Issue with two different PCjrs - looking for troublesh help

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Re: Issue with two different PCjrs - looking for troublesh h

Postby DoctorOctal » Mon Sep 14, 2020 5:31 am

bloguidice wrote:So it’s odd, but I guess both systems have shorted drives somehow.


Shorted tantalums are a common problem with old computers, not just with these diskette drives. IBM PCs and compatibles almost always have tantalum electrolytic capacitors, typically rated for 16V. Notably, the PCjr motherboard uses aluminum electrolytics rated for 50V instead of tantalums (presumably as a cost-cutting measure) and has few capacitor problems. The Alps diskette drives also use aluminum electrolytics.

Gremlin5 wrote:Odd I have acquired about 7 Qume142 drives and they have all worked perfectly.


Obviously there are plenty of PCjr owners out there with working drives, most of which will be Qume drives. I can think of several possible reasons that your drives are working:

  • They’ve been turned on and used occasionally (rather than sitting idle for 30 years). Tantalum capacitors aren’t supposed to wear out, but experience with vintage computers suggests that they do deteriorate with age. They can self-heal to some degree, however, so powering them up before they go too far may renew them. Also, spinning disks in the drives from time to time may keep the lubricants from gumming up too much.

  • The bad ones got weeded out. Drives that break tend to be fixed or tossed. When you buy machines on eBay or in person, you probably favor those in working condition. My experience, on the other hand, is mostly with a pile of used and NIB drives that have been sitting in a warehouse for 30 to 35 years.

  • You got lucky. I’ve bought two PCjrs on eBay. One happened to have an Alps drive in perfect working order (and the previous owner had used it recently). The other Jr had a Qume drive that couldn’t spin a diskette at all until I lubed the drive thoroughly. Presumably the previous owner had not used it recently, as the motherboard had a bad RAM chip that caused the POST to fail with two beeps and no video.
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Re: Issue with two different PCjrs - looking for troublesh h

Postby Gremlin5 » Tue Sep 15, 2020 1:10 pm

DoctorOctal wrote:[*]You got lucky. I’ve bought two PCjrs on eBay. One happened to have an Alps drive in perfect working order (and the previous owner had used it recently). The other Jr had a Qume drive that couldn’t spin a diskette at all until I lubed the drive thoroughly. Presumably the previous owner had not used it recently, as the motherboard had a bad RAM chip that caused the POST to fail with two beeps and no video.


Well in I went to use one of my Qume 142 that I had used before and had worked fine but this time the tantalum right in front of the molex connector on the drive blew up in a cloud of smoke, guess thats karma from my previous post.
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Re: Issue with two different PCjrs - looking for troublesh h

Postby antongale » Tue Sep 22, 2020 10:42 pm

This is my first post on here, but I recently got a batch of 8 PCJrs. 6 of them exhibited the issue discussed in this thread. The tantalum capacitors next to the <olex connector on the 5.25" drive were shorted across the 12V line, simply replacing them with some electrolytic 4.7uF 50V capacitors I had laying around fixed the issue. As the 12V line is shorted there will be no video as the 5V regulator (VR1) for the video signal is fed from the 12V line.

Here are 7 of the units: https://twitter.com/antongale/status/1307748599808356352
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Re: Issue with two different PCjrs - looking for troublesh h

Postby Gremlin5 » Thu Sep 24, 2020 2:00 pm

antongale wrote:This is my first post on here, but I recently got a batch of 8 PCJrs. 6 of them exhibited the issue discussed in this thread. The tantalum capacitors next to the <olex connector on the 5.25" drive were shorted across the 12V line, simply replacing them with some electrolytic 4.7uF 50V capacitors I had laying around fixed the issue. As the 12V line is shorted there will be no video as the 5V regulator (VR1) for the video signal is fed from the 12V line.

Here are 7 of the units: https://twitter.com/antongale/status/1307748599808356352


Not entirely sure why anyone would put themselves through this
That question kinda answers itself for a haul of PCJrs like you have pictured on twitter. Very nice...... :D
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