TRS-80 Repairs

About this page!

TRS-80’s that have sat around for 40 years are very likely to need some tender loving care before they are back in full form.

This page is devoted to repairs and is broken into 3 parts.

The first part is a general troubleshooting guide written by Ken B, who was a Brookner. If you see “KB” as the author of technical bulletings, that is him. This is a good place to start.

The second part covers a blown Model III/4 power supply. This happens so often that it is almost guaranteed to happen.

The third part are prior questions asked, and the answers which were given.

I, of course, take no responsibility for your own repairs. The below are just suggestions. If you wish to have someone else do your repairs, please visit the People who can repair page.

General Troubleshooting Starting Points by Ken Brookner

These systems are old, but not necessarily simple devices. If you do some homework prior to pulling out your test equipment you’ll have better luck chasing down bugs. In our technician training courses we taught these systems to the schematic level in our 6 week courses at that time we troubleshot to the component level. Understanding how the systems work made troubleshooting more efficient which translated into happier customers and increased shop revenue.

Here’s my suggested methodology and tips:

  1. Learn your machine by reading the Service and Technical Reference manuals, and get familiar with their contents before you start work.
    Read the Technical Bulletins for your machine. These bulletins detail upgrades and modifications you may run into on your particular machine that may cause it to differ from the descriptions in the Service and Technical Reference Manuals. Not all machines will have the relevant bulletins done, and you should do the ones listed as mandatory at the very least. In fact, “must” is probably a better word than “should” when it comes to the mandatory bulletins. If you find a machine with the original warranty sticker intact, the Technical Bulletins should be one of your first stops prior to putting your hands into the machine.

    Read Notes and Jumpers. This extensive two volume set lists most of the systems, boards, and peripherals and offers pictorials to ID what you have. It offers the proper jumper information and in some cases various tips. You’ll save yourself a lot of time by verifying that the machine is jumped properly before diving in. Don’t assume they are correct, or that the jumpers are making good contact after all these years.

  2. Begin with a close physical inspection. Look for burns, connectors making poor contact, anything obvious. Rat piss is conductive! So is concrete dust. As a point of maintenance, better to assume that all dust is conductive, and blow it out.

  3. Molex connectors oxidize. This wasn’t a problem 40 years ago, but it is a problem now. At the minimum unplug and re-plug these connectors to hopefully scrape some of the crud off. If you have something like Deoxit, that’s even better.

    Same with plug in cards, especially with the 8″ systems. Card edges oxidize and can make poor connection. Be sure your card edges are clean and that the sockets are tight and that your cards are fully and solidly inserted.

    Some socketed components may have corroded pins that inhibit good contact. Some may be welded in by corrosion. Usually you can spot these devices by looking for rust on the pins, or pins that appear dark with oxidation. Carefully pull these (see the “Static” comments later in these notes), clean the pins and reinsert. In the worst cases you may have to replace the device, or socket, or both. Take your time when prying the devices out of their socket.

  4. We produced extensive diagnostics for our machines and most of these are available on-line along with their accompanying documentation booklets. If your machine boots, you should run these.

  5. There often are various board revisions. Find any schematics that are specific to the revisions you’re working on. Chip numbers, test point numbers, and basic circuits can change between revisions. Having matching schematics will save you a lot of time and frustration.

  6. Don’t overlook alignments. For example we aligned the FDC (if applicable) as well as the hard drive controllers every time a system came into the shop. we didn’t do this just for fun. We checked floppy drive alignments, touched up as needed, or replaced any drive not meeting alignment specifications. Alignments drift. If your machine won’t boot or read a drive, check your alignments. Drive alignment is difficult now since alignment diskettes are almost unobtanium. These were written with special machines and some of the tracks are elliptical. You can’t copy an alignment diskette. Ask for help if you have a drive that seems out of alignment and you don’t know what to do.

  7. Power supplies: Most often these will be switching supplies and must run loaded. Without a proper load the voltages may be way out of spec or become noisy. In the worst case, you may damage the supply by running it without a load. Supplies differ in where the loads need to be and current draw. When in doubt, all of the voltage outputs should be used as normal for your machine. Some supplies will only require 5V, or 12V to be loaded, but you shouldn’t guess. Find the specs for your supply if using a modern one. We required the Astec supplies to be fully loaded during troubleshooting. You can build a load box if you want and one is detailed in the Model II Technical Reference manual.

  8. Voltages: Check them and make sure they meet spec! And this includes noise. You’ll need a scope to check noise. A noisy supply will keep your machine from working properly.. including booting.

  9. In the day we discouraged “swap till you drop” troubleshooting. Today it may be the only way to find a problem. Try not to go crazy until you have an idea of what to swap. We are seeing Z-80 family devices failing as they age, for example.

So, by this point, you will have performed a physical inspection including jumpers, and power supply voltages and noise.

  1. Next, depending on what your failure seems to be, I’d check to see if the Z80 is running. Be aware that the Z80 is very particular about reset timing. Timing caps are failing or falling way out of spec particularly in the Model 3s and 4s. If you have no activity on address/data lines, check reset timing, or if you don’t have a scope, swap the Z80.

  2. Clocks: Clocks make the computers run. If the clocks aren’t working or are out of spec you’re going to have a dead or flakey machine. Check them if you have the test equipment. Be aware that it is quite difficult to troubleshoot these machines without a scope.

  3. Scopes: Having a scope is no help unless you know how to use it. If you’re really green spend some time with the manual and bone up. You should understand coupling, what timebase does, and particularly triggering “AUTO” and “NORMA”L, as well as “SLOPE” and “TRIGGER LEVEL” if you don’t know what these are and why they’re important you have some learning to do. You need to read your manual or watch some vids online if you find yourself spinning knobs and pushing buttons trying to get your scope to display something meaningful. Set your probes to X10 and adjust your volts/div accordingly if your scope doesn’t automatically compensate for this.

  4. Freeze spray: Often the best way to find an intermittent is to freeze sections of the suspect board. You can buy cans of freeze spray, but canned air (like that for blowing dust off camera lenses) is just as good if you invert the can. If your machine starts up and then fails, freeze spray can help you find the problem area and often the specific device. Sometimes, a hairdryer will help to locate thermal problems, too.

  5. Static: Static discharges will kill these machines. What many people don’t realize about static damage is that it is CUMULATIVE. The first and second static hit may only degrade a device and it may be the third time, as an example, that finally forces the failure. If you don’t know what a static safe troubleshooting environment is do some research. Troubleshooting on a synthetic fiber carpet is not a good thing. Be aware of humidity levels – lower is worse. I suggest you use a static strap and an antistatic mat at a minimum, but this is up to you.

  6. CRTs: Be careful. Use common sense. The anode of the tube – this is the cap on the side of the tube, incorporates a bleeder resistor to drain potential from the tube at power off. THESE RESISTORS MAY FAIL OPEN (the one in my Model 16 did). If you are working on the video section you should discharge the tube by attaching a clip lead to ground and the other end to a long screwdriver. Gently insert the blade of the screwdriver underneath the anode cap to discharge the tube. You may not hear anything – which is ok. Whenever working on video keep one hand in a pocket – this keeps you from reaching in with both hands and potentially creating a short path across your heart. I use a back pocket. You should remove any rings, too! Avoiding dropping the tube :slight_smile: if it implodes you’ll have a real mess on your hands or worse. On the back of the tube in the center of the pins is the nipple. It’s easy to break. If you break it, you killed your tube.

Most Common Issue … Did you Model 3/4 Go Poof!

If you turned on your Model III/4 and smoke came out, it is possible your power supply caps blew. This is very common. To repair it yourself, please see this page.

Per that site, you can possibly find replacements:

NOTE: I do not make any representations that the information there is correct – do it at your own risk.

If you don’t mind using current components and don’t want to deal with any of that and don’t mind using current components, Jay Newirth has developed a Model III and Model 4 power supply using modern parts. You can purchase the power supply on

TRS-80 Repair

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.

All Models

Is it ok to use sewing machine oil to grease the rails on a floppy drive?

WAV’s played on my PC into the LINE IN plug from a cassette cable don’t seem to work.
First, check that the TRS-80 will load a normal cassette the traditional way with a CTR-80A or a CCR-81. If it won’t load from a regular tape deck it will never load from a PC. If it does load from the tape, then we know the cassette load circuit on the TRS-80 is working. Then it’s a case of getting the volume and tone correct from the PC headphone output jack to what the TRS-80 is expecting to “hear”. This can take some time and most people struggle with it. One site visitor reported that his issue went away when he found bad soldering near the LM3900 (Z4) chip.

I need to clean disk drive heads from an oxide build-up due to shredded disks – what can I use?

Model I

Screen garbage.
Possible Cause:
Ram errors are the most common video problem. To isolate the defective chip, first determine the ASCII code of that character that should be shown and the character that appears instead. For example, a blank screen (press CLEAR) is filled with ASCII code 32’s (the code for a “space”). If some location shows a “$”, which is ASCII 36, subtract 36-32=4. The location with the error is displaying character 36 instead of character 32 because bit 2 is stuck on (2^2=4). Replace Z46. Similarly, if pressing “B” on the keyboard displays “J” on the screen: B=66, J=74, 74=66=8, and since 2^3=8, replace Z45.
Possible Cause:
If you remove your Z71 and get a screen full of @S’s, likely you have a RAM issue.
Possible Cause:
If you remove your Z3 and get a screen full of @9’s, then all the address lines A0 – A9 are good. The remaining address lines A10 – A15 can be checked for activity with a scope.

If the screen shows “@9@9” in some areas and garbage characters in others, in a repeating pattern, the defective address line can be found easily. For example, if the pattern is alternately four characters of @9 and four of garbage, then address line A2 is stuck because 2^2=4. If the pattern is a full line of @9 (64 characters) followed by a full line of garbage, then address line A6 is stuck because 2^6=64.

I turn on the computer and it looks like this.
Possible Cause:
Assuming that you do not have an E/I connected (to remove any of those issues), the general common cause of this is a bad Z5, Z6 and/or Z57. Test them, and if you cannot test them and just want to replace, start with the Z5 first.



I turn on the computer (no E/I hooked up) and it looks like “FIRST”
and then a second later it looks like “NEXT”. Pressing keys makes no difference and nothing on the screen changes.
Possible Cause:
If the pattern is “@9”, that means the ROMs are bad. If that’s not the pattern, one user noticed that his D3 line was dropping and had success repairing this by replacing Z55 and Z76.

Screen comes on but will not center (i.e., is too far right or left and can’t be adjusted further).
Possible Solution:
First, try the questionable Model I on a 2nd monitor and see if that picture can be centered on that one, if so, then it is the questionable monitor.

Second, try a good Model I on the questionable monitor and see if the picture can be centered on that one and if so, then it is the questionable Model I.

Before proceeding unplug everything and let it sit for a long while.

If its the Monitor, you can take the back off and check that the copper deflection yoke is sitting straight on the neck of the CRT. If it has moved at some point during it’s life it’s going to be sitting on a bit of an angle, that might be you he can’t get a picture to centre on the screen. Alternatively, you can also find instructions to are instructions to build a composite video cable here.

If it’s the Model I, first swing the controls on the back to find the center point. Then open the Model I keyboard and adjust R20 and R21 so the picture is centered on the screen. If THAT doesn’t work, then look at the video sync circuit comprising of Z5, Z6 and Z57 as it’s possible one of those chips (Z5 I would guess) would be going bad.

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

I bought an Expansion Interface from eBay, but I still see my RAM at 15570.
Things to Try:
First, test every power supply on the Model I keyboard alone. If a power supply doesn’t power up the Model I, then you have a bad power supply.

If both power supplies work, then hook up the Expansion Interface, turn it on, and when turning it off, listen for a second click (not the power switch) that occurs 0.25 seconds after you power down. That is the cassette relay. If you don’t hear a click, the Expansion Interface isn’t powering up. Ian Mavric prepared a video for the click test here.

If you do hear a click, then perhaps you have the wrong cable so, you need to figure out if your Expansion Interface needs a buffered cable with DIN, buffered cable without DIN, or just a regular cable:

If either the Model I keyboard or the Expansion Interface has a DIN plug hanging out of the Expansion Interface port, then they BOTH need that. Click here for a picture.

If the Expansion Interface has no DIN plug hanging out, but has some twisted wires on the SOLDER side of the Expansion Interface, then it needs a buffered cable.

If the Expansion Interface has no DIN plug hanging out, and has no twisted wires on the SOLDER side of the Expansion Interface, then it needs a regular (straight) cable.

If I hit enter after MEM SIZE I get a ?SN ERROR and/or I can’t type anything when I get the READY prompt
Possible Cause:
This is likely a memory error. Substitute a known good set of ram chips and see if that fixes it.

Repairing A TRS-80 Model I And Expansion Interface

Model III/4

I have garbage on the screen during boot that goes away with a push of the reset button.
You probably need to replace the power on reset timing cap – an electrolytic that’s usually 22uf at 16V. These components are aging out of specification being ~40 years old now. Look on your schematic just to the left of the z80 reset* pin to find the reset circuit. This cap is C218 on the 4P, C54 on the M3, and C20 on the model 4’s.

Will a Model 4 floppy controller work in a Model III.

My Model III is all scratched up, can I paint it?
Yes. Videos by Texas Tandy Restoration can be seen here: Part 1 and Part 2.

Model III has nothing on the screen. The brightness does show raster lines, but that’s it. Powering it on with a RESET+BREAK hits does access drive 0, but nothing shows on the screen. The brightness/contrast controls were checked with a meter, they are good and have a full range of resistances. Power is -12, +12 and +5.
Possible Cause:
Initially sounds like the video output on the motherboard. A first step might be to start with a TRSDOS 1.3 disk and try to boot that, if the Drive 0 LED changes to Drive 1 LED for a moment then back to Drive 0 LED a few moments later thats a good sign that the rest of the computer is OK and the video output needs to be attended to. If you have another Model III or 4, try the computer lid from another Model III or 4 and make sure its not the video board (and if it’s the video board they are easy enough to obtain and replace).

I can’t open my Model 4/III/4D!!!
On the Model 4 (and III and 4D) there are 10 screws underneath and one in the back:
  • 3 short machine screws along the front
  • 2 long machine screws about 10cm in and recessed (as the base of the disk drive and base of the CRT)
  • 5 self-tappers around the rear, one may be obscured by a warranty label
  • 1 black screw in the middle of the back of the computer
If you count 11 screws then youhave them all out. If it still won’t open, it’s possible something in the back left corner like wires from the video board are snagging, so look through the top disk drive hole as you lift the lid to see what’s going on.
BE SUPER CAREFUL WHEN OPENING THE MACHINE – The CRT sticks out, and if you do not angle the case just so, you will damage/break it.

I am about to turn on a M3/M4 for the first time – is there anything I should do re the drives?
  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.

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

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.

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.

Click to Enlarge

I turn on the computer and it looks like this
Possible Cause:
The picture (called the raster) can be fine and normal, just turn the brightness down until it goes away. Then, turn the contrast up to full and insert a floppy disk but don’t close the door. It should state “Diskette?” on the screen and the contrast can be adjusted for the best clarity on the screen.

Click to Enlarge

A few minutes of turning it on, one of the RF started to smoke.
Possible Cause:
You can just remove the capacitor and the system will still work but it won’t be FCC compliant. That doesn’t mean you SHOULD do this. Just go to an electronics shop and ask for two 0.01uF MKT 250V mains filter caps X or X2 rating, and one 0.1uF MKT mains filter caps X or X2 rating.

My keyboard does not work … oh wait, it is slowly coming back to life. It now responds to Clear, the left and right arrows, the number 8 on the keypad, q, i, and c. However, I have to hold them down for a second before they do anything.
Possible Cause:
You need to examine the keyboard. If it has 4 lugs per key then you can remove individual keyswitches, disassemble them and clean then, which in most instances will restore them to functionality. One of the videos shows how to do this. On keyboards with the two lug switches they can also be removed and disassembled but they are much trickier and fiddly compared to the 4-lug switches. Pascal shows on his site how to fix these key switches however when we find 2-lug keyboards with failed keys we just unsolder them and replace them with known good ones.

Can internal Texas Instruments disk drives be swapped between drive 0 and drive 1?
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.

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.

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..

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.

I have a non-gate array model 4 and I want to change out every capacitor.
We certainly do not recommend doing this.

Where do I find a new CRT tube?
VDC still sells them ( Model III and Model 4 (White Tube) are codes 12VCMP4 / 12VCLP4, Model 4 and 4D (Green Tube) is code 12VCMP31

What are the significant differences between the 26-1059 student diskless version student network 4 model 4 and the 26-1069 model 4 (besides the lack of disks)?
The student workstation has 64K Ram and different Roms so it will attempt to boot from the network via the serial port. Other than that it’s a pretty normal diskless Model 4. .

If I have a 26-1059 student diskless version student network 4 model 4 that I have actually upgraded to run with disks, does the ROM need to change?
The network boot ROM should still run as a disk system so there is no real need for a change. If you feel you MUST change the ROM, you can program a FreHD auto-boot EPROM which will get it working (and all will be ready if he wants to use a FreHD in the future). Ian Mavric does sell ROMs but the cost of shipping from Australia will GREATLY exceed the costs of the ROMs..

How do you upgrade a diskless Model III or Model 4 to work with disk drives?
It is a very involved process needing more than just a disk drive and controller. Actually, the following is needed for the disk upgrade:
  • Disk drives (2)
  • Drive mounting towers (2)
  • PSU (1)
  • FDC (1)
  • Motherboard-FDC white cable (1)
  • FDC-Disk drives data cable (1)
  • Power distribution cables (3)
  • Ground cables (2)
  • Assorted RFI shields (3)
  • Screws (various sizes)

Model 4:

I have a Model 4GA with a Membrane (ALPS) Keyboard that is not working very well.
Visit here for instructions on how to clean/repair it (if you dare!)

Non-Gate Array Model 4 gives a scrambled screen when booting TRSDOS v6.
If the motherboard has a variable capacitor up near the FDC cable connector, give it a wiggle of a couple of degrees either way. After booting TRSDOS 6 and the screen should sync up. The reason the variable cap effects the “later” DOSses is the fact that M4 DOSses run in 80 column mode and Model III DOSses use the 64×16 mode of the Model III. When a Model 4 DOS loads, it switches the video to 80×24 mode. The variable cap adjusts the horizontal sync of 80 column mode.

Model 4D screen shows letters duplicated and then other letters missing. Typing a single letter on the keyboard (any letter) displays it twice then the next keystroke show only a flashing cursor next to the original set of letters. In other words, every odd character shows twice, and every even character is ignored.
Its a motherboard problem. It could be one or two of the gate array chips, the CRT controller, and/or the video memory chip. Can’t definitively be determined without actual examination of the motherboard. Note: Some of the chips are very hard to source (gate array chips for instance, are custom chips made by RS and only used in the Model 4).

Model 4P:

My Model 4P speaker doesn’t work. What can I replace it with?
Assuming it is just your speaker, you can use a 1W 8 Ohm Mini Speaker such as the uxcell a15102900ux0379.

Click to Enlarge

My Model 4P dims when the disk drives are accessed. Is this normal?
No. Your system is not getting the right voltage. The first thing to try would be to reseat all the connectors to knock the oxidization off of them. If that doesn’t work, you should check the solder on the connectors. Quite a bit of the time, that is what will cause the power variations, and more often than not, its the video board to power supply connector. If that’s the case, you would need to reflow the solder. See the image for the usual trouble points. If that’s not the problem, then you should put a meter on the 12V supply and see what the meter shows when the video dips to try to narrow down the issues AND you might want to replace the capacitors.
My 4P will not boot Model 4 disks and, instead, the screen is full of a single repeating character, which character appears more or less random. RAM test shows no errors.
Since the memory test doesn’t fail, this sounds like a disk drive problem.
One suggestion is to swap Drive :1 and Drive :0 and see if you can boot that way. If so, you drive :0 was bad.
If you still cannot boot, then the problem is likely to be either BOTH drives or the disk controller portion of the motherboard.

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?
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.

Any disk put in either of the drives results in either CRC error, ID error, or Disk error.
That is way too generic for an actual answer, so a guess would be that it is more likely a problem with the disk controller on the 4P motherboard than a problem with the disk drives themselves.
If it is the disk drives, it is likely that they are out of alignment.
The best bet is to plug in known good disk drives and work backwards.
If you are stuck with a 4P and don’t have good disk drives to test, buying an auto-booting FreHD, and forgoing using disk drives altogether, MIGHT be a solution for you.

When I plug in my Model 4P I get nothing from the drive, or CRT, but there is a faint clicking sound.
The power supply on a 4P ticks when it is overload failing, so there is a short somewhere in the system. In a 4P it’s almost always in the disk drives. Disconnect them and power up the system, let me know if you see a three language boot error on the screen with the disk drives disconnected.

Click to Enlarge

I turn on the Model 4P and it looks like this
Possible Cause:
This picture commonly indicates a bad Z80 or bad RAM (or both). You can first try reseating the RAM and see if that helps. If not, you should next try replacing the Z-80 and/or the RAM


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

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Can I use a 1990’s dot matrix printer with a TRS-80 Model III?
Yes – You should compare the parallel printer port on the Model III with that on the printer to make sure there are no “value added” incompatibilities, but, barring that, you just need to get a cable to convert the Model III’s parallel port edge connector to a centronics connector. Just make sure to printer into “carriage return only” mode or the printer may not advance to the next line when printing. Most printers back in the day had some form of DIP switch for this.

The cable should just be a 34-pin edge card (for the Model III side) and a 36-pin centronics plug for the printer. The centronics plug should have pin 1 labeled (and on the left) so make sure that the pin 1’s match.

22 thoughts to “TRS-80 Repairs”

  1. 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…..


  2. Trs 80 model 1 boots up to
    MEM size
    If I press the reset button it just keeps saying READY_ but I cannot type anything
    Any ideas on the issue?

    1. ?SN ERROR is “SYNTAX ERROR”, meaning your Command isn’t understood.
      Try typing ?MEM asking the Model 1 to Print the Available memory.
      It will respond with the amount it finds.


  3. I have a TRS-80 Model 1, and I followed the video “How to connect TRS-80 Model 1 to a composite monitor”. I made the wire, and everything is correct. I tried a composite to vga converter to hook it up to a newer monitor, it said something about unknown video signal type, and did not work for this. I also plugged it directly into a flatscreen TV which has composite input, and it gave me a similar error message. I plugged it in to the TV again to get the exact wording for you, but it just says No Signal now. I am doing this because I bought one without a monitor, and on ebay, the monitors are at least a hundred dollars, so I don’t want to have to buy one for my TRS 80 to work. If you could find a solution, it would be greatly apriciated.

  4. I am looking for a replacement cable between the tape recorder and a model 1. Do you know how I might source one?

  5. My sister gave me an old TRS-80 that belonged to her husband’s family. When I power on it stays on for about 5 seconds and the disk drives start spinning and then the pc shuts off. Could this be the power supply even though it initially starts up?

    1. What you describe sounds like a normal start up, it’s not shutting off it just turns off the disk drive after 5 seconds if it doesn’t boot.

      Turn the brightness and contrast up to full (if its a Model 3 or 4, there are wheels under the plastic near the keyboard) to make sure the screen is on, and then press and hold the and while you are holding that key down, hit the orange reset button. After you do that, you can let go of the break key.

      On a Model 1, 3, or 4 this will show a “MEMORY SIZE” or “CASS” prompt. If that shows up, then you have a system which is behaving as normal when it doesn’t have a disk in the drive.

      If not, then the computer likely has a problem


  6. I recently picked up a Model 4 from ebay and on the power supply, 8709365 Rev C, it appears the Inductor 30uf 5A, at L4, Tandy part # 8419008, leaked and I have not been able to find a replacement. Would anybody know of a current part # that can be ordered?

    1. I realize this is a late response, but L4 being an inductor would be rated at 30uH 5A, not uF (microfarads a measure for capacitors), but again I was not able to find any readily available. maybe consider changing the psu for a more modern one?

  7. On the TRS 80 Model I near R20 and R21 (video pots), there is a 6 pin part that looks like an axial capacitor, but it has 3 pins on each side. The part is marked MEKO, Glendale Calif. 55-1051-10. Does anyone know what this part is? google turns up nothing. I tried to trace the part in the schematic, but it’s throwing me off..

    1. The part you describe is located at board position K1 and is “Relay, Motor Control, 5V” – Controls the cassette. Part 450000.

  8. What a fantastic site.
    I bought my TRS-80 Model 1 from the Comp Shop in New Barnet, UK, in 1978.
    Saved up for a 4k to 16k upgrade back in the day.
    Decided to resurrect it this year. The old TV I used to use has long gone, so I saw one of those TRS-80 ‘5-pin DIN plug to TV Monitor SCART plug’ things on eBay and bought one.
    Hooked it up – nothing.
    My TRS-80 powers-up, Red light On, but nothing on my Sony TV.
    I’m not sure if it’s the TRS-80 or cable or TV or…?
    Any advice appreciated!

  9. I have a Model 4P that boots up and runs but when the disk drives run it dims the screen. Is this just a filter capacitor issue or should I check something else also?

  10. I have a Model III, 2 disk system with 32K of RAM and am looking for the correct value that should be returned from ?MEM. What should the value be? My machine gives a value of 31698 when I enter ?MEM. Is this correct?
    Thank you

  11. I have a TRS-80 Model III. When powered up, it will display all @ signs. It also continuously resets itself (not a power supply click, the power supply is silent, it seems that the computer keeps resetting itself every 5 seconds as the monitor never powers off). The drive powers up and looks for a disk before it resets. If I hold the break key down while it resets, I get the Cass? and Memory Size? prompts that stay static on the screen. However, after I press enter to both, the computer goes back to all @ on the screen and continues its reset pattern. Any thoughts on what I should check first? The PS seems OK (there are two of them in this machine), and I have a known working replacement on the way, but this seems like it might be more than a PS failure…

  12. The newer Model 4P power supplies do not have a plug-in connection for the internal Fan. What 12volt positive /Negative wires should I use? Can I tap into the 12Volt lines that go to the Disk Drives?

  13. Hi I have a Model I with 48K ram, a floppy drive and a printer hooked up by switch box. When I go through the power up sequence, the Model I drive keeps running and doesn’t stop, even though all the other components are off. When I turn on the EI and keyboard, the floppy drive still keeps running and doesn’t read any disk in there. The EI’s memory is good. And typically when I turn on the computer, the floppy drive doesn’t run until I turn on the PC, then click the reset button to boot up into DOS.

    Any thoughts? Thanks in advance.

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