Tandy TRS-80 Model 4

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 [1]) as a M3 one, but electtronically very different. [1] 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 didn't find that necessary. There are not that many small parts. 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 or so). 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! -tony