I have a model III I'm trying to resurrect. It turns on, but has garbage on the screen. The floppy lights come on, but they do not try to spin up. I'm getting the proper voltage to the memory and CPU, and I've removed all but the first 16k of RAM (and swapped chips around) as well as swap out the CPU. Not sure where to look from here. ANy suggestions would be awesome. Thx.
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Repairing Model III
- Thread starter pfworks
- Start date
ldkraemer
Veteran Member
pfworks,
You may have created more of a problem by reducing the RAM to 16K. If you haven't properly re-strapped the Memory Size Jumpers, you have created a self-induced problem.
I'd recommend you properly set the Jumpers back for 48K and then replace the pulled Memory. Or, make sure the Jumpers are set for 16K, and then verify what memory
is detected.
To properly bring up any Radio Shack Model 1, 3, or 4, you should really hold the BREAK KEY Depressed, and Turn "ON" Power. When you see the CASS ?? displayed on
the screen release the BREAK KEY, and hit the ENTER KEY at least twice. Next type in "? mem" without the quotes. The system will respond with the memory it has detected.
If the Memory Size is correct, then insert the Boot Floppy, and hit the RESET Button. Your Model 3 should boot from the floppy, assuming it is a good Boot Floppy. In either case
Drive 0 will spin for about 5-10 seconds, while it searches for the Boot Sector.
If you have some BAD RAM, and don't have a Boot Floppy it's going to be harder to get a program such as Babyroot loaded to test the RAM. I've got the source for Babyroot
that was originally written for the Model 1, but I've patched it so it works on the Model 3 & 4 too. There is a companion program to test each bit (0 thru 7) for the exact RAM Chip
that is defective, once you know there is a BAD ADDRESS located for the RAM.
If you PM me with a valid email, that accepts .ZIP files I'll forward you the Babyroot code and documentation. I'll also attach the Model 3 Service Manual & Tech Reference manual
so you can determine where the RAM jumpers are located, and how they are strapped.
I'm also assuming you still have the Original Tandon TM-100 Full Height Drives in the Model 3. Is that correct? With power off the Model 3, slip a floppy into the first drive.
Carefully close the drive latch. See if it feels normal and then slowly open the latch. Repeat for second drive. Both should feel the same. What you are looking for is the
pins the door pivots on in the latch. Later production drives were modified to use two White Nylon Pins, versus one one long, solid brass pin across the latch. You will know
from the feel of the latch when closing it, if it's a problem. The White Nylon pins don't let the Head assemble load onto the floppy properly. I use a piece of Brass Brazing
Rod (the largest oversized rod .002 or .003) from my Welding Supply house to cut new full length Piviot Pins and slip them into the drive latch. That has fixed my problem
for sure on several machines. The Brass Rods are 1/8" if I remember correctly. I just carried my Micrometer and looked through their stash for the largest oversize that I
thought would fit the latch.
Another thing you need to do is to get some Dry Slide which is a Motorcycle product for lubrication of cables, and carbs. It doesn't attract dirt, and works well to lubricate
Drive and Scanner rails. A few drops goes a long way, after shaking the can to mix the lubricant. I clean the rails the best I can with a cotton swabb dipped in Alcohol.
Then drop a few drops of Dry Slide on the Drive Rails. I then carefully cycle the rails from end to end. You will notice a lot less effort to move the head assembly.
Same method works on Flat Bed Scanners.
www.drislide.com/
I'll try to help more as you respond.
Larry
You may have created more of a problem by reducing the RAM to 16K. If you haven't properly re-strapped the Memory Size Jumpers, you have created a self-induced problem.
I'd recommend you properly set the Jumpers back for 48K and then replace the pulled Memory. Or, make sure the Jumpers are set for 16K, and then verify what memory
is detected.
To properly bring up any Radio Shack Model 1, 3, or 4, you should really hold the BREAK KEY Depressed, and Turn "ON" Power. When you see the CASS ?? displayed on
the screen release the BREAK KEY, and hit the ENTER KEY at least twice. Next type in "? mem" without the quotes. The system will respond with the memory it has detected.
If the Memory Size is correct, then insert the Boot Floppy, and hit the RESET Button. Your Model 3 should boot from the floppy, assuming it is a good Boot Floppy. In either case
Drive 0 will spin for about 5-10 seconds, while it searches for the Boot Sector.
If you have some BAD RAM, and don't have a Boot Floppy it's going to be harder to get a program such as Babyroot loaded to test the RAM. I've got the source for Babyroot
that was originally written for the Model 1, but I've patched it so it works on the Model 3 & 4 too. There is a companion program to test each bit (0 thru 7) for the exact RAM Chip
that is defective, once you know there is a BAD ADDRESS located for the RAM.
If you PM me with a valid email, that accepts .ZIP files I'll forward you the Babyroot code and documentation. I'll also attach the Model 3 Service Manual & Tech Reference manual
so you can determine where the RAM jumpers are located, and how they are strapped.
I'm also assuming you still have the Original Tandon TM-100 Full Height Drives in the Model 3. Is that correct? With power off the Model 3, slip a floppy into the first drive.
Carefully close the drive latch. See if it feels normal and then slowly open the latch. Repeat for second drive. Both should feel the same. What you are looking for is the
pins the door pivots on in the latch. Later production drives were modified to use two White Nylon Pins, versus one one long, solid brass pin across the latch. You will know
from the feel of the latch when closing it, if it's a problem. The White Nylon pins don't let the Head assemble load onto the floppy properly. I use a piece of Brass Brazing
Rod (the largest oversized rod .002 or .003) from my Welding Supply house to cut new full length Piviot Pins and slip them into the drive latch. That has fixed my problem
for sure on several machines. The Brass Rods are 1/8" if I remember correctly. I just carried my Micrometer and looked through their stash for the largest oversize that I
thought would fit the latch.
Another thing you need to do is to get some Dry Slide which is a Motorcycle product for lubrication of cables, and carbs. It doesn't attract dirt, and works well to lubricate
Drive and Scanner rails. A few drops goes a long way, after shaking the can to mix the lubricant. I clean the rails the best I can with a cotton swabb dipped in Alcohol.
Then drop a few drops of Dry Slide on the Drive Rails. I then carefully cycle the rails from end to end. You will notice a lot less effort to move the head assembly.
Same method works on Flat Bed Scanners.
www.drislide.com/
I'll try to help more as you respond.
Larry
I'll try switching jumpers. Even holding break key, I can't get the machine into a state where there is anything but garbage on the screen, even with the RAM fully populated. I originally thought power issue, as pin 11 on the cpu had 0 volts, but after unplugging and replugging all the power connections (while looking for damaged components), the CPU gets power now. So, I'm not quite to the state of worrying about the floppy drives yet. They seem physically sound, and a floppy slides in and out and the latch works fine. They just don't try to spin.
thanks
thanks
Audronic
Experienced Member
@pfworks
Try reseating the 3 roms ( 1 at at time ).
Ray
Try reseating the 3 roms ( 1 at at time ).
Ray
TRS-Ian
Veteran Member
Hi Pf,
I would (and do) start by disconnecting the FDC and RS232 board from the main board (unplug the 2 white cables), confirm that the white twisted cable from the processor to the (?) top middle of the motherboard is intact (computer won't run without it), remove all 24 ram chips. Remove CPU. Install a known good CPU and a bank of 8 known good ram chips in the top row of the ram banks. Don't play with any of the jumpers!
You now have what is known as a 16K diskless Model III and powering it up should give you Cass? on the screen. If you are still getting nowhere with known good ram and cpu, you need to poke around the address bus and memory refresh lines with a logic probe. At this point you need to decide whether your skill set permits you to further diagnose what is wrong or consider a replacement motherboard.
Check the outputs on your power supplies - the motherboard requires each of the 4 pins from the power supply: 5VDC, 12VDC, -12VDC and Gnd. The 12VDC and -12VDC are used by the type 4116 ram chips, and if either one of those voltages are missing you will be presented with a screen of garbage.
Hope this helps,
Ian.
I would (and do) start by disconnecting the FDC and RS232 board from the main board (unplug the 2 white cables), confirm that the white twisted cable from the processor to the (?) top middle of the motherboard is intact (computer won't run without it), remove all 24 ram chips. Remove CPU. Install a known good CPU and a bank of 8 known good ram chips in the top row of the ram banks. Don't play with any of the jumpers!
You now have what is known as a 16K diskless Model III and powering it up should give you Cass? on the screen. If you are still getting nowhere with known good ram and cpu, you need to poke around the address bus and memory refresh lines with a logic probe. At this point you need to decide whether your skill set permits you to further diagnose what is wrong or consider a replacement motherboard.
Check the outputs on your power supplies - the motherboard requires each of the 4 pins from the power supply: 5VDC, 12VDC, -12VDC and Gnd. The 12VDC and -12VDC are used by the type 4116 ram chips, and if either one of those voltages are missing you will be presented with a screen of garbage.
Hope this helps,
Ian.
OK. There are 3 connectors it seems coming onto the motherboard. Two on top and one on the side. I see -12V coming off the smaller, 4 pin connector on top, 5V coming off of a voltage regulator in front of the memory, but only 3V from the bigger power connector on top. Should that one be 12V? If so, I guess I should start replacing caps on the power supply?
thx
thx
TRS-Ian
Veteran Member
Does your Model III have 2 power supplies or just one? Photos?
Typically the Model III has 2 power supplies - the one behind the disk drives powers the motherboard, RS232, CRT, and the one on the disk drive tower powers the 2 disk drives and FDC. Both have the same outputs so if you suspect one is not outputting the correct voltages you can just plug the power leads into the other one.
THE LAST batch of Model IIIs made in early 1983 had a single power supply mounted on the disk drive tower, same as on the soon-to-be-released Model 4.
I don't have any power supplies here at work with me but I can check the pinouts of the power supply and what you should be getting at each pin, or someone from the forum can chime in with what the pins are supposed to be outputting.
Ian.
Typically the Model III has 2 power supplies - the one behind the disk drives powers the motherboard, RS232, CRT, and the one on the disk drive tower powers the 2 disk drives and FDC. Both have the same outputs so if you suspect one is not outputting the correct voltages you can just plug the power leads into the other one.
THE LAST batch of Model IIIs made in early 1983 had a single power supply mounted on the disk drive tower, same as on the soon-to-be-released Model 4.
I don't have any power supplies here at work with me but I can check the pinouts of the power supply and what you should be getting at each pin, or someone from the forum can chime in with what the pins are supposed to be outputting.
Ian.
Chromedome45
Veteran Member
Pull the power cable off of one of the Floppy disk drives. The 2 inside connector (Black) are ground. Check the opposite ends. One should be 12V the other 5V. Let us know what you find. That will let you know that the floppy Power supply is good. Then could swap with the Main power supply and see if the Mod III starts up. Make sure you disconnect from mains power before swapping. If it does start then you know where to start looking. :D
Chromedome45
Veteran Member
Pictures would help immensely. Does the power supply mount on the side of the floppy drive towers? If so you have a late model Model III. The same power supply used in the Model 4. The two connectors in question are they small and Brown in color? Most likely also do they have 4 position connectors? The one with 3 wires would go to the floppy controller it carries Ground, +5 and + 12volts. If memory serves. The one with 4 would go to the RS-232. It has the same voltages except the extra wire for is for -12V I think.
TRS-Ian
Veteran Member
Photo?
Is the power supply an Astec AC9335-01 ? If so it has a wiring loom connected to TB2 which splits into 5 connectors... 2 are molex for the disk drives, 1 is has only 3 wires, thats for your FDC, the other two have all 4 wires BUT they are wired differently - one for the RS232 and one for the motherboard - make sure you have the correct one of these connected to the motherboard.
Check and confirm.
Ian.
Is the power supply an Astec AC9335-01 ? If so it has a wiring loom connected to TB2 which splits into 5 connectors... 2 are molex for the disk drives, 1 is has only 3 wires, thats for your FDC, the other two have all 4 wires BUT they are wired differently - one for the RS232 and one for the motherboard - make sure you have the correct one of these connected to the motherboard.
Check and confirm.
Ian.
Chromedome45
Veteran Member
As Ian said be careful. Can't remember which goes to the Mainboard off hand. But sounds like it's there already.
TRS-Ian
Veteran Member
Yep its a model 4 power supply, try your spare one and see if it makes any difference.
Chromedome45
Veteran Member
Ok so what was the voltage readings on the floppy power cables? Should be 5 and 12 volts.