TRS-80 Has Random Characters on Startup

[Tempest]

My beloved TRS-80 Model III has taken a turn for the worse. It was working when I put it in storage a few months ago, but now when I turn it on I get a screen full of random characters. The disk drive turns on briefly, but nothing else happens. The reset button also works. I've been told that this could be a symptom of bad RAM, a bad CPU, or a bad power supply.

 

Is there a way to tell which of these it is? How can I go about repairing it? Like I said, it was working a few months ago, I find it hard to believe that it magically just died on me.

[jlanier]

Frankly, it could be anything. Cold solder joint, oxidized pins on socketed chips, bad ROM, CPU, RAM, character RAM, bad TTL chip or PAL, bad power supply... The service manual for the TRS-80 is excellent, assuming you can find one. I don't know where mine is unfortunately.

 

If it helps, some generic hardware tips: Old electronics, anything can fail including silicon, but the most likely failure points are 1) capacitors, especially in the power supply, 2) parts under the most stress during normal operation, such as voltage regulators and transformers, 3) bad solder joints, since most old electronics were not made with eutectic solder and can develop bad joints over time.

 

Unfortunately, the Model III is one of those old first-gen 8-bit computers that uses 4116 DRAM. These chips are notorious for spontaneously failing. They also require a triple power supply (-5, +5, +12), and part of their spec is that the -5 line MUST be the first on and the last off, otherwise internal parasitic diodes cause them to die almost instantly. What this means is, if the -5V rail fails independent of the other two supply rails, you can kiss all 24 chips goodbye - they're dead.

 

If that happened, your best bet for repair would be to mod the motherboard to accept single-rail DRAM. Somewhere on the forums here are instructions on how to do this for the ColecoVision, which was also a Z80 system with 4116 DRAM. That could probably be adapted for the Model III with some minor effort.

 

Your best path to repairing the system is to get a voltmeter and oscilloscope, the service manual (or at the very least, schematics), start diagnosing from the power supply first and work your way backwards through each system until you find the problem. The fact that you have a raster and character display is a good sign; at least you won't have to troubleshoot any of the high-voltage parts associated with the display.

 

If you have a spare Z80 you could try swapping it out as a quick first random try. Do NOT try swapping any RAM until you have verified all power supply rails are clean and the correct voltage, and if possible verify that the supply sequences correctly (-5 on before +5/+12, also -5 last off). If the supply isn't working or sequencing correctly, swapping RAM will just blow out whatever you put in.

 

Since the Model III supported 16K, 32K, and 48K configurations, if the system fails to boot due to memory failure you can try removing/swapping banks of memory to temporarily downgrade to a smaller memory configuration. If low memory fails the system can't start beacause the OS uses low memory. Also, the Z80 stack is in high memory, but this is less likely since if the top bank had failed, the boot ROM should have just adjusted the memory size down automatically. The test is not thorough, though, so it is possible to have a high memory failure that passes the memory scan but fails in use.

 

Hope that helps. It's been quite awhile since I repaired a TRS-80, they are generally very solid and reliable. Good luck with your repair.

[Tempest]

Yeah I'm going to have to do a little detective work on this one. I have a voltmeter but no oscilloscope.

[Keatah]

I agree with all the previous suggestions. In fixing my classic Apple 2 series equipment I've always found that re-seating chips in their sockets, measuring supply voltages, inspecting solder, re-seating connectors, and downgrading/swapping ram, all that, has generally served me well. This can be done on the cheap.

 

Once you've got the basics out of the way then you can start with the troubleshooting manuals. Thing with manuals though is they will ask you to look at timing and waveforms, and that will require an o'scope. Sometimes a logic probe can help too.

[jlanier]

If you don't have a scope, you can still use the voltmeter to check the power rails. The DC voltages under load should generally be within a few percent under the target voltage, I.e. the +5V should be no less than 4.8 volts, and no higher than 5. You can also use the AC mode on the voltmeter to measure RMS ripple; theoretically a perfect reading would be 0 volts but as long as the ripple isn't much more than a tenth of a volt the rail is probably OK. These older regulators aren't nearly as good as a modern switched mode power supply, but the Model III had a fairly ample supply due to the fact it had to supply power for all internal devices. If I recall it actually had a couple of power supplies, the one for the floppy controller and drives was separate in case you bought the system without drives. Anyway, if you are seeing excessive AC on any rail, or if any rail is sagging too far below, it should be fairly obvious on the voltmeter. One thing you can't check without a scope is sequencing. I don't recall how they handled that; some designs just left -5 on as long as the unit was plugged in, others used relays or other logic to sequence the rails. A schematic would help figure that out. If -5 is live all the time and the rail is good then you don't have to worry about sequencing and you could start swapping RAM.

[Tempest]

I don't suppose there's anyone close to MI who knows how to fix one of these? That might be the easiest way given my lack of electronics skill.

[jlanier]

In case this helps, I located a PDF of the service manual: https://docs.google....R1lNMUl4RVc3NVU

 

Looking at the schematic, it would appear that there is no sequencing; apparently the design counts on simultaneous power up/power down on all rails. That is generally bad design for 4116 DRAMs but the Model III apparently also uses a switched mode power supply, which is more reliable than linear. It's very likely that the load on the +5V/+12V rails alone guarantees that it would be impossible for the -5V rail to be out-of-sequence.

 

Unfortunately, it also looks like the power supply is +5, +12, -12 rather than +5, +12, -5. So it looks like Tandy cut some corners with the -5V supply and used a 7905 -5V linear regulator to generate the -5V rail for the DRAM. These linear regulators are notoriously flaky. In this case, the regulator will be on the motherboard. VR1. It is highly suspect, as are C102 and C103, the filter caps on the front and back side. I would replace all 3 if any one were bad. The regulator and two capacitors are located just to the left of J4 and J5, where the power enters the motherboard, just above the DRAM area. To check the -5V line you will need to measure the output from pin 3 of VR1, which is easiest to get at from pin 1 of any of the DRAM chips.

 

If by off chance the -12V input is bad rather than the -5V output, it is worth pointing out that the two 40W power supplies in the Model III are in fact identical. The motherboard and the RS-232 board need -12V, but to my knowledge the floppy controller and drives do not, therefore it would be possible to swap them if one had a defective -12V rail. I would not recommend that as a long-term solution though.

 

I was surprised to locate a source for 4116 DRAM at a reasonable price; these are getting harder to find but still apparently not impossible. If you find that the -5V was bad you will likely need to replace all your DRAM after repairing the power supply, since as I said 4116 DRAM usually self destructs when the -5V fails. Jameco still has stock on 4116 DRAM at the eminently reasonable price of $0.99 each: http://www.jameco.co..._10001_41339_-1 That's a much better deal than, say Mouser wants for the NTE equivalent part at $15.74, which is what I'm used to seeing 4116 DRAM go for these days: http://www.mouser.co...67i2UBxWsO%2bg=

 

If you can check all the power rails for DC and AC from both power supplies as well as the output from the 7905 as a starting point maybe I could help more. If you give up and/or can't locate anyone locally who can help you repair it let me know, I might be able to help. My TRS-80 is in a box somewhere - I'd need to find it before I could service yours. but if you really can't get any service anywhere else I'd be willing to help try and fix it, possibly by just shipping the motherboard rather than the whole unit which would be painfully expensive and bad for the CRT. Also you might check on TRS-80.com, sometimes there are people giving them away for free, you might just be able to get another Model III instead for parts.

[DaNDeE]

A little bit late, but I read somewhere that you get this kind of random char trash on you screen, as long as you don't have disk to boot from inserted into the drive. Hit the break key and you should get to the promt. At least that's what was told about the model I with expansion unit.

 

cheers.