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TRS-80 Model 4 drive trouble

J

jltursan

Veteran Member
Joined
Sep 24, 2010
Messages
840
Location
Madrid, Spain
Hi!

Recently my Model 4 has started to show a flaky behaviour. After a couple of hours working I found the display shaking badly, it was something more related to frenzy fine scanlines shaking than the usual tube distortions,...panic!. I quickly switched off the machine and after some seconds and switching again the machine, it showed again a clean screen with the usual "Diskette?" message; but....
The drive 0 is now always spinning and no boot disks seems to work. I've tried swapping drives and both of them shows the same symptom. I've tried several resets without succes, no heads noise, the disks simply spins and the message "Diskette?" apears again.
I've also tried to boot into Model III BASIC and type a test program that I found in the TRS-80 google groups. Doing outs, it seems to control at some degree the drive (I can stop the endlessly spinning); but the basic testing it does seems to return a chaotic picture of the drive subsystem. So...

Can anyone point me out about this problem?. I'm pretty sure that the disk controller has blown up;but, having no idea about how it works (and I'm also pretty noob about electronics), how can I trace the "always spinning and no heads reading" symptom?, has anyone spotted it before?.

Thanks in advance,

Jose
 
You may alternatively want to address the monitor problem. There is a simple fix to that shaking/warbling. I have read that this symptom was fixed by Radio Shack with the addition of a diode in an easy spot. The solution is in an early Computer News 80 ( I don't have a copy handy) and I am sure elsewhere. Anyone know what I am talking about?

The bad drive spinning may be a controller problem, have you tried swapping the controller from the other system?

Bill
 
Is there any memory mapping for devices like drives on the Model 4? If so could it be a failed RAM chip? My Osborne showed the same kind of symptom (Disks spinning, light on) and I eventually found one of the RAM ICs had failed.

Tez
 
Thanks for the replies!

@billdeg: The monitor has not shown again the warbling problem. Not a long time a ago I was in need to adjust some pots to correct the display; but after that, the monitor has worked flawlessly until that day. I suposse the strange monitor behaviour and the drive problem were related because they appeared together (maybe a power surge peak?). The monitor was fine again after the power cycle, the drive never come back...:-(
I'll try to find that "Computer News 80" and seek for more info about the extra diode you've point me out, thanks!
About swapping controllers, I also have a Model 4P; but, are the controllers swappable?. If so, I can try easily....if they're not...I'm doomed as I've no spares :cry:

@Tezza: No idea about device RAM mappings in the Model 4; but I think that a memory test launched from the Model III BASIC can do the trick and find any possible RAM fault. It's an unexpanded gate array model; so only 64KB are present. Does anyone knows a simple memory test program to type in?.

Thanks!
 
When I used to own a model 4, the "shrinking" display issue was caused by the power supply (the power supply in the model 4 was red lined out the door; any way to save a buck, I guess). I had swapped out the radio shack power supply with one from jameco (higher wattage; beefier supply). That cured the display problems for me. The only bad thing was that the new power supply had to be mounted outside the case (sometimes I miss that old setup :)

-Chris Noyes
 
Fortunately, the machine seems to be in pretty good shape (except the unexpected drive fault), dust & rust free inside. Anyway I also checked the PSU voltages and fuses and all looks fine. I also noticed that it's not the gate array model as I stated before, it's the non gate array one, it not only has the 128K empty RAM sockets, there's a U72 socket that I think that must be filled with a, PAL chip?. Need to find more info about this topic...

I'm still waiting for some spare time to try the controllers swapping, it's my first and only option to get some results.
 
jltursan said:
it's the non gate array one, it not only has the 128K empty RAM sockets, there's a U72 socket that I think that must be filled with a, PAL chip?. Need to find more info about this topic...
Click to expand...

Upgraded my Model 4 recently: the non-gate array version is a catalog # 26-1069 (on the bottom), and would need 8 - 64K X 1 chips in U85 to U92, and one PAL (Tandy # 8075468 or Tandy PAL16L8CN 833IV) in U72.
 
Lorne said:
Upgraded my Model 4 recently: the non-gate array version is a catalog # 26-1069 (on the bottom), and would need 8 - 64K X 1 chips in U85 to U92, and one PAL (Tandy # 8075468 or Tandy PAL16L8CN 833IV) in U72.
Click to expand...

I'm thinking about doing this to mine sometime in the future Lorne. I have the chips. Was there ever any software that took advantage of this extra memory though, apart from use as a RAM disk?

Tez
 
tezza said:
I'm thinking about doing this to mine sometime in the future Lorne. I have the chips. Was there ever any software that took advantage of this extra memory though, apart from use as a RAM disk?

Tez
Click to expand...

Double Duty is the only one I have. It lets you run two programs at once, switching between them. 128K is required, and their pitch at the time, was that it was like having two Model 4's in front of you at once.
 
If you swap controllers look out for the "Flex" cable that connects the Motherboard to the Floppy controller. They are notorious for failure. And trying to get a replacement is nearly impossible. Usually wind up making my own cable up using a Hard drive cable and removing the edge connectors for the flex and replacing with a 20 pin connector. Easy to do if your good with a soldering iron and a solder sucker. Just recently did this to my model III.
 
Lorne said:
Upgraded my Model 4 recently: the non-gate array version is a catalog # 26-1069 (on the bottom), and would need 8 - 64K X 1 chips in U85 to U92, and one PAL (Tandy # 8075468 or Tandy PAL16L8CN 833IV) in U72.
Click to expand...

You know, I would love to get my hands on that PAL and run it through my chip tester and see if it crosses to a current chip. I haven't unboxed my Model 4 since I got it (it's a 128K) to see if it's GA or NGA so I'll have to do that, someday.
 
If you swap controllers look out for the "Flex" cable that connects the Motherboard to the Floppy controller. They are notorious for failure.
Click to expand...

Indeed, it looks weak. Just to be sure I checked the continuity (and swapped it with the one from the RS232 card) and the cable passed all my tests :)
An emergency replacement could be a standard floppy or IDE data cable, peeling every single wire and applying tin to make them more rigid. A lot of work tho...
Your solution, replacing the connectors, is much cleaner if you dare to use a solder iron over the machine ;)

About the upgrade, those PAL chips (Tandy # 8075468 or Tandy PAL16L8CN 833IV), are they easy to get or replace?. I've found threads like Source for Model 4p (GA) RAM?; but it's not clear for me if it finally can be replaced.

EDIT: I've found some info related to the replacement of the PAL chips by means of GAL chips (maybe a bit more easy to find and use), has anyone some more info about this topic?
 
Last edited:
It's easy just remove the motherboard, which you would have to do anyway to get at the floppy controller then remove the connectors from the floppy controller and motherboard. Replace with 20 pin header pins and use an IDE cable. Just cut the long part of the Cable off and use the shorter piece to connect the 2 together. A little involved but it will last a lot longer. I use IDE because it is 20 pins across. Just use one side of cable. Also make sure the pins aren't to long! :)
 
Druid6900 said:
You know, I would love to get my hands on that PAL and run it through my chip tester and see if it crosses to a current chip. I haven't unboxed my Model 4 since I got it (it's a 128K) to see if it's GA or NGA so I'll have to do that, someday.
Click to expand...

Actually, I had a 16K Model 4 motherboard out of a classroom arcnet network terminal and it's the GA type, so, all I have to do is add 16 64K x 1 chips to bring it up to 128K. It even still has the Arcnet remote card in it where the FDC card would go. Hopefully, the other Model 4 has the PAL and I can run it through the chip tester.
 
Druid6900 said:
Actually, I had a 16K Model 4 motherboard out of a classroom arcnet network terminal and it's the GA type, so, all I have to do is add 16 64K x 1 chips to bring it up to 128K. It even still has the Arcnet remote card in it where the FDC card would go. Hopefully, the other Model 4 has the PAL and I can run it through the chip tester.
Click to expand...

Mine's back in it's box right now too (while I try to clean up and make space).
If yours doesn't have the PAL, you know where to get hold of me.
 
I've finally found some time to disassemble the Model 4P and after some dozens of screws...the disk controller at last:



Woot!?....; where's the controller board?...

Well, seems that it's merged with the mainboard; so there's no possibility to swap controllers :mad:
Given that it's nearly impossible to find a replacement, my only hope is try to fix the faulty controller, a daunting task. Maybe I'll try first replacing capacitors and socketing the TTL ICs.

Has anyone experience fixing one of this boards?
 
You know you kind of confussed me when you said Model 4 I thought you ment the desktop version. Maybe next time specify Model 4p it would be more helpful to us.
 
I own both machines, a Model 4 and a Model 4P ;)

The faulty one is the Model 4, luckily the 4P is working better than ever (once opened I'd find that the 4P is very easy to upgrade, both memory and drives. I'll open a new thread to show some photos). I wasn't able to swap controllers,...the reason above :(

As for the controller board, I've been socketing ICs and until now I've replaced all SN7416N with no changes. Next ones are SN74LS123N, SN74LS04ND and SN74LS02N.

Btw, seems that the machine tends to ruin the floppies used to test it, even the protected ones. Maybe the drive head does something awful over the floppy surface...
 
Oh ok my bad. Sorry. Could be the heads are bad and chewing up the floppies?. If you are working on the Controller board I would look at the Western Digital chips. I had one of those go bad on me before. Luckily it was socketed. What is the part number of your board? I think there were 2 versions that I am aware of.
 
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