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Commodore PET 4016 CRT Problems??

twistedpneumatic

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Apr 9, 2017
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N Bellmore, LI
I went back to approach fixing my PET again. This is where I'm at currently.

I installed 3 new ram chips and their sockets.
PETTEST2.BIN is installed.

As you claim it should...the signals are all 5V until about ~6s after it boots of which VSYNC is about 4.4, HSYNC is around 3.5V, and 0.3V on Video-IN. Seems normal enough...so I think I can conclude the board is atleast partially functional. Maybe V-in should be higher...but it seems as the CRTC is being initialized so I don't think its much of an issue. I could always disconnect the resistor to the grid to see if a raster forms.

Anyway...based on that I measured around the CRT board. Got proper voltages on the cathode(glows), HYOKE and VYOKE coils, along with proper voltages on E18, E30, E400, and the -115V rail once HSYNC kicked on. The only voltage so far that has come up wrong is Q761(by R762 on zimmers-321448 schematic), where the base never comes back down to 12.9V, rather stays at 18V. Other transistors on the board do similar but kick down once HSYNC comes in and the HOT can start generating voltages.

So the tube currently does this: The tube starts glowing...a dot appears on the screen, eventually gets quite bright over a second or two. The beam drops down and a raster that makes the entire width and about half the height flashes then it dies. I've found the tube to make a sorta crackling sound...I had an old trinitron that did this so I think its the HV discharging? Also some slow motion video on my phone....I heard the while from the CRT since it was slowed down by 8x, and it was stable for a teeny bit, then start to wharble or something...sounded...unhealthy? Anyway the raster appears(1 run) and the picture dies. No more dot even. I'm suspecting loss of HV....though the grid voltage is on which would also cause that.

I'm lost on where to go now from here. I'm still betting on the CRT being bad....I showed the video of it running to someone at VCF and they said caps on the tube. I checked them with my multimeter and they mostly seemed good. I'm confused on where to go from here though.
 
I disconnected the video signal while leaving them all intact. Got a good raster and no crackly sound. Now is the video causing HV failure(ie...no wierd sound), or is the CRTC outputting garbage video. Given that HSYNC and VSYNC are being properly generated...I see no reason why there shouldn't atleast be SOME picture, right?
 
The video and the H/V shouldn't interact with each other - unless the E30 Voltage is being compromised in some way?

Dave
 
Once HSYNC kicks on, E30 seems to be rock solid. Also checked many of the caps on the board with my multi meter in capacitance mode...they seem to check out for the most part.
 
I did read that in your previous post regarding E30 though.

The problem with using a capacitance meter is that the capacitor could break down when subjected to a voltage across it (especially in the higher-voltage stages, but not exclusively there).

Dave
 
The video signal is only reading 0.3V, shouldn't it read something higher...PETTEST2 should be increasing the voltage on screen. Could the part between the CRTC and the video out be bad? The VIDEO signal drops from 5V to 0.3V when HSYNC starts.

Would recording a video of the machine with and without VIDEO be helpful?
 
I checked more of the circuit on schematic 8(dot gen & even display), clock seems okay, data appears to be flowing into UA2 from UA3, though I'm not sure anything is coming out. Input 12 of UC2 seems a rather low 0.4V, figure it should be atleast over 1V if there is a video signal.
 
Okay, I injected a separately generated test signal from an Arduino into the video line and got an expected pattern. I can safely assume the CRT is fully functional. Also given that the CRTC is creating a proper HSYNC and VSYNC after a very repeatable ~6-7s, I think the CPU, RAM, CRTC and ROM have to be properly functional. From here...it should be somewhere in the character rom/dot gen part of the circuit, right?
 
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Are your 4 video RAMs (2114) on sockets? If you remove them, you should get a checkerboard pattern on the video data line. If you disconnect the J7 Connector for protect the CRT, check the data line for about 2.5VDC average on your meter.

But I do not see how any signal on the video line since it is a TTL signal can disrupt the raster scan.
 
I checked more of the circuit on schematic 8(dot gen & even display), clock seems okay, data appears to be flowing into UA2 from UA3, though I'm not sure anything is coming out. Input 12 of UC2 seems a rather low 0.4V, figure it should be atleast over 1V if there is a video signal.

No, if the screen is primarily blank, I think one will see a very low voltage on the video line (mostly zeros shifting out).
 
That seems right because every once in a while when I turn off the PET, i get a few dots of green on a line and then it shuts off. I'm assuming this is the grid shutting off before the heater is fully cooled down so a few electrons spit out before it dies. Also an injected video signal works just fine...so it must just be outputting a low signal.

Sort of side note: I know that those V-RAM chips go bad often, but is it possible that one of the additional 74 series chip went bad...maybe the shift register??
 
I know that those V-RAM chips go bad often, but is it possible that one of the additional 74 series chip went bad...maybe the shift register??

Sure, and it is also possible that a bad RAM or ROM is keeping the the PET from running properly and it is outputting a blank screen.

Can you summarize what is happening when the video data line is connected to the video board along with the proper timing signals? Is the screen blank or ??? How about when the video line is open? I am not understanding the problem. How did you 'inject a video pattern? was it in sync with the timing signals? Was it a TTL level signal or ?
-Dave
 
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Okay, there are 3 scenarios:

1) The connector is plugged into the board, the CRT is fed HSYNC,VSYNC and Video from the PET. Raster flash then nothing. 0.3V at the video pin and nothing on the display. 30V at the grid.

2) The connector has jumpers attached, the CRT is fed HSYNC, VSYNC, and NO VIDEO(left open, not grounded). Boots up to a solid green screen.

3) The connector has jumpers attached, the CRT is fed HSYNC, VSYNC, and VIDEO from the ARDUINO. Literally I disconnected the jumper from the PET main board and stuck it in pin 13 of my Arduino. The Arduino is outputting a random square-wave at a few dozen kHz. Striped lines appear on the CRT as expected. Video responds to changing timing on the Arduino(ie...higher frequency, narrower stripes, and vice versa)
 
If you set the CRT at maximum bright, you should see some green nebula and diagonal lines. If not, probably the CRT isn't working.

I had the same problem with both my 8032: a broken resistor in the high-voltage line, because of a bad electrolytic.
If it's your case too, you should download the the 40xx manual from Zimmers and take a look to the high voltage section (page 76 of the pdf manual from Zimmers).
Check the resistors and if you find any broken, check the capacitors close to them.
 
I did notice that the brightness changed the picture size, which AFAIK is a dead give away for something wrong in the power circuitry, likely caps. Which cap/resistor did you replace?
 
Also, if the video signal is completely dark...ie grid on full, shouldn't the brightness be worthless since the electrons are already being blocked?
 
Okay, there are 3 scenarios:

1) The connector is plugged into the board, the CRT is fed HSYNC,VSYNC and Video from the PET. Raster flash then nothing. 0.3V at the video pin and nothing on the display. 30V at the grid.

2) The connector has jumpers attached, the CRT is fed HSYNC, VSYNC, and NO VIDEO(left open, not grounded). Boots up to a solid green screen.

3) The connector has jumpers attached, the CRT is fed HSYNC, VSYNC, and VIDEO from the ARDUINO. Literally I disconnected the jumper from the PET main board and stuck it in pin 13 of my Arduino. The Arduino is outputting a random square-wave at a few dozen kHz. Striped lines appear on the CRT as expected. Video responds to changing timing on the Arduino(ie...higher frequency, narrower stripes, and vice versa)

We have to make sure the HSYNC and VSYNC from the PET are correct or you may damage your CRT/ video board. Can you make the adriano into a counter to see if H is 20 KHZ and V is 60 Hz?
 
This is an interesting fault.

Post #14 item (2) clearly indicates to me that it is the video signal from the PET to the monitor that is the problem. Disconnecting the PET video signal causes the monitor to pull the video signal to +5V (white level) via a resistor. I would temporarily stop looking at the monitor for now.

There was something strange you said back in post #1 about the PET taking ~6s to "start-up". This does seem to be a bit long to me... Is the RESET line holding things up - or is the ROM not working properly and 'eventually' the CPU manages to initialise the CRTC to get H/V drive I wonder? If this is the case, what has the PET tried to display on the screen? If it has managed to clear the video RAM - we will see a "black cat in a coal cellar at midnight"... Need to check through the ROM source code - but it may be that the EDIT ROM initialises the CRTC and clears the screen but (if page 1 RAM is faulty - the stack) the subroutine return from the EDIT ROM back to the KERNEL causes the CPU to crash'?

You say in post #7 that you are seeing data between UA2 and UA3 and that you see a clock on the shift register. Do you have the load signal as well? It is no good having the clock to shift the bits if it didn't load them in the first place.

Just thinking of scenarios...

Either work forwards from the character generator through the logic chain to see where the video signal disappears or work backwards from the video signal to see where it appears.

Dave
 
There was something strange you said back in post #1 about the PET taking ~6s to "start-up". This does seem to be a bit long to me...

Daver2,
I think our OP is using the old pettest2 which tacks on the 6545 CRTC initialization at the end of the first pass of the Eudi's program. It could very well take 6 Seconds to get to the initialization. Bad programming (mine!), I did not have have a 6502 assembler handy when I did it. I just tacked some machine code to the end of the binary code and then jumped to the beginning.

I told our OP in a private message to switch over to your (pettest2kV4) in the EDIT socket, but he reported he does not have access to a prom programmer.

-Dave
 
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