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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 17th Jan 2021, 6:25 pm   #141
SiriusHardware
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Default Re: Non-working Commodore PET 3016

If your meter indicates '0' when it sees infinite resistance that is.. rather unconventional, but at least we know that now. I'm wondering now if we misunderstood you when you seemed to be saying you had 'zero resistance' between UG5 pin 7 and UG5 pin 8.

Before you go looking for another meter could you do that voltage check I asked for... UG5 pin 7? 0V? or 5V? or 2.5V?

You said quite a while ago that you thought you might be able to borrow a scope, that would be another way to verify what is happening on various signal lines and as you progress it will be more and more likely that we will need to do that, so if you can put out some feelers for one, that would be good.

Last edited by SiriusHardware; 17th Jan 2021 at 6:37 pm.
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Old 17th Jan 2021, 6:32 pm   #142
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Default Re: Non-working Commodore PET 3016

Quote:
Does this look like it might do the job?
Possibly, but try the voltage measurement first.

The QD output (pin 7) should be producing a 50/50 square wave. If you put a voltmeter on that signal, the meter will be too slow to react to the DC voltage changing between 0V and 5V a million times a second, so what it will do instead is show the average voltage, around 2.5V.

If you see 2.5V on that pin, that will be telling us that the pin is actually outputting a 50/50 waveform. If you see 0V or 5V, it isn't. While you are at it look at the DC voltages on UG5 Pins 2, 3 and 6, which we know definitely are producing 50/50 waveforms.
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Old 18th Jan 2021, 1:59 pm   #143
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Default Re: Non-working Commodore PET 3016

Hi. Post #141. When the meter is switched to resistance mode and no probes are touching anything, the reading on the screen is "0.L MΩ". That's what I meant when I said zero resistance. Apologies if this is confusing you. When I have had readings before that are non-zero, the L disappears off the screen. Unfortunately the manual does not specify anything about what the L means...

UG5 pin7 voltage reading is 2.3V

Post #142
UG5 pins
2 - 2.4V
3 - 2.3V
6 - 2.5V

Thanks.

Colin.
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Old 18th Jan 2021, 3:25 pm   #144
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Default Re: Non-working Commodore PET 3016

O.L - My guess? = 'Over Load' (the resistance exceeds the highest Mega-Ohm value which the meter can display).

Well, those readings are interesting.

As you can see you have readings of just under half of the supply volts on the three pins which we -know- are producing steady 50/50 pulse waveforms, and you are getting the same on pin 7 which -should- have a pulse waveform on it. This suggests that you may in fact have your 1MHz signal coming out of UG5 pin 7, but why your meter would refuse to see it is a complete mystery.

We need some alternative method of looking at the output from pin 7, this could be another frequency meter, a logic probe or, ideally, a scope?.
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Old 18th Jan 2021, 3:45 pm   #145
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Default Re: Non-working Commodore PET 3016

Would it be worth trying to measure the frequency on the lowest address lines of the 6502 at this point, to verify the 6502 is acting on the 1MHZ clock?
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Old 18th Jan 2021, 3:47 pm   #146
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Default Re: Non-working Commodore PET 3016

Hi Mark - how do I do that please.

Both; I'm looking at this - do either of you have any experience with one of these cheaper scopes? Thanks.

https://www.amazon.co.uk/DSO-Shell-O...&rps=1&sr=8-19
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Old 18th Jan 2021, 3:48 pm   #147
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Default Re: Non-working Commodore PET 3016

One more thing which may be absolutely nothing. I accidentally measured UG5 black Pin7 to Red pin6 and got a steady 1Mhz. is this just some happy accident and I;m actually measuring pin6 with this measurement?
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Old 18th Jan 2021, 4:27 pm   #148
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Default Re: Non-working Commodore PET 3016

Quote:
Originally Posted by ScottishColin View Post
Hi Mark - how do I do that please.

Both; I'm looking at this - do either of you have any experience with one of these cheaper scopes? Thanks.

https://www.amazon.co.uk/DSO-Shell-O...&rps=1&sr=8-19
Try measuring frequency with black on pin 1 or 21 of the 6502, and red on pin
9-A0, 10-A1, 11-A2.

Iím only looking at 6502 pinout from wikipedia as I have a problem looking at schematics posted earlier in zip files on the ipad.

Something like 6022BL might be better as it doesnít require assembly and has a 20 M sample per second.
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Old 18th Jan 2021, 4:34 pm   #149
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Default Re: Non-working Commodore PET 3016

I almost did suggest this, but it just seemed too crazy to suggest- try measuring the frequency on pin 7 with black on pin 7, red on pin 8 and see what you get then?

Mark's suggestion is a good one, I was looking to try to find some place where the 1Mhz was divided by two, in which case we could have looked at the output of that divider. I'll let Mark explain what he meant by looking at the lowest address line.

Edit: Crossed with Mark.

Quote:
Something like 6022BL might be better as it doesnít require assembly and has a 20 M sample per second.
Here, I think Mark is referring to an alternative model of scope?
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Old 18th Jan 2021, 6:19 pm   #150
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Default Re: Non-working Commodore PET 3016

I think Mark means this, a box which provides the front end electronics of an oscilloscope but uses your PC as the display / control panel.

https://www.amazon.co.uk/HANTEK-6022.../dp/B075YPZ44D

Available cheaper in other places, most likely. You can get 'Toy' oscilloscopes for about £20 upwards but for something which will serve you as an essential and versatile electronics tool for this and other projects and repairs you really need to spend a bit more.

If you can borrow a proper scope from somewhere, that's obviously the cheapest option of the lot.
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Old 18th Jan 2021, 6:26 pm   #151
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Default Re: Non-working Commodore PET 3016

re post #148

Sorry Mark but I'm being dense. When you say A0/A1/A2, I can't find them on the motherboard......

Does this look ok (6022BL) - it looks to me like it needs a PC to display the graphs?

https://www.amazon.co.uk/Analyzer-Os...0993986&sr=8-5

re post #149

If I measure black pin7 to red pin 8, I get a 0 mhz reading with a 'true' 0Ω response i.e. 000.0Ω on the screen, not 0.L
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Old 18th Jan 2021, 6:35 pm   #152
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Default Re: Non-working Commodore PET 3016

Sorry Colin, there are conflicting terms in that last sentence.

If you are measuring in frequency mode (with power on) you should see a symbol for Hz, KHz or MHz on the screen, not Ω.

If you are measuring in resistance mode (always with the power off) the reading should be in Ω, KΩ or MΩ.

You're making it sound as though there is a short between pin 7 and pin 8 after all, but if there was you wouldn't be seeing 2.3V out on pin 7 as you were earlier - unless you have a short which constantly comes and goes.
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Old 18th Jan 2021, 6:52 pm   #153
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Default Re: Non-working Commodore PET 3016

We asked you a while ago if you would try repeating the resistance measurements, especially between UG5 pins 7 and 8, with your old, original meter but I'm not sure if you did?
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Old 18th Jan 2021, 7:02 pm   #154
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Default Re: Non-working Commodore PET 3016

#152
OK - here goes and hopefully I'll make more sense.

Black pin 7-> Red pin8 UG5
Frequency Measurement - 0hz
Resistance measurement - 000.0Ω

Pin7 voltage measurement is now 2.15V steady

#153
Old meter shows 1 when probes are connected to nothing. When connected Black pin 7 Red pin8 UG5, it changes to -1 (minus 1).
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Old 18th Jan 2021, 7:10 pm   #155
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Default Re: Non-working Commodore PET 3016

And can you confirm that when making the resistance measurements there is NO power to the PCB or to UG5?

Check this by measuring the voltage between UG5 pin 8 - (black) and UG5 pin 16 (red).

You can see why we are having trouble here. Your new meter says there is a short between pin 7 and pin 8 but it also says there is 2.1V on pin 7 - impossible if there is a short to pin 8, which is 0V.

At the same time, your old meter is saying there is no short between pins 7 and 8 and that weird -1 reading is something I might expect if I was trying to measure resistance on a powered circuit (which should never be done).
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Old 18th Jan 2021, 7:51 pm   #156
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Default Re: Non-working Commodore PET 3016

I am a danger to myself sometimes. I apologise yet again for my inconsistencies.
Frequency power on, resistance power off. I'll try to remember....

Updated numbers:

Black pin 7-> Red pin8 UG5
Frequency Measurement - 0hz (power on)
Resistance measurement - 0.850.0Ω (power off)

Pin7 voltage measurement is now 2.15V steady

I have my old meter available. Before I measure, can you please take a look at the dial and let me know which setting I should be using.

Thanks very much.
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Old 18th Jan 2021, 8:19 pm   #157
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Default Re: Non-working Commodore PET 3016

Quote:
Originally Posted by ScottishColin View Post
Hi Mark - how do I do that please.

Both; I'm looking at this - do either of you have any experience with one of these cheaper scopes? Thanks.

https://www.amazon.co.uk/DSO-Shell-O...&rps=1&sr=8-19
I have one of these scopes, quite useful subject to its limitations. The most severe limitation is the upper frequency limit of 200KHz which would rule it out for most of the measurements you will need to do.
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Old 18th Jan 2021, 8:27 pm   #158
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Default Re: Non-working Commodore PET 3016

With a fixed range meter the guideline is that you choose the range whose maximum value is the closest one -above- the measurement you are expecting / hoping to see.

Checking a 1.5V battery - use the 2V DC range
Checking a 3V battery - use the 20VDC range
Checking a 9V battery - use the 20VDC range
Checking a 12V battery or supply - use the 20VDC range

For assorted resistor values:-
0Ω (a short) - use the 200 Ohm range
56Ω - use the 200Ω range
110Ω - use the 200Ω range
180Ω - use the 200Ω range
220Ω - use the 2KΩ range
560Ω - use the 2KΩ range
1.5KΩ - use the 2KΩ range

You can see how it goes. If you know what you are expecting to see, you choose the appropriate range, but if you don't see the measurement you were expecting then you dial up or down ranges until you get a meaningful reading.

In the special case where you are looking for or checking for a short (0Ω) you either use the lowest resistance range (200Ω) on the meter or you use another feature called 'continuity' in which the meter will buzz or beep if the leads are joined together either directly or via a low resistance.

When looking for voltages on that PET PCB, in most cases you will need the meter set to its 20V range.

As stated previously and acknowledged by you a couple of posts ago, all voltages to be measured when the power is ON (otherwise there is no voltage to measure!) and all resistance measurements to be made with the power OFF, otherwise the currents flowing in the circuit will confuse the resistance meter.

Your last results are still quite baffling / contradictory. If you really do have a resistance of 0.8Ω between pin 7 and 8, it would be highly unlikely for there to be 2.1V between those points as well.

Last edited by SiriusHardware; 18th Jan 2021 at 8:33 pm.
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Old 18th Jan 2021, 8:38 pm   #159
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Default Re: Non-working Commodore PET 3016

Please take a resistance reading between UG5 pin 7 and UG5 pin 8 with the old meter on 200 ohm range and of course, the system power off.

In addition you can try (with power on) Mark's suggestion of looking for activity on the microprocessor's Address 0, Address 1 and Address 2 lines using the frequency meter feature of your new meter. The pins in question are pins 9, 10, and 11 of the 6502 CPU IC.
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Old 18th Jan 2021, 9:09 pm   #160
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Default Re: Non-working Commodore PET 3016

I'm starting to doubt myself now, as I'm sure you are...

#158
IC in breadboard
Black pin7->red pin 8 UG5 - 1.50Ω
IC in socket on motherboard power off
Black pin7->red pin 8 UG5 - 1.21Ω

#159
I cannot get a reading on the old meter set at 200 Ω. At 2MΩ I get a solid reading of 1.05

With regard to pins 9/10/11 of the 6502, please can you tell me where the black probe should be?

Does any of this make any sense to you?

Your help and patience is massively appreciated.
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