Re: Non-working Commodore PET 3016
 

We have what look like good clock / clear input signals to both halves of UH1 but no apparent output from UH1, either part.

Could you just verify that UH1 pins 2 and 12 both have steady high states on them? Also look for any activity on UH1 pins 5 and 9.

Re: Non-working Commodore PET 3016
 

From #1119 inputs to UH1 you should have activity on UH1-8 and UH1-6.

To be a little more certain please confirm pins 2, 4, 10 and 12 of UH1 are constant high.

Then See if there is any activity on UH1-5 and UH1-9.

Re: Non-working Commodore PET 3016
 

UH1/2 and UH1/12 both high - 5.3V.

UH1/5 - slight uptick measured as 0.6V on a meter, but no waves. UH1/9 goes high - 5.4V on the meter.

Colin.

Re: Non-working Commodore PET 3016
 

UH1 2/4/10/12 all go high and stay there.

Colin.

Re: Non-working Commodore PET 3016
 

I think its time to get the solder sucker going again.

Do you have a 74S74? It might also work with a 7474 or 74LS74, though the timing will be slightly different it would at lease confirm the 74S74 was faulty.

Re: Non-working Commodore PET 3016
 

Are we thinking dud UH1? An awkward one, 74S74. Are they available? Must be a specific need for that type as they used 74LS74 elsewhere. Crossed with Mark.

I think Colin may have a 74LS74 already, but Commodore surely used an 'S' for a specific reason.

Re: Non-working Commodore PET 3016
 

I bought one from post 807.

It's marked as SN74LS74AN.

That be ok?

Colin.

Re: Non-working Commodore PET 3016
 

I would try the 74LS74AN, maybe try and get a 74S74 later. We’ll need to keep it in mind in case of any strange issues later.

Make sure its socketed.

Re: Non-working Commodore PET 3016
 

Cricklewood have the 74S74 if I need that specifically?

Colin.

Re: Non-working Commodore PET 3016
 

So - what's the difference between a 74S74 and a 74Ls74 then? What does the L mean?

Colin.

Re: Non-working Commodore PET 3016
 

It should probably show different behaviour from the apparently dud original UH7, but best to try to source a proper replacement if possible. I'll have a look around and see if there are any historic instances of this having been replaced with other flavours. Bit of a long shot.

What's so exotic about the 'S' series? Slothie will know.

If Cricklewood have the 74S74 I am beginning to think they must be PET parts specialists, so yes, if you can get one please do.

Re: Non-working Commodore PET 3016
 

Wikipedia description of the 74LS series which suggests that the difference between 74S and 74LS is lower switching speed in 74LS, traded for lower power consumption.

Re: Non-working Commodore PET 3016
 

Ordered. Is there likely to be a problem if I pop a socket in and try the 74LS74 first?

Colin.

Re: Non-working Commodore PET 3016
 

As in a chain reaction in which all the other ICs burst into flames? Possibly not. Since the right IC is available, I personally would be Mr. super-patient and wait until it arrives, but you are free to do as you wish.

Ironically, when I gave you that short list of ICs to order just in case, I deliberately sidestepped that 74S74, hoping we wouldn't end up needing to find one.

A while ago I joked that the alternative repair method would be just to re-chip the whole PCB. It's almost heading that way. I can't imagine how this could have ended up in this state with such a high and seemingly almost random chip mortality rate.

Re: Non-working Commodore PET 3016
 

I'll get the socket in tomorrow. Probably Tuesday before the new IC turns up.

I hope none of the duff chips have been due to my ham-fisted testing. I'm still determined though.

Colin.

Re: Non-working Commodore PET 3016
 

I don't think you are responsible for anything which has happened here. The only thing we can definitely say is that the machine lived in a very damp environment for a long time, but I can't really imagine how that would cause such large scale chip death, I would usually expect that the hardware (tracks, vias) would be the first to suffer under those circumstances.

Re: Non-working Commodore PET 3016
 

The S series was 3 times as fast as LS (3nS delay vs 10nS, I think) but had 10 times the power consumption. There were other later series with different trade offs, like ALS (Advanced Low power Schottky). Maybe the high speed of S was needed in a specific location but ALS or AS are reasonable fits too if the LS doesn’t work.

Re: Non-working Commodore PET 3016
 

Thanks for the info. They obviously needed that quickness in this specific location. As the ever dependable Cricklewood seem to have the 74S, I think we may as well fit the real thing.

Re: Non-working Commodore PET 3016
 

I suspect this pet had some overvoltage or static discharge that stopped it working before it went into storage.

For the 74LS74, I don’t think we need to worry about setup time as D is held high and there looks like plenty of recovery time from reset to clock. Propagation time from clock or reset to /Q could affect timing between RAS and CAS, which could be a reliability issue for the dynamic ram.

I don’t think there is a risk of the 74LS74 causing any further damage. I’d be tempted to try it, but its not my pet.

Re: Non-working Commodore PET 3016
 

For the avoidance of doubt / fear, I don't think it will cause damage but it may cause misleading behaviour which could send us off on a wild goose chase, unnecessarily so when the right IC is actually available. If you want to try it, I'm quite happy to observe from a safe distance.

I did wonder if possibly the PSU has a mind of its own and likes to lash out every now and again - if possible maybe eventually put some high wattage 'just above rail voltage' zeners on the supply rails so that if the regulators do go doo-lally it won't do too much damage.