Using ZX Microdrives on the QL

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Bloodnok
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Using ZX Microdrives on the QL

Postby Bloodnok » Wed Apr 05, 2017 8:01 am

I've wondered for a while why the original, standalone ZX Microdrives, refused to work with the QL. They usually read fine but, apart from the odd occasion, formatting and saving were out.

There are no significant mechanical differences as far as I can see, and for the read/write hardware the circuit is almost exactly the same. That is apart from R4/R5 in the ZX schematic and R34/R36 in the QL. The only reference to their function I’ve found is in the Interface 1 and Microdrive service manual which has them setting some sort of asymmetrical bias current (my interpretation) to overcome the effects of the DC erase.

Here’s the resistors I’m referring to:

r4-r5-r34-r36.JPG
Schematics


In the ZX Microdrive these values are 150 and 180 ohms respectively while in the QL these are both set at 330 ohms.

Out of interest, I decided to try the QL values out in a ZX Microdrive and was surprised to find that it now worked just fine with the QL. Format performance was equivalent to the QL internal drives for the same cartridge. What’s more the drive worked fine on a Spectrum with no apparent side effects.

Intrigued by this, I then tried a range of values for R4/R5. These were both set to the same value for testing.

Before I could test effectively I needed to bypass the QL’s cartridge rejection feature. The QL guarantees a minimum of 100K (200 sectors) for a formatted cartridge. It does this by simply rejecting any that don’t make this minimum (that’s not quite how the sales literature put it). The Spectrum Interface 1 does not set any minimum and therefore it’s quite easy to see if the mechanism is not quite right, or a cartridge is getting past its use by date, as the formatted sector numbers reduce correspondingly.

For the QL, if the cartridge doesn’t make the 200 sectors then it simply fails. It also fails if sector 0 can’t be found as this is needed for the sector map. To overcome this in testing, I made a version of the Minerva rom with the minimum sector threshold removed. That way the QL would report on actual sectors formatted rather than just failing mysteriously.

I tested using the same cartridge (a known consistent performer) performing three passes for each resistor configuration and on each system. I then calculated the average formatted sector count.

This is what I found:

Microdrive Sectors vs R4-R5.JPG
Results


As can be seen, when connected to the Spectrum, and its lower data speed, just about any resistor value within the range of 150 ohms, to over 390 ohms, produced no statistically different result. When connected to the QL the acceptable range was more limited, as expected, but had a marked overlap with the values that also worked on the Spectrum. The original ZX asymmteric combination of 150/180 ohms failed to produce any usable sectors on the QL but worked fine of the Spectrum. Interestingly, setting R4/R5 to both the same low values, of 150 and then 180 ohms respectively, gave good results with 180 ohms being perfectly acceptable.

This lead to the question of why the choice of values for R4/R5 on the ZX Microdrive. Out of all the combinations I tried the only one that failed on the QL was the very one which was chosen for the ZX. Why would that be? Be great if someone could confirm what I’ve found.

In the meantime, try changing both R4 and R5 on a spare ZX Microdrive to 330 ohms and you may find that it now formats and saves OK on the QL.

Charles


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Re: Using ZX Microdrives on the QL

Postby Timbucus » Wed Apr 05, 2017 7:40 pm

Interesting findings - if as you say the converted ones work OK still on the Spectrum I will modify one of mine that I know will read on the QL but, not format, and see what happens - My mdv2_ is shot so the Spectrum one is needed as my second drive but, as you say it is not reliable. I have recently swapped C1 and C2 for ceramic caps (they were Polyester) on mine and that seems to improve them somewhat (in fact got one going) with a nice clean signal on the scope during a read.


Bloodnok
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Re: Using ZX Microdrives on the QL

Postby Bloodnok » Wed Apr 05, 2017 9:16 pm

I'm sure this must have been found before by someone but can find no mention of it. Let me know how you get on - any value between 220-330 ohms for both resistors seems to work fine.

Charles


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Re: Using ZX Microdrives on the QL

Postby Silvester » Mon Apr 10, 2017 12:41 pm

Just checked pair of ICL OPD drives I have (used on QL & ZX), both use pair of 220ohm resistors.


Bloodnok
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Re: Using ZX Microdrives on the QL

Postby Bloodnok » Tue Apr 11, 2017 9:39 am

Interesting. I wonder if anyone has a copy of the ICL OPD schematic?


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Re: Using ZX Microdrives on the QL

Postby RWAP » Tue Apr 11, 2017 10:39 am

Bloodnok wrote:Interesting. I wonder if anyone has a copy of the ICL OPD schematic?


I don't think I have any of the schematics - I have a folder given out to the ICL / BT engineers for troubleshooting, so will have a look through that.

I have permission for the whole folder to be scanned in and pu ton the net (so long as addresses are blanked out).... Derek? - it is mainly loose leaf A4 sheets.

I also have several spare OPD microdrive sets and boards if anyone would find them useful.... It might also be worth digging out the Quanta articles on using the OPD microdrives with the QL


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Re: Using ZX Microdrives on the QL

Postby Derek_Stewart » Tue Apr 11, 2017 10:54 am

RWAP wrote:
Bloodnok wrote:Interesting. I wonder if anyone has a copy of the ICL OPD schematic?


I don't think I have any of the schematics - I have a folder given out to the ICL / BT engineers for troubleshooting, so will have a look through that.

I have permission for the whole folder to be scanned in and pu ton the net (so long as addresses are blanked out).... Derek? - it is mainly loose leaf A4 sheets.

I also have several spare OPD microdrive sets and boards if anyone would find them useful.... It might also be worth digging out the Quanta articles on using the OPD microdrives with the QL



Hi Rich,

If you care to send me the documents, I can scan them.


Regards,

Derek
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Re: Using ZX Microdrives on the QL

Postby RWAP » Tue Apr 11, 2017 7:32 pm

I shall have a look when I am next in my loft - I think I might have sent them to Dan Abbott as they were not where I expected earlier


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Re: Using ZX Microdrives on the QL

Postby Silvester » Wed Apr 12, 2017 11:51 am

RWAP wrote:.... It might also be worth digging out the Quanta articles on using the OPD microdrives with the QL

Modifying OPD drives for QL use:
Quanta June 1988 page 13
QL World June 1990 page 14


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Re: Using ZX Microdrives on the QL

Postby Outsoft » Fri Apr 14, 2017 9:41 pm

Ta daaaaaaaan!!!

I’ve bought 2 NEW MD units for the Speccy on eBay (never used before, serial stamped in green colour, so the last series).

One of them…connected on my QL…WORKS at 100%!!!

Read better than the internal units (less error in first reading) + Format and Write PERFECTLY!!!!!!!!!!!

This is the FIRST and ONLY ONE Speccy unit 100% compatibile with a QL that I've got in my hands!!!

Without and tricks or mPort connected!!

I’ve to test another 1 unit that I bought with this one that seems made on the same period...and another unit that I have here new for my Speccy too.

I absolutly need an MD cable inverted for use this unit in FRONT of me.

Wow!!!!!!!!!!!!!!!!!!!!!!



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