal and lifetime specifications.
The higher the spec the higher the price!
e.g. 12000Hrs@105C, 2000Hrs@85C
Generally, the hotter they're run the shorter their lifespan.
The giant S-100 Bus power supplies filter caps which are about the size of small drink cans are damned expensive so to prevent these exploding reformed them.
This is quite easily done with a laboratory power supply that has both voltage and current limiting controls and a suitably sized bleed resistors or programmable DC Load.
It's also recommended to measure the open circuit output voltage from the transformer. The transformers sold at Applied Technology had relatively high open circuit and loaded voltages relative to their specified voltages.
e.g. 16V output often measured up to 25V open circuit and 20V under load.
The higher output voltage means that the downstream regulators run hotter to dissipate the unneeded power.
One also needs to ensure that the accompanying components such as filter caps are sufficiently rated.