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Jeff Brace
Vintage Computer Federation Vice President
I'm repairing/restoring a later model MITS 8 inch floppy drive. I can not
seem to find schematics for it. It is
the same as Bill Degnan has here:
https://vintagecomputer.net/MITS/88-DCDD/
You can see in his photos (and mine match) that the _plus_ 24 volt DC supply
on the board uses a 7924 (note the
9. negative) voltage regulator, with a small heat sink, directly on the
circuit board. Along with a couple of 7805s,
there is a .Motorola 2n6045 screwed to the large black heat sink. All three
of the devices drop into sockets on the
circuit board to allow the heat sink to be easily installed/removed. On my
unit, the socket for the 2n6045 was burnt
to a crisp. I have replace the socket, the 2n6045, the 7924 and the
electrolytic caps. When I test the +24 volt rail
with a dummy load, it measures +41 volts.
I don't understand this circuit. I know MITS was notorious for its power
supplies. Does anyone have the schematics
for this version of the 8 inch floppy? Any suggestions?
Thanks,
Bill S.
--
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>Message: 26
>Date: Sat, 1 Feb 2025 18:13:17 -0500
>From: Nigel Johnson Ham <g4ajq1(a)gmail.com>
>Subject: [cctalk] Re: RS232 then and now
>
>On 2025-02-01 18:11, Chuck Guzis via cctalk wrote:
>> One mystery to me was why did the industry stick with the EIA-232 levels
>> for terminals and whatnot long after differential EIA-422 was
>> introduced. Higher-speed, better noise immunity, single-ended power
>> supply... Seems that the popular places were Appletalk and ST506 data
>> lines. But not on DTE/DCE. If you were using full-voltage (±15-±25)
>> at high speeds (>500Kbps), the slew rates were ridiculous.
>>
>> Inertia? My old 80286 motherboard had junper-selectable 232 or 422.
>>
>> Still have a couple of tubes of 422 drivers/receivers.
>>
>> --Chuck
>>
>DEC had RS422 available on their DLJ11J four-port interface, but I never
>saw it used in the field.
>
The Otrona Attache has two serial ports, DA-15 female, that are jumper selectable between RS-232C, RS-422 or RS-423.
Bob
In the early 70s my employer used stand alone data entry tape units in our remote locations. We started with NCR units but then switched to Tally. In the late afternoon the data would be transmitted to the home office in Chicago for processing. Also using NCR/Tally units. We used Racal-Milgo modems. These things were about the size of a large home stereo receiver. We started at 2400 baud. The units could do 3600 or 4800 (I don’t remember which) if a circuit board strap was moved.
We had locations in many cities/towns all the way out to Montana. It was decided that we needed to have the ability to switch between 2400 and the faster speed when possible. We did not want to have the locals power off the modem, open it up, remove a card and move the strap. So I got volunteered to come up with a plan.
I found a place in the modem where I could mount a switch. Luckily the modem had a sliding panel on the front so the locals would not have to open the box top. Went to our fleet garage where they had the tools I needed. I made a bunch of aluminum brackets for a tiny toggle switch. Added wires to the switch.
It was also decided that we could not trust the locals to do the switch install. So part of the plan included me going to all the locations and doing the install. I did not have a car at the time so I got a fleet car. Headed south to Kentucky then turned west. Don’t remember all the states I hit but I believe the last place I hit on the way home was Rockford. I remember being on a back road in Montana and I took a picture of my speedometer. I was doing 65. This was when there was a countrywide speed limit of 55. I can’t be limited by the Man. :)
Back at the home office we had 6 NCR/Tally machines and only 4 modems. I built a plug panel box that had everything wired to it and used telephone jack plugs to connect the Tally’s to an available modem. The box also had switches for the modem speed.
I now forget how long the whole process took. Put my DeVry AAS degree to good use. :)
Microsoft, or is it Micro-Soft or MICROSOFT, is a 50 yr. old trans-national
corporation. Whether it has been good or not-so-good for society is
debatable but on the financial-side its been very kind to investors(the
earliest ones anyways). Capitalism certainly owes Microsoft a good nod!
Happy computing.
Murray 🙂
Follow-up:
Vendor now on eBay for Fairchild 75325DC in lots-of-five; they accepted my
Best Offer.
https://www.ebay.com/itm/333675901157
So now I can get back to making some progress on repairing the H222A in my
MM11-DP :-}!
From: pbirkel(a)gmail.com
Sent: Friday, March 11, 2022 10:01 AM
To: General Discussion: On-Topic Posts <cctech(a)classiccmp.org>
Subject: Seeking a MC75325L Dual Memory Driver
I have here in my hands a DEC H222A (16Kx18), part of a MM11-DP, that took a
blow at sometime in the past. In consequence there are a number of small
parts damaged (snapped diode, crushed axial electrolytic, chipped mica
capacitor, cracked/broken SIP resister net) but those all appear to be
relatively easy to replace.
What's not so easy to replace is the MC75325L Dual Memory Driver (L =
Ceramic) that was de-lidded in the process :-<.
I am wondering whether anyone has one of these ICs in their spare parts
drawer that I could acquire?
I do see a MC75325P (plastic) on eBay at littlediode_components for ~20USD,
plus a surprisingly modest shipping charge (Royal Mail International).
UTSOURCE claims to have a supplier of the ceramic part "new", with a
significantly higher shipping charge.
Before I go with the ceramic part (IMO not the sort of packaging that gets .
remarked) I thought that I would check here for alternative sources.
Thank you,
paul
I prefer to use the Daily Digest mode. Then you can quickly glance at the topics and decide whether it is worth reading any particular messages. The pain comes when you may want to contribute something, as then you need to reconstruct enough of the message of concern in a new composition and reply that way.
Kevin Anderson
Dubuque, Iowa
