Archive for March, 2009

Disk II interface update #7

Friday, March 27th, 2009

With a tweaked RC oscillator calibration setting, the UART is behaving and I can format a disk that can be completely copied by an Apple ][. Read performance seems to be around 1 soft error in 1000 sectors read. This is about 1 soft error in 2.7*10^6 bits. I’m not sure what is expected with one of these old drives using old media, but I’m declaring success (at least for now).

I’m running out of things to work on with this first proto board.

I need to start working on proto ][ with more memory, USB interface and an improved low drop out voltage regulator. I’m likely to switch to a crystal based oscillator since the built in RC oscillator calibration setting seems too course for this application. Seems like there is only 1 calibration value that works reliably. and I’d like a little more precise clock.

Disk II interface update #6

Wednesday, March 25th, 2009

I’ve mostly recovered from the power supply disaster. Instead of replacing the ATMEL AVRISP programmer, I purchased a USBtinyAVR kit. I figured that I would be able to repair any damage instead of doing a replacement, if I burned another programmer up. Well the kit works alright, but isn’t nearly as well integrated into AVR studio software as the ATMEL programmer was. Anyway, I’ve been able to get back to work finishing the format and write functions, well sort of.

Now I am working with a regulated bench power supply jumpered to the mother board with alligator clips (I guess I don’t learn). The wall wart is on the back burner until I can find a better 12 volt regulator solution.

Since the great power supply disaster I have had issues with dropping RS-232 characters being sent from my proto-board. I also found that I couldn’t fit all 16 sectors on a track during the format process. Tweaking the speed of the drive solved the formatting issue, but this clearly isn’t an acceptable workaround. What is really happening with both these issues is that, for some reason, the processor clock is now running way too slow. I need to recalibrate the internal RC oscillator that I’m using to clock this device. That will hopefully solve both issues.

Disk II interface update #5

Friday, March 20th, 2009

Ok, I’ve done some additional analysis of the power supply, while waiting for the new flash programming device to show up. Here is the situation. The power draw is between .25 and .75 AMPs depending upon whether the motors are running or not. The LM7812 voltage regulator can supply enough power, but it has a specified drop out of about 2 volts. Therefore I need a .75 AMP at 14 volts, DC, in order to supply enough power for this application. The wall wart I was using is rated for 1 AMP at 12 volts. It couldn’t support the .75 AMP at 14 volts. I’ve been looking for a higher power wall wart. I did find a 15 volt 1 AMP surplus unit at for only 2.95 that might work. However after thinking about it, I think it would be better if I could lower requirements a bit. This would allow me to find a more common source for power, such as a 13.8 volt DC supply meant to replace a car battery. I have one of these at home, and it’s not quite up to the job, due to the large 2 volt dropout with the LM7812, even though it is rated at 2.5 amps. In order to solve this, I’m going to investigate low drop out 12V DC regulators. An example might be the LM2940 from National, that reduces the dropout to .5 volt typical or 1 volt maximum. I found another example implementation on the web that used op-amps controlling a FET. The dropout on this design is far less: .1 volt, but it requires a few more parts.

This is kind of interesting to me, since I’ve had some fun with op-amps in the past.

Now that I know overall power consumption, I also plan to power the 7805, 5 volt regulator off of the 12 volt regulator, instead of unregulated input power. This should lower the power consumption (and heat output) of the 7805.

Disk II interface update #4

Sunday, March 15th, 2009

Well I confirmed that I have a power supply issue. I’m seeing a nearly 2 volt drop in the 12 volt supply when stepping. The onboard regulators are doing their job, but the wall wart I’m using doesn’t appear to be able to deal with the instantaneous load of the stepper motor. I’ve confirmed this by connecting a bench power supply with alligator clips and seeing a much more stable 12 volt supply during head movement operations. Unfortunately in the process, I somehow fried my AVR flash programmer. Until I build or buy a new one, further debugging will be delayed. I will be able to work on designing the USB interface while I sort out the flash programming issue, though.

SRT flash contact repair/adjustment

Thursday, March 12th, 2009

I received this tip some time ago from Herbert Kroiher, but never had time to add it to my web page.

“I have bought a later model of the 303b (without FP-sync) in ebay few weeks ago. The flash doesn’t fire reliable. When the flash fires it happens when the second curtain has finished his travel => much too late. This defect happens at all shutter speeds except “B”. At “B” the flash fires when the first curtain has travelled to the end position. But then the flash fires again an again because the electrical contactor doesn’t come free.

I have not found any advices in the internet about this defect.

I have studied the service manual. The flash contactors are located in the very inner of the camera body. These contactors are not accessible only by removing the top cover or the bottom plate. You have to disassemble nearly the complete camera to reach these contactors (bad design).

I have sent the camera to a repair shop. The repair costs are about 180,- Euros, so I ordered the camera back unrepaired.

Then I tried a relative simply attempt. I removed the bottom plate and the battery housing. Under the battery housing is a hole of about 5 millimeters, see page 14 of the repair manual (appendix of this mail). Here you can see part 2113 (cross slotted nut, made of brass). This nut tensions a friction brake. A lever above the brake triggers the X-contactor. I compared the position of this nut with another SRT. The nut was tightened too much.

The service manual says: “Next tighten the brake spring adjusting nut until the nut may be located in the same plane with the bound stop shaft.”

I used a little home made wrench and turned the nut counterclockwise two and a half revolutions. The nut can only turned in quarter steps, because there is not enough room to manoeuvre. Be careful. One of the shutter curtain spools is very close.

After that the x-contact defect was fixed.”

service manual page14

Disk II interface update #3

Monday, March 9th, 2009

Format is having issues, when moving between tracks. The first sector written after moving to next track isn’t reliably readable. A rewrite of the track, always works. I’m going to have to review my power supply, as I’m guessing that I’m not providing enough juice to cleanly power the stepper motors. This is something I’ve suspected before, but haven’t taken the time to fully investigate. Now appears to be a good time, since it is getting in the way of finishing the format function.

Disk II interface update #2

Friday, March 6th, 2009

Ok folks, I have found the issue with the format track function. I was writing a preamble of one 0xff followed by all zeros, due to a programming error. With that error, it’s surprising that I could find any sectors at all. Fixed that and the first track is written cleanly and I can read it back reliably. However I must have a bug in the verify track portion of the format function, as the format disk routine quits after writing one track. As RosanaRosana Danna would say – “it’s always something”

Disk II interface updates #1

Friday, March 6th, 2009

I’m moving my Disk II to PC interface blog from my Disk II page to this page, in order to give visitors a chance to add in their comments. Visit the disk II interface page if you want an overview of what this project is about.
The initial blog entries about this project are on that page.