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Pet Chicklet 2001 9" Monitor Dead

Pet Rescue

Experienced Member
Joined
Dec 15, 2010
Messages
484
Location
Staffordshire, England
Hi All it's been a while, I decided to dust off my 2001 Chicklet and have a go at repairing it.

The story so far has been a toughie!! I have repaired it, it runs for a while then fails. I repair it again and something else fails.

I don't think it likes being switched on, anyway my first lot of repairs involved the dreaded Mos 6550 Rams. A few failed and I shuffled them around and then half an hour later another failed etc.I bought two old 2114 replacement Ram boards and these worked a treat. Sure enough though on about 20 mins and it fails, this time it was the 74ls244's that had gone faulty. I replaced those two and it worked again so I left it on around 30 mins and it failed.I made an adapter to test the 6540 Roms and found one faulty. I built an adapter to use a 2716 but it didn't alter so it must have another fault as well. In my wisdom I decided to make extension wires so that I can take the mainboard out and run it outside the case for better fault finding. (bad idea!)

Unfortunately I mixed two of the wires up on the main plug J8, the one with two reds two browns and a black (got a red and brown the wrong way around). I turned on the pet and instantly blew the 0.75 A fuse. seeing what I had done wrong I put the wires back correctly. Replaced the fuse and turned it on again but Pop! it went again, from the initial fault I created upon checking the board, it had blown the middle Diode of the three in a row near J8. I replaced the damaged diode but when it turned on the transformer hums and the tube at the neck glows but nothing is on the screen. I got out my scope and checked at the Video plug J7 for the Hor, ver and vid signals and they looked fine compared to the sheet on zimmers.

So I made a composite out adapter and put that on to J7 and although it wasn't very stable it had a picture of garbage displayed on my TV.
Therefore the monitor itself no longer displays anything! Could the short have damaged the video board?

I tested the transformer and have 15VAC there and 12VDC on the voltage reg on the video board and that's as far as I have got.

It's hard enough repairing them as it is, without adding more faults myself!

Have you guys any pointers where to test on the video board or what I may have blown from my error?
 
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Hello,
first of all, you should determine what version of video PCB you have. On zimmers there're three schematics, though they are very similar.
The 9" video circuit is very simple, so if you have an oscilloscope, you should quickly determine in what area the fault is. Having worked in a few of these video circuits, I suggest you to completely remove the CRT and PCB from the metal case, to have more room when probing around.
Of course, pay attention to the lethal voltages inside a typical CRT monitor! If unsure, leave it alone or find someone who is comfortable in working on those circuits.
First of all, I suggest you to check all semiconductors on the video PCB with all powered off and unplugged. Use the multimeter "diode" check mode. Check all diodes and transistors (B-E and B-C junctions can be tested as diodes).
If all junctions look ok, you should check if you have -30V on C22 and +85V on C27. Those voltages are generated by the horizontal oscillator that needs the horizontal sync signal from the PET mainboard, so either you need the PET mainboard connected when you troubleshoot the CRT pcb or you need to make a separate generator of compatible TTL syncs and video signals (well, I guess most people will stay on the first option).
If you have those two voltages, it means the horizontal circuit is mostly ok, so we can start checking elsewhere.
HTH
Frank IZ8DWF

PS: look at my avatar image, does it look like something you know? ;)
 
Hi Frank, thanks for the reply.

Here's the board:-

IMG_6696.jpg

Well I was probing the board with my multimeter, testing diodes and transistors using the first board type schematic off Zimmers, the one with the picture of the PCB with component location.

I tested each diode and transistor etc and they all tested fine, I then powered up and tested the voltage out of the voltage reg and it was at 12v, I saw the test points underneath marked E1 and E2 and put the voltmeter on them and coincidentally the display came to life. (relief that the added fault causing a shorted diode did not kill the screen.)


It now displays perfectly but turning it off, now and again it fails to come on.

As I lifted the pet top it came on and if I tapped the side it went off then came back on again!

It reminded me of the 70's when we used to tap the TV to get it to come on. :D

Now I have a picture here is the screen I now have from the pet that is just garbage. When it first powers up I initially get the same characters all over the screen i.e. all "?" and then a scrolling garbage screen but not garbage in the form of letters more a kind of noise, see photos.

First initial second I get:-


IMG_6711.jpg

then this:-

IMG_6703.jpg

It is really annoying after repairing the 74ls244's it worked perfectly but now I don't even get random garbage just a strange rolling pattern.

PS just remembered when the screen plays up it seems collapsed into a vertical line in the centre at first then displays a full screen.(don't know if that's relevant)
 
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Whoa... first of all I would dust off that video board, just to see what's under it :)
Interrmittent faults might be bad solder joints, so you should inspect the video board for cracked solder joints also.
The all ? on the first screenshot make me think about faulty video ram chips (sorry, both of them in this case) or bad LS244 video RAM transceivers or a combination of these chips.
If the screen collapse into a vertical line, it's because of missing horizontal sync, but that will shut down the entire video board too anyway.

Frank IZ8DWF
 
Yes it is a bit dusty! :D

I think you are right regarding a dry joint on the video board when I clean it I'll give it a close inspection.

Regarding the RAM's I'm sure they are definitely ok but I have substituted them for a 2114 Ram board to make sure.

I tested the ROMS again and found H1 and H7 faulty so I burnt a test EPROM and put it in H7 to eliminate the ROM and RAMs.

But it's still the same scrolling screen rubbish.

When I take out the RAMs and ROMs I get a check board pattern but with artefacts scrolling over them so something is amiss.
 
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