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PET 2001-8 Video Display Dead

wbochar said:
2V/Div x1 probes, zero line is the middle of the grid vertically.
Click to expand...

Wolf,
This is curious. If your DSO scope is working correctly, it says you have zero on the input and output of both inverters.

Check that you have good Vcc on pin 14 and ground on pin 7. Double check with voltmeter.
-Dave
 
dave_m said:
Wolf,
This is curious. If your DSO scope is working correctly, it says you have zero on the input and output of both inverters.

Check that you have good Vcc on pin 14 and ground on pin 7. Double check with voltmeter.
-Dave
Click to expand...

Pin 7 has good ground and 15 has 4.92V using a DVM.

I guess I will have to just dump that mini scope, its causing more headaches than help.
 
The oscillator circuit on the top right of the schematic is having a hard time. Maybe a bad crystal (Y1) or shorted capacitors C3 or C4. Let's see what others think. Make sure there are no solder splashes between pins on the D9 inverter chip. Of course pins 9 and 10 should be connected. Let's see if others think it good to cause a quick short to ground on pin 9 to try to start the oscillation.
 
dave_m said:
Let's see if others think it good to cause a quick short to ground on pin 9 to try to start the oscillation.
Click to expand...

It won't likely hurt, and might be worth a shot. I've forced an oscillator to start with a signal generator, and that works too but isn't really the way to go either.

I would replace the capacitors, and if it still doesn't oscillate, replace the crystal. Either one is as likely as the other to fail at this point, but the capacitors are easier to obtain.
 
KC9UDX said:
I would replace the capacitors, and if it still doesn't oscillate, replace the crystal. Either one is as likely as the other to fail at this point, but the capacitors are easier to obtain.
Click to expand...

I agree. First, with power off, ohm across each capacitor looking for a low reading and replace that one first.
 
dave_m said:
I agree. First, with power off, ohm across each capacitor looking for a low reading and replace that one first.
Click to expand...

I looked at the capacitors, but I have a few questions:

1. most of them have no markings, but the few that do have "Z .1 10V" and are disc shaped ceramic (the majority that seem to isolate the power rails). The smaller ones that are around the NPN power transistors are marked 103 which are 10000pf. So the flat Ceramic ones marked Z .1 10V are they .1 Micro Farad? I also assume these ones have no polarity, correct?

2. I don't have a capacitor mode on my DVM, I've seen some writing on the net about using Ohm measurement around set around 2k. But I am not sure, what a successful measurement is. The DVM picks up a few spikes from the capacitors then stops. I assume this is the discharge from them, if there is no discharge then that's bad. Correct?
 
wbochar said:
1. most of them have no markings, but the few that do have "Z .1 10V" and are disc shaped ceramic (the majority that seem to isolate the power rails). The smaller ones that are around the NPN power transistors are marked 103 which are 10000pf. So the flat Ceramic ones marked Z .1 10V are they .1 Micro Farad? I also assume these ones have no polarity, correct?
Click to expand...
Did you check the schematic? C3 is shown as 22 pF and C4 at 10 pF. Oddly the voltage is listed at 500 V, but anything over 15V should do.

2. I don't have a capacitor mode on my DVM, I've seen some writing on the net about using Ohm measurement around set around 2k. But I am not sure, what a successful measurement is. The DVM picks up a few spikes from the capacitors then stops. I assume this is the discharge from them, if there is no discharge then that's bad. Correct?
Click to expand...

After it stops, replace the lowest Ohms first. a good capacitor should show a high reading. We are just hoping it is one of the caps as they are easier to find than an 8 MHz crystal.

EDIT: I just realized that C4 has a 470 Ohm resistor in parallel so it will read less than 470 so the test is not valid. Just replace them both and hope that they are the problem.
If the problem is the crystal we will need something like this: http://www.mouser.com/ProductDetail...=sGAEpiMZZMsBj6bBr9Q9aQ6SZfIlWoiSivervqkPV0g=

We will need a little advice on the correct type of crystal to use.
 
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Boy, we may be on the way to setting a record for number of posts in a thread, at least in the Commodore area ;-)

Are we positive that there is no clock signal at E2-6 ? Sometimes sticking a scope probe into an oscillator like this can affect its operation.

Also, can that scope even handle an 8MHz signal? I thought we'd seen signals elsewhere, which would suggest that the master clock is running, wouldn't it?
 
MikeS said:
Boy, we may be on the way to setting a record for number of posts in a thread, at least in the Commodore area ;-)
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Not really, I checked and the record for the Commodore Forum is 354 messages from October 2010:
http://www.vintage-computer.com/vcf...ndom-Characters-Issue-on-a-COMMOODRE-PET-2001

Are we positive that there is no clock signal at E2-6 ? Sometimes sticking a scope probe into an oscillator like this can affect its operation.

Also, can that scope even handle an 8MHz signal? I thought we'd seen signals elsewhere, which would suggest that the master clock is running, wouldn't it?
Click to expand...

The signal was dead even at the buffered output (E2-6). I hope the OP is using a probe. The DSO gadget is only good for 5 MHz bandwidth, but I had hoped he could catch some high frequency signal to indicate a clock. The DVM readings were about 1.3 V on the inputs/outputs of the two inverters in the circuit. The 'clock' photos he was posting were noise level signals. What would be a good thing to check? I'd hate to have him replace a good oscillator crystal.
 
dave_m said:
Not really, I checked and the record for the Commodore Forum is 354 messages from October 2010:
http://www.vintage-computer.com/vcf...ndom-Characters-Issue-on-a-COMMOODRE-PET-2001
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Pshaw! I'm sure we can beat that!

The signal was dead even at the buffered output (E2-6). I hope the OP is using a probe. The DSO gadget is only good for 5 MHz bandwidth, but I had hoped he could catch some high frequency signal to indicate a clock. The DVM readings were about 1.3 V on the inputs/outputs of the two inverters in the circuit. The 'clock' photos he was posting were noise level signals. What would be a good thing to check? I'd hate to have him replace a good oscillator crystal.
Click to expand...
Sorry, I haven't been following in detail for a while; do we have a clean TTL-level signal anywhere on the board? Wherever it is, it's presumably derived from the 8MHz clock.

I offered to have a look but Wolf understandably prefers to work through it himself and learn along the way; what I might be able to offer is a working board that he could use to compare (and see if his scope can actually display these signals), but that'd be next week at the earliest.
 
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MikeS said:
Sorry, I haven't been following in detail for a while; do we have a clean TTL-level signal anywhere on the board? Wherever it is, it's presumably derived from the 8MHz clock.
Click to expand...

Apparently not at the moment, but for a short while after he replaced a counter, there was some clock on the horizontal Drive as the CRT lit up for a moment with some high voltage sounds and then smoke. We had him disconnect the CRT connector (belatedly).

If you recall there was some +5V issues and clock issues we were troubleshooting. Now the power seems stable (although I don't like that there is only 4.93 on some rows, but no clocks at all. Curiouser and curiouser...

That's a nice offer of the loaner main board. And he is looking for a good scope, but it takes time to find the right one at the right price.
-Dave
 
I'm back with a tektronix 465 in tow :)

I've redone the scans and double checked things. Before I started I ran the scope with function generator to make sure everything is crisp/sharp.

E2 - PIN 06 - Flat @ 5.9V
C9 - PIN 11 - 1 mhrz
C9 - PIN 12 - 500 khrz
B5 - PIN 14 - 500 khrz
B5 - PIN 12 - 250 khrz
B5 - PIN 09 - 125 khrz
C9 - PIN 05 - Flat @ 5.9V
C8 - PIN 06 - 1 mhrz
C9 - PIN 14 - 1 mhrz
C9 - PIN 10 - Flat 0V
C5 - PIN 09 - 500 khrz
C5 - PIN 08 - 15 khrz
C5 - PIN 11 - Flat 4.5V
B5 - PIN 11 - 31 khrz (Weird Looking)
C5 - PIN 13 - Flat @ 5.91V
C5 - PIN 10 - Flat @ 5.91V
C5 - PIN 14 - Flat @ 5.91V
C6 - PIN 14 - Flat @ 5.91V
C7 - PIN 14 - Flat @ 5.91V
C5 - PIN 05 - Strange Waveform (not even pattern)
C5 - PIN 06 - Strange Waveform (not even pattern)
C5 - PIN 02 - Strange Waveform (not even pattern)
E2 - PIN 05 - Flat @ 4V
D9 - PIN 08 - 8 mhrz (top of wave goes 4v)
D9 - PIN 09 - 8 mhrz (top of wave goes 4v)
D9 - PIN 10 - 8 mhrz (top of wave goes around 3.5v)
D9 - PIN 11 - 8 mhrz (2v width of waveform, but starts 1v up)
D9 - PIN 14 - Flat @ 4.91V
D9 - PIN 07 - Flat @ 0V GRND

HORIZ LINE - Has some Pulses on it
VERT LINE - Flat @ 4.5V

I have some pics for the weird pulses (the 31/15khrz ones)
 
MikeS said:
Weird; if there's no clock signal at E2-6 how can we have any signals anywhere else? 5.9V ???

Did I miss something?
Click to expand...

Yes, he has made a number of wrong measurements. He was probably on the wrong pins on some of them and I hope we do not have 5.9V on anything. On two connected pins, he has the correct 8 MHZ (D9-8 ) and +4V (E2-5)?? But overall not bad for the first time with a new scope. When you see a waveform with skinny negative pulses, use a negative edge trigger and turn the timebase to see about two or three periods. It may help to brighten the persistence. The strange waveforms are probable OK. They will not be square waves. A period is one high time followed by the low time. They may not be symmetrical. It should then repeat. Also the Vertical Drive will be a very skinny negative pulse every 16.67 mS (60 Hz). Without careful syncing on the negative edge and setting the time base to 5 mS, it might appear to be just a high signal. Try some of those again. The Tek 465 is the perfect tool for these measurements.
 
MikeS said:
Weird; if there's no clock signal at E2-6 how can we have any signals anywhere else? 5.9V ???

Did I miss something?
Click to expand...

OK. In a previous part of the thread -- When I asked about the positioning of the chips and the grid reference. I read over it again and now I can see me thinking one thing and you pointing at another.

ROW E has seven chips, starting in slot #2 through to slot #9 in the middle of this row there is a SN74L00N taking two slots horizontally.
So is the first chip at the bottom of ROW E (Near Slot 2)(which is a 74LS08N) is E1 or E2?, I have been treating it as E1. When I re-read the schematic, it has E2 as 74LS08, so E1 Doesn't exist. The schematic skips over it.
Like the rest of the Row E8 and E9 are listed in the schematic as 74LS20's and there is no E7 as E6 is listed at the SN74L00N in the schematic.

Sorry for the mess up, I totally screwed that up.

Here are the E2 Values:
E2 - PIN05 - 8Mhz
E2 - PIN06 - 8Mhz
 
I checked the VERT pin, which I listed as flat -- it does a have a waveform. I switch to .5V/Div and then I could see a waveform (after messing with the the trigger).
 
wbochar said:
I checked the VERT pin, which I listed as flat -- it does a have a waveform. I switch to .5V/Div and then I could see a waveform (after messing with the the trigger).
Click to expand...

This is good. Tell me that the period is 16.67 mS which would be 60 Hz. Measure the period of the Horizontal Drive signal. Is it about 64 uS? If so, we are getting close to hooking up the CRT. We need to also check the video data line (J7-1) for high frequency pulses. Each pulse will become a green dot and gets shifted out at 8 MHz.
 
The Horizontal is around 64uS.
The Vertical is around 17
J7-1 is one of the RAM chips, which has .5V wide waveform. Sawtooth like.
 
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