Commodore PET 4016

[SiriusHardware]

It may come down to which one is more available, as these are both going to be fairly uncommon devices now.

Did you do the checks in #255?

[ScottishColin]

All measurements taken below with my scope.

UD3/14 - 5.15v
UD3/7 - 0v

UD3/1 - moves up to 1.13v and stays there
UD3/13 - moves up to 1.13v and stays there

UD3/5 - 4.58v but no frequency
UD3/9 - 4.58v but no frequency
UD3/2 - 4.58v but no frequency

All previous frequency measurements taken with my scope.

Colin.

Originally Posted by SiriusHardware

This is all very strange - when the video output failed you diagnosed the 74(LS)86 faulty because you had clear activity on the inputs to that IC, but now you have an apparent failure which couldn't be any further from that fault if it tried. I hope this doesn't mean there has been some sort of catastrophic wipeout going right through the machine.

It's not looking good for UD3, but just check a couple more things first.

Look for:
Supply volts on the power pins of UD3 (14=5V, 7=0V).
A logic 'high' state, on UD3 / 1 and UD3 /13
8MHz on UD3 / 5
4MHz on UD3 / 9
2MHz on UD3 / 2

I had better ask, how are you measuring these frequencies? On your original PET we got a bit sidetracked when one of your multimeters, which did have a frequency range, was happy to show 8MHz, 4MHz and 2MHz but bizarrely not 1MHz on pin 12 - until we provided the meter with a half-supply reference point to put the black lead on so the meter thought it was looking at an 'A.C.' signal. Then we found out that the 'missing' 1MHz had been there all along.

..Just in case that's what's happening again.

[ScottishColin]

£2.00 each from Cricklewood. Ordered.

Colin.

Originally Posted by SiriusHardware

It may come down to which one is more available, as these are both going to be fairly uncommon devices now.

Did you do the checks in #255?

[ScottishColin]

Would you like screenshots of my freqnecy tests to ensure I haven't got it wrong (again)?

Colin.

[SiriusHardware]

UD3/1 - moves up to 1.13v and stays there
UD3/13 - moves up to 1.13v and stays there

OH.

No, wait, they should both be at a decent logic-1 level. The chip is most likely being held in an OFF state.

I don't have time to look, does that version of the PCB have 2 x 5V regulators and if so are you getting +5V out of BOTH of them?

[ScottishColin]

So there's two different types of voltage regulator (See attached photos).

I've not tested the ones that are VR2, VR3 and VR4 before - is it simply a matter of using one of the legs that have nuts on for testing?

Colin.

[SiriusHardware]

The two 7805 regulators (barely identifiable through the rust) are the two closest to the transformer, the one furthest away from the transformer on that heatsink is the +12V regulator and the smaller black regulator in the corner is the minus voltage regulator.

Measure on the 'blank' (non-banded) ends of diodes CR6 and CR7 where you should have +5V on each of those points.

If either of them are NOT at +5V move to the other end of the relevant diode and measure the voltage at that end.

[julie_m]

When you're ready for it, here's an updated ZIP file, with both the quick and full tests:
testrom_06.zip

Contents:

• testrom_06-quick.bin (2048) -- ROM image for quick test
• testrom_06-full.bin (2048) -- Full test, with read-modify-write to screen memory
• testrom_06.6502 (35242) -- Source code

[ScottishColin]

Both read +5.12v.

Colin.

Originally Posted by SiriusHardware

The two 7805 regulators (barely identifiable through the rust) are the two closest to the transformer, the one furthest away from the transformer on that heatsink is the +12V regulator and the smaller black regulator in the corner is the minus voltage regulator.

Measure on the 'blank' (non-banded) ends of diodes CR6 and CR7 where you should have +5V on each of those points.

If either of them are NOT at +5V move to the other end of the relevant diode and measure the voltage at that end.

[SiriusHardware]

Find R28 (1K) and R2 (1K) and measure the voltage on each end of both of those resistors (use a meter, to be on the safe side).

R28 is the INIT line pull up resistor, R2 is the PULLUP_1 pull up resistor. The fact that both of those (independently pulled up) rails are reading low is what made me suspect you had lost one of the +5V rails. So this is to see if they do, or do not, have +5V on their 'hot' end.

[ScottishColin]

R2 has +5.1v at one end and +0.96v at the other.

R28 has +5.07v at one end and +0.96v at the other.

Colin.

[SiriusHardware]

So no problem with the +5V supply to the tops of those pull resistors.

I am unfortunately quite busy tonight and then away for the weekend - those two voltages which are sub- 1V should be a lot higher, you need to find out why they are so low. When it's been on for a while have a look with your thermal camera to see if any of the ICs look unusually warm.

Maybe for reassurance, swap the 4032 PCB back in and measure the voltages on those points, including UD3 pins 1 and 13, to see what they should be on a working system. I am sure they should both be up at logic-1 level and as they are not, a lot of the ICs including UD3 will currently be held in an inactive state.

[SiriusHardware]

I've just noticed something. Both times when you have measured the voltages on UD3 pins 1 / 13 and R2 and R28 'cold' ends - which are the same circuit nodes measured at a different point, the voltages were identical to within 100th of a volt. For both of those nodes to have exactly the same voltages on them is quite unlikely.. unless they are shorted together.

To rule that out, with power off, measure the resistance between UD3 pin 1 and UD3 pin 13.

[ScottishColin]

Working 4032 readings:

R2 - +4.936v one end, +4.901v the other end
R28 - +4.927v one end, +4.896v the other end

Resistance between 4016 UD3/1 and UD3/13 - 141 ohms.

Colin.

[SiriusHardware]

That resistance seems rather low to me, although not as low as I expected. The same resistance, measured, power off, between UD13 pins 1 and 13 on the 4032 PCB is...??

(When making the measurement have the red and black leads the same way around, so red to pin 13, black to pin 1 for both measurements).

It is looking as though possibly one of the ICs which has a connection to both the Pullup_1 line and the INIT line has failed, making that low resistance connection between them. UD3 is one of the ICs which is connected to both lines, so it could yet be the culprit.

INIT goes absolutely everywhere, PULLUP_1 not so much, so if you follow the PULLUP_1 line and note every IC that that goes to, then see how many of those are also connected to INIT, that may narrow it down.

[SiriusHardware]

Sorry, this may not have been clear.

(When making the measurement have the red and black leads the same way around, so red to pin 13, black to pin 1 for both measurements).

When measuring resistances of semiconductor devices you will typically get different readings with the probes one way around, then the other way around. When you measured the 141 ohms between UD3 pins 1 and 13 you had the probes a certain way around: When making the same measurement on the 4032 UD3 pins 1 and 13, measure it with the probes the same way around as they were when you measured on the 4016 UD3 pins 1 and 13.

[julie_m]

And if in doubt, measure both ways anyway; but be sure to say which is which, e.g. "1.62k? with red on IC1 pin 1 and black on IC3 pin 4, crept up from 1.1M? to OL with black on IC1 pin 1 and red on IC3 pin 4".

[ScottishColin]

OK - red probe to pin 13, black probe to pin 1 of UD3.

4016 - 131 ohms

4032 - 2k ohms

I measured with the probes the other way round and got the same results on both motherboards.

Colin.

Originally Posted by SiriusHardware

Sorry, this may not have been clear.

When measuring resistances of semiconductor devices you will typically get different readings with the probes one way around, then the other way around. When you measured the 141 ohms between UD3 pins 1 and 13 you had the probes a certain way around: When making the same measurement on the 4032 UD3 pins 1 and 13, measure it with the probes the same way around as they were when you measured on the 4016 UD3 pins 1 and 13.

[ScottishColin]

Here's the ICs that are common to both INIT and PULLUP1 as far as I can see (happy to be corrected):

UE7
UE6
UD3
UB1
UB2
UC1
UE1

UE7, UE6 and UD3 are all 74177 devices according to the BOM. All are connected to Pin 1 for PULLUP1 and pin 13 for INIT

UB1 UB2 are 74LS74 according to the BOM and UC1 and UE1 are 74S74 according to the BOM.

All of UB1, UB2, UC1 and UE1 are connected to multiple pins (up to 4) for INIT/PULLUP1 - I have the notes if needed.

Colin.

[SiriusHardware]

That's fair enough, so you can see from those differences that there is something definitely up there - the 131 ohms is almost certainly being measured through a failed chip which is connected to both of those normally pulled-up rails.

As said earlier my suggestion to try to narrow down the source of this unwanted low resistance between those lines is to follow the PULLUP_1 line wherever it goes to on the circuit, noting the IC numbers of any ICs it visits, and then look to see how many of those ICs are also connected to the INIT line. This may narrow down the range of suspects.

UD3 is one that we already know about, but can you find any others?

Edit: I see you are ahead of me. Quite a long list! I suppose it's fair to assume none of them are currently in sockets?