TRS-80 4-p repair help

[Zion]

I expect the dip shunt to be set the same. In TRS-80 Model 1, 3, 4 4P, 4D, and CoCo, Tandy did the drive select in the ribbon cable connected to the drives ( removed the pins for the other drive selects), making each cable connector unique. It's possible the seek error occurs when there is a dead or missing floppy (I've not run through all possible scenarios, and I do not have a working 4P apart right now).
Neither drive in this setup has a terminator. I think the 4 series added termination/pullup at the main board (a little wrong, but simplifies things. This is an external pair of TM50s I had available).
You'll notice the cable connector is missing pins, that's how drive select is managed. (yours should be similar)

None of my three drives have a terminator, resistor, I believe you are correct. That is how my ribbon cable is as well. I also have tested the voltage going to the floppy drive and it is 11.86 and 4.82 which is within spec.

I powered on the 4-p without the drives attached and got the same message. I am leaning to thinking this is a problem with the 4-p not the drives.

Here is what I know

Ribbon cables selects the drives
Termination resistor is on main board?
Voltage going to drives is good
Both drives heads spin
The drives LED will light up for a fraction of a second before going dark
Drives will not move their heads at all
After the old drive shorted it will only show “seek error” when turned on. Unless I do a different command. (Memory test ect…)

Thoughts on this? I could be completely wrong in my reasoning here.

What else should I be checking to narrow it down?

My next steps
- replace the bad caps when my new ones come in
- install the gotek when the cables come in (thanks TUC)

 

[ldkraemer]

OK, Let's see if the drives will step back to the Home Position. This will require the
Write Signal to the FDC to get the Drives to step. Since they are already at the
Outward most position you will need to physically move them.

With Power off, quickly remove the Logic board(s) from the Floppy Drive enough
that you can move the head carriage assembly 3/4 the way towards the center hub
for both drives. Then, replace the Logic board, and any other cable(s) you have
removed.

Then Turn on Power and see if you her the Floppy's stepping to the outward most position.
Just don't turn Power ON or OFF with a floppy Inserted. Power ON, then Insert the floppy.
In this case you won't be required to insert a Floppy.

If they stepped to the Home position every thing is good for stepping to the HOME position.



Larry

 

[Zion]

OK, Let's see if the drives will step back to the Home Position. This will require the
Write Signal to the FDC to get the Drives to step. Since they are already at the
Outward most position you will need to physically move them.

With Power off, quickly remove the Logic board(s) from the Floppy Drive enough
that you can move the head carriage assembly 3/4 the way towards the center hub
for both drives. Then, replace the Logic board, and any other cable(s) you have
removed.

Then Turn on Power and see if you her the Floppy's stepping to the outward most position.
Just don't turn Power ON or OFF with a floppy Inserted. Power ON, then Insert the floppy.
In this case you won't be required to insert a Floppy.

If they stepped to the Home position every thing is good for stepping to the HOME position.



Larry

Thank you! I will do that,

 

[ldkraemer]

Zion,
In the Photo IMG_8353.jpg the Top FDC Connector is for Drive 0, and the Bottom one is for
Drive 1. And the floppy drive(s) do not require the Terminator, since the Motherboard has
the 150 OHM Terminators Installed. The Cable has specific Pins removed to Select Drive 0 or
Drive 1 when they are plugged into the Floppy Drive.

I downloaded the TRS-80 Model 4P Manual to my PCloud Account.

https://u.pcloud.link/publink/show?code=kZQPlFJZz5uvzcpQHgFUcYVVDl18y7Rd4QC7


Larry

 

[Zion]

Zion,
In the Photo IMG_8353.jpg the Top FDC Connector is for Drive 0, and the Bottom one is for
Drive 1. And the floppy drive(s) do not require the Terminator, since the Motherboard has
the 150 OHM Terminators Installed. The Cable has specific Pins removed to Select Drive 0 or
Drive 1 when they are plugged into the Floppy Drive.

I downloaded the TRS-80 Model 4P Manual to my PCloud Account.

https://u.pcloud.link/publink/show?code=kZQPlFJZz5uvzcpQHgFUcYVVDl18y7Rd4QC7


Larry

Thank you very much, Larry,

That is going to be very helpful with diagnosis. My current next steps are to hook up the flash floppy to my TRS-80 , I got the card edge connector, and the power from TUC, they already came in! I actually did a unboxing video on my channel just for fun.
I am currently working on wiring my own USB cable in to reprogram the flash floppy. This week I am very busy, hopefully I’ll get some time to work on it. I also for some reason decided to pick up an apple //e this Monday.

 

[Zion]

Update

I replaced the bad capacitor on the original drive board; I checked the other two caps, and they were good. Still the same symptoms (seek error)

I tried to reflash the Gotek with flash floppy firmware, but couldn't get it to work, so I plugged it into the trs 80 to see if it was broken, and it has no signs of life. I checked that power is going to the board, and it has power but no light or anything; pretty sure it is dead. Will do more diagnostics in the future

Next steps
- buy a frehd from Tuc (before this, it would be great to know if the same control path runs both the floppy and hard drive)
- I don't really know; I could buy another Gotek, probably will at some point, but I don't see how that would be the problem.
- I need to find out what would be damaged by the short on the floppy drives; it used to show "floppy drives not ready" and other messages.

I want to thank everybody for all the help! Please share any advice and help in the future - it will be gratefully recieved!

 

[gary245]

If you buy another gotek, get one with the oLED. it can provide helpful diagnostic info. You set drive select correctly on the gotek?
FreHD uses the 50pin connector on the back of the computer. Gotek uses the floppy drive cable, both require a 2nd power connector.
The floppy you replaced the capacitor on no longer loads down the power supply? Of course connecting the ribbon cable upside down makes the drive select continuously & erases any diskette in the drive at the time.
Tantalum cap short on the power supply usually holds down the power supply. cap failures elsewhere on the drive cause other problems, depending on what circuit they are part of.
Properly connected drive 0 should light up for a few seconds ( and drive 1 dark) on power up. If they both light up for only a split second, that's power up, they probably do the same thing if only the power is connected(no ribbon cable) during power up.

 

[Istarian]

@Zion While it is possible that your Gotek device is kaput, the reality is that "no signs" of life doesn't necessarily mean that much.

Many modern electronic devices simply do not include much in the way of diagnostics or clear status indictators. They either work properly for the intended purpose, have a minor issue, or present as a useless brick. A single toasted MOSFET can make a $1000 laptop appear to be stone cold dead.

I think the primary reason is that so few devices these days have any analog functionality, moving parts, or lights/sound that directly correlate with their function.

With respect to power/electricity you often need to verify that it is making to the actual components. The power may be good right up until the actuals device... and it could be okay but never reach the MCU --- And even when power gets everywhere it needs to, a failed component in the signal path can easily prevent signals from getting where they need to.

No light might mean that the LED has burned out or that power isn't actually being applied to it. If the LED is controlled by the MCU/uC, then it may never be illuminated if the programming is borked or something causes it to halt execution.

 

[ldkraemer]

Another reason to start with some know hardware, and see if it will step the floppy's
Head Carriage Assembly back to the home position. Simple test, and if it works, we
know it might also work in the other direction.

OK, Let's see if the drives will step back to the Home Position. This will require the
Write Signal to the FDC to get the Drives to step. Since they are already at the
Outward most position you will need to physically move them.

With Power off, quickly remove the Logic board(s) from the Floppy Drive enough
that you can move the head carriage assembly 3/4 the way towards the center hub
for both drives. Then, replace the Logic board, and any other cable(s) you have
removed.

Then Turn on Power and see if you hear the Floppy's stepping to the outward most position.

Larry

 

[Zion]

@Zion While it is possible that your Gotek device is kaput, the reality is that "no signs" of life doesn't necessarily mean that much.

Many modern electronic devices simply do not include much in the way of diagnostics or clear status indictators. They either work properly for the intended purpose, have a minor issue, or present as a useless brick. A single toasted MOSFET can make a $1000 laptop appear to be stone cold dead.

I think the primary reason is that so few devices these days have any analog functionality, moving parts, or lights/sound that directly correlate with their function.

With respect to power/electricity you often need to verify that it is making to the actual components. The power may be good right up until the actuals device... and it could be okay but never reach the MCU --- And even when power gets everywhere it needs to, a failed component in the signal path can easily prevent signals from getting where they need to.

No light might mean that the LED has burned out or that power isn't actually being applied to it. If the LED is controlled by the MCU/uC, then it may never be illuminated if the programming is borked or something causes it to halt execution.

Thank you. I will keep on trying to troubleshoot it but my time is limited right now. and my skills at newer hardware are sadly lacking.

 

[Zion]

Another reason to start with some know hardware, and see if it will step the floppy's
Head Carriage Assembly back to the home position. Simple test, and if it works, we
know it might also work in the other direction.



Larry

The floppy drive I got was a tested and working drive from the seller, I don't have another way to test drives, only other computers I have are the apple2 and Commodore 64. ( unless you count an oddesy 2). all three drives do the exact same thing when plugged into either of the two spots. spin up, don't show light, don't step or move heads at all, and throw the seek error (although this happens when no drives are attched or not, ever since the short)

 

[Zion]

Did some probing, pin 20 on the shugart is supposed to drive stepper, and it is holding steady at .25 volts
also any id on this card? i assumed it was an internal modem card but wanted any more info on it,

 

[Zion]

Probed the FD1793 chip (U13) and the voltage going to is good, 5v to the pin fifteen, this should be the step line, then it apears to go though some other logic chips before going to pin 20 on the shugart.

 

[ldkraemer]

Yes, it's a Modem card with the Line connections.

If Pin 20 is ALWAYS LOW there is something wrong with that signal. If you Turn the Model 4P ON the
STEP Line Pin 20 should be HIGH (+5VDC). It should only PULSE LOW for a STEP Signal.
I've attached that section of the 1793 FDC.

Double check what you are seeing on Pin 20. If it is low on Power UP, then Power OFF and REMOVE
the 1793 FDC, and turn ON the Model 4P and test Pin 20 again. It should be at +5 VDC. Same for
Pins 18 and 24.


Larry

 

[Istarian]

Did some probing, pin 20 on the shugart is supposed to drive stepper, and it is holding steady at .25 volts
also any id on this card? i assumed it was an internal modem card but wanted any more info on it,

The MC1488P is an "RS-232C quad line driver" and the MC1489P is "an RS-232C quad line receiver". You may occasionally see EIA-232 as well, which naming references the standards body. The MC14412 is a CMOS LSI circuit and the datasheet calls it a "Univeral Low Speed Modem (0-600 bps)". So, you got lots of clues right there even without the telephone jacks and the DB-25 which is probably carries RS-232 serial.

P.S.

Electronic Industries Alliance - Wikipedia

en.wikipedia.org

^ this EIA, which is defunct as a organized body
^ aka 'Electronic Industries Assoication' (pre-1997)

 

[Zion]

Yes, it's a Modem card with the Line connections.

If Pin 20 is ALWAYS LOW there is something wrong with that signal. If you Turn the Model 4P ON the
STEP Line Pin 20 should be HIGH (+5VDC). It should only PULSE LOW for a STEP Signal.
I've attached that section of the 1793 FDC.

Double check what you are seeing on Pin 20. If it is low on Power UP, then Power OFF and REMOVE
the 1793 FDC, and turn ON the Model 4P and test Pin 20 again. It should be at +5 VDC. Same for
Pins 18 and 24.


Larry

Thank you, Larry! Got some information not sure how helpful.
With the U 13 chip removed, all of the lines that should read 5 V read .25 V - previously pin 24 was reading 4.7 pin and pin 18 was 4.7 and pin 20 was .25.
Now all three are .25

 

[Zion]

Going into the U 56 chip. Pin one has continuity with Pin 15 on the U 13 chip. Pin has continuity with Pin 20 on the shugart cable output. U 13 is inputting 4.56 into
Pin one. However, Pin two is only outputting a quarter of a volt. That is what I read at Pin 20 of the shugart cable.
Thanks

 

[Zion]

The MC1488P is an "RS-232C quad line driver" and the MC1489P is "an RS-232C quad line receiver". You may occasionally see EIA-232 as well, which naming references the standards body. The MC14412 is a CMOS LSI circuit and the datasheet calls it a "Univeral Low Speed Modem (0-600 bps)". So, you got lots of clues right there even without the telephone jacks and the DB-25 which is probably carries RS-232 serial.

P.S.

Electronic Industries Alliance - Wikipedia

en.wikipedia.org

^ this EIA, which is defunct as a organized body
^ aka 'Electronic Industries Assoication' (pre-1997)

Thank you! Wow!

 

[ldkraemer]

OK, with U13 REMOVED, Pin 20 is GROUND and Pin 21 should be +5.00 VDC. See what voltage is
on Pin 21. There must be something pulling down the +5 VDC. If Pin 21 is not +5 VDC.
Disconnect the Power Plugs to both floppy drives. And disconnect the 34 Conductor Ribbon cables
to both floppy's. Do you now have +5 VDC on Pin 21 of the FDC IC socket?


Larry

 

[Zion]

OK, with U13 REMOVED, Pin 20 is GROUND and Pin 21 should be 5.00 VDC. See what voltage is
on Pin 21. There must be something pulling down the +5 VDC. If Pin 21 is not +5 VDC.
Disconnect the Power Plugs to both floppy drives. And disconnect the 34 Conductor Ribbon cables
to both floppy's. Do you now have +5 VDC on Pin 21 of the FDC IC socket?


Larry

I reinstalled the FDC IC, and I have 4.68 volts on pin 21 with both floppies completely disconnected. Should I test without it in as well?
With the floppies having power reattched i get 4.72 volts at pin 21 of the FDC IC.
With the floppies attached, with power and the 34 ribbon cable reattached, I get 4.71 volts
Pin 20 of the FDC IC has continuity to ground