Adjust It!

By Garry Francis


Issue 25

Jan/Feb 87

Next Article >>

<< Prev Article



Did you check out SPEED CHECK last issue? If you have problems with your drive Garry Francis now tells you how to adjust it.

As a follow on from last issue's Speed Check program, let's take a look at the symptoms of an incorrect drive motor speed, ways of testing it and ways of adjusting it.


As you are probably aware, an Atari formatted disk consists of 40 tracks. Each track is divided into 18 sectors of equal size. Each sector has 128 data bytes - this is the data that you normally read or write to disk. What you may not know, is that there are a number of extra bytes at the beginning and end of the data bytes. These are not accessible to us, but are used by the drive's disk controller for indicating track number, sector number, cyclic redundancy checks (similar to checksums) and various dummy bytes to ensure the integrity of each sector. In addition, each track has an extra very narrow 'sector' used as an index to define the start of the track.

All this information is written to the disk when it is first formatted. However, if the disk is spinning too fast some of the important housekeeping information' may be overwritten. This results in improper formatting which might not be detected at formatting time. This will show up at a later date with the occurrence of Device Done Errors (ERROR 144). A fast drive may also have trouble writing to a disk formatted at a slower speed, as it will overwrite the space allocated on the disk.

On the other hand, if the drive speed is too slow, the data will be packed closer together and becomes hard to read by a drive operating at normal (or fast) speed. You may not be aware of this problem until somebody with a faster drive has trouble trying to read your disks (although slow drives can usually read disks formatted at faster speeds). The inside tracks (i.e. closer to the centre) are the first that are likely to fail due to the closer packing of data.

Other errors may also occur, but these are the most common.


You should test the speed of your drive at least once a month. If you use it a lot, then increase the frequency of the tests. A commercial software developer should probably test it once a week!! It is also important to check the speed before a formatting session, as this is when it is most critical.

Keep in mind that drives tend to spin faster when they are first turned on and slow down slightly as the internal mechanisms warm up with use. You should therefore test the drive's speed at a time most appropriate to your usage habits.

I am aware of 3 different ways of testing the disk motor speed. The first method is the one used by Atari as outlined in their service manuals. For example, see pages 7-9 to 7-10 of the Atari 810 Service Manual. This method requires some fancy equipment (such as an oscilloscope) which makes it impractical for the average user. It measures the disk speed indirectly by measuring the current travelling, from the drive motor's generator. The measured value is then compared with the optimum value listed in a table. If it differs, then the speed needs adjusting.

The second method is to use a stroboscope. The manufacturers of disk drives often include this on the flywheel of the drive. In the case of the 810, if you remove the top and bottom covers of the drive and turn it upside down, you will see a large circular hole in the metal baseplate. Through this hole, you can clearly see the large flywheel that the drive belt passes around. Attached to the flywheel is a stroboscope with two concentric rings of markings. The innermost ring is marked '50' for 50Hz power supplies such as Australia and the U.K. The outermost ring - is marked '60' for 60Hz power supplies such as the United States. You will note that they differ in the number of markings. The 50Hz scale has 20 markings and the 60Hz scale has 24.

The stroboscope works by flashing a light on it while the drive is spinning. The flash rate should be the same as the power supply (i.e. 50 times per second for Australia and the U.K. or 60 times per second for U.S.A.). A fluorescent light is usually good enough to do the trick. If the scale appears to be stationary, then the speed is correct. If the scale appears to be slowly moving, then the speed needs adjusting.

Unfortunately, in the case of the 810, some quick calculations reveal that the stroboscope is totally useless. The reason for this is that the drive was originally designed to spin at 300 r.p.m. (and the stroboscope designed to suit), but Atari have adapted it to run at 288 r.p.m.

I tried making my own stroboscope to test for 288 r.p.m, but it proved to be a huge flop.

The third method is, of course, to use a software driven speed tester such as Speed Check from last issue. You don't have to disassemble your drive, it does not require any extra hardware, it is reliable and it is remarkably simple to use. See last issue for complete details.

If any of the tests indicate that, your drive is operating at the incorrect speed, then you'd better correct it quickly. There are two options open to you. You can take it to your nearest Atari Service Centre and get it fixed by a qualified technician or you can fix it yourself.

Adjusting disk speed

Adjusting the drive speed is a ridiculously simple job. If you follow the instructions below, you should have no trouble. There are already thousands of users adjusting the speed of their own drives.

There are only two points to keep in mind. Firstly, adjusting your own drive may void any warranty you have on it and secondly, neither the author nor PAGE 6 will accept any responsibility for damage you cause if you do the wrong thing. Use your common sense. If you don't feel confident of adjusting the speed, then don't do it! Take it to an authorised Atari Service Centre.

The Atari disk drive has had a turbulent history, in which it has gone through a number of changes. The original Atari 810 was plagued with problems from the day it was introduced. To Atari's credit, they made several modifications to improve the drive's performance, including a revised file management system (DOS 2.0), the addition of a data separator board and the Revision C ROM chip. If you have one of these early drives, see the instructions given under 'Early Atari 810'.

After a couple of years, an extra printed circuit board was added to the framework above the read/write head. Its function was to regulate the disk motor speed. This was in recognition of one of the drive's most common problems - it tends to drift from the correct speed. If you open up your drive and find that it has one of these boards, then you should use the instructions given under 'Late Atari 810'.

In the Morgan era, Atari replaced the aging 810 with the lower profile 1050. If you have one of these drives, then use the instructions given under 'Atari 1050'.

Before you start, you will need:

A pointy-bladed knife
A medium sized Philips head screwdriver
A medium to small sized slot screwdriver
A copy of Speed Check
About 10 minutes of your precious time

Ensure that all tools are free of magnetism. (If a screwdriver can pick up a single pin or staple, then it is magnetised.)

Early Atari 810

1. Prepare a clean, dust-free environment to work in. Stray bits of dust, hair and other foreign particles can cause damage to the disk drive.

2. There are 4 circular self adhesive tabs covering the screwholes at each corner of the top cover of the drive. Remove these using a pointy-bladed knife or similar pointed object. Upon removal, these tabs take on a life of their own. They love sticking to shirt sleeves and anything else that comes within range of them, so put them aside in a safe place.

3. Use a medium sized Philips head screwdriver to remove the screws in each of the 4 holes.

4. Remove the cover by lifting it straight up and place it to one side.

5. While the cover is off, take a good look around inside to familiarise yourself with what things look like and where they're located, but do not touch anything unless you know what you're doing.

6. Lying flat at the back of the drive is a printed circuit board known as the rear board or power board. Locate the potentiometer labelled R142 in the back left hand corner of this board. It is a plastic disc about 15mm diameter with a slot through the centre. It is usually milky white in colour, but some sources have quoted models that are blue.

7. Run Speed Check. It's okay to run the drive with the cover off, just be careful not to touch anything.

8. Carefully place the slot screwdriver in the slot of the potentiometer without touching any other components. Turn the potentiometer clockwise to slow the drive down or anti-clockwise to speed it up. You only need to turn it in tiny increments.

9. Check the test results being displayed on the screen by Speed Check If the results are not consistently in the green, region, then repeat step 8 until they are. 10. Replace the drive cover and check the test results one more time before screwing it in place.

Late Atari 810

1-5. As for the early 810.

6. These drives are identified by the extra printed circuit board mounted above the drive mechanism. Locate the potentiometer labelled R104 which should be to the left of the only integrated circuit on this board. It is usually green with a small screw head protruding from the top.

7-10. As for the early 810.

Atari 1050

1. Observe the precautions outlined in 1 above.

2. Remove the power and I/O cords.

3. Turn the drive over and remove the four sunken screws at each corner together with the two at the front holding the front plate.

4. Turn the drive right way up and remove the top cover by lifting it from the back and sliding it forward to disengage the front plate. It is important to move the front panel forward as you may otherwise break the plastic lugs connecting the front panel to the top.

5. Follow the precautions in 5 above.

6. The potentiometer is a small blue upright box on the left (as you look towards the rear) side of the circuit board at the back of the drive. It is labelled VR2. There is a small screw on the top which may be covered with some sort of sealant. If so, you must chip this away carefully with a sharp screwdriver or knife. Be very careful. The sealant is quite hard and it is easy to slip and damage other components. Take your time. This is the only step where a heavy hand could cause damage.

7. Carefully re-insert the power cord and I/O lead, switch on and run SPEED CHECK.

8. If the results of Speed Check show a variation of speed from 288 r.p.m. insert a small screwdriver in the screw on the potentiometer to adjust the speed. Turning anti-clockwise will slow the drive down and clockwise will speed it up.

9. When you are satisfied, switch off the drive and replace the top cover by placing it on the drive with the front overlapping and then sliding it back, ensuring that the two lugs on the bottom of the front panel engage with the slots on the case. Finally turn the drive over and replace the screws.

If on any model you find that you cannot achieve the correct speed or the speed varies or is consistently slow shortly after adjustment, then you may have a hardware problem. This could be due to a stretched or incorrectly tensioned drive belt, a bad write protect circuit or drive motor circuit, the drive motor tach line is out of place or the spindle bearings are freezing. In any case, take your drive to the nearest Atari Service Centre for repairs.

Good luck!