Atari XF551 floppy driver / power switch

[_The Doctor__]

1050 tranformer blocks are straight up center tapped step down transformers. Unless something akin to winding burning up... it won't be weaker at any age. The wire coming from it could have a break or something making poor contact etc. but other than that or a blown fuse... they are bulletproof.

Edited March 10 by _The Doctor__

[Dropcheck]

11 minutes ago, _The Doctor__ said:

1050 tranformer blocks are straight up center tapped step down transformers. Unless something akin to winding burning up... it won't be weaker at any age. The wire coming from it could have a break or something making poor contact etc. but other than that or a blown fuse... they are bulletproof.

It's the 'but other than that' that he needs to check.  I've had a 1050 power brick that looked good, but wouldn't power the drive.  Replaced the power brick with another one and it worked.  It's a quick and easy check, so it's worth a few minutes time.  

[macsonny]

On 3/10/2024 at 3:35 PM, Dropcheck said:

When you supposedly have the power switch turned off, are you getting either +5V or +12V at the drive power cable?  Of course you shouldn't have either voltage with the switch off, but the switch can be failing or my footprint is wrong. 

 

I don't see any other issues with the power circuit design itself.  It's now matching Mytek's design, so any problems would be related to soldering and possible part failure or wrong part orientation. 

 

I think you said you had assembled two boards at least to the point of the drive power cable and power leds.  I would recheck the power circuit solder flow on all parts on both the boards you have assembled.  Apply the bodge wire on the other board and see if there is any difference.  Remember that the wire needs to be thick enough to carry not just the voltage but current as well.   If one has +5V at the drive power cable and not the other then you may have a bad regulator on that board.  I would also double check the orientation of the +5V regulator in the board. 

 

If both are still well below +5V, then I would replace the +5V traco regulator with a bog standard 7805 positive TO-220 linear regulator on one board.  You can run it without a heat sink for a little while for testing purposes.  Basically we are testing the ability of the traco converter to handle lower than normal +5V input level.  There's not much margin to work with here.  A max of 18V minus 12V, minus 4/6 diode drop out voltages.  If you hit +5V with the 7805 installed, then the traco just can't step up the voltage enough to compensate for the low input voltage. 

 

The Traco regulators are nice units, but I haven't used them very much.  I don't know how they react to the possible marginal input voltage despite what the datasheet says.   

 

Let me know how it goes.

Ok. Bit of an update. I replaced the 5 VDC regulator with a standard TO-220 regulator. Same results - low power on the 5 VDC rail.

 

Tried a different power supply - same result again.

 

So I don't think it's the TACO.

 

I've attached a few photos of the board including bodge wire.

 

See anything obvious?

 

 

 

[Dropcheck]

Okay not good.  🤕

 

Let's see. 

 

Did you reflow all the solder joints on the power supply circuit?

 

Did you check if you are getting both voltages on the drive power cable connector with the power switch off?  Shouldn't get either one. 

 

Do you have another 1050 power supply to substitute?  I suspect that we just don't have enough voltage/current getting to the +5V regulator.

 

 

[Dropcheck]

Sorry missed the line on trying a different power supply. 

 

I'm not seeing anything obvious.  Let me retrace my steps in design and see if I missed something else.  Don't know how, but if I was perfect I would be only 30 years old and living in Utopia.  Not here in po-dunk Oklahoma pushing 66.

 

Mouser and Digikey seemed determined to do away with TH parts altogether as fast as they can.  Sometimes progress ain't.  I noticed that Digikey seems to have discontinued a couple of parts.  Mouser still has them for now. 

 

 

Edited March 11 by Dropcheck
added info

[_The Doctor__]

I see half empty solder and over filled joints, I see icicles. I also see dirty legs on components, that appear to have needed cleaning before soldering in. In this day and age it is not uncommon to have items come with gunk or fine layers of corosion 'patina' or other impurities. I'd suggest making sure everything it clean before using them.

since the pcb is populated already, it makes it hard to trace things out remotely.

A full corrected schematic against a clean pcb would be most helpful.

then a comparison against a fully built out and working PCB could shed some light for future build outs.

It's hard to observe all that should be, as schematic isn't exactly pcb layout in this case.

It also would make sense to build out using components that have worked or have been known to have worked from the spec or BOM first- before updating to something else.

Edited March 11 by _The Doctor__

[Dropcheck]

6 minutes ago, _The Doctor__ said:

I see half empty solder and over filled joints, I see icicles. I also see dirty legs on components, that appear to have needed cleaning before soldering in. In this day and age it is not uncommon to have items come with gunk or fine layers of corosion 'patina' or other impurities. I'd suggest making sure everything it clean before using them.

since the pcb is populated already, it makes it hard to trace things out remotely.

A full corrected schematic against a clean pcb would be most helpful.

then a comparison against a fully built out and working PCB could shed some light for future build outs.

It's hard to observe all that should be, as schematic isn't exactly pcb layout in this case.

It also would make sense to build out using components that have worked or have been known to have worked from the spec or BOM first- before updating to something else.

Substituting parts by a builder is a reality that anyone who has released their designs to the public has to deal with up to a point.  If the builder has matched the spec sheet of the original BOM items, it should work.  But should and does doesn't always match.  That's why I ask him to substitute a bog standard 7805 regulator to see if his substitute was not able to do the job.  It apparently did not correct the issue, so something other than the substitution is going on.

 

Substitution by the designer is a reality when parts go obsolete, but simple like for like form substitutions should not affect the function of the design.  Mytek's 1088XLD power supply circuit is fairly well tested, so adding the flexibility of a different power switch wired correctly should not affect the function.  His 1088XLD uses the same 1050 9VAC power supply as the XF551 original design.  The only question might be 50Hz vs 60Hz.  I don't remember which the Aussies use, but I am assuming that it works normally on a standard 1050/XF551. 

 

Soldering skills are always going to very, but perfection is not required for the board to work.  As long as he has reflowed the joints I asked him to and we don't have a parts failure or incorrect orientation, he should be getting correct voltage to the right places.  Definitely the parts legs should be clean and rust free.  That can cause a problem with voltage transmission.  The bodge wire should have corrected the only obvious error in the schematic design and it appears to have been properly installed from what I can see.  That is still a weak point, but did restore +12V that wasn't working before.  A full +5V is still missing. 

 

It would help to only assemble the board in parts, ie power supply first, test to verify correct voltages, then assemble all but track display circuit, test for function.  There's not really anything between the first assembly part and the second.  The track display circuit is not needed, but everything else is for a functioning board. 

 

Trying to troubleshoot remotely is never a fun thing, too many factors you don't control and can't visually check.  Or probe.  But that is the deal with DIY.  I'm willing to help as much as I can. 

 

 

 

 

[sup8pdct]

1st question i would ask is what voltage is going to the 5v regulator??

 

[macsonny]

Ok - some positive progress made tonight. I checked all the soldering and did a few touch ups. Most seemed ok.

 

But then just by fluke I touched the clock crystal and noticed it was incredibly warm. I removed it and spun around and hey presto - 5 VDC to the board!!!

 

I'm not sure if the clock crystal is kaput but I had a spare and dropped it in just in case. This time in the correct config. Floppy drive powered up!

 

Dropped in all the IC's being an 8050, original XF551 Atari Rom REV 7.4 and a 1772.

 

Powered on Atari and nothing. Spent about 1 hour swapping jumpers, trying different SIO ports, checking chip seating and even tried a HXPER+ ROM and Single Patch ROM. Also changed orientation of cable going from floppy to main board. Also tried changing the floppy IDs.

 

Same result - computer boots to desktop with zero connectivity to the drive.

 

I'm going to call it a night but feel at least I've made some progress. Rookie error having the crystal in the wrong way around!

 

I'm also a little suspicious of the 8050 (from China) an awaiting on a genuine 8040 to arrive. That might make a difference.

 

 

Edited March 12 by macsonny

[Dropcheck]

Glad to see some life😃.  I thought about a short later in the circuit, but didn't think very hard or long.  Fixated on the problem with input voltage instead.  😔

 

One thing to keep in mind now is that a multitude of a things maybe going on.  Because a multitude of things are going on.  Try to get methodical.  Write down what you change every time, test.  note response, if no go,  change it back.  Change something else, test, note response, no go, change it back.  I tend to get in a hurry and try several things at once.  Not good.  I quickly lose the thread of what changed and what the results were if any and end up chasing my tail for an hour or two. 

 

Do you have a way to check the clock crystal substitute for proper 8.3333MHZ speed?  If it's off by too much, the drive will appear to not see the Atari commands.  Off just a little and it may read, but can't write stably. 

 

The sad state of affairs with chips not sourced from a functioning drive in your possession is that it's very easy to get a bad one.  Especially the 1772PH chip.  If the replacement 8040 doesn't work then try another SIO cable too,  I've had them break internally after years of use. 

 

[macsonny]

21 hours ago, Dropcheck said:

Glad to see some life😃.  I thought about a short later in the circuit, but didn't think very hard or long.  Fixated on the problem with input voltage instead.  😔

 

One thing to keep in mind now is that a multitude of a things maybe going on.  Because a multitude of things are going on.  Try to get methodical.  Write down what you change every time, test.  note response, if no go,  change it back.  Change something else, test, note response, no go, change it back.  I tend to get in a hurry and try several things at once.  Not good.  I quickly lose the thread of what changed and what the results were if any and end up chasing my tail for an hour or two. 

 

Do you have a way to check the clock crystal substitute for proper 8.3333MHZ speed?  If it's off by too much, the drive will appear to not see the Atari commands.  Off just a little and it may read, but can't write stably. 

 

The sad state of affairs with chips not sourced from a functioning drive in your possession is that it's very easy to get a bad one.  Especially the 1772PH chip.  If the replacement 8040 doesn't work then try another SIO cable too,  I've had them break internally after years of use. 

 

A little more progress - kinda.

 

I was able to confirm then 1772 is ok as tested in my working XF551 and it boots fine. The 8050 however seems to be an issue as will not work in my XF551 even when the jumper was changed from 8040 to 8050 setting.

 

I'm going to wait for the 8040 to arrive and do more testing.

 

I have no way of testing the crystal substitute but I bought off a guy who sells heaps of them from Poland. I think he's reputable so I'm fairly confident the crystal is fine.

 

Last time I built up a XF551 board I go the resistor array around the wrong way. Just wanted to check that the writing side of the array should be towards the J4 connector right?

 

I'm going to check solder joints tonight and see where I get to.

 

I forgot to ask but did you ever resolve the issue where the jumpers were almost set for opposite. On the XF551 board I built from your design, to get the 8040 processor running I had to set the jumper for 8050 and also the OSC 2 jumper. Did you change this on your newer boards?

 

Edited March 13 by macsonny

[Dropcheck]

That's good.  If the WD1772PH is good, that's half the battle.  They are so hard to find nowadays.

 

The jumper setting for the 8040/8050 controversy has cleared up in the last year or two.  Normally keep the jumper setting on the 8040 side.  The only time you need to move to the other side is if you know the 8050 you have is one preprogrammed by Atari.  In that case the source drive would not normally have had an eprom installed. The 8050 has the ability of 4k storage builtin and Atari tried to reduce costs by programming the OS into that storage and eliminating the eprom chip.   Not many drives were produced that way, so the chances are slim yours is one of those. 

 

The OS jumper is simply to allow you to burn two OS's into one 2764 eprom and then choose which one you want to use.  The jumper just selects which bank of the eprom is active. 

 

The resistor array should have an indicator for pin one at one end on the lettering side and of course that should go into the only square pin hole(pin 1) on the array footprint on the board. 

 

Let me know how the new 8040 goes.

[macsonny]

On 3/13/2024 at 10:50 PM, Dropcheck said:

That's good.  If the WD1772PH is good, that's half the battle.  They are so hard to find nowadays.

 

The jumper setting for the 8040/8050 controversy has cleared up in the last year or two.  Normally keep the jumper setting on the 8040 side.  The only time you need to move to the other side is if you know the 8050 you have is one preprogrammed by Atari.  In that case the source drive would not normally have had an eprom installed. The 8050 has the ability of 4k storage builtin and Atari tried to reduce costs by programming the OS into that storage and eliminating the eprom chip.   Not many drives were produced that way, so the chances are slim yours is one of those. 

 

The OS jumper is simply to allow you to burn two OS's into one 2764 eprom and then choose which one you want to use.  The jumper just selects which bank of the eprom is active. 

 

The resistor array should have an indicator for pin one at one end on the lettering side and of course that should go into the only square pin hole(pin 1) on the array footprint on the board. 

 

Let me know how the new 8040 goes.

Ok. So a bit of an update. Two 8040's received and tested in my XF551 just to make sure they wee ok. They tested fine.

 

Tried in the board I built up with no joy. The floppy powers up and heads move but when connected to the Atari and turn on, the Atari makes a "squirting" noise and goes to desktop in about 2 seconds. Very quick.

 

I tried moving jumpers, etc. and got a bit frustrated. Decided to build up a second board.

 

So bored built. The WD1172PH, 8040 and ROM are all good as tested on XF551. Two green power LEDs. Looking good so far.

 

Turn on Atari - "squirt" noise and at ready prompt in 1-2 seconds. If I move the drive ID number I get the unit making the normal bumble bee sound when no drive is connected.

 

So I now have two completed boards with virtually the same issue.

 

What's the next testing step?

 

Oh, I'm collecting a clock crystal tester tomorrow to make sure I'm getting the right speed.

 

Edited March 29 by macsonny

[Dropcheck]

Okay if we are getting normal XF551 startup at power on when it is not connected via SIO to the computer, then that is somewhat verification that the XF551 is able to load the eprom instructions and is waiting for instructions from the computer on what to do next.  That usually means that the crystal is working enough to do the self tests.  But we might still be having an issue with the ROM chip.  What is the size of the ROM chip?  Is it an Atari original or a burned eprom?   Can you verify it in a working XF551?

 

Next step would be to troubleshoot SIO communication errors.  It can be as simple as the wrong drive ID setting on S2.  Try all four settings one by one.  Both up, both down, one up, two down, and finally one down and two up.  Normally if it's the only drive in the system, both dip switches should be down.  Is this the dip switch called for in the BOM?

 

Next is a SIO cable failure.  Verify that the SIO cable you are using is working with the good XF551 drive.  SIO cables can break internally, work with a particular bend in the cable and then when you disconnect and reconnect to another drive you disturb the physical placement of those wires and now ti doesn't. 

 

Next steps are definitely more involved.  I wanted to do the easy ones first.  

 

 

[macsonny]

9 hours ago, Dropcheck said:

Okay if we are getting normal XF551 startup at power on when it is not connected via SIO to the computer, then that is somewhat verification that the XF551 is able to load the eprom instructions and is waiting for instructions from the computer on what to do next.  That usually means that the crystal is working enough to do the self tests.  But we might still be having an issue with the ROM chip.  What is the size of the ROM chip?  Is it an Atari original or a burned eprom?   Can you verify it in a working XF551?

 

Next step would be to troubleshoot SIO communication errors.  It can be as simple as the wrong drive ID setting on S2.  Try all four settings one by one.  Both up, both down, one up, two down, and finally one down and two up.  Normally if it's the only drive in the system, both dip switches should be down.  Is this the dip switch called for in the BOM?

 

Next is a SIO cable failure.  Verify that the SIO cable you are using is working with the good XF551 drive.  SIO cables can break internally, work with a particular bend in the cable and then when you disconnect and reconnect to another drive you disturb the physical placement of those wires and now ti doesn't. 

 

Next steps are definitely more involved.  I wanted to do the easy ones first.  

 

 

The ROM is a Hyper+ and yes' it's been tested in the working XF551 so fairly sure it's not the ROM. 

 

I've played a little around with the ID setting switch but will try again today. FYI - ithere is no S2 listed in your BOM - just S1 and S3. What is the actual part number for S2?

 

I'll do a little more testing over the next few days. I know I asked this before but want to double check. PIN 1 of the resistor array is the left hand pin on the writing side of the resistor array right?

 

I've got 3 SIO cables and all exhibiting same result - so suspect it's not the cable.

 

Thanks for your help. Slowing narrowing down the issues 🙂

[Dropcheck]

1 hour ago, macsonny said:

The ROM is a Hyper+ and yes' it's been tested in the working XF551 so fairly sure it's not the ROM. 

 

I've played a little around with the ID setting switch but will try again today. FYI - ithere is no S2 listed in your BOM - just S1 and S3. What is the actual part number for S2?

 

I'll do a little more testing over the next few days. I know I asked this before but want to double check. PIN 1 of the resistor array is the left hand pin on the writing side of the resistor array right?

 

I've got 3 SIO cables and all exhibiting same result - so suspect it's not the cable.

 

Thanks for your help. Slowing narrowing down the issues 🙂

The dip switch I called S2 would be the Drive ID switch on the rear of the board.  Probably labeled S3. 

 

You are correct on the resistor array pin 1. 

 

So you've tried both OS jumper settings?

[macsonny]

10 hours ago, Dropcheck said:

The dip switch I called S2 would be the Drive ID switch on the rear of the board.  Probably labeled S3. 

 

You are correct on the resistor array pin 1. 

 

So you've tried both OS jumper settings?

So what is the part number for S2? Go the part from a different BOM here: https://au.mouser.com/ProductDetail/774-1952MST S3 is shown as the rocker power switch https://au.mouser.com/ProductDetail/611-7201J1ABE2 which is not the ID select switch. 

 

I have tried multiple settings for the DIP switch with no real change.

 

Checked the crystal oscillator and that's running perfectly at 8.3333MHz.

 

I've also changed the position of the OS jumper with no change. Have swapped the ROM from HYPER+ to an original XF551 ROM from Atari - no change.

 

Any other ideas?

 

 

Edited March 30 by macsonny

[Dropcheck]

41 minutes ago, macsonny said:

So what is the part number for S2? Go the part from a different BOM here: https://au.mouser.com/ProductDetail/774-1952MST S3 is shown as the rocker power switch https://au.mouser.com/ProductDetail/611-7201J1ABE2 which is not the ID select switch. 

 

I have tried multiple settings for the DIP switch with no real change.

 

Checked the crystal oscillator and that's running perfectly at 8.3333MHz.

 

I've also changed the position of the OS jumper with no change. Have swapped the ROM from HYPER+ to an original XF551 ROM from Atari - no change.

 

Any other ideas?

 

 

S1 is the original XF551 power switch, S3 would be the alternate.  S2 would/should be the Drive ID Switch.

 

You've indicated that

 

A.  Same Atari 8bit computer used to test both working XF551 and newly built up XF551_B v 6.0.

B.  Verified SIO cables with same Atari computer

C.  Verified 8040/8050, WD1772PH, ROMs, and OSC.

D.  Two built up XF551_B v6.0 pcbs are behaving as normal when not connected to Atari 8bit SIO port.  IE normal boot up behavior.

 

The only other easy steps would be to swap out the drive unit itself with a known good one and ohm out the data drive cable for bad connections/shorts at the connectors on both ends.  If you can double check that, that would be last easy steps to troubleshooting this problem.   I'm assuming that you are connecting +5 and +12V and GND to the drive unit's power input.  The previous pictures posted didn't show a drive power cable installed at that point.  

 

The next troubleshooting steps are pretty involved and even if they work will leave you with a bodged up pcb.  Only four SIO signals are needed from the computer to control an XF551, GND, CMD, DataIn, and DataOUT.  That would imply a bad/wrong/bad orientated part, bad solder joint or a bad routing of those four signals on the pcb I released.   If you have double checked your soldering and part orientation, that leaves bad/wrong part or bad pcb routing.  Since you've built up two boards and now both are behaving the same, that kind of leaves bad pcb routing. 

 

Since I'm going to have to re verify the XF551_B v6.0 board anyway by doing a new pcb, it's probably best that we halt troubleshooting here until that's complete.  I need to verify the misc parts I have are good here and then have the new board manufactured.  About two weeks time, give or take a few days.  If it turns out to be a pcb routing issue, I'll post you at least three, hopefully four bare boards with the corrected routing.  That should allow you to move most parts to the new boards.  I wouldn't waste my time de-soldering the sockets, just use new ones on the new board.  Even the resistors/diodes/transistors are usually fairly cheap to replace. 

[macsonny]

10 hours ago, Dropcheck said:

S1 is the original XF551 power switch, S3 would be the alternate.  S2 would/should be the Drive ID Switch.

 

You've indicated that

 

A.  Same Atari 8bit computer used to test both working XF551 and newly built up XF551_B v 6.0.

B.  Verified SIO cables with same Atari computer

C.  Verified 8040/8050, WD1772PH, ROMs, and OSC.

D.  Two built up XF551_B v6.0 pcbs are behaving as normal when not connected to Atari 8bit SIO port.  IE normal boot up behavior.

 

The only other easy steps would be to swap out the drive unit itself with a known good one and ohm out the data drive cable for bad connections/shorts at the connectors on both ends.  If you can double check that, that would be last easy steps to troubleshooting this problem.   I'm assuming that you are connecting +5 and +12V and GND to the drive unit's power input.  The previous pictures posted didn't show a drive power cable installed at that point.  

 

The next troubleshooting steps are pretty involved and even if they work will leave you with a bodged up pcb.  Only four SIO signals are needed from the computer to control an XF551, GND, CMD, DataIn, and DataOUT.  That would imply a bad/wrong/bad orientated part, bad solder joint or a bad routing of those four signals on the pcb I released.   If you have double checked your soldering and part orientation, that leaves bad/wrong part or bad pcb routing.  Since you've built up two boards and now both are behaving the same, that kind of leaves bad pcb routing. 

 

Since I'm going to have to re verify the XF551_B v6.0 board anyway by doing a new pcb, it's probably best that we halt troubleshooting here until that's complete.  I need to verify the misc parts I have are good here and then have the new board manufactured.  About two weeks time, give or take a few days.  If it turns out to be a pcb routing issue, I'll post you at least three, hopefully four bare boards with the corrected routing.  That should allow you to move most parts to the new boards.  I wouldn't waste my time de-soldering the sockets, just use new ones on the new board.  Even the resistors/diodes/transistors are usually fairly cheap to replace. 

So I'll double check my soldering later today but as you say, if I'm getting the same issue it's probably not soldering 🙂

 

FYI - I bought two seperate batches of components and I double check resistance, impedance, etc. prior to placing on the board is I'm relatively confident it's not a component issue.

 

I did double check the oscillator. They cost me a bit of money to import from Poland so super happy they proved to be ok.

 

I might hold off on further work until you've had a chance to check the board over. FYI - I put the bodge wire on the second board just in case you were wondering.

 

I'm ok with bodge wires on the board just as long as it works 🙂

 

I'll let you know how I go after a bit for checking. 

 

I look forward to seeing if you find any errors in the board that we can work on diagnosing.

 

[macsonny]

On 3/31/2024 at 11:34 AM, macsonny said:

So I'll double check my soldering later today but as you say, if I'm getting the same issue it's probably not soldering 🙂

 

FYI - I bought two seperate batches of components and I double check resistance, impedance, etc. prior to placing on the board is I'm relatively confident it's not a component issue.

 

I did double check the oscillator. They cost me a bit of money to import from Poland so super happy they proved to be ok.

 

I might hold off on further work until you've had a chance to check the board over. FYI - I put the bodge wire on the second board just in case you were wondering.

 

I'm ok with bodge wires on the board just as long as it works 🙂

 

I'll let you know how I go after a bit for checking. 

 

I look forward to seeing if you find any errors in the board that we can work on diagnosing.

 

 

I doubled checked the soldering today and also reflowed a few points. Swapped the ROMs for stock Atari v7.4, Single patch and Hyper+ which by the way all work in my current XF551 fine. Doubled checked the 8040 and WD1772PH and also moved jumper to try different combinations. I finally have changed the ID settings but I'm not 100% sure I have the right ID selector switch. Ordered a new one from MOUSER per your par number just to be sure.

 

The floppy drive is a working mechanism as tested in XF551 fine.

 

I've not populated there track display components as figured they aren't critical at this point.

 

Swapped SIO cables just to be sure also. Same results.

 

Now I'm a bit stuck. 

[+DjayBee]

Erhard on the ABBUC forum is currently also fighting with this board and stumbled over the missing connection.

 

He also found this:

U6 (EPROM) PIN26 (A13) is open, while A14 is connected to Vcc.

The open line A13 causes system crashes when using an EPROM, ROM (...) which uses at least A13 due to the memory size.

 

[sup8pdct]

Sounds like timing isn't right or sio in or out isn't quite right in some way when you get the Squirt sound

The crystal also affects sio timeing. It can be a little out but beyond a limit, strange things happen.

One thing it could be is the highest address line from the 8040 not connecting properly, unlikely but you never know.

Latest std rom is 7.7

 

If you can, boot dos with another drive, then try a status command or percom read or write block with drive you are having trouble with when drive switches are set to squirt position. use std atari rom.

 

 

[macsonny]

1 hour ago, DjayBee said:

Erhard on the ABBUC forum is currently also fighting with this board and stumbled over the missing connection.

 

He also found this:

U6 (EPROM) PIN26 (A13) is open, while A14 is connected to Vcc.

The open line A13 causes system crashes when using an EPROM, ROM (...) which uses at least A13 due to the memory size.

 

Is Erhard on this group?

 

And do you know what the correct connection for PIN26 and A14 is?

 

 

Edited April 1 by macsonny

[+DjayBee]

1 minute ago, macsonny said:

Is Erhard on this group?

 

Nope. This is why I am relaying.

Or you register on https://abbuc.de/forum/ - he speaks Englisch. He is ABBUC's floppy disk expert.

[Dropcheck]

53 minutes ago, macsonny said:

Is Erhard on this group?

 

And do you know what the correct connection for PIN26 and A14 is?

 

 

The design does not take into account using a bigger eprom size than 2764, ie two distinct XF551 OSs switchable by the A12 line using the JP2 jumper.  So pin 26 is NC per design.  If you want to use a 27128 or bigger, you will need to bend pin 26/1 out of socket and use the normal hack to allow for the bigger size eproms.  Actually pin 1 is the VPP pin.

Edited April 1 by Dropcheck
Added more info