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Commodore 4032-12 dead screen

rschissler

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It took me awhile, but I finally got back to this.

I installed the new 6845, but with no change. The computer still chirps, but the screen is totally dead. I checked pin 40 and it came to .02 volts.

I checked J7 and got these results:

1 - 2.29-3.15v, it constantly changes
2 - .02v
3 - 5.10v
4 - .02v
5 - 3.66v
7 - .02

Any ideas?
 

dave_m

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1 - 2.29-3.15v, it constantly changes
2 - .02v
3 - 5.10v
4 - .02v
5 - 3.66v
7 - .02

This looks like bad luck. You changed the 6545 on our recommendation and still no apparent Vertical Sync. VS has a skinny negative pulse every 16.6 mS so it should be slightly less that 5V (pin3).
 

dave_m

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Here is another reading to add to my last post.

I checked Pin 7 at UB14, the 6502, and it reads 3.21 volts.

Pin 7 of the CPU is Sync and should pulse with every instruction fetch. Normally it would have a DC average of about 1.2V. The 3.2 V indicated it is stuck somewhere. Last time you measured it it was 1.2 V before the CRT Controller replacement. Can you re-measure?

It is the 0.02 V on pin 40 of the CRTC (6545). It indicates no Vertical Sync, just like the old chip. While other signal on the 6545 are OK which indicates chip is being initialized. I wonder if there is a short on the pin 40 circuit trace. With power off, use ohmmeter to measure between pin 40 and ground. Is it very close to zero ohms?
-Dave
 

rschissler

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Pin 7 of the CPU is Sync and should pulse with every instruction fetch. Normally it would have a DC average of about 1.2V. The 3.2 V indicated it is stuck somewhere. Last time you measured it it was 1.2 V before the CRT Controller replacement. Can you re-measure?

It is the 0.02 V on pin 40 of the CRTC (6545). It indicates no Vertical Sync, just like the old chip. While other signal on the 6545 are OK which indicates chip is being initialized. I wonder if there is a short on the pin 40 circuit trace. With power off, use ohmmeter to measure between pin 40 and ground. Is it very close to zero ohms?
-Dave
I rechecked Pin 7 of the 6502 and it still came to 3.2v. One screwup I made while desoldering, was I got confused with the board turned over and started to desolder the 6502. When I realized my error, I hadn't got to the point where any of the pins there were unsoldered, and made sure to resolder them just to be sure.

I checked the ohms on pin 40 and ground for the UB13 and got like .001 ohms.
 

dave_m

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I rechecked Pin 7 of the 6502 and it still came to 3.2v. One screwup I made while desoldering, was I got confused with the board turned over and started to desolder the 6502. When I realized my error, I hadn't got to the point where any of the pins there were unsoldered, and made sure to resolder them just to be sure.

I checked the ohms on pin 40 and ground for the UB13 and got like .001 ohms.

Let's not worry about SYNC for the moment. As for the 6545/6845, note that pin 1 is ground and is the pin to the left of the notch. Pin 40 is the pin to the right of the notch when looking at the top of chip.
 

rschissler

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Let's not worry about SYNC for the moment. As for the 6545/6845, note that pin 1 is ground and is the pin to the left of the notch. Pin 40 is the pin to the right of the notch when looking at the top of chip.
With the power off, and with the black lead on pin 1 and the red lead on pin 40, I got 190K.
 

rschissler

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Something is amiss. One test had pin 40 to ground at 0.001 ohm. Later pin 40 to pin 1 (ground) at 190K Ohm?
-Dave
The .001 reading was using a bare metal part of the case for ground. That has always been my interpretation of ground. When you mentioned pin 1 as ground, I retested using that, and got 190K.
 

dave_m

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The .001 reading was using a bare metal part of the case for ground. That has always been my interpretation of ground. When you mentioned pin 1 as ground, I retested using that, and got 190K.

I was not being clear. UB13 pin 1 should be ground for the 6545 chip, and should read the same as chassis ground because digital ground and chassis ground are connected in the PET.

So it is possible that Vertical Sync is shorted to chassis ground and/or the 6545 is not grounded properly at pin 1 (bad solder or bad circuit trace?). You must do some detective work.
 

rschissler

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I checked the pins on the 6845 and resoldered pin 1 and any others that looked weak. I checked resistance to pin 40 to pin 1 and got 190K ohms. I checked pin 40 to the case ground and also got 190K. So, that is good, right. Either pin 1 wasn't soldered correctly, or I previously measured wrong. One thing that confuses me, is that when checking resistance in resistors it doesn't make any difference what lead is touching each end. With the 6845 however, the black lead has to be on pin 40 and the red on the ground, otherwise I get nothing.

Anyway still no video.
 

dave_m

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I checked resistance to pin 40 to pin 1 and got 190K ohms. I checked pin 40 to the case ground and also got 190K. So, that is good, right.

OK we know that Vertical Sync is not shorted to ground. So how come no signal? One possibility is that Register 4 in the 6845 which stores Vertical data can not be addressed because of a problem with A2. Get a DC average of this address line (pin 11 of 6502). If it is zero, the address line may be stuck low.

One thing that confuses me, is that when checking resistance in resistors it doesn't make any difference what lead is touching each end. With the 6845 however, the black lead has to be on pin 40 and the red on the ground, otherwise I get nothing.

There are diodes in the 6845 that may affect resistance readings.
 

dave_m

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Pin 11 of the 6502 reads 2.05 volts. Not sure what you mean by "average".

By taking a DC voltage reading of a pulsing signal one will get the average voltage. 2 V implies the signal is pulsing around 50% and therefore not stuck high or low. Check all 16 address lines, looking for signals stuck at ground. A logic probe would be best for this, but we'll get some info from the voltmeter.
 

rschissler

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The 2v reading I took on pin 11 of the 6502 was constant. There was no change or pulsing.

"Check all 16 address lines, looking for signals stuck at ground." Sorry, buy you will have to explain that in better detail.
 

KC9UDX

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The 2v reading I took on pin 11 of the 6502 was constant. There was no change or pulsing.

It must be pulsing, else you have a voltage divider somewhere, which is not good. Pulsing there is normal. To measure a constant 2V when it is pulsing that way at that frequency is also normal.
 

dave_m

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The 2v reading I took on pin 11 of the 6502 was constant. There was no change or pulsing.
2V DC is the DC average of a 50% duty cycle pulse switching between 0 and 4 volts.

"Check all 16 address lines, looking for signals stuck at ground." Sorry, buy you will have to explain that in better detail.

There are 16 address lines in the 6502 CPU (A0 thru A15). Check each one with the voltmeter looking for those that read very close to zero volts. Those may be stuck/shorted.

You may be at the limit of your troubleshooting capabilities having replaced the 6545 CRTC unfortunately to no good result.
 

dave_m

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What I am saying is that the 2v didn't change. It didn't go from 0 to 4 volts with the multimeter.

Is A0 to A15, pins 1-16?

A multimeter is not an oscilloscope. What you get is an average reading.

Here is sheet 1 of the PET schematic
showing the pinout for the 6502. Check the address bus pins shown there as AB00 to AB15. I'm not sure you have enough background in electronics to fix a computer with this problem. You did good replacing the 6545. It was worth the try, but since that didn't fix the problem, the troubleshooting needs to go much deeper.
 

Pet Rescue

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What I am saying is that the 2v didn't change. It didn't go from 0 to 4 volts with the multimeter.

Is A0 to A15, pins 1-16?

If you imagine that the voltage is constantly changing from 0 to 4v very very quickly this is too fast for the multimeter to keep track of so it just sees the average of the two 2v. A bit like movies are a series of still pictures put together very fast to show movement.
 
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