Problem with dual floppy drives on Master 128

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quokkaz
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Problem with dual floppy drives on Master 128

Post by quokkaz »

I recently acquired a BBC Master 128 on eBay with a dual disk drive. It was sold as spares/repairs and the seller said that the last time it was used there was a bang and puff of smoke. However, it was booting up and displaying text on the monitor.

Since I've had it, I've replaced the battery pack and it boots successfully to ADFS or DSF 1770. However, I cannot get it to consistently work with the floppy drives. I can only use disk 1 - trying drive 0 just hangs with both drive lights on. Using drive 1 - on one occasion I managed to format a floppy and read/write a short BASIC program (DFS). Another time I got it to load the welcome disk (ADFS). Next time I tried again it failed to even do a *CAT of the same disk/drive.

In the meantime I found another dual drive on eBay and tried that but it displays exactly the same behaviour.

Here's what I've set the CMOS to (for DFS) :-
*CONFIGURE NOBOOT
*CONFIGURE NODIR
*CONFIGURE MODE 7
*CONFIGURE LANG 12
*CONFIGURE FILE 9
*CONFIGURE FLOPPY
*CONFIGURE FDRIVE 0

I've searched everywhere for a similar drive 0 problem but haven't found anything. Hopefully I'm missing something obvious but I'd be grateful for any advice/ideas.

Thanks
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Re: Problem with dual floppy drives on Master 128

Post by DutchAcorn »

Did you have a good look at the disc drive port on the Master? No bent pins or other possible connection problems?

I assume the two different disc drives have their own cables so that an issue with the cable can be ruled out?
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

DutchAcorn wrote:Did you have a good look at the disc drive port on the Master? No bent pins or other possible connection problems?

I assume the two different disc drives have their own cables so that an issue with the cable can be ruled out?
Good point. Both had their own cables. I'll take a closer look at the connector on the Master tomorrow. Thanks
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

Disc connector looks OK. Took off the keyboard too see underneath and found a broken capacitor in the vicinity:
Image

Another one further away:
Image
Not sure how relevant they might be.
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Re: Problem with dual floppy drives on Master 128

Post by 1024MAK »

I recommend desoldering or cutting off those damaged disc ceramic capacitors even if you currently don't have anything to replace them.

Damaged disc ceramic capacitors can fail short circuit causing all kinds of strange trouble. Having said that, it is unlikely that they are the cause of your strange disk drive fault as they are only being used to decouple the power rail.

There are many types of suitable replacement. Acorn often used 33nF and 47nF. These days 100nF are very commonly used with logic and computer circuits.

More relevant is to measure the pitch between the legs / PCB pads and get a replacement with the same pitch. Saves having to bend the leads to fit.


Mark
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Re: Problem with dual floppy drives on Master 128

Post by DutchAcorn »

Just a thought; the battery pack you removed was leaking and that was next to the disc drive socket. Can it have caused collateral damage to the under side of mother board there?

You mention selecting drives 0 and 1. Do drives 2 and 3 (sides 2 of the physical drives) display the same behaviour?
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Re: Problem with dual floppy drives on Master 128

Post by station240 »

The bang and puff of smoke is from noise filter caps in the master PSU blowing up. Should be about 10,000 threads about that around here, it's a very common failure.

The drive problems could (partially) also due to dirty heads. To be expected with 30 year old gear people sometimes store in attics. You can get special cleaning disks, but they are hard to find. Nothing beats getting into the drive and cleaning the head directly, gets fiddly if you have to unscrew the heads (not recommended).
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Re: Problem with dual floppy drives on Master 128

Post by george.h »

Just out of interest, do you know if the drives are single or double sided?

I'm just wondering, if double sided, what happens if (using DFS) you try using drives 2 and 3?
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

Thanks for all your input so far.

I've had the motherboard out and it looks pretty clean underneath - no signs of damage from the battery leak but a fair bit of corrosion around some of the contacts on the top side, which I've tried to clean up.

The drives are double-sided and I've had them out to clean the heads, which was pretty simple. These ones are Mitsubishi MF503A and the heads can be accessed by releasing a spring.

Testing again using DFS, Drives 2/3 behave just like 0/1. It tries to read drive 3 but fails. Drive 2 puts both drive lights on.

I've also swapped the drives physically (cable) and using the jumpers and the behaviour is unchanged, i.e., it doesn't look like a problem with the actual drives.
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Re: Problem with dual floppy drives on Master 128

Post by 1024MAK »

Okay, what test gear do you have?

Multimeter?
Logic probe?
Oscilloscope?

Or the very poor (wo)mans tool, a red or green LED and a resistor (470 ohms to 680 ohms) and some wires...

The drive (device) select outputs from the computer are on pins 10 and 12 of the 34 way IDC disc drive connector.
Pin 10 is /DS0 (Acorn calls this "/S0"). This signal comes from IC2 pin 3. Also check pin 2 (active high).
Pin 12 is /DS1 (Acorn calls this "/S1"). This signal comes from IC2 pin 6. Also check pin 5(active high).
IC2 is a 7438 type chip. Note that the outputs of this chip are "open collector". It cannot drive it's outputs to logic high (1). It relies on the disk drives having pull-up resistors (to the drives +5V rail). So it's outputs have to be tested with the drives connected.

The signals for IC2 in turn come from IC4, a 74LS174 latch containing six D-type flip-flops.
The signal for IC2 pin 2 comes from IC4 pin 2.
The signal for IC2 pin 6 comes from IC4 pin 5.

Note that some drive manufacturers start counting from zero, while others start counting from one.
Also note that signals prefixed with "/" are active low signals. This means when "active" they are logic low (0V to 0.8V), at all other times they should be logic high (2V to 5V).

The following taken from http://mdfs.net/Docs/Comp/BBC/Disk/WD1770

Code: Select all

Drive control register
======================

  This controls various functions that the 177x is unable to:

  Master drive control:
        Bit       Meaning
        -----------------
        7,6       Not used.
         5        Double density select (0 = double, 1 = single).
         4        Side select (0 = side 0, 1 = side 1).
         3        Drive select 2.
         2        Reset drive controller chip.
         1        Drive select 1.
         0        Drive select 0.

  Note that only 1 drive select bit line should be set at a time.

Addresses of registers:
=======================

   Computer        Location of disk chip & chip  Location of drive control
   ---------------------------------------------------------------------
   BBC Master          &FE28 - &FE2B    WD1770             &FE24

If you disconnect both drives, and disable both DFS and ADFS, it should be safe to poke any test values to &FE24 to test IC2 and IC4 (unless the MOS gets in the way). Otherwise be careful that you don't try to do something silly like activate both drives at once! :mrgreen:

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Re: Problem with dual floppy drives on Master 128

Post by station240 »

Also test:
pin 16 (motor)
pin 32 (head select)
both of which go via IC1 (7406)
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

1024MAK wrote:Okay, what test gear do you have?

Multimeter?
Logic probe?
Oscilloscope?

Or the very poor (wo)mans tool, a red or green LED and a resistor (470 ohms to 680 ohms) and some wires...

Mark
I have a multimeter or can set up wires/LEDs if required - also might invest in a logic probe.

While this isn't my usual field, I'm pretty clear what needs doing but (forgive me if this a daft question) what's the practical solution to checking results while typing in the pokes given that IC2 and IC4 are under the keyboard? :|
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Re: Problem with dual floppy drives on Master 128

Post by 1024MAK »

Having four hands helps! :lol:

Yeah, I know, it's not easy :(

If you can get you hands on another keyboard cable and some suitable insulated solid core wire, you can bodge together an extension lead.

On a Memotech MTX computer, users have a similar problem. One user uses an old style IDE cable (the more modern cables are not suitable as there are internal ground connections linking various pins together in the connectors). But I have not tried this with a Beeb. If you do try this, ensure you have not mismatched the connections.

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Re: Problem with dual floppy drives on Master 128

Post by station240 »

quokkaz wrote: While this isn't my usual field, I'm pretty clear what needs doing but (forgive me if this a daft question) what's the practical solution to checking results while typing in the pokes given that IC2 and IC4 are under the keyboard? :|
The trick is to attach wires to the IC pins, then put everything back together (carefully). You can get proper test probe sets, or just bodge it by say sticky taping wires in place.
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Re: Problem with dual floppy drives on Master 128

Post by CMcDougall »

quokkaz wrote:Testing again using DFS, Drives 2/3 behave just like 0/1. It tries to read drive 3 but fails. Drive 2 puts both drive lights on
in your intro with pic of master insides, there is another unknown rom in the bank below WORD, and maybe something else? a *ROMS list may help. If it's a MMC interface (user port) rom, then weird things do happen :shock:

top drive should be 0 & 2, bottom 1 & 3

also, make sure the discs are DFS or ADFS format, preferably DFS so that with your current config of FILE9, then DFS is selected at boot =D>
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Re: Problem with dual floppy drives on Master 128

Post by poink »

station240 wrote:The trick is to attach wires to the IC pins, then put everything back together (carefully). You can get proper test probe sets, or just bodge it by say sticky taping wires in place.
Cheap test clips are available cheaply, and will save much hassle.

You can use the IC hook style of those and banana plugs to make up some test leads; or for a little more, just buy some premade ones (edit: Direct from China is obviously cheaper)
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

CMcDougall wrote: in your intro with pic of master insides, there is another unknown rom in the bank below WORD, and maybe something else? a *ROMS list may help. If it's a MMC interface (user port) rom, then weird things do happen :shock:
Here's the list of ROMs

ROM F TERMINAL 01
ROM E View 04
ROM D Acorn ADFS 50
ROM C BASIC 04
ROM B Edit 01
ROM A ViewSheet 02 unpluggef
ROM 9 DFS 79
ROM 8 SPELLMASTER 00 unplugged
ROM 7 INTER-WORD FF
ROM 6 INTER-WORD FF unplugged
ROM 5
ROM 4 unplugged
ROM 3
ROM 2
ROM 1
Rom 0 unplugged
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

Looks like I'll be investing in some clippage too, thanks.
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Re: Problem with dual floppy drives on Master 128

Post by CMcDougall »

quokkaz wrote:Here's the list of ROMs
those are all fine for not messing about with disc drive/s weirness, so must be something else in the drive/s

myself would start with known good discs in another drive and work from there, or just use 'new'/old stock PC 31/2" drives (not Amiga as probably worn to death) and discs with one hole.
See here for conversion:
viewtopic.php?f=3&t=678#p2703
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

1024MAK wrote:If you disconnect both drives, and disable both DFS and ADFS, it should be safe to poke any test values to &FE24 to test IC2 and IC4 (unless the MOS gets in the way). Otherwise be careful that you don't try to do something silly like activate both drives at once! :mrgreen:
Mark
I now have a probe and have found a way of using the keyboard:
Image
Only time for a quick test to check it works - on IC2 I get a LO signal from pins 1 2 4 5 7 - nothing from 3 6
I'll try a bit more later tomorrow.
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

This morning I've taken some readings with the logic probe with both drives connected which I've summarised here:
Image
Column B shows the status at startup, then C and D are after doing a *CAT on each drive.

Drive 1 works consistently and I've been able to read from 40 and 80 track DFS disks.

If I swap the drives over using the jumpers, the behaviour is exactly the same but with the drives reversed.

I'd be grateful of any suggestions or other tests I can do.
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Re: Problem with dual floppy drives on Master 128

Post by station240 »

Pin 5 on IC2 and IC4 is where the fault lies, hence IC2 pin 6 is not producing a correct output. Fun part is figuring out which of the two ICs is faulty.
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

station240 wrote:Pin 5 on IC2 and IC4 is where the fault lies, hence IC2 pin 6 is not producing a correct output. Fun part is figuring out which of the two ICs is faulty.
Thanks. That's sort of what I'd figured but wasn't sure if the fault was actually in one of these ICs. Is the only way to swap one at a time? They seem to be easily available.
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

I've moved the goalposts slightly...

I saw another Master on eBay that was being sold as faulty but looked like the motherboard might be OK - it looked in better condition than the one I've got. I bought it and it arrived earlier today.
Image
Inside is as bad as it looks - many of the keys are actually rusted solid.
Image
But the board does look clean.
Image
I did a straight swap in mine and it immediately booted up. :D
Image
Beyond there, I've reconfigured to boot to DFS and tried one of the twin drives. Sadly, I seem to have swapped one problem for another: doing a *CAT on either drive makes it spin but no lights on either drive and no response. :(
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

Interesting probe results from IC2 and IC4 with the new board (previous results from original board on the right):
Image - Image
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Re: Problem with dual floppy drives on Master 128

Post by quokkaz »

It's been a while but I finally got round swapping ICs to try and fix the floppy problem, after also replacing the 2 broken capacitors, and started with IC4. Removing the old one was proving difficult until I bought a desoldering iron which made short work of it. I found a broken trace underneath (though that might have been me :oops: ) but recreated the link on the underside of the board while soldering in the replacement IC.

Good news is that it now all works perfectly :D , so thanks for all the advice.
Image
I've learned a lot in the process and hope to use this on the second, faulty Master 128.
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Re: Problem with dual floppy drives on Master 128

Post by 1024MAK »

Well done =D> =D> =D>

I do like a repair story with a good ending :D

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