💾 Archived View for uscoffings.net › retro-computing › collection › trs80-4p captured on 2023-09-08 at 16:09:28. Gemini links have been rewritten to link to archived content
⬅️ Previous capture (2023-01-29)
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[date: 2013-03-21]
Bought off ebay for $69.98 + 18.35 shipping. I was actually screwing with the high bidder, because I didn't want to see it go for his price, but I didn't expect to win either. How about that.
Unpacked (thanks for the good packing, youresocoolbrewster). Cleaned case. Cleaned drive heads. Attempted to boot from 3 DOS disks; each returned seek errors. Next step is to open up, clean and inspect drives, and perhaps as a short-term fix swap drive 0 with drive 1.
Cleaning drive heads allowed it to boot occasionally. Opened up and swapped drives; still booted only occasionally. Plugged in an external drive that I had on hand, which also booted only occasionally. This suggests the problem is not the drives, but the motherboard. Opened the case, cleaned, reseated chips, no change. Now wondering about capacitors on the motherboard. Ordered a new batch from Jameco.
Replaced the 11 22uF 50V radial capacitors, the 10uF 16V radial, and the two 22pF disc caps connected to the 16Mhz crystal. Now can reliably boot the original DOS disk on the second drive. Not sure whether I fixed anything, or just finally recognized the functional combination. Regardless, now it seems likely that the motherboard is functional but the other drives need to be adjusted. Also installed an additional 64k of RAM.
Finally opened this back up, with the assistance of a wee bit of Jameson's whiskey. (Some things, you do not undertake lightly.) Note to self: Need to make cable extenders for the front connectors; impossible to poke at the motherboard while it is on otherwise. But at this point, I am suspecting and hoping that any remaining flakiness is confined to the Tandon (model TODO) floppy drives. I cleaned each drive carefully (pulled fuzzies out of the upper pad; swabbed the lower head with a Q-tip dipped in rubbing alcohol, examined and cleaned the belts and pulleys). The Tandons have 50 and 60Hz timing markings on the bottom but I do not have a strobe light to check their RPMs. The only possible adjusting pot I see is R15, bottom front. The more difficult drive has corrosion on the large drive pulley; cleaned this as well as I could but can't imagine that any minor bumplies would affect the rotation speed. More worrying: its read/write head did not slide easily on the shaft. It should "step" due to the stepper motor, but for the first third of its travel, it was stiff and did not have any specific steps. This may have been the cause of the seek errors. The shafts had thickened oil on them, which I cleaned off with a Q-tip and rubbing alcohol. After all this cleaning and fiddling, both drives are functioning properly.
Perhaps also of note: Even after the cleanup, neither drive was able to read a TRDOS disk I copied with my Catweasel using a 1.2M drive. 360k (40 track) drives write physically wider swaths, and this may be critical to be readable on the 4p.
Clean the rails, so that the head has clearly identifiable "steps" for each track.
RPMs: Tandon drives have 50Hz and 60Hz markings on the bottom. Need a strobe light. The 4p wants these at 50Hz (300 RPM).
Possible (TODO: verify) RPM adjustment via R15 on bottom front.
TODO: alignment?
The site http://www.trs-80.org/model-4p/ says the screen was originally black and white, but later were green. How early is early? My serial number "432" is green.
Some of the serial numbers I have seen on ebay:
sn where? notes 000432 Mine! inverted "T" arrow keys; green screen 011134 ebay separated arrow keys 011708 ebay 2015 separated arrow keys; white screen; metal case clips; mint! 030397 ebay inverted "T" arrow keys; green screen 039381 ebay 2013 inverted "T" arrow keys
But then the extremely helpful and friendly Matthew Reed of trs-80.com pointed out that green screens were a common upgrade:
There's always the possibility that Radio Shack was using green screens
in the Model 4P inconsistently, but I think it is more likely that you
have a Model 4P with a replacement CRT. Replacing the CRT was a pretty
common TRS-80 upgrade. The most popular way was using the Soft-View,
sold by Langley-St. Clair.
<http://www.trs-80.org/soft-view-crt/>
Radio Shack also offered green screen upgrades at one time in their
Computer Centers.
location specs Jameco part #
-------- ----- -------------
C201 22uF 50V radial 93739
C202 22uF 50V radial 93739
C203 22uF 50V radial 93739
C204 22uF 50V radial 93739
C205 22uF 50V radial 93739
C206 22uF 50V radial 93739
C207 22uF 50V radial 93739
C208 22uF 50V radial 93739
C209 22uF 50V radial 93739
C210 22uF 50V radial 93739
C211 22uF 50V radial 93739
C215 10uF 16V radial 330692
10pF disc (3)
22pF disc (3)
33pF disc
100pF disc
220pF +/- 10% disc
.001uF disc
.022uF disc (2)
Jameco sells 16 pin DIP 64k DRAM 4164-150, part #41662, which will upgrade the 4P from 64K to 128K. These are pulls, but I've had great luck in the past will pulls from Jameco. Update: my "pulls" are Samsung, and look like new-old-stock. Jameco rocks my retro world.
50V 22uF
C201 C206 C209 C208 C205 C204 C207 C203 C210 C211 C202
10uF 16V
C215
+ 100 10pF 3
+ 22 22pF = .022nf 3
+ 33 33pF
+ 101 100pF
+ 221K 220pF +/- 10%
+ 102 1000pF = 1nF
+ 223Z HS 22000pF = 22nF = .022uF 2
<a href="http://z80cpu.eu/mirrors/oldcomputers.dyndns.org/rechner/tandy/docs_from_www.trs-80.com/software/model4/">mirror of trs-80.com</a>
Many operating systems were available for the TRS-80s.
TODO
<a href="http://http://madsoft.lonestar.org/catalog/4prom.html"/>M.A.D. Software</a><br>
<a href="http://www.trs-80.org/model-4p/">trs-80.com/model-4p/</a><br>
<a href="http://www.old-computers.com/museum/computer.asp?st=1&c=1083">old-computers.com</a><br>
<a href="http://z80cpu.eu/mirrors/oldcomputers.dyndns.org/rechner/tandy/docs_from_www.trs-80.com/manuals/hardware/">tons of docs</a><br>
<a href="http://www.tim-mann.org/misosys.html">Tim Mann: Miosys Software and Documents</a><br>
<a href="http://www.classiccmp.org/cpmarchives/trs80/mirrors/www.discover-net.net/~dmkeil/software/trs-DOS.htm">DOS images at classiccmp.org</a><br>
<a href="img/">images</a><br>
<a href="doc/">documentation</a><br>