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{
    "title": "TRS-80 Model 4P",
    "tags": ["retrocomputing"],
    "date": "2013-03-21"
}

[Back to my retrocomputing collection](..)

## The System



[![TRS-80 4P](20130323-4p-sm.jpg)](20130323-4p.jpg)

## Log

### 20121014

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.

### 20121027

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.

### 20121104

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.

### 20121111

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.

### 20130321

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.

## Floppy Drive Adjustments

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?

## Serial Numbers

[trs-80.com](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.

<table border=1>
    <tr><th>SN</th><th>where</th><th>notes</th></tr>
    <tr><td><tt>000432</tt></td><td>mine!</td><td>inverted "T" arrow keys; green screen</td></tr>
    <tr><td><tt>011134</tt></td><td>ebay</td><td>separated arrow keys</td></tr>
    <tr><td><tt>011708</tt></td><td>ebay 20150216</td><td>separated arrow keys; white screen; metal case clips; mint!</td></tr>
    <tr><td><tt>030397</tt></td><td>ebay</td><td>inverted "T" arrow keys; green screen</td></tr>
    <tr><td><tt>039381</tt></td><td>ebay 20130609</td><td>inverted "T" arrow keys</td></tr>
</table>

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.

## Motherboard Capacitors

<table border=0>
    <tr><th>location</th><th>specs</th><th>Jameco part #</th></tr>
    <tr><td>C201</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C202</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C203</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C204</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C205</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C206</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C207</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C208</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C209</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C210</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C211</td><td>22uF 50V radial</td><td>93739</td></tr>
    <tr><td>C215</td><td>10uF 16V radial</td><td>330692</td></tr>
    <tr><td></td><td>10pF disc (3)</td><td></td></tr>
    <tr><td></td><td>22pF disc (3)</td><td></td></tr>
    <tr><td></td><td>33pF disc</td><td></td></tr>
    <tr><td></td><td>100pF disc</td><td></td></tr>
    <tr><td></td><td>220pF +/- 10% disc</td><td></td></tr>
    <tr><td></td><td>.001uF disc</td><td></td></tr>
    <tr><td></td><td>.022uF disc (2)</td><td></td></tr>
</table>

## 64K RAM Upgrade

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

## Software

<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>

### Operating Systems

Many operating systems were available for the TRS-80s.

TODO

## References

<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>

## TODO



## Local

<a href="img/">images</a><br>
<a href="doc/">documentation</a><br>