Someone stole my MB-H50's capacitors!

Supporter (11)

01-03-2023, 17:56

I purchased a set of rather hurt-looking MSX1s from the Japanese auctions (I felt bad for them) and among the group is this Hitachi MB-H50 which I've fallen in love with. Although it's keyboardless, that doesn't keep me from testing it... but this does.

Someone has robbed C54, C60, and C62 from my system!

I can probably guess at what values these are supposed to be, but I would be very grateful if someone else with an MB-H50 could open theirs up and tell me what it has. :)

By Daemos

Prophet (2061)

02-03-2023, 19:26

They look like smoothing capacitors. If so the bigger the better. nowadays that size can easily hold 10mF. If you know the secondary output multiply it with squareroot 2 and choose that voltage as your max. Be carefull though that the bigger you go the higher the inrush currents so a NTC resistor can help you out there. To my experience the bridge rectifier holds up to 20mF. Thats a overkill value anyway since they propably use a voltage regulator to smooth things out and otherwise the old fashion transistor way. Looking at the size id say 32 or 24 volts 3,3 mF. The small ones 24 or 16 volts 1 mF.

By barbeque

Supporter (11)

07-03-2023, 04:36

Update on this one: After flipping the board over, it became obvious that these were bypass caps for +5, +12, and -12V rails. I stuffed a 2200µF/25V cap into the big hole and then put some 470µF/35V caps into the other two because that's what I had on hand. The previous owner also somehow damaged the wire jumper you can see at the bottom of the picture, so I had to reinstall it.

The machine still isn't working but it's not the fault of these caps – it is pulling zero amps from the wall. I am guessing this is the fault they were trying to repair with what appears to be a very hasty recap. I haven't had much time to diagnose, but I've eliminated the power switch, fuse, and power cord as suspects. I suspect it's likely either the bridge rectifier or some component feeding it from the transformer – there's just not much else on this board! I'll post again when I figure out what the problem was.

Thank you for the reply!

By barbeque

Supporter (11)

26-03-2023, 05:15

I figured it out.

While I was replacing the capacitors and fixing broken traces, I figured I would also change the input on the transformer from the "100V" pin to the "127V" pin as I live in Canada. I didn't want to open up the computer twice, since getting the board out of the case is very difficult, so I figured I would change two things at a time.

This was a mistake, as it turns out that there is infinite resistance between the 127V pin and the other input pin of the transformer. It seems like they didn't actually wire up a tap for that in the transformer: the top of the transformer only says "100V," so maybe the factory planned to have multiple regions of transformer and just drop the appropriate one in at the time of assembly (there is also an unpopulated PAL-related oscillator on the board.)

I set the tap back to 100V and the computer works fine. I still need to build a keyboard, so that's the next step. Thanks for your help Daemos!