A310 unable to access FDD

[nicf82]

I have an A310 machine here which I thought was working fine! Just opened it up, chucked out the old batteries and gave the contacts a clean and replaced, DEL-power on and everything else looked great!

It had no FDD. So I took one out of a donor, and when I tried to read a disc, the FDD LED lit up and stayed on. The machine became unresponsive but the mouse still moved ok, a reboot fixed it. I tried another FDD and the same thing happened

So I got out the oscilloscope and noticed that when I probed pin 1 (CS) of the 1772-02, I saw a decaying voltage followed by the machine crashing. My theory is that either IC46 or the 1772-02 are goosed and have such a high impedance that the probe is drawing the current? Not sure if that makes much sense but maybe somebody can tell me if that's possible!

Anyway I got a logic analyzer as I wanted to check IC46 in case that was at fault.

I attached probes to 1 (/PRE) and 2 (/PWE) and 3 I wanted to see if 3 was driven HIGH when either input went LOW. It didn't it was just permanently HIGH. So I tried 9 (/S1) and 8 (since 10 is 5V), again 8 is permanently HIGH.

I did also try attaching pin 6, the output to the 1772-02's /CS expecting the machine to crash like it did when I put the oscilloscope on there but it didn't, it just stayed HIGH too.

I am going to break out these pins to a breadboard and replicate the function of the IC46 NAND chip using a 74LS00N I have here and see if it works as it should. Probably not necessary, but I am just learning here and want to make totally sure I don't pull out a good chip.

Cheers!
Nic

[nicf82]

So I ended up pulling out the old IC46 74HC00 and replacing with a new 74LS00, and some progress at least. The machine no longer crashes when I try to read a floppy disk, the FDD LED illuminates and then i get a `Drive Empty` dialog. The machine keeps running properly though and I am able to shutdown cleanly etc.

Will hopefully get another look at it tomorrow and see what is going on with the FDC.

Cheers!
Nic

[nicf82]

I have had a chance to do a bit more investigation on this and may have found something.

It seems as though IC29, the flip flop that controls the FDD (selection and mechanics I gather), may not be quite right. It seems like the INUSE signal that lights up the LED is working ok, but the MOTORON is not being set correctly.

The trace below shows that although /INUSE takes the value of BD6 when it is triggerd by the CLK, /MOTORON does not seem to be getting set LOW. I find it a bit strange that the Flip Flop would be 'partly' working, but looking at the schematic, there is not anything else I can find that could be pulling the MOTORON line up.

Trace

Relevant part of the schematic

Any suggestions much appreciated, I am thinking pull out IC29 and replace it?

Cheers!
Nic

[nicf82]

Replacing IC29 does seem to have solved the immediate problem, so it was just the MOTORON Flip-Flop (6D->6Q) not working while the others were! The disc is now spinning and making the right noises, but it wont format a disc. I have tried 2 drives (one I trust less than the other!) and neither work. The next common denominator I guess would be the FDC so I will probe what it is doing on the WRITE pins while the format operation is running next...

[nicf82]

Probing the FDC read and write pins showed that the chip was working, so I went back to experimenting with the 2 drives. I noticed that I stated to get a slightly different error from one of the drives which made me focus more on the drives themselves. To cut a long story short one drive is broken, and the other needed a good clean and the worm gear greased and now it is working perfectly!

As for the other one, I am pretty sure I broke it a while back trying to clean it and it will need a replacement head, if such a thing exists! I will keep my eyes peeled for a broken F63W drive on the auction sites.

All in all though the A310 is now working perfectly

[nicf82]

I have now got another A310 showing very similar symptoms to the one earlier in the thread! This one was really very rusty when I opened the case but have been amazed at how well the board has worked. It was only when I thought it was all good and i reassembled everything that I put an FDD in and it would not work!

Although this one is without the crashing behavior of the earlier one, it does not activate the FDDs' LED before returning a "Drive Empty" dialog. So maybe not quite as bad as the last one.

Since the last procedure worked I have decided to follow it again, so analysing the interesting pins on the 74HC00 NAND at IC46 I am getting the following:

IC46 Logic

74HC00 NAND at IC46

I'm reading this as it is all OK, since following the 70HC00's truth table, its outputs will be H unless both inputs are H. I can see here that only when pins 8 and 3 go high, does 6 go low. That should enable the 1772-02 FDC correctly. I will have a look at the logic on the other relevant chips next

[nicf82]

Looking at IC29 now, I can see that the INUSE (FDD LED) and MOTORON lines are not being driven correctly, they stay High despite BD6 and BD5 both being Low as the CLK ticks back to High. I will try replacing IC29 and re-test.

IC29 Analysis

IC29 Diagram

[nicf82]

Hmm replaced IC29 now getting a red screen on boot with

VIDC Sirq (Sound Interrupt) timing failed
VIDC Virq (Video Interrupt) timing failed
IOC register test failed
CMOS RAM checksum error
Long memory test performed
Self-test due to power-on



Edit: I have checked the I2C pins 5 and 6 on the Clock Chip and they appear to be putting out some binary data right after a reset, so I think that is OK.

Edit: I managed to track the problem down to a broken track on the BD7 line, after fixing it the machine is booting OK But accessing the FDD just resulted in a grinding, so seems maybe the drive is bad. Tried another drive which I know at least tries to work (it has a misaligned head) but with this drive in, the machine crashes when trying to access a disk. Deja vu from the previous A310 in this thread again!

I'll have to put this away for tonight and look again soon.

[nicf82]

I have been doing a bit more investigation into this, and by probing the floppy disc interface cable, I can see that the STEP signal is never activated and this seems to be the same at the FDC (before the IC38 inverter).

FDC and IC30

I don't not want to believe that the FDC is broken because it sounds expensive to replace so to exhaust all other options I turned to IC30 which seems to control only DDEN (double density selection) and MR (master reset) of the FDC. I am wondering if MR is what would normally cause the drive to step back to TRACK0? Or maybe it just needs to be called to init the FDC?

I probed MR, from switching on the machine all the way through to trying to access a disc and it is never brought low. To compare I have done the same on my working A310 and I can see that it does bring MR low during boot, which makes me wonder if IC30 could be broken.

The good A310 during boot

The BAD A310 during boot

EDIT: I have just replaced IC30, it does now appear that the MR line is being pulled low at boot

The BAD A310 during boot (after replacing IC30)

But, I still cant read a disk same symptoms of before, drive motor and LED come on, but no movement on the stepper motor and the machine just hangs although the mouse cursor still moves

I have also just tried this FDD in the good A310 and was able to read write and format a disk ok. Starting to run out of ideas, other than replacing the FDC which is a last resort!

[TimoHartong]

I had a bit of strange issues with an A310 FD as well. Replacing IC31 ( 74HCT138 ) did the trick for me.

[nicf82]

Yes I think I have managed to eliminate that one in this case, since it drives IC29 and IC30 and they both seem OK now. I am tying to get ahold of a VL1772 to try in there, I think it is the only thing it could be at this stage