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Model 4P display

robdaemon

Member
Joined
Jun 7, 2018
Messages
24
I have a TRS-80 Model 4P, one of the earlier ones with the white screen.

When you first power it on, the display is very bright - lines across it and everything. After about two minutes, the lines go away but the display is a little dark / fuzzy. If I turn up the brightness, it gets the lines back.

Are there replacement displays I could do for it? Or something else I can try?

I just replaced the power supply - the old one started started acting up and the display started pulsing brighter and the fan was spinning up and slowing down. I turned it off and replaced the PSU before any damage could occur.

Any ideas?
 
Here's what the screen looks like, starting with initial power on, about 5 minutes later, and then going into CP/M.

The lines get less and less predominate, but the screen just gets dark, and if I turn up the brightness at all, the lines remain.
 

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It looks like the brightness is up way too high in the first photo. Tune the brightness
down so you don't see the retrace lines. You can vary the contrast some to get the
characters to display better.

Even in the third photo there is some remains of the retrace displaying. if you
turn the brightness way down it most likely will be a typical Model 4P.

Larry
 
It seems that way, but it starts as it's too high and then reduces on its own.

If I turn it down too much it gets too dark to use.
 
OK, that sounds like you need to start by checking the Power Supply Voltages to
make sure they are regulated properly on startup, and after being on for a length
of time.


p. 5 Brightness & Contrast Controls
p. 19 Point #7 - Check Power Supply Section 5.4
p. 143 Power Supply Check Section 5.4
p. 146 There are 4 secondary output voltages to the auxiliary Supply:
+5.05 VDC, +12 VDC CRT, +12 VDC Floppy, -12 VDC.
p. 147 After setting V1 +5.05 VDC it must stay withing +/- 3%
V2 +12 VDC CRT must stay withing +/- 5%
V2 should have no more than 150mV Peak to Peak Ripple

From the looks of the Power Supply on p. 162, it has the typical row of Pins
on the end of the Power Supply. It's also a TANDY supply, and so it needs to
be removed, so you can check each pin's connection to the PCB with a good
magnifying glass. Those pins will show a small ring around them where the
connection to the PCB is broken. You should reflow the soldering to each
pin with some good Kester 63/37 Rosin core solder. I've seen lots of these
bad solder connections on the TANDY Supplies.

p. 178 is the Schematic of the Video Board.


Larry
 
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