Sawtooth on 12vdc

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Please take a look at his old power supply schematic. I am getting the expected 12VDC on the collector of Q113. But in addition to that I get a 150mv sawtooth riding on the 12VDC. I assume that is not normal? What would be causing that?



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In addition I am getting some noise off of the output of the 7805.

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The supply is almost 40 years old and has never had the filter caps changed.


Here is the block diagram of the power supply.

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Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
So what part of that circuit provides regulation? The newer boards supposedly have a 7812 as the regulator. I am trying to purchase a newer "analog board" as they call it.

Believe it or not this 810 is from the early 80s.

I am also confused at what I am seeing on Q113. The transistor is a TO220. It is mounted in a connector. From the schematic the middle pin of the connector is the base. Isn't the middle pin usually the collector on the TO220?
 

rphare

Joined Nov 20, 2015
9
I'm having trouble sorting out the AC return path because of the connector and incomplete schematic, but the doubler is a classic half-wave circuit that has a natural ripple at the incoming line frequency (60Hz), not 120Hz. So I don't think it's the bridge, I go with MrChips that it's one of the caps, C148, 146, 149 in order of suspicion. The device's age also suggest this.
Can you measure the waveform or ripple voltage across C148 and C146?
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Odd configurations can be checked. Start from the load and see where does the noise come from. Maybe something amplifies the noise.

I am not seeing that triangle wave on pin 20 of the connector. Getting some noise will post that image later. I did find out this was a horrible design. It was the first rev. They had tons of problems and most drives lasted only 2 years. A new drive might be in order.


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Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
Well I guess it is normal because I got the disk to boot! I lifted the drive mechanism to inspect underneath. First thing I noticed was it was manufactured in My of 1981! I tried a boot and noticed thebelt appeared to slip. So I flipped it over, tried again and voila! It booted. An almost 40 year old system brought back to life.!

Wow it sure is slow. ;) But back then it was super fast compared to loading from cassette.

Thanks to everyone for their input.


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MisterBill2

Joined Jan 23, 2018
6,383
For starters there is a whole lot of the circuit missing and so an actual analysis is not possible.The waveform shown as the sawtooh is exactly what you get with a single diode charging a capacitor that has a load discharging it for a good part of the cycle. But what is drawing the current is not shown because we only see a small part of the circuit. So what you have is a bunch of best guesses based on what is not seen. And a best guess may not be at all correct, but if the rest of the circuit is missing it is all you will get.
 

Thread Starter

spinnaker

Joined Oct 29, 2009
7,835
For starters there is a whole lot of the circuit missing and so an actual analysis is not possible.The waveform shown as the sawtooh is exactly what you get with a single diode charging a capacitor that has a load discharging it for a good part of the cycle. But what is drawing the current is not shown because we only see a small part of the circuit. So what you have is a bunch of best guesses based on what is not seen. And a best guess may not be at all correct, but if the rest of the circuit is missing it is all you will get.

Ah see above. It is working. And the PSU part of the circuit was posted. The rest is motor control for the floppy.
 

MisterBill2

Joined Jan 23, 2018
6,383
Ah see above. It is working. And the PSU part of the circuit was posted. The rest is motor control for the floppy.
And so the problem was a belt slipping, which nobody had guessed. and all of the best guesses were rather off the target. And also the ripple was probably also on the emitter of that transistor..
But now it is working, great. I hope that you have the software and the disks because that sort of materials are difficult to find these days.
Someplace I have some of those 8 inch floppy drives, but they have synchronous motors that I was hoping to use for an audio tape deck that I was going to build. But that didn't happen in a timely manner.
 
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