Commodore C128 PS repair HELP PLEASE!

[Serge125]

Hi all

    I have a PS to fix and the other place that I have asked don't seem to care to help me out so I HOPE that over here I can finally get somme. I need help with this c128 power supply. I would like to test it with my meter but don’t know how or where to put the probes. I would like to know how to do this very much so that I can save this ps. I have changed the 2 caps but doesn’t have power. So how to start checking?? Yes I know I know HIGH VOLTAGE and it can kill you, but if you tell me where exactly to put the probes then no problem. Ok up to now in my "visual" the traces are great, 2 fuse ok, no burnt components, no bad smell and as I said both caps have been changed. I have done some non voltage checks and I have noticed something odd and not normal but this is where I need help and information on this. On the PS trace pic I have a black and white round for the 120v input and when I check with my meter on either the black or white and I touch all the joints in the yellow square I have beep I have provided a vid on YouTube. I feel this isn't normal. Now when I check for voltage on the male plug (pic included) I get; Pinout; #1 +5v = 0v on my side, #3 9vac = .006vac, #5 9vac = .032vac. This has just been red with LIVE voltage. I have provided schematics, both front and back of PS, vid on YouTube and pinout pic. So I hope that all the info I provide will help you to help me. THANKS for all informations!!! VIDEO; https://youtu.be/Le3Lw_jYwkQ

[Chris56000]

Hi!

Can you read me the type number of the regulator control IC – it's hidden underneath a dark shadow on your photo!

Chris Williams

[LateLesley]

Right the 9V AC , you need to measure that in AC mode between the two AC pins, as these are not referenced to ground, they are a direct connection to the transformer secondary coil. So measuring between pin 3 and pin 5 with the meter on ACV, should show you a voltage around 9 AC. It won't be exact, but should be within a couple of volts of it's stated figure.

As for the 5V side, the first thing to measure would be across the capacitor you replaced, nearest the heatsink, in DCV mode. This is the smoothing capacitor after the diode bridge rectifier (The 4 block diodes 1N5... [I can't read the rest] ). It should be more than 6V DC I would expect. If it isnt, power off, discharge caps, then test the 4 diodes.

That's a starting point.

Edit : oh and the reason the primary side beeps, is you'll be measuring across the transformer winding, which is a coil of wire, so will be a low resistance, and probably trigger the low resistance beeper on your meter. If you test with Fuse F1 removed, that will disconnect the transformer primary, and it should show open. (It may initially beep and count up the display as it charges capacitors on the primary side. )

[Serge125]

Hi!

Can you read me the type number of the regulator control IC – it's hidden underneath a dark shadow on your photo!

Chris Williams

[Serge125]

Right the 9V AC , you need to measure that in AC mode between the two AC pins, as these are not referenced to ground, they are a direct connection to the transformer secondary coil. So measuring between pin 3 and pin 5 with the meter on ACV, should show you a voltage around 9 AC. It won't be exact, but should be within a couple of volts of it's stated figure.

Ok getting .046vac

As for the 5V side, the first thing to measure would be across the capacitor you replaced, nearest the heatsink, in DCV mode. This is the smoothing capacitor after the diode bridge rectifier (The 4 block diodes 1N5... [I can't read the rest] ). It should be more than 6V DC I would expect. If it isnt, power off, discharge caps, then test the 4 diodes.

Ok for checking the cap how do I plug the probes? One on the + on the cap and the other on ground? As for the diodes I think that I need to remove them to check right?

That's a starting point.

Edit : oh and the reason the primary side beeps, is you'll be measuring across the transformer winding, which is a coil of wire, so will be a low resistance, and probably trigger the low resistance beeper on your meter. If you test with Fuse F1 removed, that will disconnect the transformer primary, and it should show open. (It may initially beep and count up the display as it charges capacitors on the primary side. )

Ok I removed the fuse and the bottom part and the black wire input doesn't beep no more BUT the 2 transformer input still beeps, ok?

THANKS for your help and patients!!  GREATLY APPRECIATED!!

[LateLesley]

Yeah, the transformer should still beep, because it's a coil. You should get an ohms reading though, it should be a few ohms. You can also test between the transformer primary and earth, it should have no reading, otherwise the winding could be shorted, or one of the primary side caps (though these should be fail safe IE open caps). C2 and C3 will mean it'll take the reading a few seconds for the reading to settle.

*** REMEMBER DOUBLE CHECK THE MAINS ARE DISCONNECTED/UNPLUGGED BEFORE MEASURING OHMS. LIVE MAINS ARE DANGEROUS ***

you have been warned

[Serge125]

Right the 9V AC , you need to measure that in AC mode between the two AC pins, as these are not referenced to ground, they are a direct connection to the transformer secondary coil. So measuring between pin 3 and pin 5 with the meter on ACV, should show you a voltage around 9 AC. It won't be exact, but should be within a couple of volts of it's stated figure.

Ok getting .046vac

As for the 5V side, the first thing to measure would be across the capacitor you replaced, nearest the heatsink, in DCV mode. This is the smoothing capacitor after the diode bridge rectifier (The 4 block diodes 1N5... [I can't read the rest] ). It should be more than 6V DC I would expect. If it isnt, power off, discharge caps, then test the 4 diodes.

That's a starting point.

Ok for checking the cap how do I plug the probes? One on the + on the cap and the other on ground? As for the diodes I think that I need to remove them to check right?

[Serge125]

Yeah, the transformer should still beep, because it's a coil. You should get an ohms reading though, it should be a few ohms. You can also test between the transformer primary and earth, it should have no reading, otherwise the winding could be shorted, or one of the primary side caps (though these should be fail safe IE open caps). C2 and C3 will mean it'll take the reading a few seconds for the reading to settle.

Yep no reading in ohms. BTW where is c2 and c3? Is it the blue boxes?

*** REMEMBER DOUBLE CHECK THE MAINS ARE DISCONNECTED/UNPLUGGED BEFORE MEASURING OHMS. LIVE MAINS ARE DANGEROUS ***

you have been warned

Check the mains disco/unplugged do you mean the plug that you plug into the wall?

THANKS!

[LateLesley]

Yes, Make sure it is all powered down, disconnected from the wall, and all caps discharged, before making any measurements. The only time you do powered on measurements is to check voltages, and even then, you should set up the measurement, get clip probes and clip on to things, THEN power the circuit with you well away from it. On the secondary side, the risk is lower, but if anything can have a direct path to live mains, you keep yourself WELL AWAY from it.

Anyway, safety not aside, C2 and C3 yes they are the blue "boxes", probably X rated capacitors. (They might have an X2 printed on them somewhere, along with their capacitance and voltage rating) These types are meant to fail in a safe way if they go faulty.

the 9V AC voltage, that looks faulty. To test that circuit, you would need your multimeter on ACV, connected to pin 3 and 5, and then power the circuit for 30 sec/1 min. before you test it with power, you would make sure F1 and F2 (two fuses) were good (Showing short with ohms reading) and installed properly (good connection) if they both check out, you can then test pin 3 and 5, and briefly power the circuit, and see if you get an AC Voltage reading. If you do, that winding on the transformer is good. If not, your transformer may be bad. With the circuit powered off again, you could check the ohms reading between pin 3 and 5. IF you get an OL or very high reading, then some part of the circuit, or winding could be open.

Lets make sure that side of things is working first, before we progress to the more complex 5V side of things.

EDIT : Could you provide a picture of your meter setting when you are trying to measure between pin 3 and 5? Just as a double check for newbie mistakes?? (We all do them, even experienced folk)

[Serge125]

Anyway, safety not aside, C2 and C3 yes they are the blue "boxes", probably X rated capacitors. (They might have an X2 printed on them somewhere, along with their capacitance and voltage rating) These types are meant to fail in a safe way if they go faulty.

Gee I never saw those types of caps, maybe I did BUT didn't know that they were caps, nice to know! I didn't see any X2 but I do see voltage (250V) but can't really see the value. BTW can this be checked with an ear meter?

the 9V AC voltage, that looks faulty. To test that circuit, you would need your multimeter on ACV, connected to pin 3 and 5, and then power the circuit for 30 sec/1 min. before you test it with power, you would make sure F1 and F2 (two fuses) were good (Showing short with ohms reading) and installed properly (good connection) if they both check out, you can then test pin 3 and 5, and briefly power the circuit, and see if you get an AC Voltage reading. If you do, that winding on the transformer is good. If not, your transformer may be bad. With the circuit powered off again, you could check the ohms reading between pin 3 and 5. IF you get an OL or very high reading, then some part of the circuit, or winding could be open.

Lets make sure that side of things is working first, before we progress to the more complex 5V side of things.

EDIT : Could you provide a picture of your meter setting when you are trying to measure between pin 3 and 5? Just as a double check for newbie mistakes?? (We all do them, even experienced folk)

Ok here are the pics for ACV and Ohms.

[LateLesley]

Yay! OK - 10.45volts AC - Your 9 Vac supply is ok, which also means your Primary side is ok. Thats the first part of the PSU checked.

So now we are on to +5V Supply. First you probably want to check the transformer winding for the 5V supply - so power off, discharge everything, and in the back side of the PCB you will see 4 solder pads where the secondary windings connect from the transformer.(Top right, above the text in your first picture of your OP)
The inner two connections are the 9V AC winding we've just tested, you'll see the tracks run across the top of the board, one via the F2 solder pads, to the PSU output cable.
We now want to test the ACV on the transformers other secondary, thats the outer two pads. you'll see they have slightly thicker tracks, and head off to 4 solder pads - this is the connection to the 4 diodes which make up a diode bridge rectifier, which changes the ACV into DC volts.
So -power off, everything discharged, put your meter on ohms, and take an ohms measurement of the outer two pads on the secondary of the transformer - you should get quite a low ohms reading, similar to the last measurement in your last post.

If that checks out, next we want an ACV measurement, from the same two pads. You will have to do this with the power on - SO REMEMBER EVERYTHING IN THE YELLOW BOX WILL BE LIVE! and stay WELL away from that part of the PCB.
Power it up, and from the side of the board away from the live side, take an AC volt reading of the outer two secondary transformer connections.  You should get an ACV reading above probably 7V AC.

If that checks out, Set the meter to DC Volts, then measure the back of the capacitor, inside the blue box I've drawn, I've marked the + and -. It's under the Diode Bridge Rectifier. Power OFF once you've taken your measurements.
Lets see what results you get here.

[Serge125]

Yay! OK - 10.45volts AC - Your 9 Vac supply is ok, which also means your Primary side is ok. Thats the first part of the PSU checked.

So now we are on to +5V Supply. First you probably want to check the transformer winding for the 5V supply - so power off, discharge everything, and in the back side of the PCB you will see 4 solder pads where the secondary windings connect from the transformer.(Top right, above the text in your first picture of your OP)
The inner two connections are the 9V AC winding we've just tested, you'll see the tracks run across the top of the board, one via the F2 solder pads, to the PSU output cable.
We now want to test the ACV on the transformers other secondary, thats the outer two pads. you'll see they have slightly thicker tracks, and head off to 4 solder pads - this is the connection to the 4 diodes which make up a diode bridge rectifier, which changes the ACV into DC volts.
So -power off, everything discharged, put your meter on ohms, and take an ohms measurement of the outer two pads on the secondary of the transformer - you should get quite a low ohms reading, similar to the last measurement in your last post.

If that checks out, next we want an ACV measurement, from the same two pads. You will have to do this with the power on - SO REMEMBER EVERYTHING IN THE YELLOW BOX WILL BE LIVE! and stay WELL away from that part of the PCB.
Power it up, and from the side of the board away from the live side, take an AC volt reading of the outer two secondary transformer connections.  You should get an ACV reading above probably 7V AC.

If that checks out, Set the meter to DC Volts, then measure the back of the capacitor, inside the blue box I've drawn, I've marked the + and -. It's under the Diode Bridge Rectifier. Power OFF once you've taken your measurements.
Lets see what results you get here.

Hi I'll be back sunday morning with the results because I'm not home right now to do this. I was thinking last time that I checked for voltage on the pins 3-5 for 9V I was just getting .046vac and now I'm getting 10vac?? ODD... Maybe a cold solder, bad output wire (broken inside near the plug)? Before I do the test I will resolder all the pads JUST to make sure before moving on to the next 5v test. So YAY for the 9v side of things!!! Again THANK YOU SO MUCH for you time helping me with this it is VERY APPRECIATED!!!!

[aqibi2000]

You could have easily had the multimeter in DC mode therefore read the voltage incorrectly last time.

[Serge125]

You could have easily had the multimeter in DC mode therefore read the voltage incorrectly last time.

Hi there, yeah I could of done this. Ok at home for now I didn't know that I would have time to work on this but I just couldn't wait! Ok now for the pads on the transformer in ohms I get .6ohm. Now for power on the same transformer pads I get 22.18vac. Yep way too much here. At the cap in dcv I get 30.24vdc (obviously). So what is next? 

Serge

[LateLesley]

OK! So we know that your DC supply rail is good (And it may not be too high, it will drop there under load), so now we have to tease apart how the rest of the circuit works.)

Here we need a combination of the IC datasheet (I've attached it here), and the PSU Circuit diagram you posted in the OP.

Now is seems it is a PWM controller of some sort, with external series pass transistors in a Darlington configuration to increase the current handling capacity. It seems the IC itself would be good for up to 1.5A, but the circuit needs to provide at least 2.5A for the C128 (Found from specs online) - so they've added additional components to increase its current handling capacity.

Now in these type of supplies, favourites for dying are the pass transistors, diodes, the IC itself which drives the transistors, And sometimes the low value current limiting resistors.

So, first you want to check all the first part of the circuit, which all seems to do with the IC power supply (Vcc) and current limiting.

This is made up of R1, R2 (Which I think is a loop of wire), C5,  V5 (which I think is the small transistor next to the diode bridge and fuse), and R3,R4, and C6. There is also aditionally R14 between ground and the current limiting circuit.

So power off, I would check that Transistor V5 isn't shorted, and that R1, R4, R14 are intact/correct value.

If they seem to measure correctly, then i'd measure the voltage between pin 13 or 5 of the IC and Ground (pin 3 or 11). This is to check the power supply voltage to the chip. Belt and braces step, check it's getting power. :-)

Lets see if you can identify each component and measure them.

Now we're getting into the more complex bits, I don't understand all parts of the circuit, I only have an idea, and might be a bit weak at explaining how it works. So I may need help from others. :-)

But it seems it'll turn the transistors off and on, in a PWM fashion, to regulate the output voltage, at the far side of L2. V8 diode and capacitor are part of the PWM, and conduct when the transistors turn off. (It's what's called a buck converter, see HEREhttps://www.electronicshub.org/switch-mode-power-supply-smps/ ). C9 smoothes the output from the PWM (probably the other big electrolytic) and R10, C11, R11 (the horseshoe variable resistor) and R12 look like an adjustable voltage divider to provide feedback to the IC so it can regulate the voltage.

R13, V9 and V10 confuses me, I do not know it's purpose, but suspect its some sort of overvoltage clamp.

L3, C10 will just be ripple smoothing, it's a high current, low pass filter.

Anyway, lets check the first part, and make sure the IC has power, then we'll move on to the switching transistors and clamps and stuff.

[Renate]

R13, V9 and V10 confuses me, I do not know it's purpose, but suspect its some sort of overvoltage clamp.

It is. It's a "crowbar".
Like throwing a big steel crowbar across the output terminals of a power supply.
It shorts things dead and hopefully a fuse will blow.

If the pass transistors on that power supply blew, you'd throw 20-odd volts into the 5V computer.
To prevent that we have the crowbar.

[Serge125]

So power off, I would check that Transistor V5 isn't shorted, and that R1, R4, R14 are intact/correct value.

Ok tested it and I had to take it out because was getting all kind of readings that I knew it wasn't right. This was a thyristor (pic) I had to check it with this Chinese gadget and was able to do the test needed and it passed, so V5 has checked ok! YAY. BTW I couldn't find any data sheet for this thyristor (TGA8504) so that is why I checked it with that gadget so I can see what I was dealing with so that I can check it properly. Ok next thing later tonight. Should I check the ic chip or Rs?

Thanks

[Serge125]

So power off, I would check that Transistor V5 isn't shorted, and that R1, R4, R14 are intact/correct value.

Ok tested it and I had to take it out because was getting all kind of readings that I knew it wasn't right. This was a thyristor (pic) I had to check it with this Chinese gadget and was able to do the test needed and it passed, so V5 has checked ok! YAY. BTW I couldn't find any data sheet for this thyristor (TGA8504) so that is why I checked it with that gadget so I can see what I was dealing with so that I can check it properly. Ok next thing later tonight. Should I check the ic chip or Rs?

Thanks

Oh boy messed up I didn't check the right V5 I think that I checked V10 (but it's good). Gee the company should of written the parts# on the board to identify them better and less searching. I think that V5 is the little transistor right besides the 2 big ones. R1 I think is the resistor next to the green resistor. Looking at a Shem and applying it to the real board is puzzling (but fun!).

Thanks!!

[LateLesley]

OH! OK, that wasn't V5 then - that was V10 you checked. It's part of the crowbar circuit. Good to know it's alright. :-D Like you though, I was unable to find a datasheet for the thing.

V5 is the small black transistor NOT on a heatsink. It's sat right next to Fuse F2.

It's handy you have the tester, That'll be handy to test some of the semiconductors.

If it checks out ok, then next would be the resistors, because without them being ok, the IC will not be powered properly.

I've attached pics below. Check the soldering on R14, because it looks a bit iffy. Do you know resistor colour codes at all?

R4 and R14 are low ohm resistors, so they will show pretty much as a short if ok.

If they all check out, then will be time to move to Volts DC, and check the IC is getting proper voltage, when powered up.


EDIT : You caught the error just as I was writing my reply. :-) That's good, and yes, you are right. I've attached photos IDing some parts.

[Serge125]

OH! OK, that wasn't V5 then - that was V10 you checked. It's part of the crowbar circuit. Good to know it's alright. :-D Like you though, I was unable to find a datasheet for the thing.

Gee good think this part isn't dead because I wouldn't been able to find one or I could find one on eBay...

V5 is the small black transistor NOT on a heatsink. It's sat right next to Fuse F2.

Ok the V5 is good, had to take it off circuit to give it a fair test.

It's handy you have the tester, That'll be handy to test some of the semiconductors.

Yes it is for what it's worth it helped me identify V10 so I think that it's a good tool to check some parts when you're not sure.

If it checks out ok, then next would be the resistors, because without them being ok, the IC will not be powered properly.
I've attached pics below. Check the soldering on R14, because it looks a bit iffy. Do you know resistor colour codes at all?

Thanks for the pics!!! I don't know the colours by heart but I do have a chart that I can look up.

R4 and R14 are low ohm resistors, so they will show pretty much as a short if ok.

Yep they do check out fine as you said, short.

If they all check out, then will be time to move to Volts DC, and check the IC is getting proper voltage, when powered up.

Ok getting 29.95 VDC on pin 13 of the chip. So what's next?

THANKS!!!

[LateLesley]

OK! So the chip is getting powered. I think first, check the output, and see if it's providing +5V, just before we go digging any further.

If it isn't, lets look at the next easily suspect components, V6, V7, and V8. V6 and diode V8 will probably be on the big heatsink, and I suspect V7 will be the wee black transistor near the IC, next to the heatsink, it's tucked in there, not far from R1, so hard to see. We will assume the IC is good for now, since I don't think you have an oscilloscope to see if it's output is switching/oscillating.

Also double check V7 is connecting ok - it looks like a pad is missing from the board, and i'm not sure it's connected.

[Serge125]

Ok I just did ALL resistors and all is good. Now soldering them back before I check for 5Vdc at plug. Ok done now checking for 5Vdc.. Ok just getting .001 Vdc on pin 5.

[Serge125]

OK! So the chip is getting powered. I think first, check the output, and see if it's providing +5V, just before we go digging any further.

Getting .001Vdc.

If it isn't, lets look at the next easily suspect components, V6, V7, and V8. V6 and diode V8 will probably be on the big heatsink, and I suspect V7 will be the wee black transistor near the IC, next to the heatsink, it's tucked in there, not far from R1, so hard to see. We will assume the IC is good for now, since I don't think you have an oscilloscope to see if it's output is switching/oscillating.

Also double check V7 is connecting ok - it looks like a pad is missing from the board, and i'm not sure it's connected.

Ok will do this tomorrow and should take about 10mins to do. With my new electric desoldering pump the job is done way faster!!! I am planing to get an oscilloscope in a few months and after that a hot air (only) rework station.

So until tomorrow!!!

Ok I do have a VERY OLD oscilloscope but didn't know if it worked and gee it does so tomorrow I can try this BUT there is so many dials on this you may need to tell me what settings and where on the ic to plug this. I know the ground clip goes on ground and the the other on the part you need to see the signal. So here's a pic of the old dinosaur.

Later!!

[Serge125]

Ok here's the old dinosaur!!

[Serge125]

Ok got the heatsink with the 2 transistors BUT one is a Schottky diode but this one only has 2 legs so when I read it with the + on the left and the black on the right I get  .OL when I reverse it I get .179 so is it good? The other transistor is ok. So right now I'm ready to test the ic with the oscilloscope but I will need to know where on the chip and what I need to do on the oscilloscope's settings. So I'll be waiting for your answer.

Later

Serge

BTW the Schottky diode # is MBR1045