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Hazeltine/OSI 1420 blowing fuses

falter

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Got a mint, possibly NIB OSI branded Hazeltine 1420 recently and finally unboxed to check out. On power up it immediately blows the 1A fuse and there is a puff of smoke from the general area pictured. I didn't quite catch exactly where but the circled cap appears to be shorted.. could a little thing like that be blowing the main fuse? Or is it likely something else is going on?

https://drive.google.com/file/d/1-iqnJ9-s7x0f-XrWsa7pYVRcHqqQVn80/view?usp=drivesdk
 
Got a mint, possibly NIB OSI branded Hazeltine 1420 recently and finally unboxed to check out. On power up it immediately blows the 1A fuse and there is a puff of smoke from the general area pictured. I didn't quite catch exactly where but the circled cap appears to be shorted.. could a little thing like that be blowing the main fuse? Or is it likely something else is going on?

https://drive.google.com/file/d/1-iqnJ9-s7x0f-XrWsa7pYVRcHqqQVn80/view?usp=drivesdk

That little thing is a tantalum capacitor. Those are known for making very good short circuits after some years.
You should just remove that capacitor and check if it's shorted indeed.
Frank
 
Yeah it seems to be shorted alright.. I hope the smoke wasn't an IC getting the wrong voltage.
 
Tants just go short circuit...

I assume the 1A fuse is on the low voltage side and not the mains?

Dave
 
I believe it would be considered mains. It is right at the power switch.

This terminal is otherwise absolutely mint. Doesn't look like it was ever used.
 
If it's the mains side fuse blowing then I'd check around that area also including the transformer.
You can just pull that tant cap and check it with a meter, but it does a little discoloured to me. It's only a filtering cap so not going to affect the operation of the PSU that much for testing purposes. Is there a voltage selector switch at mains input ? Check that is set correctly also, I guess you have already anyway.
 
When the mains fuse blows promptly, I begin by checking that side of the power supply. In particular, make sure that that TO-3 (2N2955) hasn't shorted, along with all of the large capacitors and diodes on the mains side. It would be unusual, but not unheard-of, for a small tantalum on the low-voltage side to take out the mains fuse.
 
I'm with Chuck on the Motorola 2955. Easy to check with your VOM - just pull from the board and do 10 to 1 front to back ratio resistance check between all legs (emitter-base-collector).
 
Falter - Did you make any progress on this - I just got a Hazeltine 1420 - when I plug it in I get an arcing / crackle sound near the on/off switch so very quickly switched it off. I’m hoping that it’s just a loose power lead or a dirty switch….CF9341DD-7C91-483C-B79C-E2055675918D.jpeg
 
4B77962F-5E2F-418D-A733-066EDC93DFA1.jpegIts alive ! I put some deoxit in the power switch, and the safety interlock, and reseated the molex power connections. Cleaned the mainboard to remove 40 years of dust, Seems to be good now.
Connected to a raspberry pi running “getty”, runs zork1 but with some garbage codes.
 
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Those "garbage codes" are terminal-specific control codes; namely ANSI/VT100. The 1420 has its own (wretched) set of control codes that are completely incompatible. See the OEM manual for details. Maybe you can find a terminfo/termcap setting corresponding to this terminal to set things right. cf. Linux $TERM variable.
 
Yep, spent the last couple of hours trying to make sense of terminfo/termcap references. -hz switch for the Hazeltine tilde symbol issues.
Anyway, it works enough to move on to part 2 which is getting an MSI6800 to talk to it.
Msi6800
 
Yup, in the 1963 version of ASCII, hex 7e was ESC (hex 1b was S3, one of 8 separators), hex 7c was ACK (hex 06 was RU--I gather that it was a query to address a specific receiver; see WRU).
All of that changed in 1967, with ESC and ACK moved to the lower 32 control character space. We still have hex 7f as DEL; sometimes called Rubout because on a 7-level paper tape, all holes are punched and are ignored by the receiver--PTP leaders often consist of strings of rubouts.
So the Hazeltine code implementation isn't all that weird from a historical standpoint; just a bit anachronistic.
(I know, I know, more that you wanted to know--but I used to work with these things).
 
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