exxos's Atari, Amiga & retro forum

andresv wrote: 09 Nov 2021 14:56 That makes sense?

Thanks in advance,

Agv

Yes that is definitely what I would do also. And yeah - if I'm not 'fully re-capping' the board, I always want to do the 4700uf at a bare minimum (as well as the reset capacitors) Please let us know if it fixes the problems.

Also there is a chance you could be having memory problems, might be a good idea to run YAART to make sure you are all good in that department.

Thanks for your comments! I also thought about the RAM and run YAART several times over a few days so I guess I'm ok on that topic.

Will do these last recaps and report back.

Again thanks!

Agv

I'd love more information on the bus pullup resistor change outlined in #12 as it pertains to C070115 Rev. 2. I've looked at one schematic https://ia800206.us.archive.org/19/item ... c_1985.pdf which is for C070231 (which looks roughly similar).
[edited]
I was fooled by some unused resistors, ultimately this is the map I sorted for this board for data and address resistors:

RP#	1	2	3	4	5	6	7	8	9	10
RP1	+5v	A2	A1	A6	A7	A5	A4	A3	?	?
RP2	+5v	?	A15	A13	A14	A8	A10	A12	A11	A9
RP3	+5v	D0	D1	D2	D3	D4	D5	D6	D7	?
RP5	+5v	A16	A17	A18	A22	A23	A20	?	A19	A21
RP6	+5v	?	D8	D9	D10	D11	D12	D13	D14	D15

All the above are 10k Ohm. RP4 (4.7k Ohm) appears to be for assorted other bus signals, do you recommend changing that as well?

Can you recommend the best place to sample bus signals, I'm assuming checking for overshoot/undershoot, rise/fall times, signal levels? Anything else? I'm wondering exactly where it is that an improvement is expected so I can look at the signals before and after and see whether what I have done has made anything better (or worse). Would the levels at the processor data and address outputs be fine as test points?

I found as well on my 520ST that my WD1772-PH 00-02 failed sometime during a ~250hr burn in with 10k pullup resistors on DAL0 through 7. Possibly it was heat maybe this is just too much current. I'm a little nervous about applying a pullup again, if I do so I might add a heatsink. It's a shame to break parts that are getting increasingly rare.

10k adds such little current it's almost nothing. Sounds more like your PSU may be the cause after such a long run time. Or the heat inside your ST has built up enough to cause the machine to malfunction.

You have to understand these machines are barely functional to start with. Swapping the CPU or ROM or any chip for another is enough to break them. Certainly heat will likely break them as well. The fixes are there to counteract all the known problems with the machines.

exxos wrote: 21 Apr 2022 11:18 You have to understand these machines are barely functional to start with.

And that, ladies and gentlemen, should be the very first words of worth written in any ST repair manual. :lol:

exxos wrote: 21 Apr 2022 11:18 10k adds such little current it's almost nothing. Sounds more like your PSU may be the cause after such a long run time. Or the heat inside your ST has built up enough to cause the machine to malfunction.

You have to understand these machines are barely functional to start with. Swapping the CPU or ROM or any chip for another is enough to break them. Certainly heat will likely break them as well. The fixes are there to counteract all the known problems with the machines.

0.5mA isn't a lot, but it's something. I'm using a modern PSU and the machine wasn't getting particularly warm. Maybe it was a freak occurrence but the chip is well and truly dead now. I don't have a thermal camera to see what things get hot in the Atari but the old finger test says the WD1772 gets hotter than most other components. :)

v836 wrote: 22 Apr 2022 14:21 I don't have a thermal camera to see what things get hot in the Atari but the old finger test says the WD1772 gets hotter than most other components. :)

The CPU would ordinarily be the hottest chip on the board. The VL1772 Does get hot, but I don't remember the WD1772 Getting particularly hot unless overclocking it etc. I doubt the pullups cause that problem anyway. That fix has been in pretty much all my machines for the past several years and indeed the standard fix on the remake motherboards which are my daily drivers now. So I don't know how you can get from installing the pullups to killing the 1772.

mrbombermillzy wrote: 21 Apr 2022 19:27

exxos wrote: 21 Apr 2022 11:18 You have to understand these machines are barely functional to start with.

And that, ladies and gentlemen, should be the very first words of worth written in any ST repair manual. :lol:

Amen to that.

v836 wrote: 20 Apr 2022 21:44 I found as well on my 520ST that my WD1772-PH 00-02 failed sometime during a ~250hr burn in with 10k pullup resistors on DAL0 through 7. Possibly it was heat maybe this is just too much current. I'm a little nervous about applying a pullup again, if I do so I might add a heatsink. It's a shame to break parts that are getting increasingly rare.

A word of warning, if I seem to be a bit blunt here, sorry, I've had a bit of a day.

My first question would be "Why on earth are are you doing a 250 hour burn in on an ST?" For what reason would you ever need one to run for that long?

These machines are home computers and 30 odd years old, and even back in the day, they would probably never run continuously for much more than a 10th of that. I expect the Atari burn in was "does it switch on and get to the desktop and switch off again afterwards".

The ICs are going to get hot when run for that long without cooling of any sort, and they were never designed to do that, they are a home computer, not an industrial machine, built to a budget and now suffering deterioration due to age.

I sincerely doubt that the 10K resistors are the reason for the WD1772 failing, with a 10K pullup and 5v the current of .5ma is negligible, I think you'd have a very long wait for those to get warm, and if they are, it is more likely thermal coupling from the WD1772.

I'd suggest the WD1772 failure was entirely thermal due to the length of time in operation and the age of the IC, which may have been marginal in the first place.

I have several machines with this mod installed and don't have any issues, but then I don't do 250 hour burn ins either...

If you want to run for that long, then you'll need some sort of active cooling, a passive heatsink just won't cut it.