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TRS-80 Repairs

by @ 12:14 pm on December 2, 2011.
[Model I]      [Model III] TRS-80 Repairs     [Model 4]      [Model 4P]


Prior Q & A

Any hardware which has sat around unused for a few decades may have developed problems in the interim. Solder joints may have detached, belts broken, disk drive heads frozen in position, etc. The following are some questions which had been asked in the past and the answers given. While YMMV, these may be of use. Of course, ANYTHING AND EVERYTHING YOU DO IS AT YOUR OWN RISK, EVEN IF YOU ARE FOLLOWING THE SUGGESTIONS BELOW.

Model I:
     Issue: Garbage on the entire screen. Screen contents change as directed by a program/OS/typing but just change to different garbage
     Possible Cause: One of the 7 video memory chips is bad.

Model III/4:
     Question: I am about to turn on a M3/M4 for the first time – is there anything I should do re the drives?
     Answer: [1] open and close the door; lubricate if needed.
[2] Make sure the head moves back and forth with little resistance; lubricate the rails if needed.
[3] Check that the drive belt is not too lose or missing; replace if needed.
[4] Inspect the drive head; clean with rubbing alcohol and cotton swabs if needed.
[5] Check the felt pad on single sided drives; if missing replace it.
[6] Clean the solder covered edge connector with a pink pearl eraser.
[7] If all is well, hook it up and try to format; if it fails check RPM (should be 298). If still cant read, perhaps the logic board is bad or the disk is out of alignment.

     Question: If the drives are disconnected from the controller board, but everything else was working, would it still say CASS?
     Answer: Yes.

     Issue: Disk door is open and it says “CASS?” instead of “DISKETTE?”
     Possible Cause: Bad FDC or interconnect cable between the motherboard and FDC. Try removing both disk drives, cleaning their edge connectors and trying to boot from both drives, one after the other.

     Issue: M3 boots but crashes when running a BASIC program or a self booting game.
     Possible Cause: Bad memory. If PRINT MEM produces a number much different than 48082, it is bad RAM.

     Question: Can internal Texas Instruments disk drives be swapped between drive 0 and drive 1?
     Answer: Yes as TI Drives in a M3/M4 have no jumpers; drive selection is by cable, and because the internal cable is so short, drive 0 has no terminating resistor.

     Issue: M3 has a screen full of random characters. Disk spins on power-up but won’t do disk operations.
     Possible Cause: Power supply, RAM, CPU. CAREFULLY check the power supply to ensure it is giving proper voltages. If yes, then remove the bottom row of ram (=48K) and see if the problem goes away. If not, remove the middle row also (=32K) and see if the problem goes away. If not, remove the top row of RAM (=16K) and fill it other chips you removed, and see if its ok. If at any point it gets better, you have narrowed down which chips are bad. If permuting and combining RAM doesn’t help, it may be the CPU.

     Issue: M4P runs M3 software perfectly and won’t run M4 software.
     Possible Cause: On a non-gate-array M4P, the C210 adjustable trimmer cap may be out of sync. To get at the trimmer cap, the mainboard needs to be folded down, but when its folded down you can’t plug the disk drives in (cable not long enough)! So, the way to adjust it is have a known good TRSDOS 6 disk in drive 0 and alternate between reset button and adjusting the trimmer until the disk boots. Initially it will be fuzzy on the screen and adjusting it further will sharpen it up. Further than that it will go fuzzy again, so find the sweet spot and you are done.
     Possible Cause: On a gate-array M4P, check to see if one of the gate array chips has stopped working or has become loose..

     Issue: Non-gate Array model 4 won’t boot consistently, tossing out a variety of errors. If Drive 0 -or- Drive 1 is used, same issue.
     Possible Cause: Non-gate M4 has a separate FDC board. If getting the exact same problems on both drives then look closely at the FDC board, and try and procure a good used one. There are two types and neither are more or less reliable than the other. The early analogue one from the Model III and early Model 4 has potentiometers which need to be properly calibrated. The later digital board in theory is better but we have seen failed FDC chips, PLL chips etc.

     Issue: I have a non-gate array model 4 and I want to change out every capacitor.
     Answer: We certainly do not recommend doing this. However, here is a spreadsheet supplied by Christopher Eddy of the capacitors.


Model 4P:
     Question: My Model 4P just let off a pop and a puff of smoke. I am pretty handy with replacing electrolytic capacitors. Anything I should think about?
     Answer: Yes, 4P has the same power supply as the M4, so suffers the same filter cap smoke-out issue.
The 4P is even easier than the M4 to fix because after the outer case has been removed, the power supply is mounted to a metal panel directly above the CRT – easy access without the possibility of breaking the CRT neck like on a M3 or 4.
If the power supply is Astec then C1, C2 and C13 need to be replaced.
If the power supply is Tandy, then C32 and C33 need to be replaced.
In both cases use yellow Suntan MKT X2 275V caps
It is EXTREMELY rare for any caps on the 4P motherboard to need replacement, so don’t ‘t change anything on the motherboard unless there is an obvious fault.

Printers:
     Question: Can a DW-2 printer be hooked to a PC?
     Answer: Yes, but there may be issues with line feeds/carriage returns not being processed properly.



TRS-80 Repair Videos

Ian Mavric, who is also the Australia source for TRS-80 disk requests and who repairs TRS-80s, has been making YouTube videos to assist in certain repairs..



TRS-80 Repair

For those with broken TRS-80′s who have been looking for people who might be able to repair them, this page is a listing of people who have advised that they are able to do repairs. I have not met, used, spoken with, vetted, etc., anyone on this page, so you must make your own decision as to whether you wish to hire the person and send off your TRS-80. I cannot stress strongly enough that I take no responsibility. I am merely passing on the identities of people who say they can do the repairs.
Country Name E-Mail Address City/State What They Can Repair
Australia   Ian Mavric ianm@trs-80.com Melbourne, Australia TRS-80 Models I/III/4, 4P, Cocos, Tandy 1000 and 2000, Hard drives

USA Charles Higgins charles.higgins52@yahoo.com Holly Springs, Mississippi TRS-80 Models 3 and 4

USA Jeff Flanagan Jflanagan@epix.net LeRaysville, Pa TRS-80 Model I

France Pascal Holdry pascal.holdry@sfr.fr Paris, France TRS-80 Models 1, 3, and 4

One Response to “TRS-80 Repairs”

  1. chris zeigler says:

    Does ANYONE do Model II’s?? I need the legal program as well. How does one hook a 5 1/4 inch disk to a model II? I have a trs dos program on 8 inch disks…..

    help?

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