Suspected psu problem on 1570 disk drive

[roadrash]

I was just using my Commodore 1570 disk drive that has been sitting for a year or two and it was working fine for about 15 minutes. Then I left it to format a disk and when turned round to check on it the drive lights were all off.
This thing is real old fashioned transformerror power supply. I've disconnected the power outputs and i am getting 5 volts and on the other output I get 19volts but when I plug the power output to the drive motherboard the 5 volts remains stable while the 12 volts drops to 0.5 volt. Can someone give me a idea what to suspect. I have a service manual for a similar drive and it has the same psu as this one. You can get it from my Dropbox here:
https://www.dropbox.com/s/kme7z0bweh...85%29.pdf?dl=0

[CapLeaker]

Re: Suspected psu problem on 1570 disk drive

Either the output capacitors in the PSU are bad and can't handle the load, bad LM340 cant handle current anymore, or there is something shorted on the 12V rail inside the drive, loading it down. Measure the resistance between the +12V rail and GND and the 5V rail on the disk drive. Maybe there is a short?

[stj]

Re: Suspected psu problem on 1570 disk drive

if it's anything like the original 1541 then i would replace all the thermal compound on the heatsinked parts - it's going to be dust by now.
then recap it.

i used thermal pads on mine - didnt want paste all over my fingers!

[roadrash]

Re: Suspected psu problem on 1570 disk drive

Thank you for your help. Ive had a good look at it and traced the 12v rail through the PCB and listed all the components it travels to. There is surprisingly few mainly resistors and few ceramic caps and 2 transistors. I hope its one of these because otherwise it would be a faulty IC. I just today found a schematic for the drive. Can someone take a look at it and suggest where its most likely to have happened. The two Transistors I was talking about are Q2 & Q3.
Schematics attached

[R_J]

Re: Suspected psu problem on 1570 disk drive

Have you tried a different load on the 12 volt line of the power supply to confirm it is not the problem? IE 25 ohm 5 watt resistor, should draw about 1/2 amp
with the drive connected did you check the voltage going into the 7812/LM340 regulator?, if it is droping the output to .5 volts the reg. ic would be getting very hot also.

I cant make out the second schematic
I found the schematic on line and I doubt q2 OR Q3 could cause the problem, the base's are connected to the 12 volts through 4.7k resistors, a short after the resistors would not kill the 12 volt supply
You could have a shorted motor, can you disconnect CN7 plug?
You could have a shorted U7, disconnect one end of L3

[roadrash]

Re: Suspected psu problem on 1570 disk drive

Thanks RJ that helps a lot. I will have to go search out a 25ohm 5w or more resistor. I was wondering, if I dont have one can I use a 12v light bulb? I have lots of those 5w to 75 watt.

[R_J]

Re: Suspected psu problem on 1570 disk drive

A light bulb should work, If the power supply can output about 1/2 to 1 amp It should be ok. I would think the output should be close to 12 volts unloaded, if you measured close to 19volts I think that regulator ic is bad

[roadrash]

Re: Suspected psu problem on 1570 disk drive

Yeah I was right I have no resistors like that unless I rob one from something else.
What sort if wattage bulb should I use? I was kind of hoping it was a voltage reg or something rather than a motherboard problem.

[R_J]

Re: Suspected psu problem on 1570 disk drive

try the 12 volt 25 watt lamp first, if you want to draw more current add a second one in parallel

[roadrash]

Re: Suspected psu problem on 1570 disk drive

I tried a 12v 25w bulb on the 12v output and it did not illuminate the bulb. However when I tried it on the 5v output it made a very good glow for 5v.

[kc8adu]

Re: Suspected psu problem on 1570 disk drive

replace the 7812.
i have seen them fail in these back when these drives were popular.
the 19v reading is the big clue that its bad.

[roadrash]

Re: Suspected psu problem on 1570 disk drive

This is great news as i really didn't want it to be any to serious. I have a 1541and a 1541-2 and they have both been reliable. It was just how quick this failed when it gave no warning of any kind. One minute working then turns off. Thanks for that will get that 7812 replaced.

[roadrash]

Re: Suspected psu problem on 1570 disk drive

Thank you everyone The new 7812 Transistor arrived today. I fitted it and tested the output with the meter and 12v bulb and it showed it as working as bulb lit bright. Connected it to the drives motherboard and it powered up just how it should. Its amazing how some things can be working one minute then die suddenly with no warning. But thanks to you guys on here its all working again THANK YOU everyone...