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Heathkit H77

userman73

New Member
Joined
Oct 5, 2020
Messages
1
Location
Bel Air, MD
I recently acquired a Heathkit H77, which has been sitting for years and years unused. I went to power it on today, and I heard some crackling/sparking going on inside. Then I saw white smoke coming out of the drive bays. I instantly powered it off. I probably should have done some pre-flight checks... I opened it up, and couldn't feel anything hot. I didn't see any burnt/charred/black areas. There is a Fuse on the back of it. I am guessing that the Fuse was no good, and it fried the thing. Not knowing electronics too well, and not even sure where online to post this here, I thought here might be a good starting point. What do I do? How do I fix this? Is it recoverable?
 
The power supply for the H-77 is very simple. I would suggest unplugging the floppy drive power connectors. Using an ohm meter checking for continuity between the prongs in the A/C power cord. If there is a low resistance, you could have a short in the transformer.
I have seen voltage regulators fail also and short out.

ED-100 Power Supply Circuit Board.jpg
 
i had this happen to me, also. turned out it was not the power supply but one of the drives gave up.
look for any charred components in the disk drive(s).
Alex B
 
The power supply for the H-77 is very simple. I would suggest unplugging the floppy drive power connectors. Using an ohm meter checking for continuity between the prongs in the A/C power cord. If there is a low resistance, you could have a short in the transformer.
I have seen voltage regulators fail also and short out.

View attachment 1037461
Hi, any chance you have the full service manual for the h77. I’m seeking the voltage ratings for the small tantalum’s of the h77 psu. Thanks.
 
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