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Pet 8296 blown fuse with monitor connected, and no chirp sound

I try to post photo of the two difference of this +5v rails on the scope with two probe
 
This is situation with yellow probe on +5v ROM rail, and with cyan on +5V Logic rail, with scope at Vdiv 200mV and Tdiv 20ns..
 

Attachments

  • VID_20240308_095915.mp4
    6.7 MB
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This is the big cap on the board, and now i have a question to the 220v input. I have removed the filter cap on 220v input because is it in short, it's a problem todays without this filter on AC input?


1709892409121.jpeg
 
No problem without the mains input filters.

If it is the 'big capacitor' you should observe 100 or 120 Hz ripple on the INPUT side of the voltage regulators.

High frequency noise indicates either:

1. A lot of decoupling capacitors have failed on the board.

2. Depending upon the peak-to-peak reading of the noise (as indicated on the oscilloscope screen) it is normal for that type of machine - providing it is within the accepted tolerances for the devices.

3. You are not measuring the noise correctly; and some of it is 'self induced', either by your measurement technique or a ground loop.

Dave
 
How have you currently measured it?

Where did you attach the probe ground clips for example?

Also, is the PET and the oscilloscope plugged into the same mains outlet or different mains outlets?

If plugged into different mains outlets, you could have a ground loop.

Dave
 
The scope and pet are in same net line 220/240v

The probe ground are connected on 7805 GND pin and i have connected the two probe , yellow on +5v pin of Kernal ROM and other cyan on +5V of 74ls74 logic.
 
I have remaked this misuration at this point signed Yellow is pioint 1 and cyan point 3 of power supply:

1709914594935.png

1709914622691.jpeg
 
To rule out any type of ground loop contribution, leave the probe earth clip where it is connected and connect the probe tip onto the point where the probe earth is connected, or directly onto the probe earth clip itself. There should be zero signal. If a signal is seen it represents the results of current flowing in the probe's earth connections & cable between the computer & scope's grounds.
 
Ok, at this point I also checked and changed the capacitors to be safe, I actually have no ripple, so I rule out power supply problems, so I have to wait for the 74S288
 
Hello Dave,

i confirm the problem are the PROM 74S288, i have changed it and now all is PERFECT.

Now the problem is programming this device because my prommer support it but with an adapter
 
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So, because this is a very weird fault, (and we did track it down in the end), it would be helpful for future visitors to know exactly what the problem was.

Can you use your oscilloscope to look at the outputs from the '288 and document the good and bad traces?

Remember, 2 channels and use channel #1 on VDRIVE as a trigger and channel #2 to take the measurements. You did record the oscilloscope settings you used didn't you?

It would also be interesting to compare the '288 internal logic with what it should be. I am sort of expecting that a fuse has regrown, thus changing the internal logic. But this is purely an educated guess on my part.

Dave
 
demonlg said:
Hello Dave,

i confirm the problem are the PROM 74S288, i have changed it and now all is PERFECT.

Now the problem is programming this device because my prommer support it but with an adapter
Click to expand...
So you changed the 74S288 , but you have not programmed it yet ?

But everything is now perfect ? How can that be if it is not programmed correctly yet ?

Or you programmed a new 74S288 with an adapter ? But you don't have an adapter yet?

Or did you substitute a programmed 74s288 from another vintage machine, and this is why your computer is now working ?

I am completely confused by what you have said.

These sorts of OTP proms can be difficult to program, if the programmer does not support them.

Ideally the programmer would support the specific part, and therefore they would not require any form of adapter.
 
I have moved working 74s288 from another board i have to repair before this and now it's working correctly

I have a very old DOS programmer MQP model 200, but when i select 74S288 it says to fit an adapter type 20 in zif socket, but i don't have it, and yes i have buyed two new not programmed 74s288.

Then i have readed on internet this PROM is replaceble with a GAL, but i don't have idea to have it.
 
daver2 said:
So, because this is a very weird fault, (and we did track it down in the end), it would be helpful for future visitors to know exactly what the problem was.

Can you use your oscilloscope to look at the outputs from the '288 and document the good and bad traces?

Remember, 2 channels and use channel #1 on VDRIVE as a trigger and channel #2 to take the measurements. You did record the oscilloscope settings you used didn't you?

It would also be interesting to compare the '288 internal logic with what it should be. I am sort of expecting that a fuse has regrown, thus changing the internal logic. But this is purely an educated guess on my part.

Dave
Click to expand...

Hi Dave, sostantially is the video 221922 UC3 pin 17 on post 220, with good PROM the signal not have the fluctuation ramp visible on the video, now i try to repost video with problem and without problem

Attached video first 221922 is UC2 pin 7 or UC3 pin 17 with defective PROM UC2, then second video 143336 is with PROM UC2 working.

I want to clarify for everyone that this machine initially did not start and the first component replaced because it was very hot was UC3 74LS374, found to be short circuited, probably this also damaged the prom?
 

Attachments

  • VID_20240306_221922.mp4
    6.2 MB
  • VID_20240310_143336.mp4
    5.5 MB
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