Question about '80 computer smps / diskdrive / video problem - Page 1
Forum section: electronics / projects

author: Sky_hawk
author_rank: Contributor
timestamp: April 08, 2012, 12:07:33 am
content: I got a classic '80 computer (MSX2) that I replaced the diskdrive in, it has a 5v,-12,+12v, fairly standard UC3842 smps.Now the old diskdrive used the 12v for the drive-motor, but the new one is 5v only.Now disk-activity causes horizontal distortions in the video, specially when the stepper motor is moving the head to a new track.I want to solve this in a few that I can fit in the original housing of the computer.The diskdrive is rated 1.25A continuous when in use, 1.7A peak current.First I tried to understand my problem.1) Is it that the smps cannot deliver the 5v peak-current?, so 5v is lowered causing problems for the RGB output levels in the video circuit?2) Is it that the drive motor is causing noise that is introduced into the video signal (maybe by crosstalk or over the 5v line)?To test if 1) was the problem, I tried adding a 1000uf and 2000uf electrolytic cap near the smps 5v out, it did not seem to effect the problem, which led meto believe it was 2).I hooked up my trusty TDS210 scope and took a look at the 5v line, and found the motor causes quite large distortions on it.(1.5vtt spikes, and a sort of ringing effect, each time the motor moves the head.)Of course this is not very surprising considering that each time head needs to move the motor is 'powered', steps a few millimeters, and is powered off.I decided to route the diskdrive power over a breadboard some I could experiment serveral solutions.I tried adding caps of 100n, 1uf, and a few other values on the breadboard, about 10cm from the drive, on the scope, it seemed to havea positive effect but the video distortions where not visibly effected.Then I thought, if I cannot suppress the noise, maybe I can prevent introducing it in the video circuit.I found that the RGB output levels depended directly on the 5v line, (I have schematics of everything, the smps, the video circuit etc.)so the best thing might be to take the diskdrive motor off of the 5v lineI thought I'd try a 12v->5V converter to power the drive could help solve this.I tried a L7805CV, which is rated 1.5A max, plus again a 1000uf elco.The 7805 heats up quite a lot, even which a proper heatsink, again not so much to my suprise, because it's rated 1.5A.But, on the positive side, it seems to solve the problem!So, now my question(s):My first conclusion was based on the assumtion that the capacity of the 2000uf elco would compensate ofthe lack of currect the smps 'might not' be able to deliver in time. This might have been pre-mature, maybe serveral head-moves in quick sequencewill still drain the elcos, or the peak current is very fast so the internal resistance of the caps is preventing them from solving anything??So my first question actually is:1) how can I tell if the problem is actually that the smps is not able to provide the peak-current _or_ that the diskdrive is introducing noise on the video signal directly.(or even: a bit of both?)2) If the problem is that the diskdrive is introduction noise onto the 5v line, are there other/better ways to filter it then addingvarious values caps somewhere near the drive motor (is 10cm too far away?)3) If I have to resort to 12v->5v conversion to solve this, I'd like an elegant solution that does not produre (much) heat. A DC-DC converter comes to mind, but a 1.7Ampone will be expensive I imagine? Any low-heat alternatives?Greetings from the netherlands!Jan

author: SeanB
author_rank: Super Contributor
timestamp: April 08, 2012, 07:10:37 am
content: Try using a LC filter in the power line. Best is to use a old PC power supply and use the output choke from it, and put the heavy windings on it ( used generally on the 5V rail) in series as the choke. This will give a good amount of isolation to the current spikes from the drive, and if you use a 4700uF 25V low ESR capacitor on the output it will do the job. Otherwise you will have to either get or build a simple switch mode PSU to make the separate 5V rail you need. Easiest will be to go get a cheap external drive enclosure, as there have a wall power supply that generates 12V, and have a internal switcher to make 5V for the drive that you place inside. You should have room to fit the PCB inside the case, with the 3 leads to the right places.

author: Sky_hawk
author_rank: Contributor
timestamp: April 08, 2012, 09:40:41 am
content: Sean,Thanks for your reply! how do I identify the output choke? is it the round ferrite thingy (http://www.uploadarchief.net/files/download/Ferriet.jpg) ?What do you mean by ' put the heavy windings on it ' should I try to do +10 windings? or just a few with +0.5 mm cable ?Also what is the best place to put it, near the source of the noise, or near the circuit i'm trying to protect from the noise.Greetings,Jan

author: SeanB
author_rank: Super Contributor
timestamp: April 08, 2012, 10:19:28 am
content: Any of the big ones will do. Wind about 30 to 40 windings of 1mm enamelled wire on as a winding ( or more if it will fit), and place in the positive lead to the drive. Place the 4700 uF capacitor near to the drive, and connect the negative lead to the power return from the drive. Best is to use a inline fan lead ( the ones that you plug in series with a drive cable, with a female Molex socket and a male one connected by 4 short wires - yellow black black red) and cut the red lead to insert the choke, and connect the 2 black leads together with the capacitor negative. This should provide a lot of noise reduction to the video.You might also consider replacing the output capacitors in the power supply with new ones, as they are very likely to be on the way out if they are the original ones. Use low ESR units as replacements, and if you can use a higher voltage rating on them if the units will still fit. i often have changed these, and the originals were often bulging on top, or were blowing out the bottom seal, as well as being discoloured from heat. With the age of the unit this will be a good thing to do, and will help with the noise on the video as well.

author: Simon
author_rank: Unknown
timestamp: April 09, 2012, 09:31:35 am
content: be careful with seperate power supplies on 5V as that voltage is shared in the signal lines too. I ran my hard disks off a separate ATX power supply once and all went well but I found that my motherboard connector was burnt ! a slight voltage different across the signal wires etc can cause some nasty currents unless all signals are isolated but I doubt it