Microdrive Motor Adjustment

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tofro
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Re: Microdrive Motor Adjustment

Postby tofro » Thu Aug 18, 2016 6:51 pm

Tomas,

I don't really know the RPM specifictions, but 7s loop time for the tape is spot on.

Tobias


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Re: Microdrive Motor Adjustment

Postby tcat » Fri Aug 19, 2016 7:08 am

Hi,

Thank you all for your continued support in my quest.

Here is the mod documented in QL Service Manual, I am curious what does it actually rectify in electronic sense. Is it to do with powering MD units, or rather to do with head frequencies used?
improve_mdv-iss6.png
QL Iss#6 main board


Many thanks
Tomas


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Re: Microdrive Motor Adjustment

Postby Outsoft » Fri Aug 19, 2016 8:21 am

tofro wrote:Tomas,

I don't really know the RPM specifictions, but 7s loop time for the tape is spot on.

Tobias


I think the same: 7s loop is ok.

When I make a dump if the cartridge is perfect in 7 seconds the loop is complete, maximum 8 seconds.

It will be great to adapt the motor speed of the Spectrum MD for use it on the QL. :-)


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Re: Microdrive Motor Adjustment

Postby tcat » Mon Aug 22, 2016 5:24 pm

Hi,

I have found this piece of information in QL Concepts Manual.

Physically each Microdrive cartridge contains a 200 inch loop of high quality video tape which is moved at 28 inches per second. The tape completes one circuit every 7.5 seconds.


200" / 28"/s = 7.14secs

Tomas


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Re: Microdrive Motor Adjustment

Postby Bloodnok » Mon Aug 22, 2016 11:19 pm

Hi,

The 7.14s correlates very closely with the other method of determing the duration i.e. a nominal 225 sectors @ 31.8mS per sector = 7.16s so I think we've got as close as we can to the theoretical motor speed (close to 2260rpm). The tape gets stretched over time (hard to tell by how much) so the duration will extend giving more sectors as long as the tape holds up.

As I've found during practice the speed can be critical (on the QL more than the Spectrum as the format algorithm on the QL is much more stringent) and is made even more difficult to set given the variation in cartridge condition that is to be expected with equipment of this age.

I'm sure all this info must have been published somewhere but technical details on the Microdrive seem almost non existent these days (the Minerva source and Adrian Dicken's QL Advanced User Guide are probably the best sources) . I don't know about you but reverse engineering this stuff is great fun.

Charles


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Re: Microdrive Motor Adjustment

Postby tcat » Tue Aug 23, 2016 10:08 am

Hi Charles,

Thank you, I believe with my tools, I can set the loop time in between 7.0 - 7.25, as the precision of my measurement currentlty is <0.25 secs.

A small advantage is, I measure it with a real motor load on real cartridges.

I would also like to make a jig as per ink pen drawing, but I lack a better scan where all the dimensions would be clearly legible.
jig.png
MD Motor Adjustment Jig


Last not least, I still miss the answer to the QL main board modification (a few posts earlier), how does that influence the loop speed, R/W performance, etc?

Many thanks so far.
Tomas


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Re: Microdrive Motor Adjustment

Postby Outsoft » Tue Aug 23, 2016 3:56 pm

tcat wrote:Hi Charles,

Thank you, I believe with my tools, I can set the loop time in between 7.0 - 7.25, as the precision of my measurement currentlty is <0.25 secs.

A small advantage is, I measure it with a real motor load on real cartridges.

I would also like to make a jig as per ink pen drawing, but I lack a better scan where all the dimensions would be clearly legible.
jig.png


Last not least, I still miss the answer to the QL main board modification (a few posts earlier), how does that influence the loop speed, R/W performance, etc?

Many thanks so far.
Tomas



Interesting the tests with the Tomas specifications on my QL:

MDV1_ secs 7 rpm 2310

MDV2_ secs 7 rpm 2310

MDV3_ secs 7 rpm 2310

MDV3_ is a Spectrum Microdrive connected to the QL ;)

MDV3_ can read a QL DIR and a complete cartridge written by MDV1_ that I've just copied by myself without any problems!

but…but…but…he can’t FORMAT an MDV and he can’t write any files on a pre-formatted cartridge too.

The cartridge written by MDV3_ is totally empty and they got error at the end of the copying files ;)

So the problem seems not the SPEED of READING on MDV3_ (ZX Microdrive) but the the way that the ULA of the QL check it on WRITING ;)


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Re: Microdrive Motor Adjustment

Postby 1024MAK » Tue Aug 23, 2016 5:43 pm

tcat wrote:Hi,

Thank you all for your continued support in my quest.

Here is the mod documented in QL Service Manual, I am curious what does it actually rectify in electronic sense. Is it to do with powering MD units, or rather to do with head frequencies used?
improve_mdv-iss6.png


Many thanks
Tomas

I think this is an electrical noise reduction attempt rather than anything to do with actually controlling the Microdrives. But I would have to study the board layout, the tracks and the schematic to be sure.

Mark


QL, Falcon, Atari 520STFM, Atari 1040STE, more PC's than I care to count and an assortment of 8 bit micros (Sinclair and Acorn)(nearly forgot the Psion's)
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Re: Microdrive Motor Adjustment

Postby tcat » Wed Sep 21, 2016 7:31 pm

Hi,

I have an interesting observation, out of 4 MDV units I have for tests here, I measured below resistance for R/W and Erase Heads.

Code: Select all

Unit    Iss#No  R/W Upper       R/W Lower       Erase
1       5       16R             15R             714R
2       5       15R             15R             755R
3       6       14R             14R             725R
4       6       14.6R           15R             96R


I am surprised to see such a big difference for Erase head of Unit#4.
What values I should be actually expected to see?

Many thanks
Tomas


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tofro
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Re: Microdrive Motor Adjustment

Postby tofro » Fri Sep 23, 2016 9:41 am

Isn't it obvious? ;)

The service manual doesn't say anything what the static resistance of the erase head should be like. The values you have measured are pretty much in the range of measurements you'd normally see with M/C tape heads (typically spread widely, 100< R <1000R).

The last one is definitely off. As it is the erase head, it probably doesn't even matter (does it? Do you have an idea whether erase still works on that drive?)- The erase head is only used to, well erase the tape - the actual resistance does not really matter so much as with the read/write head.

Tobias



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