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Model 4P - No Video and I'm SO disappointed...

V

VirtualTed

New Member
Joined
Feb 23, 2011
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9
Well, my hopes were high as I opened the Ebay box today on my "as-is" Model 4P. I had a Model I, then a Model III, and I had always wanted to get a 4P but the parents never went for it. Outside of some yellowing of the case, it was in good cosmetic shape and had the 64k upgrade. Finally decided to get it myself 30 years later.

Unfortunately, plugging it in gets me no video. I'm hoping someone can help me out on some troubleshooting, but my fear is that it's just never going to work again. Here is what I see...

* Reportedly worked 25 years ago, was boxed and never turned on again. (hmm..)
* Warranty Sticker still in tact when I opened it up.
* Inside of case and internal components are remarkably dust-free. A couple tiny spiders/cobwebs on outside edge of screen make me believe it truly hasn't been messed with.
* Adjusting internal pots or brightness/contrast on front result in no change in video - monitor is dead-black. No lines, no flicker, no square box visible on screen.
* Disk Drives light up one at a time as if it's looking for a disk to boot (I don't have any to try.)
* Neck of CRT lights up when powered on.
* Both fuses I found (PSU & on Video board) are good.
* Attempted to unplug and replug all power connectors on back of CRT tube, and leading to power/video boards. No luck there either.

Boy was I excited. I had the 5.25" disks on order, all the website material memorized for transferring from PC back to TRS80 floppies, and was ready to go. Unfortunately, it wasn't to be.

Any ideas? I saw someone mention reseating chips might help, but I'm not getting any video at all, which seemed to be a prerequisite for that suggestion.

ANY help is appreciated. I've read every thread related to video problems and I'm not seeing anything of help for this particular case, although I'm sure I'm not the first.

Thanks!

-T
 
Last edited:
Anode problem? Or maybe the tube is cracked somewhere or leaked.
 
Mos6502 said:
Anode problem? Or maybe the tube is cracked somewhere or leaked.
Click to expand...

If the tube gassed out, the heaters would burst into flame and turn to dust in seconds.

The idea of re-seating the chips is to see if you can get the screen to come back on, but, I doubt it is as easy as that. It's been a lot of years since I service a 4P, but I think I have a schematic of the little beastie around and I'll dig it up when I get the chance, unless someone beats me to it.
 
That would be a great help. I've heard other people mention in other threads regarding screen issues that the caps might have worn out. I visually inspected everything and it all looks in really good shape, but I'm willing to start replacing those if anyone thinks it might help.

I might reseat the chips just to cross it off the list...
 
Check HV Power Supply

Check HV Power Supply

DANGER! High voltages can kill!!!

My experience with retro systems is the power supplies are often the culprit.

I would first check (CAREFULLY) that I had high voltage.
 
Most common cause of broken 4p video is the PLL circuit on the motherboard. If you have the model III mode floppy, you can confirm this by trying to boot from it.

The trimmer cap is near the back, but it's a bit risky to play with when the power is on, but you have to do it with the power on to adjust it...

Though typically with this problem you'll at least have angled scanlines typical of a loss of vertical sync if you crank the brightness.
 
K5XS said:
DANGER! High voltages can kill!!!

My experience with retro systems is the power supplies are often the culprit.

I would first check (CAREFULLY) that I had high voltage.
Click to expand...

You can check for high voltage by running the back of your hand across the face of the CRT. If the hair on your hand stands up, you have it.

Alternately, if you have one of those neon bulb voltage testers, hold the bulb end near the flyback transformer and if the bulb lights up you have high voltage.
 
Can you hear the high pitched "deflection whine" from the monitor? Can you feel the HV on the back of your hand like Druid mentions? Cranking up the brightness all the way should give you angled retrace lines on the screen. I'm not 100% familiar with the monitor in the 4P, but it's probably like all the other monitors from this era... Usually there are two fuses on the monitor's chassis, one for power input and one for the B+, The B+ fuse will be smaller, and probably in the 300ma range. I'll have to locate the service manual to give you more detailed things to check.

-Ian
 
The monitor is truly dead. I've had plenty of tube-style monitors in my life, and I can hear a monitor going (with no sound) a room away. The hair test, the whine, and an attempt to crank brightness and other controls as far as they can go produce no effect. The screen is absolutely pitch black w/ no scan lines and visible increase in brightness to screen. I've tested this in a dark room as well to see if I can see anything at all, but there is no effect.

I'm going to try and get a model 3 floppy with the appropriate ROM from someone, as I have none to use at the moment. If the drives are disconnected from the motherboard upon bootup, then they don't respond in any way, but if they are connected when I turn the computer on, the light up one at a time (drive 0, then drive 1) as though they are searching for a floppy the way I remember they used to when I was a kid.

Both fuses tested good on my DVM.

My next step was to start desoldering caps and testing them on the video control board (since voltage on the power supply looks good - 12v / 5v appears normal and within range). Otherwise, I'm just shooting in the dark.

Thanks again for all the help!
 
I had the same problem

I had the same problem

First, hello guys, I am new.

My 4p was in the same shape. No video at all, no whine that you could hear or feel. My problem was that the flyback transformer had some cracked solder spots and after fixing those it still had a problem. It had 3 white blocks going down the center of the screen. A little further re soldering on the power board wire plug in connectors and those three white blocks stretched out into "Drive is not ready" in English and two other languages.

Too bad the disks that I have are all warped and I have no software to go any further. :-(

This may not help you but it might help someone else.

Thanks guys,
Chad
 
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