Troubleshooting external floppy drive on an Amiga 500

[rubeon]

Hi, Amiga experts

 

In short:

 

I may have zapped my external floppy port by connecting/disconnecting drives while the machine was powered up. That's just a guess. The external drive powers on but is not recognised, neither with the floppy drive installed nor a Gotek.  Can anyone suggest which components to check on the motherboard or the port itself?

 

This is the what the connector looks like inside the case:

 

 

I don't see any obviously damaged components, and that row of capacitors passed checks for continuity with the multimeter.

 

Context:

 

Recently I got my first Amiga 500, and was having fun getting to know the machine.  I ordered an external floppy and a Gotek drive, and was able to make a couple of Workbench and game floppies to play around with, booting from the internal drive.

 

At some point during my experimentation, the external floppy drive stopped working.  It powers up with the Amiga, but won't recognise the Gotek or real floppy drive. The Gotek works fine when connected to the internal connector, as does the mechanical drive. I've swapped the ribbon cables between internal and external as well to check those, and they both work fine on the internal but not on the external.

I sounded out all the pins from the external floppy's connector to the ribbon cable connector inside the enclosure with my multimeter, and everything goes through.

 

I've come to the conclusion that the issue must be between the port on the back and the CIA somewhere.

[Daedalus2097]

For most of the signals, you should be able to do a full continuity check between the contact on the Gotek / external floppy and the CIA. This will take the cable, connectors and everything else in between the two into account.

 

Possibly too obvious, but have you checked that the 5V supply from the external port is making it to the Gotek?

 

The two CIAs are identical, so you can troubleshoot by swapping them over and seeing if the problem moves elsewhere. The floppy functions largely correspond to the parallel port on the other CIA.

[rubeon]

3 minutes ago, Daedalus2097 said:

For most of the signals, you should be able to do a full continuity check between the contact on the Gotek / external floppy and the CIA. This will take the cable, connectors and everything else in between the two into account.

 

Possibly too obvious, but have you checked that the 5V supply from the external port is making it to the Gotek?

 

The two CIAs are identical, so you can troubleshoot by swapping them over and seeing if the problem moves elsewhere. The floppy functions largely correspond to the parallel port on the other CIA.

Thanks for the tips! I checked the 12v and 5v pins on the connector with the multimeter, and both are working.  The Gotek boots up, and also recognises when the floppy cable is turned around incorrectly, so enough pins are working for that to happen.

 

I swapped out the CIAs and problem persists; external Gotek turns on but doesn't seem to get recognised.  

 

I'll go about beeping out the signal pins end-to-end to see if there's an obvious break, thanks again!

 

[+OLD CS1]

This is pretty strange.  The 8520A CIA at U8 controls the selection lines for drives 0 through 3.  Only select 0 and 1 go to the internal port, while 1 through 3 go to the external.  What happens if you remove the internal drive?  Since select 1 goes to the internal, there could be something screwy there.  You said the mechanical drive and the Gotek work internally; what happens with the Gotek internally and connect the external drive?

[rubeon]

9 hours ago, OLD CS1 said:

This is pretty strange.  The 8520A CIA at U8 controls the selection lines for drives 0 through 3.  Only select 0 and 1 go to the internal port, while 1 through 3 go to the external.  What happens if you remove the internal drive?  Since select 1 goes to the internal, there could be something screwy there.  You said the mechanical drive and the Gotek work internally; what happens with the Gotek internally and connect the external drive?

With nothing on the internal connector and either drive on the external, I get the same misbehavior.  I also tried with a DF0/DF1 selector installed and set to boot from DF1, and that did not work.

 

With the Gotek on the internal connector and the original Amiga floppy attached to the external port, nothing changes, gotek works but external doesn't.

 

With the commodore floppy on the external port, when you pop a disc in the floppy spins it a half-revolution, but the OS doesn't seem to be polling it.

 

When I boot into Sysinfo with both drives connected, it only shows 1 drive. I don't know if Sysinfo usually reports empty drives, so it doesn't tell me if it thinks it's empty or just not there.

 

I'll need to figure out which signals do what in the steps that activate and use a floppy drive. It seems that one of them is not getting through to the OS.

 

 

[+OLD CS1]

With the exception of the EMI pass-through capacitors at the external port, there is no connection difference from the CIAs to the internal or external ports.  However, part of Gary (5719 in U5) controls the motor lines _MTR0 and _MTRX.  These are the motor control lines, 0 goes to internal port and X goes to the external port.

[JamieP]

Was there a solution to this?  I have exactly the same problem - bought a GoTek with an External drive cable and when connect it to the External drive port, no power

[SviRu]

On 10/4/2022 at 7:03 AM, JamieP said:

Was there a solution to this?  I have exactly the same problem - bought a GoTek with an External drive cable and when connect it to the External drive port, no power

A500 rev3 identical issue on mine. Internal works / external does not. Does anyone have a clue? Checked continuity from port to EMI filters. External powers on but is not recognized.

[amiman99]

Looking at the schematics, GARY chip also controls floppy functions, maybe you can replace the GARY chip. Do that if you already swapped CIA chips and did not fix your problem.

[SviRu]

8 hours ago, amiman99 said:

Looking at the schematics, GARY chip also controls floppy functions, maybe you can replace the GARY chip. Do that if you already swapped CIA chips and did not fix your problem.

Changed gary. No luck. Internal is fine external is not. CIA chips also swapped. I think it might have to do something with MTR signal or SEL signal from gary and other chips. Have no idea where to look further.

After installing DF0 DF1 selector - nothing changes. External drive is still not recognized. 

Edited October 28, 2023 by SviRu

[amiman99]

I still have on A500 motherboard (it's used for parts now) that had floppy errors and I could not fix it by swapping chips. Maybe its time to learn how floppy drive works. stuff like, how amiga knows that floppy drive is connected, when the disk is inserted, how the drive is selected, etc.

[SviRu]

Just now, amiman99 said:

I still have on A500 motherboard (it's used for parts now) that had floppy errors and I could not fix it by swapping chips. Maybe its time to learn how floppy drive works. stuff like, how amiga knows that floppy drive is connected, when the disk is inserted, how the drive is selected, etc.

it is all about SEL signals and MTR MTRX etc. 

[amiman99]

8 minutes ago, SviRu said:

it is all about SEL signals and MTR MTRX etc. 

Probably a small logic chip is bad.

[SviRu]

idk...maybe u36 u37 has something to do with this issue.

[amiman99]

U36 NAND gate, if you have logic probe, easy to check. You could use volt meter too.

U37 reset circuit, I don't know.

[SviRu]

4 minutes ago, amiman99 said:

U36 NAND gate, if you have logic probe, easy to check. You could use volt meter too.

U37 reset circuit, I don't know.

It looks like U36 does something with important MTRX MTR0 signals. I think Gary is deciding what signal to trigger. Either MTRX for external or MTR0 for internal. Going to this logic gate. Maybe indeed it is damaged. I do not have logic probe. I will use volt meter if possible.

Edited October 28, 2023 by SviRu

[SviRu]

On 10/29/2023 at 12:27 AM, amiman99 said:

U36 NAND gate, if you have logic probe, easy to check. You could use volt meter too.

U37 reset circuit, I don't know.

Problem solved. Signal SEL1 was missing on external and internal port. It was little corrosion dot on the pcb near the CIA pin  Now works perfect.

[amiman99]

8 hours ago, SviRu said:

Problem solved. Signal SEL1 was missing on external and internal port. It was little corrosion dot on the pcb near the CIA pin  Now works perfect.

Yey!

Good work.