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PET 2001 8k, a new, unespected video problem

G

giobbi

Veteran Member
Joined
Dec 23, 2012
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987
Location
São Paulo country, Brazil
hello guys,

my PET 2001 started today with an odd video problem. At first boot things seem to be stable for some seconds, than it starts to show the problem in the video below.

At first the problem seemed a vertical sync problem, with the image waving up and down a little, so I tried to change D8 and E9 (74ls00 and 74ls20) that should be the ICs for the vertical drive and the video signal (I was in doubt if I should try to change also D9, that should be the horizontal drive if I'm right, but I decided to wait for more info).


I'm not even sure if it's a CRT problem or a logic problem.

Any advice?

--Giovi

P.S. I've just noticed the LED on the board doesn't light up (I can't say if it worked before... maybe not). I checked it and it works. Should it be on?
 
I'll check your video out presently.

The LED lights when the four (4) keyboard drive signals are 1011. This is clearly a function of the firmware itself. Interesting question though as to what DS1 signifies!

Dave
 
My gut feeling is that your problem is monitor related.

I think what you need to do is to scope the horizontal and vertical drive signals to see if they are stable. If they are, stable it is the monitor.

Dave
 
daver2 said:
My gut feeling is that your problem is monitor related.

I think what you need to do is to scope the horizontal and vertical drive signals to see if they are stable. If they are, stable it is the monitor.

Dave
Click to expand...

If only I powered on at least a couple of neurons.... LOL

I have a 3032, that share the same monitor... so I plugged the 2001 board in the 3032 monitor, and the issue is still there. So it's the logic. Good for me: I hate to try to fix CRT....

The behavior is strange: if you wait for some minutes before to switch it on again, it takes some seconds to show it. It become waving a little, then a little more, until you get the issue in the video.

TTL or capacitors?
 
Solved. Unbelievable.

I was replacing the RAM sockets (I should say: rebuilding... I'm going to open another thread about that) and I noticed there was a bad contact on the video connector. I cleaned it removing every cable from the plastic shell and sanding it (it was really oxidized). I also sanded the PCB connector, but it seemed one or more solder joints were unreliable. So I soldered it again, and I needed to add some more flux.
I always carefully clean the excess of flux (because I had some issues in the past due to the flux), but I had to stop for a while and when I came back I totally forgot that. I didn't ever remember I had soldered the connector. So the excess of flux was still there, and was giving the above issue.

Now I was trying to understand in which exact moment the issue started and I suddenly remembered about the flux. Silly mistake. This is why that kind of thing works better if you do at night, when everybody else is sleeping and nobody distracts you LOL

Dave, as usual, thanks for your help!


P.S. BTW is there any keyboard sequence that lights the LED? Really strange that LED there!!!
 
All of these signals levels are TTL - so I would be surprised if conduction was a problem (unless there was some bits of solder mixed in with the flux of course). But, if it works...

I generally write out a list of what I am going to do. A step by step process. And then tick each step off as I go. That way you can get distracted by a higher priority interrupt (aka the wife) and pickup where you left off. Perhaps this is a side-effect of the industry in which I work in?

I did that when we stripped down our old projectors in our Church for some repair and maintenance work to extend their lives a bit. I noted down exactly where I got the screws, bolts etc. from and what order I took them out in. After swapping the faulty bits, assembly was a breeze. Even more impressive was that I had no parts left over at the end and the projector worked first time. My other workparty colleagues just disassembled everything they could see and put the bits in a big pile. They were quicker at disassembling than I was - but they were much slower at assembly than I was - and had bits left at the end. From my instructions I was able to tell them exactly what bits they had left, where they went and (more importantly) how much they had to disassemble again to install the missing part(s)!

Dave
 
daver2 said:
All of these signals levels are TTL - so I would be surprised if conduction was a problem (unless there was some bits of solder mixed in with the flux of course). But, if it works...
Click to expand...

I have had troubles using this kind of flux. Many times a not working board (after some fix) come back to life simply washing it. I should use some other flux, but it's hard to find a good quality product here in Brazil, so I prefer to continue with a known behavior (even if bad) rather than change with an unknown brand, that could be worse (or better, of course).
This time was my fault, indeed.

daver2 said:
I generally write out a list of what I am going to do. A step by step process. And then tick each step off as I go. That way you can get distracted by a higher priority interrupt (aka the wife) and pickup where you left off. Perhaps this is a side-effect of the industry in which I work in?

I did that when we stripped down our old projectors in our Church for some repair and maintenance work to extend their lives a bit. I noted down exactly where I got the screws, bolts etc. from and what order I took them out in. After swapping the faulty bits, assembly was a breeze. Even more impressive was that I had no parts left over at the end and the projector worked first time. My other workparty colleagues just disassembled everything they could see and put the bits in a big pile. They were quicker at disassembling than I was - but they were much slower at assembly than I was - and had bits left at the end. From my instructions I was able to tell them exactly what bits they had left, where they went and (more importantly) how much they had to disassemble again to install the missing part(s)!

Dave
Click to expand...

You're right on the wife LOL
I always take pictures of everything; many, many pictures of everything (screws, bolts, wires, etc.) and I always can assemble it flawless. This time the problem was my old brain, I wasn't supposed to forget that!
 
I didn't look that far before replying. Flux is always a potential issue, especially near the reset 555 chip. Also, never mix different types of flux. Water soluble and rosin flux will make a mess if mixed that is hard to clean up.
Dwight
 
Dwight Elvey said:
I didn't look that far before replying. Flux is always a potential issue, especially near the reset 555 chip.
Click to expand...

you're absolutely right! First time I had an issue related with flux was exactly a problem on the /RST line. I only discovered it later, when somebody suggested me that. Until then, I wasn't concerned about excess of flux.


Dwight Elvey said:
Also, never mix different types of flux. Water soluble and rosin flux will make a mess if mixed that is hard to clean up.
Click to expand...

Good to know! Again, I didn't suspect that.
 
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