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Pet 2001-N PCB 320350, Stuck in reset.

D

dabone

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I just bought a 4016 9" with a 320350 motherboard, and I'm having a time getting anything out of it.

Occasionally it would run PETTESTERV4, but not anything else, and sometimes it would take leaving it on for 30 mins or so before anything runs.

I washed the board yesterday, and tonight I got out the probe, and pin 3 of the 555 @ a3 (the reset circuit) never goes low, but always stays high.
I didn't leave it running for a long time to see if it ever comes out of reset today.

I don't have any 555's in stock, so recommendations before I order more parts?

Thanks.
 
Replace all the capacitors around the 555 before changing the 555 itself out. I had a stuck reset on the same board and it was actually changing one of the ceramic disks that unstuck it.
 
C68 (0.1uF) starts the reset process off.

C67 (1uF) is responsible for the length of the reset pulse.

Put a normally-open pushbutton across C68 (between the 555 timer pins 2 and 1). This means you can manually initiate a reset cycle.

Dave
 
.......here is the image.

There was only one minor defect I could see with this circuit. There should really have been a 1N4148 diode placed across R16 (cathode to positive) the reason being that with rapid power cycling, it is better that when the +5V power rail in the PET collapses, the 0.1uF trigger capacitor is rapidly discharged to reset the circuit to its off state condition, rapidly depleting the charge in the 0.1uF cap C68, if not a trigger pulse to initiate the timing cycle might not occur and the PET fail to get a proper 1 second reset, this way. With the diode, the PET would always get a proper reset pulse after a series of rapid power cycles. Generally it would be customary to have the diode for any kind of reset circuit based on an R-C time constant, so I'm not sure why they didn't use it. But it is not likely that the PET would be super rapidly power cycled , unless say there was an intermittent connection on its line power feed. But if your PET was controlling the cooling system or control rods on a vintage Nuclear Reactor, I would put the diode in.
 

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So, C68 was bad.. Which is the only one I had in stock, so that worked out well.

I did go ahead and replace all the 40 pin sockets, because this board had a ton of dirt, bugs (I think I found a hatched spider egg sack under the 6522), and black corrosion on the pins and sockets.

It was running PettestV4 and the PET RAM/ROM/VRAM test ok, but I couldn't get it to boot to basic.

9 times out of 10 it would just boot to a black screen. The other time to the monitor, and had a fast flashing cursor with random characters appearing (No keyboard attached)

I finally removed the 6522 and both 6520s.

When I did that it always came up to the basic screen (no cursor, of course.)

I ended up plugging in each chip one at a time, and it ended up being a bad 6522.

So, my non working 4016 needed a .1uf cap, a 6522 via, some sockets, and some tlc for the board.

Now I'm seeking a left shift key and stem for the keyboard.....

Thanks for the help!
 
Well this sucks.... I got around to the keyboard, and it was heavily liquid damaged. About half the screws had gone to rust and broke off in the plastic.
And the PCB has a ton of traces eaten away...

And I never noticed till I was working on it that it had the Japanese Kanji keyboard.

I've done about 12 patches so far using kynar wire, but I'm not sure if I'll ever get thing together again even if I can patch the pcb flat enough to reinstall because of the broken screws.
 
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dabone said:
Well this sucks.... I got around to the keyboard, and it was heavily liquid damaged. About half the screws had gone to rust and broke off in the plastic.
And the PCB has a ton of traces eaten away...

And I never noticed till I was working on it that it had the Japanese Kanji keyboard.

I've done about 12 patches so far using kynar wire, but I'm not sure if I'll ever get thing together again even if I can patch the pcb flat enough to reinstall because of the broken screws.
Click to expand...

There is some particularly good German conductive colloidal silver paint that has a very low resistance and doesn't tend to crack or split and can be used for pcb repairs. It is better than some of the carbon graphite products;

Silver Conductive Varnish | Jaycar Electronics

This silver conductive varnish adheres well to glass, PCB material, ceramics etc. Repairs printed circuits, window antennas, window alarm loops etc. V...
www.jaycar.com.au www.jaycar.com.au

You could repair the missing tracks with this without adding any significant thickness. It may also be possible to repair the plastic damage too, with a number of techniques, depending on exactly what needs to be done.

(this paint also is good for repairing damaged external Aquadag on CRT's and I have also used it to re-coat the Leyden Jars (capacitors) on my Wimshurst Machine).

Jaycar also sell a conductive pen:

Silver Conductive Pen | Jaycar Electronics

PEN PCB SLV CONDUCTIVE 5G
www.jaycar.com.au www.jaycar.com.au
 
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Well, after I was done being mad at it, I did sit down... for 2 days working on that keyboard.

This style keyboard is made where the pcb has a lot of free space above it, so I did a ton of patches to the top and quite a few jumpers on the bottom.

Here's a pic of some of the traces.

WIN_20220725_20_54_26_Pro.jpg

Ended up with about 25 patches between the top and running wires on the underside also.

Now to figure out which speaker mod to do, and wire up one of my petdisk maxes...
(And find and purchase that missing left shift key).
(And order solder mask to cover all the exposed copper...)...

sigh...
 
dabone said:
Well, after I was done being mad at it, I did sit down... for 2 days working on that keyboard.

This style keyboard is made where the pcb has a lot of free space above it, so I did a ton of patches to the top and quite a few jumpers on the bottom.

Here's a pic of some of the traces.

View attachment 1244054

Ended up with about 25 patches between the top and running wires on the underside also.

Now to figure out which speaker mod to do, and wire up one of my petdisk maxes...
(And find and purchase that missing left shift key).
(And order solder mask to cover all the exposed copper...)...

sigh...
Click to expand...
Marine grade polyurethane spar varnish is great to brush over tracks to protect them. I use Feast-Watson brand, I have also tested it to 15kV on high voltage transformer windings, it has very similar dielectric properties to transformer varnish.
 
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