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TRS-80 Model 1 repair issues.

litterbox99

Experienced Member
Joined
Aug 8, 2010
Messages
103
I recently acquired a Model 1 26-1001.

PCB ver. 1700069D
One Rom Z33; Motorola "Basic 1" 7814
Not cuts or hacks the the PCB

The video output to the original monitor
is not correct. It's like the horizontal hold
is messed up, no change is seen with keyboard
input or a reset.

The monitor works, I fed it a composite sig
out of a security camera and it displays correctly.

I have not ruled out the analog video output section
but the divider chain is outputting the correct freq(s)
based on the info I have.

Voltages are good as well.
Data/Add lines on the CPU buz with activity.

Before I start chasing my tail, I'm not sure
if the Z3 & Z71 jumpers are correct. From
the docs I've found on the net, there is a
conflict from what I've read.

My PCB is jumper'd as follows;

Z3

open
closed
open
open
open
open
closed
open

Z71

open
closed
open
closed
open
closed
open
open

Are these jumpers documented ?
 
If horizontal is out, check your 74C04s and the 74C00 they commonly stop working. Better still post up a picture of what you see on the screen and I can better advise you where to start looking.

Cheers,

Ian.
 
Ok, here's what I have.

Composite out from Trs-80;
<not right>

bad.jpg

Composite out from a security camera
that the monitor displays correctly.

good.jpg

Trs-80 output to monitor
<not right>

tv.jpg

I've been short on time, but I think
I'll replace the two transistors in the
final output stage and see what happens.

Todd
 
When I get a machine which behaves like that, I go straight to Z6 and Z57 (which are the horiz and very sync chips 74C04) and and Z5 (mixes the sync signals, its a 74C00) and you should get your display back.

Ian.
 
Thanks..

I'll have to do a parts order, my store of IC's
contains only 'LS' series...

Todd
 
Thank you Ian !

Replacing Z5,Z6 and Z57 restored my video.

While I was waiting for the chips to arrive, I
recapped the PCB (about 40 or so bypass and
electrolytics)

I also replaced the kbrd cable with IDE.

I did have to replace Z4 on the keybrd as I was
missing; D,L,T,4,< and down arrow. Lucky
I had a few leftover LS368's from my S-100
stuff.


When I powered it up, I was greeted with;

READY
>

Hmm, it's been awhile, but shouldn't I've
been prompted with MEM SIZE ?

I'm not familiar with all the versions of
TRS-80 Basic.

I guess I'll play for a bit ;-)
 
Thanks for your help guys !

It's up and running.

I got bored this afternoon, so I looked
into what was needed for a 16k memory
upgrade.

What I found is the 16k chips are the
equivalent of a MB8116E (Tandy house marks
the IC's with their part number)

I did a cross to see if I had any in my spares.

I discovered that one of my S-100 memory cards
used 32 x uPD416C-200 IC's. They were a match !

I did the swap and changed the jumpers....

A PRINT MEM yields 15871


Anyone know of a simple Level 1 basic program
I can use to test the memory ? or is the print mem
a good enough test ?
 
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