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Modified Model 1 shows weird characters

whizzi

whizzi

Experienced Member
Joined
Feb 13, 2016
Messages
113
Location
The Netherlands
Hi,

I recently rescued this Tandy TRS-80 Model 1, including Expansion Interface and Tandy Monitor with a Floppy drive built in.

The Tandy is bought in 1979 and was doing nothing for the last 10 years. The old owner recently turned it on (without interface) and it worked. Then he connected the Expansion module and since then, it doesn't work anymore. I picked it up before he threw it away.

I got a ton of documentation with it. Full schematics of basically everything you can imagine and manuals. All good for one banana box of documentation. The old owner told me the Tandy was heavy modified and based on schematics, I'm pretty sure it has:
- Z80A cpu, probably running at high speed
- Ramdisk
- Expanded Memory
- High Res GFX
- RTC
- Line fixer
- Lower Characters mod
- Double Density mod

and probably a few more which I'm uncertain if it there.

Pictures say more than words:

This is what I brought home:
24285347924_ef42dc2a7d_c.jpg


This is what the screen shows (with and without Expansion Interface, with and without pressing the ESC key)
24285349844_631720ec65_c.jpg


Here's a picture of the mainboard (clickable):



And here's a video from this machine 'running'.

I really like to save this machine from the shredder. Does anyone have an idea what could be wrong before I have to remove all the mods (as far as possible, quite some chips are changed or soldered on top of the other.
 
As always your first check is that all three Power Supply voltages are within specification. (+5, +12, -5)

Then, you might be lucky and just have a faulty Z80 CPU. If not, I hope the gent kept all the modification documentation.

Larry
 
I have a schematics of everything, all mods, the entire Tandy PCB and a lot of more mods that could be made. Which makes it annoying, because it's hard to tell which mod is actually done (well, the obvious ones are quite easy to see).

The problem I'm facing is that it worked a few weeks ago and the only thing happened to it, as far as I can go back, is the Expansion Interface may have been connected wrong by the previous owner (as in, one side upside down). My guess is that's the source of the problem when it happened. So, I'm hoping that somebody can point me out what would happen if this is done and how to get the Tandy out of this state.

My only guess is that one chip who received some voltage on a place where it didn't expect it, causing the chip to break. Is this a conclusion which is possible? If so, I'll have to find the broken chip which is a whole new challenge.
 
Does it work without the expansion interface connected?

Man, that MI is modded to the max!

EDIT: ignore my question as I see you tested without the EI
 
TandyModel!_1.JPG

This is a photo of this machine working without the EI connected from a few weeks ago. As far as I know, the owner managed to wrongly connect the EI interface and then is was packed in boxes and transported to my house where I've never got this screen.
 
With the interface connection reversed, I'd start investigating every IC that was tied to that bus. It's been a while since I read up on the Model I but I can't seem to recall if any internal bus buffering existed. My suspicion was it wasn't so you might be running into a number of blown chips.
 
Start with just the Keyboard unit. Depress the BREAK KEY and continue holding it while you turn on Power. Do you get the
MEMORY SIZE Message? Depress the ENTER KEY twice. Type ?MEM and see what the response is.
Your Previous Photo is too hard to try and read the response.

If that doesn't work it's going to be time to dig out the O'Scope and start chasing CPU Control signals for one held/stuck LOW.

Pin 6 Z80 CPU Clock Signal
Pin 16 INT*
Pin 17 NMI*
Pin 24 WAIT*
Pin 25 BUSRQ*
Pin 26 RESET*

Larry
 
ldkraemer said:
Start with just the Keyboard unit. Depress the BREAK KEY and continue holding it while you turn on Power. Do you get the
MEMORY SIZE Message?
Click to expand...

Nope. Tried this already and can be seen on the video too :)

My guess so far is that because of the reverse connection with the EI, there's a fried chip somewhere (and that seems below one of the modifications holding the Z80 CPU). This bootscreen apparently is there because the Tandy can't boot from FDD, but it can't get past this, even with the 'BREAK' key. So, my guess would be that the chip controlling if the EI interface is connected or not is fried.

Some Dutch Tandy owners told me this is hardly possible since there are only data lines on the connector. That's where I'm having issues with because on one of the schematics, it shows a +5V on one of the pins (the opposite being a GND, so this shouldn't matter).

But thanks to the Dutch Tandy-scene (which I didn't knew still exists) I got the mailaddress of the guy who created the selector my Tandy is having. Perhaps he can help me out since he still goes to TRS80 Meetings (which are still held in NL apparently, :eek: ) and he probably know the rest of the mods done to this machine, which makes debugging so much easier.

If that fails, I'm going to do this Z80 test and start debugging everything step by step.
 
This bootscreen apparently is there because the Tandy can't boot from FDD, but it can't get past this, even with the 'BREAK' key.
Click to expand...
Don't even bother floppy booting until you can at least get BASIC to work.
 
EI power supply is inside yes and I hear the EI click on or off whenever I try it.

Also, the power supply inside the EI is modded to also power the Tandy. But I did try it with the original power supply and tandy only, no change there.
 
Based on the schematics / manual (http://www.1000bit.it/support/manua...e Service Manual (19xx)(Radio Shack)(pdf).pdf)
my experience with these expansion interfaces was:
1. Pull the ram, and see if it comes up (The ram would often failed this way)
2. If it is not the ram, it would most likely be one of the buffer or memory latch chips: in the expansion interface itself Z45, Z44, Z17, Z18, Z33, Z37, or Z38. Check the input and output with a scope and see if both in/and out of the chip is normal. The buffers would often pop

(I ran a Radio Shack computer service center in Bakersfield CA for 2 years)
 
Thanks for the answer. My first goal is to get the Tandy up&running as stand alone, since that's my main issue at the moment.

In about 3 weeks I'm taking this Tandy to a still existing TRS80 club in the Netherlands where there are 2 hardware guru's. One of them (the guy I have contact with) actually designed the selector mod which is inside this Tandy. According to him, this Tandy is still saveable since the characters show the basics (CPU, Power and GFX/Character chip) are still working. So, I have 2 old school hardware engineers looking at this machine in a few weeks. I'll let it know when there's a change.
 
Thought I'd give you guys an update on this. It's been a while ;-)

As said in my last post, I brought the Tandy to the still existing TRS80 club who were eager to start working on the machine:
IMG_0331(2).jpg

But after several hours of measuring the mainboard (and removal of the first modifications) there was still nothing more than it started with. They found out that the Z80 is in WAIT mode and doesn't get out of that (the signal is low and should be high according to the documentation). One of the guys, the guy who created the high resolution mod and switcher, offered to take the machine home and diagnose there with a better osciloscoop and logic analysers.

Yesterday I finally got an update on it and he found out that A10 is stuck on 0v (?) because 3 IC's are broken.
- Z51-3, A10 in on LS157 RAM adres multiplexer is 7 ohm to ground
- Z38-3, A10 driver, out LS367 is 36 ohm to ground
- Z52-2, A10 inverted with LS04 is 1.1V, despite ingang (Z52-1) on ground or +5V

So, this is the first fix to be done. It's not the high res mod since that's on A0-A7. Still doesn't explain why the Z80 is in WAIT, but we're getting closer.

BTW, this is only the Tandy itself. The Expansion Interface is part 2 of the fix (if it's broken).
 
Whizzi,
The WAIT* stuck LOW should be an easy fix, assuming none of the Mods are attached to the Expansion Port Pin 33 circuit. It could be the PULLUP Resistor
or a cold/bad solder joint, or a bad Z80. A Scope should find that easily, as the Circuit just goes out to Pin 33 of the Expansion Interface Port.

It would be interesting to know the method these experts used to locate the bad address Line A10 and how they traced the shorts to the associated chips.
This can get complicated quick, and makes troubleshooting hard. Maybe you could persuade them to share some of their methods.


Larry

Wait1.jpg Wait2.jpg
 
Last edited:
Another update from my old-school Tandy hardware guy. Z38, Z51 and Z52 replaced, instead of garbage on the screen, the Tandy is now repeating characters on the screen (@ and .)

It looks like Z36 pin 13 (also A10) is broken. Keyboard is active in adres range 3800 - 3FFF.

CAS is 18 ohm to ground. Probably Z72 and Z67 also broken.

OUT is 33 ohm to ground, Z22 or Z25 is broken (needs additional checking)

Replacing Z25 is hard to do because of the High Resolution print on the bottom. So, he's going to solder a 2nd LS32 on the Z25 with pins 8 and 9 disconnected.

Here's how he found these errors (I'm only translating it), the short version.

- 2 Z80 extra prints deleted (3x speed up + wait-state (2.5x speed increase) and selector)

The guy has a Z80 test CPU and a logic analysor. With help of these, he found the A10 layer. He agrees it could be faster using the expansion connector.

Multimeter said, low impedance (7 ohm) to ground on A10.
Z38 pin 3 disconnected, no solution.

So, 3 options left: Z36, Z51 en Z52.

Z52 pin 2 was 1.1 volt, not responding to high/low on Z52-1, so this port is broken.
With that information, Z52 pin 1 disconnecting > no solution.

50%/50% Z36 or Z51.
Z36 removed, no solution.

Z51 pin 3 removed from socket, this one was 7 ohm to ground.

Z38, Z51 and Z52 replaced and Z36 pin 13 reattached. Now repeating characters on the display.

Then he started on the expansion interface, found a broken CAS and OUT.

OUT removed from the expansion port (not required) and CAS-driver Z72 pin 9 disconnected. Z67 pin 9 and 10 shorted. No change in repeating characters.

Logic Analyzer again, CAS signal stops at Z67 pin 14.
So, the CAS driver (Z72 pin 9) and the buffer to the RAM (Z67 pin 14) and Z36 pin 13 are broken, he assumes Z25 pin 9 is broken as wel.

Next week Z22, Z36, Z67 and Z72 will be replaced and a LS32 piggyback on Z25. The computer will most likely be fixed then.
 
whizzi,
THANKS, for the update. That was very interesting, logical, and makes sense to me now.
I've been reading up on tracking shorts on circuit traces, and the various methods. Dwight,
who frequents this forum was supposed to post a detailed method he uses, but so far that
hasn't happened. He uses a Power supply to inject a low voltage, then follows the current
path around the board to pinpoint the problem in an XY plane.


Any info on the Stuck WAIT* Line. Was that also fixed?

Larry
 
The WAIT is in-use with the de-lacer (I guess that's the correct term for preventing lines on the screen) which works as expected and is it's in use on the Z80 speedup mod as well. But to make sure, the speedup print IC's will be checked.

If the last 4 IC's are replaced, the machine probably will boot as expected. I will get a more detailed explanation of what's checked and why he came to this conclusion. But things are looking up for this heavy modified and nearly 40 year old computer that it will be showing a dancing demon anytime soon.
 
I just got the word... it's aliiiiiive!

After replacing in total 8 IC's, the machine is happy asking for it's memory size again! With almost all mods in-place and with it's original Z80 CPU. The only mod not reconnected is a speedup mod.

But, another problem showed. All keys seems to work, except for the row 8 9 : ; - , . / . This shows:
8 -> @XHP
9 -> AI1Q9
: -> BJR
; -> CKS <arrow up>
- -> 4EMU <arrow right>
, -> DLT,
. -> FNV6.
/ -> GOW7/

Is this the LS05 line driver or a broken input on LS367? Any ideas here?

In about 9 days I'll be seeing this Tandy again, so yay!
 
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