It seems that nowadays you can't get *any* replacement for failing DEC
3639 aka 2N3639 transistors. All parts are obsolete and unobtanium, e.g.
2N3640, PN3640, MMBT3640 and so on.
So, what can be used instead? The most important electrical parameter is
the storage time. It needs to be *very* low, around 20-30 ns.
Does it mean that a failing PDP-8 will stay a dead PDP-8 from now on?
Christian
Hi everyone,
According to historians, and I consider myself one, let us consider what
classic/vintage computers were: The 1970s saw the three amigos: Apple II,
TRS-80 and Commodore PET and the OS was DOS and its ilk + CP/M. The 1980’s
saw the Dells, HPs and many others with MS-DOS & IBM PC-DOS from QDOS. We
saw this and behold ’bring on the clones’(I just had to say this!) The era
of old computers saw one generation building on the shoulders of giants who
designed these wayback computers(with apologies to Wayback Machine).
Today’s PCs and ARM machines are just the latest iteration of this
theory(by the way not mine).
Happy computing
Murray 🙂
I am looking for the assembler listing for a bootloader for booting from
MSCP disks like RD-disks.
Google has not been very helpful and my assembler knowledge is not enough
for me to write one from scratch. Does anyone have one they can share?
- Peter
I was reading my own Terak web page, and remembering that
I'd read Doug Gale's history of computing at Cornell:
https://ecommons.cornell.edu/bitstream/handle/1813/36810/00_TheEarlyYearsOf…
and it mentions (at page 3.23) a "how to repair a Terak" video they
made there, and he claims it was distributed to other universities that
were also using Teraks.
Has anyone ever seen this tape?
- John
> From: Jim Davis <jim.p.davis(a)gmail.com>
> Subject: [cctalk] Re: MFM Hard Drive & 8" Double Side Drives
> Was anything MFM or RLL with a removible cart build in the 80's-90's?
> I never heard of such a thing, Carts were scsi early 90's Syquest and
> others?...
>
Yup
https://en.wikipedia.org/wiki/SyQuest_Technology
The early ones were ST412 type interface
> From: Doc Shipley <doc(a)vaxen.net>
>
> Pretty sure Mr Gesswein is on this list but I don't see him post often.
> Anyway, he designed a very handy MFM drive emulator:
>
> https://www.pdp8online.com/mfm/mfm.shtml
>
Yup but try not to hawk my board. When they say they want a drive I don't
suggest alternatives.
I also get the digest so most questions have been answered before I see them.
Thanks all for the kind words.
I had forgotten that 75 years ago, Oct. 3, 1950, the transistor was
invented leading to integrated circuits making possible personal computers
and the interest of our love of computing. Where would we be without
Bardeen's, Brattain's and Shockley's invention?
Happy computing,
Murray 🙂
>
>
> Message: 6
> Date: Fri, 24 Oct 2025 20:29:07 -0400
> From: Murray McCullough <c.murray.mccullough(a)gmail.com>
> Subject: [cctalk] The transistor invention
>
> I had forgotten that 75 years ago, Oct. 3, 1950, the transistor was
> invented leading to integrated circuits making possible personal computers
> and the interest of our love of computing. Where would we be without
> Bardeen's, Brattain's and Shockley's invention?
>
> Happy computing,
>
> Murray 🙂
>
>
Did you know the role of Camp Evans in the invention of the transistor?
Find out here: https://youtu.be/6wFIVvb-5yQ
I just purchased an old Gimix computer (now if only I can get it from
Orlando to Chicago before my friend goes down there for Christmas).
It has an OMTI 20C-1 MFM hard disk controller with a IMI 5018 19MB hard
disk drive in it.
I am thinking of trying to upgrade the hard drive. The Gimix System
that I worked on had a half high 20MB drive and a Syquest removable 5MB
drive.
Does anyone have an old MFM drive laying around (half high or full
height but larger than 18MB) and/or an MFM removable cartridge drive
with cartridges?
I am also going to try to rebuild a Vector Graphic Dual Store Dual 8"
drive enclosure that I picked up at VCFMW. It suffered some dampness
damage and I'm not sure the drives will come up. If you have a dual 8"
drive setup (double sided drives, and enclosure) that you need a good
home for, please let me know.
I'm in the Chicago Area.
Thanks,
Mike
bitwiz(a)12bitsbest.com
I am trying to resolve a problem with a H7842 PSU from a DEC Rainbow. The AC OK output is not being asserted. I am looking at Tony Duell's schematic https://bitsavers.org/pdf/dec/rainbow/duell_schematics/psu.pdf and specifically at the Power OK circuit.
I am using a test load of 1R on the 5V output and 6R on the 12V output, which is within the spec of the PSU. I see that under load the 5V output is only 4.4V, so I suppose it is to be expected that AC OK is not asserted. If I (briefly!) remove the load from just the 5V output, the 5V output is 5.27V, but the AC OK output is still not asserted.
Using Tony's schematic, I looked at the input to the inductor on the secondary side (sheet marked "H7842 PSU Sheet 3") and it looks like this: https://rjarratt.wordpress.com/wp-content/uploads/2025/10/h7842-5v-secondar… I can't tell if that is correct or not. If I remove the 5V load the peaks narrow but are slightly higher.
However, the puzzling thing is the 5V Pulse signal, it doesn't pulse, it is a steady 32V. I see that the 12V Pulse signal *does* pulse. The name would suggest it is supposed to pulse and its inputs do pulse as shown in the trace pictured above. I don't see how it is possible for 5V Pulse to be at a steady 32V, even if the diode or resistor that create the 5V Pulse signal was somehow bad. I guess this non-pulsing might be the problem, but I don't really know what to check here, I have lifted the diode and tested it, it seems fine.
Does anyone have any suggestions please?
Incidentally, for those who helped me with my Olivetti M24, I had already reassembled it and got this PSU out to look at next before receiving further suggestions on the M24. Once I have the H7842 sorted out I will go back to the M24 again.
Thanks
Rob
