Microdrive Motor Adjustment

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Bloodnok
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Re: Microdrive Motor Adjustment

Postby Bloodnok » Tue Aug 09, 2016 8:32 am

No problem Tomas,

Azimuth is this context refers to the playback/record gap in the head being perpendicular to the tape travel. This gives the best possible frequency response and also means that cartridges recorded on other drives will play back correctly.
azimuth 2.jpg
Azimuth

On audio cassette machines the adjustment is performed by adjusting a screw which raises or lowers one side of the head thus changing the angle to the tape as it passes over the head.
azimuth.gif
Azimuth adjustment

The Microdrive is much the same except the the azimuth setting is factory adjusted. To ensure that the heads are calibrated they adjust the 'feet' to sit squarely on the chassis when it is set right. The front dimensions are critical and as mentioned there isn't a great deal of leeway for error if at least one of the rear feet are also seated.

Here's a side view from the Sinclair IF-1 and Microdrive service manual giving a better view of the feet than I provided earlier.
side view.JPG
Head side view


Cheers,

Charles


tcat
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Re: Microdrive Motor Adjustment

Postby tcat » Thu Aug 11, 2016 10:14 am

Hi Charles,

Many thanks.

This picture shows a situation of one of the MDs I have at the moment for tests.
I am just learning Inkscape, so not a perfect drawing, but I hope it captures the idea.

mdhead.png
Head Tilted UP
mdhead.png (5.39 KiB) Viewed 954 times

Can you please advise how to go about the head reseating, as there are solders keeping the head tilted up from the head board.

Many thanks so far.

Tomas


Bloodnok
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Re: Microdrive Motor Adjustment

Postby Bloodnok » Fri Aug 12, 2016 4:32 am

Hi Tomas,

The diagram is great. Haven't used Inkscape myself but do use Google Sketchup.

A few questions.

How does the drive perform?
If you have an oscilloscope what waveforms do you see on each data channel?
Does the drive run smoothly?

If the drive otherwise seems OK then there is likely no need to adjust anything. If it has problems then would pay to eliminate as many other possible causes before changing the head seating.

If you find that nothing else seems to be the issue then you can desolder the head pins from the head board and resolder them wih the head seated as previously described.

Regards,

Charles


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Re: Microdrive Motor Adjustment

Postby tcat » Fri Aug 12, 2016 8:29 am

Hi Charles,

I am building an MD adapter at the moment, so I could do all these tests easily then.

The drive has been used regularly, given by the wear seen at the roller, also the head face has a tiny groove visible towards the bottom tape guides, that's because head is up and therefore tape had been in contact more with the bottom part.

Will do all the tests as suggested, except oscilloscope, I currently do not have.

Could the head also be pressed down by hand to the position, will the head pins bent easily without breaking any of the internals?

Just asking to avoid resoldering.

Many thanks so far.
Tomas


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Re: Microdrive Motor Adjustment

Postby Bloodnok » Sun Aug 14, 2016 12:27 am

Hi Tomas,

Unfortunately if the head has significant wear then reseating will probably make performance worse as the tape would not tend to run flat against the head. Having the head in the correct position helps with even wearing and is easier on the tape in the long run. Badly worn heads will led to a drop in high freequency performance as the head can gap widen (dependent on the head construction). In this case I would expect the lower channel to be more prone to drop outs but only an oscilloscope can shed some light on this.

I wouldn't recommend trying to bend the pins without desoldering the head. It would require signifianct force which could lead to other damage. Given the wear on the head I would probably leave it as it is if it is working otherwise you'll need an oscilloscope to better fault find any problems.

Cheers,

Charles


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Re: Microdrive Motor Adjustment

Postby tcat » Wed Aug 17, 2016 7:03 pm

Hi,

I have first results of measuring MD speed using various reference cartridges that can read in both internal drives.

I use a small assembly routine, that reads a random sector, marks sector number and start time of H/W clock (PC.CLOCK $18000), than waits until the sector comes by again, marks end time of H/W clock. It then displays the delta, which is a time in seconds elapsed for the whole tape loop to go round.

I am able to measure it only in seconds, so for calibrating motor speed probably not enough good precision.

EDIT
I now measured on three different carts, I had corrected delta calculation, it seems drive 2 is on average by 1 second FASTER !!!

Actually both drives seem a little bit FASTER, I would have expected values around 9 seconds !!!!

Code: Select all

test#   cart#   MDV1    MDV2
1       1       7secs   6secs
2       1       7secs   6secs
3       1       7secs   6secs
4       1       7secs   6secs
5       2       7secs   6secs
6       2       7secs   6secs
7       2       7secs   6secs
8       2       7secs   6secs
9       3       7secs   6secs
10      3       7secs   6secs
11      3       7secs   6secs
12      3       7secs   6secs


Could this also account for swapping problems, some marginal carts around 200+ blocks when formated on MDV2, cannot be read on MDV1, and vice versa, when formatted on MDV1 cannot be read on MDV2 ????

I am looking for all possible explanations, many thanks in advance.

Tomas


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Re: Microdrive Motor Adjustment

Postby tcat » Thu Aug 18, 2016 10:25 am

Hi Charles, Silvester,

Still not sure about my timing results above as both drives seems a little too FAST ???

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.


When I try to calculate RPM based on this timing, I am getting these values, provided my math is right.

MDV1 speed in RPM:
5080mm / (6mm *pi) / 7 secs * 60 secs/min = 2311 rpm

MDV2 speed in RPM:
5080mm / (6mm *pi) / 6 secs * 60 secs/min = 2696 rpm

Based on the expected speed 2250 rpm, expected time for a loop pass:
5080mm / (6mm *pi) / 2250 * 60 secs/min = 7.19 secs

Based on the speed 2400 rpm, expected time for a loop pass:
5080mm / (6mm *pi) / 2400 * 60 secs/min = 6.74 secs

NOTE Silvester's argument here, 2400 RPM is likely a factory setting as seen on the motor label insignia.

viewtopic.php?f=2&t=1511&start=20#p13994

I wish I could measure it in milliseconds, but using PC.CLOCK limits me to seconds precision.

What do you think?
Tomas


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1024MAK
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Re: Microdrive Motor Adjustment

Postby 1024MAK » Thu Aug 18, 2016 12:58 pm

Do more than one pass (say 10). Count the number of passes, then divide the result...

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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tofro
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Re: Microdrive Motor Adjustment

Postby tofro » Thu Aug 18, 2016 1:19 pm

tcat wrote:I wish I could measure it in milliseconds, but using PC.CLOCK limits me to seconds precision.


Anything that typically measures time in finer granularity than 1s (the RTC granularity) on the QL is normally done by linking in a polling or scheduler loop routine (called 50 times/second) and implementing a simple counter in there.

Unfortunately, this won't help you much as I guess most of your test code is running in supervisor mode anyhow, with interrupts off. The interrupt routines won't be called, so none of the counters incremented.

I'd go with Mark's proposal and run more than one test and take the average.


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Re: Microdrive Motor Adjustment

Postby tcat » Thu Aug 18, 2016 5:23 pm

Hi,

I did a new measurement as suggested, 4 times a complete loop, than I take the average.
Only one test per cart/drive may perhaps be enough now.

Code: Select all

cart#   MDV1 secs/rpm    MDV2 secs/rpm
1       7/2310           6.75/2396
2       6.75/2396        6.75/2396
3       7/2310           7/2310
4       6.75/2396        6.75/2396


I was hoping to see clearly that drive 2 is a little faster, as it tends format fewer blocks, and sometimes have problems when swapping media?

What RPM speed should I actually see for a calibrated drive?

Tomas



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