Power problems with C8P DF drives

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lowrybt1
Posts: 89
Joined: Sun Mar 08, 2015 3:42 pm

Power problems with C8P DF drives

Post by lowrybt1 » Sun Feb 11, 2018 9:43 pm

I’m looking for some guidance diagnosing power problems with C8P DF drives. Bought the drives about 5 months ago. At the time, air-cleaned the PSU’s and mechanicals, cleaned the drive heads and lubricated motors and spindles. Powered up and tested all voltages. Everything was good. Then successfully booted up some disks. The drives were running for about 45 minutes when the glass-fuse blew and all went quiet.

I tested a known/good replacement fuse which also blew.

Any thoughts on next steps? My gut tells me to uncouple the power connectors and remove all PSU components from the case for visual inspection for frayed, melted or shorted wires. Anyone know if there is a most likely single point of failure that would trigger the blown fuse? (I did check the mains, they are fine.) And, last question, has anyone had experience replacing the 8” DF drive PSU’s with modern, switched units?

Thanks, Tom
C4PDF w. 24K, 2 x 5.25 MPI B-51 drives
C1P RevB with fully-populated 610 board and A-13. Klyball D-13.
Spares: 600 Rev.D, 610 board, 540A, A-13 with homebuilt data separator, MPI B-51 logic board (w/o data sep module) and mechanicals

dave
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Joined: Tue Sep 09, 2008 5:24 am

Re: Power problems with C8P DF drives

Post by dave » Tue Feb 13, 2018 9:17 pm

Hi Tom,

Those power supplies are pretty straightforward linear supplies. As everyone on the internet will point out, the electrolytic caps may not hold up over years of disuse, and are worth checking. If the supply was working and suddenly blew a fuse, it's more likely that the pass element (transistor) or the op amp controller failed. These are not hard to repair if you are good with a soldering iron. Do you have a multimeter and/or a scope?

I have powered OSI machines with modern switchers. For the same (or more) current, switchers will generally be smaller, lighter, cooler, and at least as reliable as the linear supplies. They also tend to have more protection against overvoltage/overcurrent than the simple linear supplies that OSI uses. If you decide to replace the supply and aren't a fanatic about authenticity (I'm not), a switcher is the way to go. Keep the old one, just in case you ever want to fully restore the machine.

Dave

lowrybt1
Posts: 89
Joined: Sun Mar 08, 2015 3:42 pm

Re: Power problems with C8P DF drives

Post by lowrybt1 » Wed Feb 14, 2018 7:39 pm

Thanks Dave. I'm going to start by uncoupling all of the power connection points on the drives, replacing the fuse and then adding power to see if it blows the glass fuse again. Since the drive spindle motors run directly off of 120VAC, I have a gut feeling that the problem may be there. Thinking that the monster aluminum capacitor linked to the motor might be faulty. I've got a copy of the Siemens FDD 100-8 installation manual with parts list but it unfortunately does not provide specs or a parts # for that capacitor. The only capacitor I've ever seen that was bigger was one I replaced in my home's HVAC compressor unit.
C4PDF w. 24K, 2 x 5.25 MPI B-51 drives
C1P RevB with fully-populated 610 board and A-13. Klyball D-13.
Spares: 600 Rev.D, 610 board, 540A, A-13 with homebuilt data separator, MPI B-51 logic board (w/o data sep module) and mechanicals

Steve Gray
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Location: Markham, Ontario, Canada
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Re: Power problems with C8P DF drives

Post by Steve Gray » Wed Feb 14, 2018 11:49 pm

Does this help?:

http://mikenaberezny.com/wp-content/upl ... ematic.pdf

I know the guy who drew the schematic and he's still active. I can get you in touch with him if you need.

Steve
C4P+D&N floppy not working, 2x C4P not working, C1P not working, Superboard not working.
505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested

lowrybt1
Posts: 89
Joined: Sun Mar 08, 2015 3:42 pm

Re: Power problems with C8P DF drives

Post by lowrybt1 » Sat Feb 17, 2018 8:14 pm

After disconnecting all power feeds to the drives and testing PSU outputs with multimeter, it looks as if the Powertec OEM II 2C24-2.3b is the problem. It is supposed to feed 24VDC to the drives but is now outputting 40.7VDC. Adjusting the VDC pot has no effect on the output voltage.

The other supply, which generates 5VDC to the the drives, is working nominally.

I’m good with soldering when I have step-by-step instructions but am a novice at diagnosing bas electronic components. Any thoughts on what component would fail such that the power supply continues to operate but with VDC well above spec?

Thanks
C4PDF w. 24K, 2 x 5.25 MPI B-51 drives
C1P RevB with fully-populated 610 board and A-13. Klyball D-13.
Spares: 600 Rev.D, 610 board, 540A, A-13 with homebuilt data separator, MPI B-51 logic board (w/o data sep module) and mechanicals

dave
Site Admin
Posts: 430
Joined: Tue Sep 09, 2008 5:24 am

Re: Power problems with C8P DF drives

Post by dave » Sun Feb 18, 2018 5:14 pm

I just pulled my 8" drive box down from the shelf and opened it up, and it has a different power supply, suppying both 5V and 24V.

I can't find the schematic for your Powertec supply, but I did find some pictures on the web showing a large transformer. Based on that, let's assume that this is a simple linear supply, without any current limiting, etc.

The 40VDC reading suggests the unregulated DC section is OK. A 28VAC transformer would produce peaks of 28*sqrt(2) = about 40V. For you to read 40VDC, the bridge rectifier circuit (either 4 hefty diodes or a thick black package with 4 pins) is OK, and probably the big filter cap as well. However, for power supply work, a scope is much more informative if you have one available, since you can see what the actual waveform looks like. If the caps are good, you will see a pretty steady DC voltage around 40V with only a bit of ripple.

A pass element will sit between the unregulated DC and the output. For supplies of that era, this would typically be a beefy NPN transistor in a TO-3 package. The pass element will be controlled by a regulator circuit, which could be a voltage reference and an op-amp, or a dedicated voltage controller IC with reference and op-amp in the same package.

Because the unregulated DC is showing up on the output, either the pass element failed (short-circuit), or the regulator failed. In the absence of misuse, it would be more common for the op-amp (or regulator) to fail first. So try replacing the op amp/voltage reg. If you see a single IC in the circuit, that would be it.

To tell which is the culprit, check the output of the regulator IC. For example:
  • If you have an NPN pass element, and the output is high, then the regulator (op amp) failed, since it should be pulling the control voltage down to try to lower the output voltage.
  • Again, for NPN pass element, if the output is low, then the pass transistor is shorted, and the regulator is working and trying (in vain) to reduce the output.
The next step would be to take some photos of the supply, and post them here, along with the part numbers of the transistor and regulator, in case they are not obvious from the photos. Then, we'll look up the data sheets and figure out what to check using the tools you have available.

Dave

lowrybt1
Posts: 89
Joined: Sun Mar 08, 2015 3:42 pm

Re: Power problems with C8P DF drives

Post by lowrybt1 » Sun Feb 18, 2018 6:53 pm

Thanks Dave for the in-depth explanation. I attached some pics. The power supply does use a TO-3 NPN (8828) and a voltage controller IC: MC1723CP. T
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C4PDF w. 24K, 2 x 5.25 MPI B-51 drives
C1P RevB with fully-populated 610 board and A-13. Klyball D-13.
Spares: 600 Rev.D, 610 board, 540A, A-13 with homebuilt data separator, MPI B-51 logic board (w/o data sep module) and mechanicals

dave
Site Admin
Posts: 430
Joined: Tue Sep 09, 2008 5:24 am

Re: Power problems with C8P DF drives

Post by dave » Tue Feb 20, 2018 1:51 am

OK, the MC1723 is the same as the LM723 from national semiconductor, and others. Check pin 10 with your DMM, with the Ground lead at pin 7. This will give you the control voltage.

If pin 10 is near 0V, the transistor is failed in the short position. If the regulator failed with output at 0V, then you'd have 0V output from the supply.

If pin 10 is greater than 1V, then the LM723 has failed (since again, if the NPN failed open, which is quite rare, you'd read 0V at the output of the supply.)

Dave

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