Model 4 Non-GA Garbage at power on

[wdatkinson]

New to me unit. Was hand delivered, not shipped. Reported to have worked fine prior to boxing it up. I picked it up yesterday and drove home. Unboxed this evening and set it up on a desk. Upon power on, I see the following:

trs80 m4 junk-forum.jpg

Subsequent power ones result in similar behavior. Every now and then drive 0 spins up like it's going to attempt to boot, nothing shows on the screen and the drive stops after a few seconds. I never see a Diskette? prompt. Reset does not work. Holding Break at power on to drop to M3 Basic also does not work.

I haven't used M3's or M4's since middle school and I just turned 47. I do repair other machines and am ok fixing this unit as opposed to returning it, just need an idea of where to start.

Thanks!

[ldkraemer]

wdatkinson,
the first thing to do is Open the case and look for a Tandy Power Supply. If there is a Tandy Power
Supply with a row of Round Pins, remove the Power Supply and then turn it over and check for
COLD Solder Joints. Look at each Pin with a magnifying glass and look for small circles around
each Pin. Re-Flow the Solder around those pins. I use Kester 63/37 Solder for the PCB's. Even
if it looks good, I'd re-flow all Round Pins on a Tandy Supply.

But, be very careful when you lift the top of the case STRAIGHT UP and then lay it on it's Left Side
so you can unhook the video cable and ground wire. You want to stand so you hover over the top
and look down through the air vents and carefully raise it straight up, while watching the neck of the
CRT so it doesn't get broke. Once you remove and replace it once you will know what to do.

If the Solder Joints are good on the Tandy Power Supply or it's an Astec, then I'd remove the metal
shield and have a look at all the IC's in Sockets. Make sure they are all seated properly, as that can
cause problems.

Next on my list would be creating a Z80 NOP tester and then replacing the Z80 with the Z80 NOP tester.
You can test all the Address Lines with it, as they route through the PCB. You will need an O'Scope for
those tests. That should check about 80% of the computer. All that would be left is the Data Bus and
it's buffers etc.

The good thing is that it has nice crisp, clear video, so you know the video section is working. It's just
displaying garbage, which is likely because the CPU is running amuck.

If you can get it to power up in Basic, then you can do a few tests to see what is going on.

Do you have the Service manual downloaded yet?


Larry

[pski]

But, be very careful when you lift the top of the case STRAIGHT UP and then lay it on it's Left Side
so you can unhook the video cable and ground wire. You want to stand so you hover over the top
and look down through the air vents and carefully raise it straight up, while watching the neck of the
CRT so it doesn't get broke. Once you remove and replace it once you will know what to do.

Since you haven't worked on a Model III or 4 before I wanted to emphasis this...watch the neck through the drive openings or vents the whole time you are lifting or replacing the top.

Also, if you have a spare Z80A lying around you can try swapping it out. I have seen Z80s go bad a number of times.

[clh333]

Having just ruined a CRT let me second the cautions about lifting the top case. The back of the CRT, where the electron gun board sits, can catch on wires running to the motherboard, which is at the rearmost of the case. If anything, when lifting up the top, which contains the CRT and its driver board etc., you want to make sure the case is moving forward slightly to clear those wires. If there is any resistance stop and reassess.

-CH-

[wdatkinson]

I've watched multiple videos on YouTube and every one stresses catching the CRT neck when removing the top case. I will be very very careful.

I do have an O-Scope. I have not downloaded the SM yet, honestly I haven't looked for it, but will give it a goog.

I'll start working down the list and report back. I just had surgery on my right hand, so I'm a bit gimpy, so I might take a day or two to get through everything.

[ldkraemer]

wdatkinson,
One other thing you need to do while you have the computer apart, is to check and see if you
have Tanton TM-100 Floppy Drives installed. If so, open the floppy drive doors, and shine a
light up into the hinge area of the door assembly and make sure there is a full length Brass
Rod for the hinge point. You might find two short white Nylon pins instead. If you have the
two short white Nylon pins, they need to be removed and a 3/32 Brass Rod inserted.
I have them available if you need one or two. They aren't expensive, but I had to
purchase a pound of Rods, and they were shipped from Florida.


Larry

[bladamson]

FWIW, I have an Astec PSU in my Model 4 NGA and it also had cold solder joints that I had to reflow.

If it still misbehaves once you check that, and all your voltages are good, I'd start poking around the CPU with an oscilloscope. There is obviously power getting to the CRT analog board (that was my problem with the Astec PSU), and the mainboard is obviously sending *something* to the analog board.

This leads me to believe that maybe the CPU isn't initializing properly. What we see on the screen might be random garbage in the video RAM, which ought to be cleared as soon as the CPU initializes. I'd check the reset line and the clock, and then poke around some address lines to see if the CPU is actually running.

Maybe it could still be a PSU issue though. Those oldskool RAM chips sometimes require some pretty gnarly voltages, so yeah.

That being said, I don't really know what I am talking about most of the time, so I may be full of crap. But if I was the one poking around at it, that's the next thing I'd check, anyway. ;P

Good luck!!!!!!

[bladamson]

I've watched multiple videos on YouTube and every one stresses catching the CRT neck when removing the top case. I will be very very careful.

You can peek in through the disk drive openings and see if you are snagging any wires, if your light is good enough.

[wdatkinson]

Ok, I've finally healed up enough I could clean off the bench and take a look at the 4D.

The power supply was an Astec. Even so, I pulled it and under the microscope found a few fractures around the pins, so I re-flowed all of them.

After an initial check, the problem persisted, so moved on to the socketed chips. I pull each, and sprayed some DeoxIT in each socket then replaced each chip.

I did not do the RAM yet. Could RAM cause my problem? I can certainly do that next. I also have a NeoLoch RAM tester, but not sure if it covers whatever chips are in these beasts.

I have several pictures, but the forum doesn't re-sized on the fly, and I don't feel like messing with GIMP to resize all the photos.

So as of now, I'm still seeing the same issue and have:
1). Reflowed pin connector on PS
2). Pulled, DeoxIT'd, and reseated all chips except RAM.

I have 2-3 C128's laying around. Do they contain the same Z80? If not, I can see about sourcing one online. I'll also do some checking for a Z80 NOP tester and see what's involved there.

[wdatkinson]

Been through all 16 RAM chips and all test OK according to my NeoLoch. The first back was Motorola ceramic chips and the second bank was NEC's.... I'm guessing due to a later upgrade.