Sudden problem with 3" drive

Started by vasilisk, 13:25, 16 February 21

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vasilisk

This morning, and without any warning signs, the drive is not reading the discs.
At "cat", is trying to load for a few seconds and then pops "bad command". I tried a spare 3" drive, and is doing the same.
Both drives have therir belt changed (last year) and parts greased. The first drive, was working without problems all week. The spare hasnt been used since the service.
If I eject the disc while is trying to load, and before the "bad command" it pops drive A is missing. Which I think is normal.
I opened the cpc, and while "cat" I noticed that the center is spining, but the shaft is not moving up and down. In both drives.
Any suggestion?

Bryce

Either a problem with the flat cable or the driver IC (74LS38) has failed.

Bryce.

vasilisk

#2
Quote from: Bryce on 13:52, 16 February 21
Either a problem with the flat cable or the driver IC (74LS38) has failed.

Bryce.


Any way to test each of them?

Bryce

You can check whether the STEP signal is being sent from the chip and if it is, trace the signal to find out where it's not getting any further.

Bryce.

vasilisk

Quote from: Bryce on 14:27, 16 February 21
You can check whether the STEP signal is being sent from the chip and if it is, trace the signal to find out where it's not getting any further.

Bryce.


Sounds complicated. Do I need any special equpiment for this?

Bryce

A scope would be best, but it's slow enough to check with a multimeter.

Bryce.

vasilisk

Quote from: Bryce on 16:00, 16 February 21
A scope would be best, but it's slow enough to check with a multimeter.

Bryce.


Is it too much to ask for some guidelines, concerning the approach with the multimeter?

Bryce

Measure the voltage between GND and pin 8 of IC208. It should be at 5V. If you do a CAT it should jump between 5V and 0V, so will show up as around 2.5V on the multimeter.

Bryce.

vasilisk

#8
Ok. Red arrow is pin 8. If I am not mistaken. GND ?

edit
If by GND you mean the calbe that connects the motherboard at the right with the metal sassi of the drive, then it is 4.77








Bryce

Ok, it should go low (0V) each time it needs to step (move) the head.

Bryce.

vasilisk

Quote from: Bryce on 17:46, 16 February 21
Ok, it should go low (0V) each time it needs to step (move) the head.

Bryce.


Voltage stays as it is and head is not moving

Bryce

Then that chip needs to be swapped.

Bryce.

vasilisk

#12
Quote from: Bryce on 18:01, 16 February 21
Then that chip needs to be swapped.

Bryce.


So this chip is responisble for the drive?
I see that IC206 uses the same chip. Is this identical to IC208?


I checked this thread also and the IC206 shows the same misreadings


https://www.cpcwiki.eu/forum/hardware-related/fixing-floppy-drives/125/

Bryce

IC206 is the same chip and also responsible for signals to the FDD, but it performs different tasks, so you can't compare pin results.

Bryce.

vasilisk

#14
Quote from: Bryce on 19:18, 16 February 21
IC206 is the same chip and also responsible for signals to the FDD, but it performs different tasks, so you can't compare pin results.

Bryce.


In the thread I mentioned, @Audronic posted some voltage changes during CAT, for the IC206. These also dont happen Maybe they need both replacement.


Can you point me where I can buy these? I see many different versions. Do they need programming?

Bryce

No programming required. What different versions are available? Post a link to the ones you want to order. Have you experience in chip swapping? If not get someone with experience to do it.

Bryce.

vasilisk

https://www.ebay.co.uk/sch/i.html?_from=R40&_nkw=74ls38&_sacat=0&_sop=15


Are any of these compatible?
Other site maybe?


I have someone experienced to do it. I wasn't going to  8)

Bryce

All of the through-pin ones in that list are fine. Buy two. If you're going to take out 208, you might as well swap 206 while you're there.

Bryce.

vasilisk

#18
Quote from: Bryce on 23:41, 16 February 21
All of the through-pin ones in that list are fine. Buy two. If you're going to take out 208, you might as well swap 206 while you're there.

Bryce.


Thank you. In an another thread, there was a discussion for fake/original chips. For this case do I have to worry about finding specific ones?
The fact that the head is not moving up/down, is responsible this chip? Could it be something else? Not enough voltage maybe? Although I tested with both the ctm644 and external psu.


Is this chip ok?


Bryce

#19
No, it's unlikely to be a voltage issue. If the 5V rail couldn't turn the motor, the voltage would drop and the CPC would crash.
As for fakes, you've no worries with this part. They are still produced in masses and are low cost. There would be no profit in cloning them.

Bryce.

Edit: I'd pay more for local parts. Shipping from China is extremely slow at the moment. You could be waiting months for them to arrive.

vasilisk

#20
Quote from: Bryce on 00:04, 17 February 21
No, it's unlikely to be a voltage issue. If the 5V rail couldn't turn the motor, the voltage would drop and the CPC would crash.
As for fakes, you've no worries with this part. They are still produced in masses and are low cost. There would be no profit in cloning them.

Bryce.

Edit: I'd pay more for local parts. Shipping from China is extremely slow at the moment. You could be waiting months for them to arrive.


Ok. But is the above code ok? In case I ask the local electronic shops
5v? Isnt the drive powered by 12v?

Bryce

#21
The only bit that matters is the base code and the case type. This is what the rest means:

SN - Manufacturer code (SN = Texas Instruments) - Base code 74 tells you it's a commercial TTL chip. The LS tells the types of gates inside (LS = Lowpower Schottky) and the 38 tells the function (38 = 4x NAND gate). The N ist the package type (N = Through pin). The other row of numbers/letters tells you where (production plant) and when (usually week and year) it was produced.

Bryce.

Edit: The motor that spins the disk is 12V, but the head motor is powered from the 5V rail.

vasilisk

These 74ls38 chips, can they be converted to socketed ones?


tjohnson

Yes, although they rarely fail so probably not worth it.

Audronic

@tjohnson


Any Chip that talks to the outside world is always suspect
For about 3-5 Dollars its worth it for piece of mind


Keep Safe


Ray
Procrastinators Unite,
If it Ain't Broke PLEASE Don't Fix it.
I keep telling you I am Not Pedantic.
As I Live " Down Under " I Take my Gravity Tablets and Wear my Magnetic Boots to Keep me from Falling off.

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