Help with disk drive problem

[Halifask]

Hi, after 40 years not having touched a BBC I recently bough a couple of Beebs, an Issue 7 and a B+.

I've also bought a GOTEK from Mark which arrived this weekend. Fitted it to the B+, it didn't work, fitted it to the Issue 7, it didn't work. Not doing well so far but assuming that Mark test his stuff before sending it out I begin checking the computers. I finally found that the 1770 upgrade to the Issue 7 had not been fitted correctly and hey presto GOTEK working on the Issue 7

Tried it back on the B+ still not working. Shift Break or any * disk commands result in a flashing cursor and silence. I found and downloaded the Service manual for the B+ and worked through the checks on page 50 and where it says "notS0 (pin 8 IC7) is low" I have a "high".

Now even when I had BBCs back in the 80s I didn't get this involved in diagnosis so I wanted to ask the collective expertise if what I'm doing is correct or am I way off mark, if it is correct would I be right in thinking that IC7 is faulty, and is there any other ways to confirm this? I like if possible to confirm things from more than one direction.

Oh, and just to give you a laugh, at some point my oscilloscope probe went faulty making me think the 1770 was faulty

Regards
Arthur

[e668ecp]

Have you tried repeatedly plugging and unplugging the hotel from the disc drive port? It often fixes simple problems

[Halifask]

It’s probably been connected/disconnected about a dozen times whilst I’ve been trying to get it to work but thanks for the suggestion.

[1024MAK]

... worked through the checks on page 50 and where it says "notS0 (pin 8 IC7) is low" I have a "high".

Now even when I had BBCs back in the 80s I didn't get this involved in diagnosis so I wanted to ask the collective expertise if what I'm doing is correct or am I way off mark, if it is correct would I be right in thinking that IC7 is faulty, and is there any other ways to confirm this? I like if possible to confirm things from more than one direction.

So, /S0 is the output to the drive connector pin 10, which is normally known as /DS0 Drive Select 0 on most drives (but some count from 1 rather than zero). This line needs to go to logic low to select the drive. The signal will only go low when the computer tries to access the drive.

Working back, IC7 (7438) is a NAND gate. Hence, if it is working correctly, the output on pin 8 will be low only when both inputs (pins 9 and 10) are high.

Now, as most B+ boards were fitted with a 1770, I'm assuming yours is as well.

The signal for pin 9 comes from IC17 (74LS174) pin 7.

The signal for pin 10 comes from IC16 (1770) pin 20 (MO or Motor On).

Mark

[Halifask]

Finally got back to work on the B+, pins 9 and 10 both going high but pin 8 staying high on disk access request so it looks like I'm replacing IC7.

I have a SN74F38N and looking at the spec sheets there doesn't seem to be much difference, would this be OK to use?

Arthur

[1024MAK]

It may work okay. Acorn used the 7438 as this was commonly used for floppy disk drive interfaces.

I suggest you fit a socket, then try your 74F38. My only concern would be, as the 74F38 is designed for faster switching speeds, its sharper, quicker output switching may generate ringing. The floppy drive interface is a relatively slow communications link that goes via a ribbon cable. Although the drive should have termination resistors fitted which are designed to reduce ringing and reflections.

Mark

[Halifask]

Good idea fitting a socket, I'll have to order some though and if I'm doing that might as well get a couple of 7438 as I wouldn't think they cost much.

Any suggestions as to the best places to go for these?

[1024MAK]

There's a topic on here that lists suppliers that members use.

It's a bit out of date now, but the vast majorly are still trading. Maplin are now a completely different company.

Be careful if buying from a trading site. Used chips are known to be cleaned up, their markings removed, and new markings applied. The trouble is, they don't apply accurate markings and don't test all the parts. Hence what you actually get may not be what you thought you were getting...
Hence where possible, buy from reputable sellers.

Note that with generic logic chips, there are multiple manufacturer's. Typically a part number is made up of the manufacturers' prefix, the series part number, the family code, the logic serial number and a suffix.
So for the SN74F38N, SN means Texas Instruments, 74 means the 74 series logic chips, F means the Fast logic family, no family code letter means the original 74 series family, in this case, the suffix N means a plastic DIL/DIP package. An A or B before the N typically means a revised or improved version. Sorry if you already know most of this.

Mark

[Halifask]

Don't worry, it's all good information.

Since my BBC days it's just been board swapping on PCs so a refresher is more than welcome.

Arthur

[Halifask]

Big smiley faces here

Socket and SN7438 arrived from Bowood Electronics today.

All fitted and now have working GOTEK

Thanks Mark for your assistance

[1024MAK]

Great news

Well done

Would be good if you can add a summary in the fault finding index complete with a link to this topic

Mark

[vexorg]

Maplin are now a completely different company.

I thought Maplin was completely gone.

[baz4096]

I thought Maplin was completely gone.

Wikipedia sums it up quite well:

"The brand of Maplin was acquired by a new company, Maplin Online Ltd., in June 2018. On 26 October 2018, this was publicly announced on Twitter. On 30 January 2019, the Maplin website was relaunched, selling a range of electronics, CCTV and computing supplies. The company was renamed Digital First Retail Ltd.—still trading as Maplin—on 31 May 2019. No physical Maplin shops were opened, and obligations of Maplin Electronics Ltd., such as warranty support, were not assumed."

https://en.wikipedia.org/wiki/Maplin_(retailer)

A bit anticlimactic for a company that was, at one time at least, so important and influential to the electronics hobbyist.

[vexorg]

It's as good as completely gone

[baz4096]

It's as good as completely gone

Companies House says 3 employees in 2023. So yes.