PET 2001-8 Video Display Dead

[MikeS]

Thanks a lot for the hand holding

Don't forget the folks following along in the background, keeping Dave honest ;-)

I'm decent with a Solder gun. But before soldering I'll check the board again with a magnifier. I have to say I've learned a lot from this and its amazing to learn new things I'll get back to you with my findings

I hope you didn't literally mean a solder gun ;-)

I assume you have a source for parts. If you need anything, let me know; if I've got it I can drop it off.

Good luck!

 

[wbochar]

Hehe, Soldering Gun is a mental holdover from older days. I use a wand type that's low powered. My Father was a HAM (which we built a lot of things together), so his was a gun type so I got used to calling soldering irons "Guns".

My Board etiquette is rusty, I did mean to everyone -- community is the true power behind this -- thanks everyone

I found a trace on the back that was damaged, so I jumpered to the points and tested that segment. It didn't fix the video issue, but it will likely help something else from causing problems.

I'll check the regular hangouts for parts, if I can't get them -- Mike, I'll take you up on your offer.

It's been fun working on this, thanks again guys

 

[wbochar]

Wolf,
OK, the clock to generate the C9-pin12 (0.5MHz) signal is fine so it's starting to look like like C9-pin12 is dead. Since it is low, there is a small chance that something is holding it down (short). Take a quick look at it loads at B5-pin14 and C5-pin9 for solder splash, etc. If nothing obvious then you will have to replace the 74LS93 counter (C9).

If you are not experienced with replacing a 14 pin IC, it will be easier if you use a small diagonal cutter to cut off all the legs as close to the IC body as you can to leave legs accessible at solder joints. Then using solder wick, remove solder and leg using needle nose, etc., one at a time using minimum heat. This will allow removal and cleanup of solder feedthroughs with minimum chance of overheating the printed circuit board and associated traces.

You might consider installing a socket first rather than installing the new chip directly in.

If this fixes the clocks, you will have good video horizontal & vertical drive signals. The horizontal drive will also be used as the switching signal to generate the high voltage for the CRT. You should then have an operating CRT. We can then go on to whatever the next issue is with the PET if any.
-Dave

I replaced chip and installed a socket with a new chip. it's not working still. C9-PIN12 is still flat lined. Maybe we should start from the begining -- as something else in the chain might have died.

 

[MikeS]

I replaced chip and installed a socket with a new chip. it's not working still. C9-PIN12 is still flat lined.

What about the rest (1,2,11,14)?

 

[wbochar]

What about the rest (1,2,11,14)?

1 1mhrz
2 flat
11 1mhrz
14 1mhrz

 

[dave_m]

1 1mhrz
2 flat
11 1mhrz
14 1mhrz

OK, pin 1 must actually be 8 MHz so they are OK.

Remove C9 for its socket and check to see if the C9 socket pin 12 is still at ground. If so, there is a short. Do you have a DVM or some kind ohmmeter?

 

[wbochar]

OK, pin 1 must actually be 8 MHz so they are OK.

Remove C9 for its socket and check to see if the C9 socket pin 12 is still at ground. If so, there is a short. Do you have a DVM or some kind ohmmeter?

pin 12 is now 15-21khrz without the chip
after pulling the chip out and in again I am getting pin 12 31khrz

 

[dave_m]

pin 12 is now 15-21khrz without the chip
after pulling the chip out and in again I am getting pin 12 31khrz

We may have some various intermittent shorts going on. C9-12 should be 500 KHz. You are going to have to power off and do some ohm checking.
15.625 KHz is at C5-pin 8 and 11, while the problem 500 KHz output at C9-12 goes to C5-pin9. Maybe a short between C5-pin9 and C5-pin8?

31.25 KHz is at B5-11. So start an ohm check between C9-12 and all those points. If using an old VOM, keep the selector on 1K ohm or less.

Take your time and continue taking good notes. You are getting close. Remember you are being Sherlock Holmes.

 

[wbochar]

Take your time; you will do fine. As crude as they are, the Commodore printed wiring boards are pretty tough and can handle some heat. Use a soldering iron rather than a gun if you have one. The old solder guns put out a lot of watts.

Well, I am getting all kinds of random things. I think I might just measure everything all over again and post the details. I am getting lost in this... my scope seems to crash now with 500k - 1 mghrz signals which is making things go slow. Arg. I opened a ticket with the manufacturer.

 

[dave_m]

Well, I am getting all kinds of random things. I think I might just measure everything all over again and post the details. I am getting lost in this... my scope seems to crash now with 500k - 1 mghrz signals which is making things go slow. Arg. I opened a ticket with the manufacturer.

OK, take your time. What is the brand of scope. Is it a gadget that plugs into a PC? Those are sampling scopes which usually have very limited bandwidth and are subject to 'aliasing' errors which can give wrong frequency information. Do you happen to have the specifications that came with the unit?

Don't worry, despite the apparent chaos, I think you are getting close to finding the offending chip.

 

[wbochar]

OK, take your time. What is the brand of scope. Is it a gadget that plugs into a PC? Those are sampling scopes which usually have very limited bandwidth and are subject to 'aliasing' errors which can give wrong frequency information. Do you happen to have the specifications that came with the unit?

Don't worry, despite the apparent chaos, I think you are getting close to finding the offending chip.

I've got a DSO QUAD, it's a standalone unit. I've got some firmware updates and another app that's a lot more stable.

Lets begin with, I'm a complete *Idiot* hehe. Mid testing, my Magnifier burnt out, which was LED based. I borrowed a friends, which ended up being tube based. So all my measurements were getting messed up, things that didn't have a signal started getting signals. I totally forgot about the noise tubes make. Which after banging my head against the wall I noticed I was getting different values at night.. Geez.

So here are the measurements again...

(No signal is --)

H Drive
--

C5
1: --
4: --
10: 234 khz
13: 527 khz

E2
6: 8 mhz (sometimes as high as 9.96)

C9
1: 9.96 mhrz
2: --
11: 1.17 mhrz
12: --
14: 2 mhrz

B5
9: --
12: --
14: 468 hrz

 

[KC9UDX]

Tubes don't 'make noise' any more than LEDs do...

 

[wbochar]

Tubes don't 'make noise' any more than LEDs do...

So why does this florescent lamp when connected and powered up cause noise on my scope? When I use the LED lamp there is no noise. I used and an induction wand and I can hear it. Maybe something to do with the ballast? I've always noticed weird things with Florescent based tube lights.

 

[dave_m]

So why does this florescent lamp when connected and powered up cause noise on my scope? When I use the LED lamp there is no noise. What about a tv tube? Use and an induction wand and you can hear it.

Wolf,
We have something basically wrong with the scope setup. You may have the vertical sensitivity set for measuring millivolts. If you are using a 1X problem set the level for 2 V/division. You reported C5 pins 10 and 13 measuring over 200 KHz when in reality they are the reset inputs of the flip flops and are held at +5 VDC through a pull-up resistor. So I'm thinking you are measuring noise. Make sure the scope input channel is set to DC not AC. You do not want to AC couple the input signal. Also make sure the scope is grounded properly. Is there a black alligator clip on the probe that you can tie to signal ground?

I checked the analog specification of your DSO and it has a effective bandwidth of 5 MHz. That means you will get erratic results with any signal over about 1MHz.

Also do you have a voltmeter or DVM? Can you check the places that you are not getting a signal. If the DC voltage reads 0V, the signal is a good low, > 4V, it is a good high. 2.5V means a good square wave, and 1.6V means the point is open.
-Dave

 

[KC9UDX]

So why does this florescent lamp when connected and powered up cause noise on my scope? When I use the LED lamp there is no noise. I used and an induction wand and I can hear it. Maybe something to do with the ballast? I've always noticed weird things with Florescent based tube lights.

You may have issues with certain ballasts. "Electronic" ballasts have a potential to be extremely RF noisy, "magnetic" ballasts less so.

I find the opposite that you do: I generally find modern LED light fixtures very noisy, most likely due to the integral switching power supplies.

 

[wbochar]

Wolf,
We have something basically wrong with the scope setup. You may have the vertical sensitivity set for measuring millivolts. If you are using a 1X problem set the level for 2 V/division. You reported C5 pins 10 and 13 measuring over 200 KHz when in reality they are the reset inputs of the flip flops and are held at +5 VDC through a pull-up resistor. So I'm thinking you are measuring noise. Make sure the scope input channel is set to DC not AC. You do not want to AC couple the input signal. Also make sure the scope is grounded properly. Is there a black alligator clip on the probe that you can tie to signal ground?

I checked the analog specification of your DSO and it has a effective bandwidth of 5 MHz. That means you will get erratic results with any signal over about 1MHz.

Also do you have a voltmeter or DVM? Can you check the places that you are not getting a signal. If the DC voltage reads 0V, the signal is a good low, > 4V, it is a good high. 2.5V means a good square wave, and 1.6V means the point is open.
-Dave

I have DVM, I'll try that out. I'll get back to you.

 

[wbochar]

You may have issues with certain ballasts. "Electronic" ballasts have a potential to be extremely RF noisy, "magnetic" ballasts less so.

I find the opposite that you do: I generally find modern LED light fixtures very noisy, most likely due to the integral switching power supplies.

Thanks for the insight, the LED light I have has power brick while the florescent doesn't. More likely the power circuitry is right beside the tube (attached to the magnifier), so when I use the magnifier (my eyes are going a little wonky as I get older ) and bring it in close the effect is stronger. The difference between light on and off is crazy.

 

[KC9UDX]

Thanks for the insight, the LED light I have has power brick while the florescent doesn't. More likely the power circuitry is right beside the tube (attached to the magnifier), so when I use the magnifier (my eyes are going a little wonky as I get older ) and bring it in close the effect is stronger. The difference between light on and off is crazy.

Unfortunately, things are getting more and more RF noisy every day. There is a push for smaller, lighter, faster, more efficient; and not enough people care about the RF part anymore. It is possible to achieve all those things without the extra noise, but no one seems to care enough.

 

[wbochar]

Values are in Volts

Chip	Pin	Sample 1	Sample 2	-	Chip	Pin	Sample 1	Sample 2
C5	1	0.71	0.71	-	E2	1	4.91	4.91
C5	2	3.1	3.09	-	E2	2	0.22	0.22
C5	3	1.15	1.15	-	E2	3	3.41	3.41
C5	4	3.54	3.54	-	E2	4	4.91	4.91
C5	5	0.71	0.71	-	E2	5	3.4	3.4
C5	6	3.53	3.54	-	E2	6	4.91	4.91
C5	7	0	0	-	E2	7	0	0
C5	8	4.91	4.91	-	E2	8	4.91	4.91
C5	9	4.91	4.91	-	E2	9	3.4	3.4
C5	10	2.06	2.07	-	E2	10	4.91	4.91
C5	11	4.28	4.28	-	E2	11	3.4	3.4
C5	12	4.91	4.91	-	E2	12	4.91	4.91
C5	13	2.06	2.05	-	E2	13	3.4	3.4
C5	14	1.71	1.71	-	E2	14	4.91	4.91
Chip	Pin	Sample 1	Sample 2	-	Chip	Pin	Sample 1	Sample 2
C9	1	1.42	1.34	-	B5	1	1.98	1.98
C9	2	0.17	0.17	-	B5	2	0.17	0.17
C9	3	0.17	0.17	-	B5	3	0.17	0.17
C9	4	0	0	-	B5	4	0	0
C9	5	4.92	4.92	-	B5	5	4.91	4.91
C9	6	2.15	2.1	-	B5	6	0	0
C9	7	0	0	-	B5	7	0	0
C9	8	1.93	1.92	-	B5	8	2.02	2.02
C9	9	0	0	-	B5	9	0	0
C9	10	2.05	2.05	-	B5	10	1.99	1.99
C9	11	1.93	1.92	-	B5	11	1.94	1.94
C9	12	0	0	-	B5	12	0	0
C9	13	1.87	1.84	-	B5	13	2.06	2.06
C9	14	1.94	1.93	-	B5	14	2.02	2.02

 

[MikeS]

Values are in Volts...

Wow!
There's gotta be an easier way ;-)