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TRS-80 Model 4 - Power on with blank screen

Win9xGuy

New Member
Joined
May 12, 2019
Messages
9
Location
United Arab Emirates
Hello everyone

Have TRS-80 Model 4 that powers on with blank screen. Floppy light red for few second on booting. CRT tube no heating or noise.

What I did:

Scrolling brightness/ contrast, not helping.
Holding break key, not helping.
CRT folk on place.

Any suggestion?
 
Is your Model 4 a Gate Array Model or Non Gate Array Model. (You can tell by the RS-232C
Port. If the RS-232C port connector is sticking straight down (towards Desk) it's a Non Gate
Array Motherboard. If the RS-232 Connector sticks towards the back of the Computer it is
a Gate Array Version.)

That should get us to the correct Video Board.


Larry
 
so if you know "CRT tube no heating or noise." then I will assume you already opened the computer.

disconnect the PSU (if a single one) and take it off the disk drive bracket, then reflow all the pins of the connector without forgetting the extra 2 that bring power to the video board, reassemble and power up and test 2 pin connector to see if power is leaving the psu for the video board, seethe technical manual below for where/how much should be coming out of pins.

M4 NGA https://drive.google.com/file/d/1eZGOEsckPiBtBDKcMdNR7AjF1KvaXg_T/view?usp=sharing
M4 GA https://drive.google.com/file/d/1AzZ7RNVQ5R12ZYtcc2kvUyfb5No38FTO/view?usp=sharing
 
Is your Model 4 a Gate Array Model or Non Gate Array Model. (You can tell by the RS-232C
Port. If the RS-232C port connector is sticking straight down (towards Desk) it's a Non Gate
Array Motherboard. If the RS-232 Connector sticks towards the back of the Computer it is
a Gate Array Version.)

That should get us to the correct Video Board.


Larry

Greetings Patrick

As you mentioned, the problem was with PSU. Pins had dry soldering.

Check picture.

Problem with CRT solved.

Thank you very much! :)
20210410_194541.jpg
 
so if you know "CRT tube no heating or noise." then I will assume you already opened the computer.

disconnect the PSU (if a single one) and take it off the disk drive bracket, then reflow all the pins of the connector without forgetting the extra 2 that bring power to the video board, reassemble and power up and test 2 pin connector to see if power is leaving the psu for the video board, seethe technical manual below for where/how much should be coming out of pins.

M4 NGA https://drive.google.com/file/d/1eZGOEsckPiBtBDKcMdNR7AjF1KvaXg_T/view?usp=sharing
M4 GA https://drive.google.com/file/d/1AzZ7RNVQ5R12ZYtcc2kvUyfb5No38FTO/view?usp=sharing

I sent reply, but not published yet.

Hi Patrick

As you mentioned, the problem was with connector pins in PSU.

Attached a picture before reflow.

Thank you :)20210410_194541.jpg
 
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