I received a TRS-80 Model 4 from Erik Klein (thanks Erik!) that needs a
little care. The machine hasn't been opened in 25 years, and it's a bit
grimy. The keyboard barely works, and the drives probably need
disassembly and cleaning too.
I have the Sams Computerfacts for the Model 3, which should be close
enough to the Model 4 for the type of work I'm going to be doing. Any
words of advise before I crack the case open and void the warranty?
Mechanincally the machine is identical to the Model 3. The difference is
in the CPU board and the keyboard. I think some later M4s have everything
(including the disk controller and RS232 port) on the CPU board, the
older ones like mine have the same disk controller and RS232 boards as a
Model 3, the CPU board is the same size (and same connectors ) as a M3
one, but electtronically very different.
 OK, there's one extra 4 pin header on a M4 CPU board. It links to the
sound board stuck to the front of the screening metalwork.
As regards dismantling it. Firstly, if you've never doen one before, take
care. The CRT and monitor PCB are fixed ot the top case, and the CRT neck
is vert close the screening metalwork in the lower case. It's very easy
to reak the CRT when reassembling the machine, it's not much harder to do
so when taking it apart!. The trick is to lift the case up vertically, in
fact pull it forawrds slightly. Then when it's clear, turn it 90 degress
anticlockwise (seen from the front) and put it down to the left side of
the base assemnbly. Then unplug the cabling.
Secondly, there are 2 SMPSU boards in there with mains all over the
place. Of course you unplug the mains cable before taking it apart, but
it's possile for stored charge to remain.
Other things. If you have the type with the separate disk controller and
RS232 boards, there are tapewires that link them to connectors on the CPU
oard. These are very fragile, and seem to be the problem in 99% of cases
when a M3 or M$ won't boot. OK, I exagerate a bit, but handle them with care.
The keyaord is a simple matrix of switches (similar to the M3 keyoard in
your manual (I assume) but with extra keys in the unused spaces of the M3
matrix. The switches can, and do, fail. What I did to my M4 was pull all
the keycaps (make a diagram of where they go back), then desolder all the
swtiches from the PCB (!). Take the whole lot apart, and then test the
switches with an Ohmmerter. Don't expect a dead short (they're contuctive
ruber contaics inside), but you can seprata the good from bad ones this way.
The swtiches do come apart. Clean the conductive rubber part and the
contacts and try aggain. This might get some of them going. The remaining
duds come apart again, and I rubed a 6B (very soft) pencil on the
conductive rubber part. That really reduced the resistance!. I then
worked out the least-used keyoard locations (in my case the number pad)
and put the dodgy/repaired switches there. I had enough good ones to
fully populate the main area with a few dubious ones in the number pad.
I plan on taking lots of pictures as I go to avoid forgetting how things
go back together. I'm also going to be cleaning the inside of the
I didn't find that necessary. There are not that many small parts.
machine as best as I can, keeping well clear of the
CRT which I
understand might try to kill me.
The danger from CRTs is, IMHO, much overrated. The CRT in the M4 is a 12"
monochrome CRT with a tension band. It's very unlikely to violently
implode (due to said tension band), it's also not that high an EHT (12kV
I would remove the CRT/monitor PCB from the top case to clean things
properly. The monotor PCB is held on by 2 scres near the bottom edge. The
CRT is held in by 4 corner nuts. Either remove them separately (if you
don't mind unplgging the EHT anode cap, etc), or together. The CRT will
certianly need a clean!. One of those CRT nuts is _hell_ to get to, I
used a very long nutdriver to get in to that corner. Oh yes, put the top
case on the edge of the bench, resting on the screen face (with the
keyoard surround overhanging the edge of the bench). And NEVER lift a CRT
by its neck!