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Help with TRS-80 Model 4 display

killr0y

Member
Joined
Oct 11, 2021
Messages
13
Hi all,

I recently got a couple early Model 4's with white CRTs and Model III video boards. I tried connecting both monitors to the one machine that has a PSU (second machine is missing the PSU and I'll need to get one - any ideas?).

With the drive open and a disk inserted I can see the "diskette?" text on the screen, but its barely recognizable and mainly looks like a bright streak across the entire display, as if the raster collapsed to one character tall. I thought maybe this was the flyback, but if that was the case, it would just be a single scan line across the center of the screen.

One other symptom is that the floppy drive makes no sound, but the light does come on. I have not yet replaced the filter RIFAs, all of which are wrecked, but I understand the machine should still operate w/o them. Any help would be much appreciated!
 
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What is the Manufacture of the Floppy Drive? Is it a Tandon TM-100 series? There are about four things
that you need to do if it is a Tandon. If the drives are Tandon's send me a PM with your email address.

I'd start by checking the Vertical and Horizontal circuits for the correct signals. You will need the proper
schematic (Service manual) for the correct version. Is your Model 4 a Non Gate array? You can easily
tell by looking at the Serial port. If the serial connector is pointed at the top of the desk, it is a Non Gate
Array computer. If the serial port points out the back it is a Gate array.

Larry
 
What is the Manufacture of the Floppy Drive? Is it a Tandon TM-100 series? There are about four things
that you need to do if it is a Tandon. If the drives are Tandon's send me a PM with your email address.

I'd start by checking the Vertical and Horizontal circuits for the correct signals. You will need the proper
schematic (Service manual) for the correct version. Is your Model 4 a Non Gate array? You can easily
tell by looking at the Serial port. If the serial connector is pointed at the top of the desk, it is a Non Gate
Array computer. If the serial port points out the back it is a Gate array.

Larry
Hi Larry, it is a non gate array version. As for the floppy drive, I'm not sure how to tell the make. Are there any photos of the different ones?
 
OK, you likely need a Power Supply that is a 65 Watt, because I assume that you have two Floppy Drives
in each Model 4. Or perhaps you have a version that has no floppy's. (That was the way I bought mine
and Upgraded it myself with two Tandon TM-100's.)

I think I have a couple of good 65 Watt Power Supplies, but the connectors may be different. You need to
have a look at the Power Supply connector in the Model 4 that has no Power Supply to see what it has for
the connector. (I don't think I have replaced the RIFA caps on the Power Supplies, or if I did I have forgotten.)
I won't be back home until after April 1.

Now try this on the working Model 4. With Power OFF, depress the BREAK KEY, and keep it held depressed.
Turn on Power, when you see any characters on the display, release the BREAK key, and tap the ENTER key
three times. Then type ?MEM and depress the ENTER KEY. You should see something like 48082, going from
memory which might be incorrect.

If you can read the characters on the display (in 64 character mode) then you need to switch to 80 character
mode and see if it is readable. There is a variable cap that can be adjusted to fix the out of sync characters.
I'll have to do a bit of searching to locate it's number. C210 is the cap.
REF: https://forum.vcfed.org/index.php?threads/trs-80-4p-video-issues.36991/
https://groups.google.com/g/comp.sys.tandy/c/sw2bw6trKzA


http://www.classiccmp.org/dunfield/trs80/m4tech.pdf

I'm still searching for the instruction to switch modes.


Larry
 
The one with no PSU has one floppy drive. It has an 11 pin clear connector to the system and a two pin connector from mains and another two pin connector to the CRT.
 

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Yes, a Tandy Power Supply will work in that position just fine. I'll have to look to make sure
I haven't used all those supplies. I'll bet I did not replace the RIFA caps yet, as I went through
them making sure they worked. (For your information those 65 Watt TANDY Supply's have a
row of Pins that the connector plugs into. THe pins are bad about not being well soldered to the
Power Supply PCB. I'll double check that it has a good flow of solder on the Pins. You will be
able to see small circles around the pins were the solder has broken loose from the PS PCB Pins.)

In the Photo above it looks like your Horizontal Sweep isn't working. You have a Vertical line
that isn't expanded in the horizontal position.

Larry
 
Yes, a Tandy Power Supply will work in that position just fine. I'll have to look to make sure
I haven't used all those supplies. I'll bet I did not replace the RIFA caps yet, as I went through
them making sure they worked. (For your information those 65 Watt TANDY Supply's have a
row of Pins that the connector plugs into. THe pins are bad about not being well soldered to the
Power Supply PCB. I'll double check that it has a good flow of solder on the Pins. You will be
able to see small circles around the pins were the solder has broken loose from the PS PCB Pins.)

In the Photo above it looks like your Horizontal Sweep isn't working. You have a Vertical line
that isn't expanded in the horizontal position.

Larry
The monitor in the pic is on its side. The bottom of the pic is the left side of the screen. You can't see it very well in the pic but it does say "diskette" it's just badly blurred.
 
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