Non-working Commodore PET 3016

[SiriusHardware]

Originally Posted by Timbucus

In this instance I would be tempted to snip the actual chip pin and bend it up as it is coming out anyway and retest the A10 drive levels on the buffer output.

Not a bad idea, as long as Colin is committed to replacing the IC anyway. If that was the intention you'd have to clip the pin slightly further away from the body than usual so that there was enough left attached to the chip to get a measurement from.

Copy that, Colin?

[ScottishColin]

Are all SN74LS244N ICs made the same? Cricklewood Electronics don't seem to have them but RS (for example)do.

https://uk.rs-online.com/web/p/line-...e-ics/0308332/

[SiriusHardware]

Yes, I trust the ones from RS but they do need to be 74LS and not 74HCT or something like that. Surprised that CW don't do them, they are (were) as common as muck in circuits like this.

[ScottishColin]

Originally Posted by SiriusHardware

Not a bad idea, as long as Colin is committed to replacing the IC anyway. If that was the intention you'd have to clip the pin slightly further away from the body than usual so that there was enough left attached to the chip to get a measurement from.

Copy that, Colin?

UB3 pin 16 snipped and retest all of the UB3 tests?

[Timbucus]

Originally Posted by ScottishColin

UB3 pin 16 snipped and retest all of the UB3 tests?

Yes but, you only need to retest that pin and maybe one other on the same chip to see if it affects how low the output signal can then go.

[ScottishColin]

Originally Posted by SiriusHardware

Yes, I trust the ones from RS but they do need to be 74LS and not 74HCT or something like that. Surprised that CW don't do them, they are (were) as common as muck in circuits like this.

I'll order tomorrow. Thanks.

[ScottishColin]

Originally Posted by Timbucus

Yes but, you only need to retest that pin and maybe one other on the same chip to see if it affects how low the output signal can then go.

Ta. Tomorrow.

[SiriusHardware]

Clip UB3 pin 16 and see if what's left of UB3 pin 16 (on the chip) then puts out a full size waveform like the others do or still only puts out that small waveform. (Crossed with Tim).

[Timbucus]

Silicon-ark also list them - they are my secret stash for reliable chips especially when you need specific manufacturers.

[Timbucus]

Originally Posted by SiriusHardware

Yes, I trust the ones from RS but they do need to be 74LS and not 74HCT or something like that. Surprised that CW don't do them, they are (were) as common as muck in circuits like this.

Maybe not so common now if they have this failure as a common event....

[SiriusHardware]

Maybe so.

Anyway, reason to be cheerful. It looks like Colin is onto a definite fault which we can chase after and with any luck it will be the buffer output which has the hard job of driving everything else which has failed, rather than one of the inputs driven by it.

[julie_m]

All the logic 0s on the green (output?) side look a bit suspect, but there is a definite smoking gun on page 6. Probing the stump of pin 16 still attached to UB3 will tell you if the buffer is alive or dead; but note that if it wasn't just old age, whatever killed it might be lying in wait for another victim .....

[Mark1960]

It might also be worth measuring resistance of A10 to ground after the buffer, before snipping the lead, and compare with the other buffered address lines to ground. Measuring before snipping would be a like for like comparison.

I like the idea of cutting the pin and see if its the buffer or something else pulling it down. It might even be possible to reconnect if the fault is found somewhere else.

I still think we should be trying to find out why there is nothing on the display. Maybe with the 6502 fitted and data bus disconnected.

[SiriusHardware]

Yes, all in good time

We can only ask Colin to do things one step at a time. The problem we had before was that there were relatively few waveforms to follow around the system because the CPU was going numb. Colin has made a couple of good breakthroughs this weekend, it will get there eventually.

If we had the machine in front of us we would now be racing ahead with one instinctive check after another based on our years of experience, but as we are doing this by 'remote access' we have to take it one slow step at a time and wait for results before taking each logical next step.

After the address buffers have been looked at and replaced, and assuming that doesn't miraculously fix the whole thing, we should probably at least do scope and resistance checks on both sides of the data bus buffers as Julie suggested.

[SiriusHardware]

First task for for today, then: Before clipping UB3 pin 16 to see what effect that has, could you please do resistance checks from UB3 pins 3, 5, 7, 9, 12, 14, 16, 18 ... and 0V, and from the same pins to +5V, all with the power off. What we are hoping you will see will be different resistance readings on pin 16, with all the rest fairly similar to each other.

After that, clip pin 16 - leave just enough on the IC so you can get a scope probe onto it, power up, still with the databus pins disconnnected, and scope CPU pin 19 and UB3 pin 16 (on the chip) to see if you still have that big difference between them.

Also scope at least one or two of the other CPU / UB3 pin pairs (as you did earlier) to see if clipping pin 16 has improved the 0V output levels on the other outputs of UB3.

[ScottishColin]

Originally Posted by SiriusHardware

First task for for today, then: Before clipping UB3 pin 16 to see what effect that has, could you please do resistance checks from UB3 pins 3, 5, 7, 9, 12, 14, 16, 18 ... and 0V, and from the same pins to +5V, all with the power off. What we are hoping you will see will be different resistance readings on pin 16, with all the rest fairly similar to each other.

After that, clip pin 16 - leave just enough on the IC so you can get a scope probe onto it, power up, still with the databus pins disconnnected, and scope CPU pin 19 and UB3 pin 16 (on the chip) to see if you still have that big difference between them.

Also scope at least one or two of the other CPU / UB3 pin pairs (as you did earlier) to see if clipping pin 16 has improved the 0V output levels on the other outputs of UB3.

Sorry again - pin 8 of the 6502 for the 5V?

And black and red probes at which end of the testing?

Thanks.

[SiriusHardware]

The +5V rail goes to the positive supply pin of every IC, it's carried by that thick track you see running past the 'notched' end of all the ICs. You could use pin 20 of UB3 as the place to pick up the +5V rail but you said you were having some difficulty getting good contact on the buffer ICs, so for now, use pin 8 of the CPU for your +5V point.

By convention, we'll say that when measuring resistance to 0V, always put the black lead on 0V and the red lead on the pins of interest. When measuring the resistance from +5V to the pins of interest, always put the red lead on +5V and the black lead on the pins of interest.

This convention is mainly to make sure that each measurement is made with a known polarity, since we already know that you will get different readings if you switch the probes around. It's just to make sure every measurement is done in a consistent way so that any variations you see are actual variations, and not just due to the measurements being done in different ways.

[ScottishColin]

Originally Posted by SiriusHardware

The +5V rail goes to the positive supply pin of every IC, it's carried by that thick track you see running past the 'notched' end of all the ICs. You could use pin 20 of UB3 as the place to pick up the +5V rail but you said you were having some difficulty getting good contact on the buffer ICs, so for now, use pin 8 of the CPU for your +5V point.

By convention, we'll say that when measuring resistance to 0V, always put the black lead on 0V and the red lead on the pins of interest. When measuring the resistance from +5V to the pins of interest, always put the red lead on +5V and the black lead on the pins of interest.

This convention is mainly to make sure that each measurement is made with a known polarity, since we already know that you will get different readings if you switch the probes around. It's just to make sure every measurement is done in a consistent way so that any variations you see are actual variations, and not just due to the measurements being done in different ways.

Thanks for the above - I'm learning all the time.

I measured the pins above on both UB3 and UC3.

Neither IC gave any result on any meter setting while testing to 0V.

Next test is for red probe on pin 8 of the 6502.

The first reading below is for UB3, the second for UC3. Meter set at 20K.

Pin 3 - 2.81 / 2.87
Pin 5 - 2.65 / 2.87
Pin 7 - 2.40 / 2.89
Pin 9 - 2.57 / 2.92

Pin 12 - 2.93 / 2.31
Pin 14 - 2.93 / 2.88
Pin 16 - 2.80 / 2.89
Pin 18 - 2.81 / 2.87

Are those variations/low readings significant?

I'll get on with Pin 16/UB3 measuring/snipping/measuring this afternoon.

Colin.

[SiriusHardware]

Originally Posted by ScottishColin

Are those variations/low readings significant?

Not as much as we would have hoped, but thanks for doing them anyway. Faults on internal elements of ICs don't always show up as differences in resistances on the pins. All we can usually say is that if one pin or line does look radically different to the others then it probably is faulty.

Conversely, to have all the readings from similar elements in an IC look the same in a resistance reading does not guarantee that the IC is OK, a fault can be buried 'in the middle' of an IC where it does not have a measurable effect on the measurements at the pins. So the IC could still be faulty.

Next job is to snip UB3 pin 16 and repeat the comparison with 6502 pin 19 / UB3 pin 16 to see if disconnecting the pin from the BA10 line makes any difference to its output, and then also do a comparative check on several other pairs of the UB3 pins to see if clipping pin 16 has changed the look of the output from other pins of the buffer.

[SiriusHardware]

Oh, hang on, just realised you took the second lot of readings from UC3. We need pins 12/14/16/18 of UB3 measured as well.