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Polymorphic BP/5 motherboard power

ef1j95

ef1j95

Experienced Member
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Oct 10, 2013
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145
Location
Philly, Yo
I'm currently working on a Polymorphic BP/5 motherboard as part of some bowenfusenwerk caused by a shorted tantalum on a card I was testing. After powering the machine back up, something in the power supply let out a burst of magic smoke.

Can anyone post a picture of their Poly-88 backplane around the power supply area on the motherboard? In particular, I'm trying to see the D3-D6 1A power diodes next to the D1, D2 rectifier set.

Thank you,
Eric
 
It should be the same as the Poly88 motherboard. Look at the picture between page 8 and 9 of the manual:

http://deramp.com/polymorphic-computers/manuals/Poly_88_Assembly_Test.pdf

They would be 1N4001 to 1N4007. Anything in that series would be fine. With the transformer disconnected you should be able to tell which diodes are bad with and ohm meter.
At the end of the manual is a schematic that should be the same as your board.
Dwight
 
Do 1N4001-1N4007 come in a DO-41 package? The diodes on my board are DO-35 style. They look like signal diodes. Which is why I was curious what is actually installed on other folk's boards.

Here's a picture:

IMG_6059.jpg
 
I've seen 1N4001s come in either glass or plastic, depending on the manufacture. These are not signal diodes but just glass cases. Notice the larger lead diameters. I don't think DO-41 specifies the material, only the maximum dimensions.
Dwight
 
Do remember, on selecting diodes, they have to withstand at least 2X the voltage of the capacitors output voltage. There are 1.5 amp diodes in similar sized packages.
Dwight
 
Super! Thanks for the leads. I'll try to get it apart this weekend to take a closer look. In the meantime, I happen to have 1N4001's on hand, so if those died it will be an easy repair.
 
The diode physical size and lead diameter look similar to 1N54 series diodes, for example a 1N5404 is a 400V 3A diode and a 1N5401 a 100V 3A version.

Sometimes I replace these 1N54 series types with 1N5822 Schottky rectifiers in the same package as they run cooler due to their lower forward voltage drop than a standard silicon rectifier, be aware that these are 40V rated (max DC blocking voltage), but that may be ok depending as Dwight points out about the diode's rating and the power supply voltage.
 
For the small diodes, the part list has 1N4003. For the large diodes it states 6 amp diodes of type 60S05. Mouser says these are 6 amp 50 volt diodes.
Dwight
 
Dwight was right. Despite the DO-35 glass cases, the diodes were 1N4003, just as the Poly-88 parts list states. (I haven't been able to find glass replacements... only the DO-41 plastic.) I think D3 and D6 were blown when a tantalum cap on the memory board I was testing shorted the +16V line to ground. D4 and D5 were unaffected. I replaced D3 and D6 with what I had on hand... two 1N4004 diodes. It is working fine now -- I'm running a VG Z80 cpu and the TDL SMB II board as a minimal system.
 
Just don't try to run a floppy drive from the +12V line. These need an extra power supply for +12V. That rail was intended for light loads, like a RS232 or bias for RAM chips.
Dwight
 
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