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Fierodoug5

Atari 1050 Slow/No spin Help!

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I had 4 broken 1050 Drives, was able to get 3 of them fixed with help from old threads here.

The last drive is giving me trouble. 1050 Tandon drive(Singapore)

When I got it, had no 12 volts at all, found shorted cap at c70. Replaced all caps, 7812 12v regulator at Q8 and CR15 and CR16 for good measure. Then had proper 12 volts (unloaded). 5 volts is fine

When you power up drive, power light full bright, Activity light dim, head seeks as normal, Drive spins very slow for a second. Close drive door, nothing other than a dim flicker from activity light. When the drive should be spinning, the 12 volts instantly drops to 4 volts.

Swapped all socketed chips with ones from known working unit. This drive has a WD2793 with a ver. K rom. Rom jumpers are in correct positions

Swapped the drive mech with one from the tested working drives. No change. Made sure nothing was bent over touching/shorting each other, checked for shorted resistors, transitors check ok with meter(in circuit)

Any ideas?

Thanx

Doug

 

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It might be the motor, or the tach feedback from the motor that is faulty.

 

Also make sure you are using a good power supply - standard issue is 9V AC, 31 VAC (about 3.4 amps). Other Atari supplies meant for computers might have lower ratings like 15VAC that are not enough for the 1050.

 

Can we rule out whether it's the electronics or the mech itself by swapping the mech from the bad drive into a known working 1050? And/or also test a known good mech in the bad 1050.

 

Edit: woops! reread and see you've already swapped other mechs in already with same symptoms. Must be a component on the PCB after all.

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Check voltage at the 7812 pins both input and output, loaded and unloaded. Post 4 results.

 

Sounds kinda like a poor job of soldering maybe? Not meant as a slam but using flux will help a lot, perhaps just reflow the regulator connections with flux? Poor supply voltage could very well be the big legs on the big caps has pulled the solder joints and those then need reflowed with flux.

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As this voltage drop only seems to happen when the drive motor starts, I think you need to look at all the

components after Q6, i.e. C44, C45, CR14 and maybe R79, although if thats gone short the activity LED would be blown.

 

You say you've replaced all the Cap's , did that include C44 ?

if you did, you might check if the polarity is correct, if so then I would look at CR14, if that goes short circuit, you only have a 1 ohm resistor to ground.

1050mod.png

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Sometimes the drive spindle or clamping cone bearings have dried out. Note when testing the cone bearing that it won't spin freely unless it is pressed down into the spindle hub (when the door is closed).

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Checked everything after Q6 and nothing showed shorted. The search continues....

I tried 3 different drive mechs, all still showed the same 12 volts pulling down to 4 volts, so problem is on the board...somewhere.

 

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I think the way forward is to try and eliminate anything that uses the +12V supply, easiest ones to start with are

the motors themselves, unplug J1 and power up etc and measure the 12V, if it still drops then plug it back and

try J15 (stepper motor), again powerup and measure.

 

I know you've change the mechanics, so they won't be the problem, but if by removing them it goes away, at least you

will know what area of the circuit you need to be looking at.

On 2/5/2020 at 1:15 AM, Nezgar said:

Also make sure you are using a good power supply - standard issue is 9V AC, 31 VAC (about 3.4 amps). Other Atari supplies meant for computers might have lower ratings like 15VAC that are not enough for the 1050.

Also, did you check the Power Supply as @Nezgar said, worth a re-check, just in case.

 

After that I think you will have to inspect the circuit board for any possible shorts across tracks or on the component

side see if any components have been 'bent' and are touching another near component.

 

Mabye take some photos, both sides and post them

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Yup, using a stock 9volt 31vac power supply from one of the known working drives. Will test with pulling J1 ,and J15 tonight. Thanx for the suggestion!

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Hi,

 

    Have you tried checking values from the defective board against values from a working board? They should be identical until they are not, if you see what I mean.

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Interesting

With J1 (drive motor) connector removed, 12 volt dropped to 8 volts.

With J15 (stepper motor) connector removed, 12 volts stayed solid and the drive motor spins perfect!

So issue is on the board somewhere upstream of the J15 connector. Checked R56 and C55, must be more upstream.

Thanx for the help so far!

Doug

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Excellent digging, kudos.

 

In the distant past someone installed J15 cable backwards and blew one or both udn5713 chips (U2,U3) into now having a dead short internally. It's probably U2 by the schematic as I read it.

 

14$ gets you ten of them here:

https://www.ebay.com/itm/QTY-10-udn5713M-ALLEGRO-8-PIN-DIP-PERIPHERAL-DRIVER-NOS/121279928551

 

Tried the usual suspects, jameco, mouser, digikey with no matches so it could get difficult to locate these all too soon.

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is there anyway to test them? I pinned them out with the meter and didnt see any shorts between the legs of those chips.

 

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It's very likely it's gone short internally, no real way to test them if you don't have other testing capabilities

i.e. a breadboard and some resistors with jump leads.

 

As @1050 says they are easily available , I would buys them and replace both U2 and U3, theres really not much else

by the look of it that could cause this problem

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might be worth checking U7 as it plays a part in both the drive motor and stepper motor activities.

is it a PIA or a RIOT chip? im struggling to confirm which from the documentation..

anyhoo, if you can - swap it into a working drive and see if it causes the error

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I had already swapped all the socketed chips one by one to a working drive and vise versa. Actually I think the chips still in this broken one are from a working one, and the working one works fine with the chips from the broken one. If that made and sense. :)

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sounds like youre thoroughly whittling the possibilities down - tenacity is a necessary ingredient in chasing these gremlins down

it should only be a matter of time - dont let it get the better of you! 🙂

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51 minutes ago, Fierodoug5 said:

I had already swapped all the socketed chips one by one to a working drive and vise versa. Actually I think the chips still in this broken one are from a working one, and the working one works fine with the chips from the broken one. If that made and sense. :)

Just ensure when you're done that the WDC 2793 chip stays with it's original board - there's 3 or four variable adjustments that are factory calibrated in each drive for each chip for reliable operation, especially double (or dual/enhanced) density. Otherwise you'll need an oscilliscope to recalibrate them...

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Replaced both 5713's at U2 and U3, no change. 12 volts drops to 4 volts unless J15 is unplugged, then have 12 volts and drive spins(no stepper motor of course since it is unplugged)

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4 hours ago, Fierodoug5 said:

Replaced both 5713's at U2 and U3, no change. 12 volts drops to 4 volts unless J15 is unplugged, then have 12 volts and drive spins(no stepper motor of course since it is unplugged)

Interesting... so it would seem you've narrowed down the stepper motor driver circuity.... I'm not one for reading schematics, but maybe investigate the following components which appear to be related to that function:

R56, C55 (you probably already replaced), U2 & U3 (5713 dual peripheral and power drivers) , U11C (LS02 Logic Gate)

 

On 2/4/2020 at 7:08 PM, Fierodoug5 said:

Swapped the drive mech with one from the tested working drives. No change.

You mentioned earlier that a known good mech from a working drive exhibited the same issue in this "bad" drive - can you also confirm the mech from the "bad" drive works in the known good 1050?

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My bad, I jumped the gun and skipped a step in proper diagnosing.

I should have told to you to double check the ohms of each winding on the stepper motor before sending you to the market to buy the wrong parts to fix it with. Those 5317s must be pretty tough.

 

From pins 5 an 6 on the disconnected  motor jack they should have 33 ohms to pins 1, 2, 3, and 4. A short should exist between pins 5 and 6.

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This drive mech came from a working drive, and the one from this broken drive is working fine in a drive I'm using on my 800 right now.

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