exxos's Atari, Amiga & retro forum

@stween I see you got some chips from my store due to the QC lable.. Though where did the other chips come from ?

exxos wrote: 07 Jan 2023 19:02 @stween I see you got some chips from my store due to the QC lable.. Though where did the other chips come from ?

I think almost all from the store.

Possibly a couple borrowed from a very old but working STF, but I wanted to keep as many working systems running as possible. I definitely bought (for example) an MFP, sound chip, MC68B50 chips from your store, even though those in the photos don't have the sticker. It's possible they were mixed into other systems by the time I got started on this project. I definitely had a couple of spare DMA chips lying around, some good some bad. SIMMs sourced from ebay.

So after extensive testing I understand why the IPL0, IPL1, IPL2, BERR, VPA, VMA, RESET, HALT, BGACK, BR and DTACK are reading as 1K4 to GND

In the diagram below let us take the following circuit for DTACK. I am seeing 1K4 on the DTACK line as there is 440R between 5V and GND.


I have the whole DTACK line not connected to anything apart from the 1K pull-up and I get the 440R. When I remove the pull-up the DTACK line is OPEN to GND so it's not the DTACK line.

With the main chips removed the board should be OPEN between 5V and GND as tested on a working board with the chips removed. All power rail caps have been checked and the whole board checked for and shorts etc.

I can only assume now that one or more than one SMT chips left on the board is fried and is producing the 440R between 5V and GND.

Icky wrote: 07 Jan 2023 23:25 So after extensive testing I understand why the IPL0, IPL1, IPL2, BERR, VPA, VMA, RESET, HALT, BGACK, BR and DTACK are reading as 1K4 to GND

I think it would be worth testing the actual voltages on those signals. With no main chips on the board, they all should be pulled high to 5V. Even with odd resistances on the 5V to 0V , that resistance would technically not be there as the 5V rail would still be 5V.

The odd resistance I think doesn't explain why only those few signals. I would assume all signals with a pullup to 5V would also measure wrong. Like the whole address and databus etc.

As we have conflicting results at this point in time, I can only assume the board has more than 1 fault. while 450R/600R/1.4k seems odd, It could just mean the DAC is blown for example. It shouldn't stop the machine from booting up. Only the reset circuit and 244/373 chips are left on the board which could cause that.

So what do we think? Is my H5 destined to be a cool looking ornament? :D

stween wrote: 18 Jan 2023 02:36 So what do we think? Is my H5 destined to be a cool looking ornament? :D

I think @Icky found the reset chip was faulty but I don't think hes had chance to do more testing yet.

Indeed, I found that IC6 as part of the RESET circuit was acting strangely. I removed and replaced and the odd impedance issue has gone away.

I just need to find time to build it all back up from the dissection I performed.

Interesting! I can confidently say I would never have tracked down something like this :D

There's no rush from me in putting it back together (I won't be able to collect it until March even if you posted it back tomorrow). I'd simply wondered if the silence on the thread meant it was game over!

stween wrote: 18 Jan 2023 21:22 I'd simply wondered if the silence on the thread meant it was game over!

Nope :) There is lots going on in the background. We will get it sorted , just takes time :)

exxos wrote: 19 Jan 2023 00:53

stween wrote: 18 Jan 2023 21:22 I'd simply wondered if the silence on the thread meant it was game over!

Nope :) There is lots going on in the background. We will get it sorted , just takes time :)

ack! I'd love to read a walkthrough/post-mortem once it's done and done. :)