QL's external MDV port

[tcat]

Hi Ralf,

I am curious, how you actually adjusted the motor speed?

Tomas

[Silvester]

There is a small hole on the bottom of the motor. Through it you can adjust preset potentiometer to change speed.

I have taken apart a spare motor. It contains a small PCB with electronic speed controller circuit. I'll try and get a photo sometime.

[tcat]

Silvester,

Many thanks, can you advise what sort of sensitivity the potentiometer has, I mean if you turn it a 1/4, to one or the other side, also clockwise means faster or slower?

Does it make sense to measure it somehow before try?

Many thanks.
Tom

[Bloodnok]

Hi,

I've looked into this to try and determine the factory setting for motor speed and as far as I know it's not mentioned anywhere. By calculation, taking into account the average number of sectors expected to be on a tape (225 + 1 for the splice), the duration of each sector (31.8mS), the diameter of the pinch roller (6mm), and the published length of the tape (200 inches /5.08 m), the expected speed for a QL microdrive is close to 2250 rpm. I'm not sure about the Spectrum drives but I'm guessing the speed isn't much different (literature mentions that the drives were faster in the QL but it's the data rate that was increased).
If the speed is too fast then the minimum number of sectors (200) won't be achieved. If too slow then the format will fail due to sectors 254 and 255 possibly being recorded (the firmware sees this as a situation that could never happen).
To set the motor speed I rigged an Arduino pro mini to work as a strobe with a high brightness white LED and painted a small white dot on the top of the pinch roller. Setting the strobe frequency to 37 Hz and then adjusting the motor speed is a simple matter.
The results were as expected although I tended to get more sectors than the 225, I guess due to the tape having been stretched (all my cartridges are second hand).

Cheers,

Charles

[tcat]

Hi Charles,

This is rather ingenious, to strobe a white spot on the roller. I was thinking of capturing a white spot on a mobile camera, then given some frames per second, I was not too sure how to measure the time elapsed on revolutions.

I am now thinking of taking a reference cartridge, and measure time when a random sector comes by again.
I could at least compare times between different MD units, and pick a good one as the reference. What do you think?

Tomas

[tcat]

These need adding per external QL Microdrive:-
TR4 / TR5 = BC184
TR6 / TR7 = ZTX551
D16 / D17 = 1N4148
C19 / C20 = 47nF
C21 / C22 = 22uF
R26 / R27 = 3.3K
R28 / R29 = 1K
R32 / R33 = 820R
R105 / R106 = 1K
D20 / D21 = LED
(Only one of each, two component ID's listed, as the QL main board has two of each, one for each Microdrive).

See the schematics (available here).

Mark

Hi Mark,

I studied the schematics, there are 14 pins with a key on the edge connector, pins 12,13 not connected.

Can you please help me correctly locating pins 1,8?

I provisionally numbered the edge like this.

QL 14 PIN MD EDGE CONNECTOR

md-edge.png (3.32 KiB) Viewed 5182 times

Many thanks
Tomas

[1024MAK]

Sorry, but it will be next week before I can follow up on this. As I'm working 12 hour shifts until Sunday...

Mark

[tcat]

Hi Mark,

Thank you, no problem, take your time.

Meanwhile I discovered in QL Service Manual, that external port is EC1.
I finally located in schematics too, PINs go 1A-7A, and 1B-7B.

Trick to select an MD seems around PINs 1,2 of HBD connector sockets, and ULA signals COMMS IN and COMMS OUT, they seem to be in a chain.

I still need to know PINs placement 1A-7A, 1B-7B on the edge connector EC1, can they be seen printed on the QL board?

As a last resort I could disassemble my only QL, and bleep signals through with a multimeter. But I need to do some programming on it so I do not want to.

Thank you so far.
Tomas

[tcat]

Hi,

I have managed to deduce the PIN/outs from circuit diagrams.

QL MD edge connector EC pinout

Still few questions left>>
1A-8A is FRONT and 1B-8B is BACK?
3A/3B really is the key / board cut out?
how to go about MD chain HDB pins 1/2?

Many thanks
Tomas

[Silvester]

tcat wrote

Many thanks, can you advise what sort of sensitivity the potentiometer has, I mean if you turn it a 1/4, to one or the other side, also clockwise means faster or slower?

Does it make sense to measure it somehow before try?

I measured the voltage fed to the motor when powered from 9V supply. The potentiometer adjustment ranged from 3.72 to 4.56V fed to the motor.

When I powered it from 5V the range was 3.74V to 4.58V.

Both tests were done with no load (no microdrive)

Switching between 9V and 5V made no audible difference to motor speed.

The potentiometer increases speed when turned clockwise (as viewed from bottom of motor). The speed range was quite limited, so adjustment isn't too critical. The angular movement is typical for a preset, about 300 degrees.

Although it would be easy to measure the current voltage setting across your motor before you make any adjustment, there is one slight problem. You need to prise off bottom of casing to get to PCB. There are three tiny tabs which have to be bent back, not easy to do neatly. For a working microdrive I wouldn't attempt it. The speed adjustment is slight enough to tweek carefully.

Bloodnok wrote

...the expected speed for a QL microdrive is close to 2250 rpm.

The label on bottom of motors is marked 9VDC(2400). I assume that means setting of 2400rpm, or perhaps the maximum.