Hi Everyone, I hope its ok to post this here, only recently joined.
I’ve embarked on a mission to restore back to working order an old Hewlett Packard 7980 9-track tape drive unit (manuf: 1988) Currently, it’s out of action with a power supply short somewhere blowing the AC line fuse.
Diagnosis continues - no obvious blowouts, bulges, scorches or any over visible signs of damage, boards look in great condition actually, however I’ve never seen so many RIFA caps on a single board in my life - No less than 9. They’ll all be replaced to avoid the magic smoke in due course before reassembly.
Now to my callout to community, so far I have struggled to find a good scan of the service manual with the schematics. The easily located copies up on bitsavers and archive.org all appear to be equally fuzzy and it’s almost impossible to read the schematics with any detail. Hoping someone here might have a better source or idea which other forums might be able to assist with a clear scan.
Thanks Adrian, I have so far checked all diodes and the MOSFETs for shorts and nothing indicating so far. The search continues, unfortunately a bit slowly due to workload in the day job and a busy few weekends.
I’ve decided to take a very methodical approach to avoid blowing up anything further and use as a learning experience as I have some other machines in the queue after this one to repair/restore. I have a reasonable working knowledge of basic electronics but troubleshooting power supply shorts still a bit of a challenge after the obvious things checked.
Done so far:
Checked all diodes with DMM diode test mode - all reading about 4.5 V drop forward, no shorts
Checked all the components mounted to the heatsink for shorts - none showing short (If I remove those onboard fuses, I do see a short with the fuse still inline)
I believe something downstream of that fuse is where short is. Fuse connects to that TDK Transformer and then the 4 diodes just nearby.
Next steps as I see (Happy to hear advice from experts here):
Have ordered an ESR meter to check all those caps in circuit, awaiting delivery
Replace all those RIFAs / X & Y filter caps and anything the ESR meter picks up.
and take it from there…
Any thoughts on running this through a dim-bulb tester if the above doesn’t work?
Im unaware of any ESR meter that can test capacitors in circuit. It is theoretically impossible to my understanding.
I would suggest all caps, including the AC line capacitor is replaced as a matter of course and safety in the case of the Line filter - be ware to replace it like for like with an X or Y series. If it is a Y series, it could be your problem.
My Sun badged HP 88780 mag tape drive is similar to your 7980 tape drive. Mine had a shorted diode in the primary AC bridge, so it also blew the AC line fuse.
You could connect the power supply to a Variac and send just a little AC voltage to the power supply. Enough voltage to find the part that gets warm, but not enough to blow the fuse.
HP sold their tape drives to lots of manufacturers, You might be able to find an HP maintenance manual from a different manufacturer.
Thanks and please excuse a slow reply. Family, travel and work ahead of hobbies I’m afraid.
Firstly the good news. I have managed to identify a failed (shorted) filter capacitor. One of the 3 big cans near the transformer secondary. This would definitely result in the fuse-blowing failure I am seeing. Not sure how I missed this in the first pass, was focusing on transistor and diode testing I guess and missed the easy diagnosis.
Now waiting on replacements to arrive and fingers crossed there is no downstream damage.
You are right about the rebadging, Looks like Sun 9914 and M4 Data 9914 or 9905 are very similar close cousins and the service manuals all look almost identical, just different layouts.
Thanks for all the help so far on here. I will report back once tape is streaming !
Good progress made this morning on the HP 7980 resurrection. Short now fixed and we have tape movement. Mechanically all seems well with one issue still to diagnose - a couple of operator panel buttons not working it seems, making it difficult to load/unload/test. With a tape loaded from the top cover open, the magical air-powered autoload even works and tape gets to BOT marker.
I can’t do anything more now until I hook up the SCSI and hopefully get some read/write testing done from the Sparc-20 (only SCSI box I have currently).
Then I guess I’m opening her up again to see if I can fix the front panel issue.
Tape air-powered autoload is a piece of engineering genius if you haven’t seen it before!
