STe PSU repair (Mitsumi 68-4231A)

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STe PSU repair (Mitsumi 68-4231A)

Postby Inks » Wed Dec 16, 2009 12:19 am

It's been a few years now since my Dad's STe gave up the ghost (2002-ish), but having gotten back from Uni this week, I'm having another go at repairing it. We managed to get it to work again the first time it died, but it popped again almost immediately.

The problem is with the PSU, a Mitsumi 68-4231A type. The problem the first time 'round was rather obvious from the scorch marks left by the diode that had blown. In typical rookie fashion, we replaced this diode with one of a similar size that was lying around. It lasted half an hour. Replacing it again, nothing worked. Yes, it's the right way around. :)

Image The 68-4231A board itself. Image provided for reference, seeing as I actually had it out of the housing. :)

Image The diode blown, marked D203, is the one I replaced, with scorch marks clearly visible above.

Now reading the related posts on this forum, it seems a good idea to do a bit of an overhaul of the PSU. I've found Simbo's guide to replacing the caps on this power supply model, but this doesn't cover the diode that's gone. I've no idea what the rating on the original diode was, as it was a bit burnt up and we've thrown it away anyway.

I'm going to replace all of the caps, the bridge rectifier, and the diode that blew. I've got the Maplin codes for the caps and rectifier, but I need to know what a suitable diode rating/code would be if anyone knows? The schematic diagram for the 68-4231A would be helpful too if anyone has it?

Cheers.
Last edited by Inks on Sun Jan 03, 2010 3:16 am, edited 1 time in total.

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Wed Dec 16, 2009 11:42 pm

On a similar power supply, the diode is a general purpose 1N5397 (1.5 Amp, 50 Volt, DO-15 case). Thus any similar diode will work. The diode that you soldered in is way too small... I can see that it is cracked (it blew and cracked it's glass case). So, the PSU will probably work when you replace the diode again. By the looks of it, this diode could be replaced with a piece of wire (I'll bet its real purpose is a fuse).

[edit]
The Mitsumi 68-4231A PSU has this diode in parallel with the output, whereas others use a series diode. So don't replace the diode with a wire on the Mitsumi 68-4231A; remove it. These diodes reflect poor design. Use a 12 volt post regulator with built in input an output protection.
Check out the MIC2940A-12WT (micrel.com)
Last edited by Hippy Dave on Wed Jun 09, 2010 6:40 pm, edited 1 time in total.

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Inks » Thu Dec 17, 2009 1:24 am

Hippy Dave wrote:On a similar power supply, the diode is a general purpose 1N5397 (1.5 Amp, 50 Volt, DO-15 case). Thus any similar diode will work.


Cheers for the reply, Dave. I'll see if I can get hold of something similar, although I'm not sure I'd want to just short over it with a wire without having a look at the schematic first... Just in case. :)

I'll post back/edit when I get it working. Any further input is appreciated!

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Inks » Thu Dec 17, 2009 6:08 pm

Ok, I couldn't be bothered to drive to Maplin today, so I soldered in another, bigger, rectifier diode I had lying around, which was double the rating of the one Dave mentioned. I believe it was an RGP30B (3.0A, 100V).

Image

Still no joy. I've got to wait for the caps to discharge a bit before I open it up again. Any ideas?

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Thu Dec 17, 2009 9:38 pm

Put a load on the 5v output. The psu may require a load to work properly.

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Inks » Thu Dec 17, 2009 10:13 pm

Hippy Dave wrote:Put a load on the 5v output. The psu may require a load to work properly.


I put it back in the STE and turned it on. No luck. :)

But now you mention it, I did test it with the voltmeter when it didn't work in there. There's nothing on the 12 or 5v rails.

There's definitely a current going in, and I'd hazard a guess that the primary circuit is working properly as there was a slight spark noise (nothing unusual) from inside the switch when I turned it on. Where the break is in the secondary circuit, I don't know. How could I diagnose this?

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Fri Dec 18, 2009 9:31 pm

Well... A few random thoughts:
Replace the electrolytic capacitors and heat stressed components
(not the power resistors, but the semiconductors and caps nearby).
I see a baked diode under three resistors.

Note that the primary side and isolated secondary side are
demarcated with a dividing line on this PCB. I don't believe
the problem is necessarily on the secondary side.

Also, test the supply without connecting it to the computer
and put an approximate 100mA to 500mA (milliampere) load
on the 5 volt output. I'm slightly suspicious that the 12 volt
output was overloaded, causing D203 to blow the first time.
If this was the case, the power supply may get damaged
whenever it is connected to the computer.

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby 1024MAK » Thu Dec 24, 2009 4:31 am

Don't think I have seen a circuit diagram for this one.
If it helps have a look at "ST internal PSU voltage fluctuating - all caps are NEW" viewtopic.php?f=15&t=18180
where I have posted two circuit diagrams of power supplies.

Did you measure any of the outputs with a meter before you renewed the diode for the second time?
Hippy Dave gives some useful advice, but it is unlikely that you can replace a diode with a wire link. If the designer wanted a cheap fuse, they either use a thin wire link or use a low value resistor. Also these are switch mode type power supplies, so some of the diodes need to be "fast" types. Not sure what the function of the diode that you changed is, so can't say for sure. Can you trace back the tracks and tell me what it is connected to?

A photo of the bottom of the PCB may help.
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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Inks » Sun Jan 03, 2010 3:10 am

I've replaced the main reservoir cap and the bridge rectifier today, I now have the components to do all the caps. Had to sit the main cap on its side, as mentioned in Simbo's post "power supplies in atari computers and periferals __repair", as it is a few millimetres too tall for the enclosure... pic below.

Image

Here's an image of the underside of the board... incidentally, there seems to be a lot of residue (flux?) on the bottom of the board, some of which I've scratched off. Hope this picture is good enough to be useful.

Image As mentioned by Dave, the primary/secondary boundary is clearly visible.

1024MAK wrote:Did you measure any of the outputs with a meter before you renewed the diode for the second time?

The far-too-small diode soldered to the underside of the board in the photo in the first post? Yes, but there was nothing on any of the output rails, and it didn't ever actually work in the STE... I think... there's been a lot of diodes in there. :?

I pulled the current replacement off of another power supply, for a TV, I think... but I've no idea if it's a fast-switching type or if that's what is required. The original one was glass, so we tried to replace it with glass ones, despite not being able to find one big enough... the current replacement is probably big enough- it has a plastic shell, but I didn't think this makes a difference?

1024MAK wrote:Can you trace back the tracks and tell me what it is connected to?


It's directly connected to the black ground rails on the output, in parallel with the large 100 ohm resistor (R205) above it in the original picture.

On the negative side of the diode there is a small orange capacitor (C207) which bridges over to a screw-hole to connect to the chassis and back to ground. On this side also: the black-wired ground (output side) rails, a couple of capacitors (C204 first, then C202) and a pin from the transformer, before a few other things... On the positive side, a wire bridge connects to a circuit with pins from an inductor (L201), the other leg of capacitor C204, and the 12V rail.

I'll try and produce a bit of a circuit diagram for the above I've tried to describe, I know it's about as clear as mud. Although my circuit-drawing skills aren't that much better... Tomorrow, possibly.

Cheers for the help so far. Much appreciated. :)

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Sun Jan 03, 2010 3:59 am

From your picture and description, it appears the diode prevents the 12volts output from going negative (don't replace it with a wire here, this psu is different). Something really wrong happened to blow the diode the first time. Either high current from somewhere forced the 12v output negative or the diode internally shorted.

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Inks » Sun Jan 03, 2010 5:24 pm

I've attempted a circuit diagram for the secondary side of the PSU, seeing as there doesn't seem to be one in the public domain... It's mostly right, I think, if a little lacking for style.

The transformer connections are an outright guess, though. I only had the 4 pins on the underside of the board to go off.

Image

It was D203 that was the originally blown component.

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Sun Jan 03, 2010 9:05 pm

Your schematic makes sense to me (draw ic201 as tl431 and d202 as two diodes).
D203 (the one that blew) looks like it is more of a problem than a requirement,
(it can be replaced with an open circuit -- arrgh). My opinion is that D203 was
baked by R205 until it shorted and blew.
I would suggest testing all diodes and transistors for shorts using an ohmmeter.
Take a good look at Q101, it would have been stressed by the event. Also,
check any low resistance resistors for unexpectedly high resistance
(including that rectangular power resistor beside C104).

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby simbo » Sun Jan 03, 2010 10:23 pm

just looking at the underside photo of the psu
i see that the center pin of the three pin rectifer package mounted on a heatsink on the low tension side of the psu
near to the gubbed diode
is disconnected

i think also this package is short

as above
cold test all parts with an ohm/multimeter

replace the diode

if you need to test the startup and high tension
do this

disconnect the psu and take it out

place the multimeter to peek hold function and 20v dc range {if you have peek hold otherwise you need a better meter}
connect it between black and red on the plug to st
and switch on the psu

if there is a problem on the ht youll get a spike of LT on the meter

problems like this are usualy a mixture of the bridge rectifier and the startup resistors
and sometimes the optical coupler is gubbed remember this yellow/white 6 pin dil package
is an optical device and uses leds etc to image earth to 0v and isolate the machine from the HT stages

usualy what happens is this
the bridge rectifier starts to leek due to spikes in the mains
this causes breakdown of solder and this is hastened by mechanical movement both by the user
and also by heat and cold stress

causing open circuit joints

also i notice that under the pcb you havent REWORKED all the component solder joints

you need to suck off the old solder and replace it
cleaning as needed to gain a good connection

please do this and then more advice can be given

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby MRAtari » Sun Jan 03, 2010 10:36 pm

does anyone know which resistors are the start up resistors on this particular Mitsumi PSU?

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Mon Jan 04, 2010 1:04 am

MRAtari wrote:does anyone know which resistors are the start up resistors on this particular Mitsumi PSU?

R102 (I can't see it, I'll guess it is about 330K ohms) is the closest thing to a start-up resistor here (runs too).
This resistor is equivalent to R2 and R3 (both 150K) on the other schematics. Note that these PSUs use a transformer
winding on the primary side to power the opto-isolater AND provide the oscillation for switching the main transistor.
This power supply starts up switching in an open-loop mode until the opto-isolator closes the feedback loop to
provide voltage regulation.

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby 1024MAK » Mon Jan 04, 2010 2:40 am

Diode D203 may have been a general purpose rectifier diode or a Zener diode. It would be good if we knew what type was originally fitted, but as we don't I suggest that you fit a BZV85C 16V 1.3W Zener Diode, order code QF58N http://www.maplin.co.uk/Module.aspx?ModuleNo=19090

If it was a general purpose rectifier diode, a possible replacement would be a
1N4001, order code QL73Q http://www.maplin.co.uk/Search.aspx?cri ... 15&menu=-2

Regarding the mess on the solder side of the PCB. It is best to clean this off. There are a number of products to clean flux off.
Maplin sell a "PCB & Flux Cleaner", their order code N64AN (I should say that I have not tried this current product, but I have used similar ones). See http://www.maplin.co.uk/Module.aspx?ModuleNo=27521. Use an old toothbrush and a clean rag.
Once the PCB is free of the mess from the flux you will be able to give the soldering a good visual inspection. If you see any cracks or very dull solder, heat up the joint, remove the old solder and then resolder the joint. Once you have finished soldering, it is good practice to clean the PCB again.

Watch out for the uncontrolled output problem.
If the circuits around the feedback system (the opto-isolator, IC201, the resistors in this part of the circuit etc) are faulty it is possible that the secondary output voltages rise above their correct values, then keep increasing until a component can no longer take the higher voltages (may be this is what caused D203 to originally die).
This may have also killed other components, so check as much as you can. If this did happen, it is best to replace both the opto-isolator and IC201. But if you have a variable power supply unit (even better if it has current limiting) you can do a basic test of the secondary side circuit. Do this test before and then after if you replace any part on the secondary side.
During this test do not connect the Atari PSU to the mains.

Connect the +V output of the variable power supply unit to the +5V output via a low value power resistor (a 10 ohms, 3 watt resistor is a good starting point). Connect the 0V / common / -V connection of the variable power supply unit to the 0V connection on the Atari PSU.
If your variable PSU has current limiting set it to about 200mA. Set the output to 6 volts.
Connect a multimeter set to voltage across the 10 ohms, 3 watt test resistor. Switch on the variable PSU, the meter will display the voltage, which should be less than 1.3V. If your variable PSU has a current meter, the reading should be less than 130mA.
If the voltage across the 10 ohms, 3 watt test resistor is higher than 1.3V, there is a short somewhere. If the voltage here is zero, there is a disconnection in the circuit somewhere.
Now disconnect your meter, then connect it across resistor R203. The voltage here depends on how much current is being used by IC201 to control the opto-isolator and therefore the transistors on the primary side of the power supply. As the "correct" value is not known, I cannot give you a value. But you should have a voltage.
Next check that you have about 4.3V on the +12V connection of the Atari PSU. This proves that current can flow from L202 / D202 / R204 connections to the +12V line via D201 and L201.

Let us know how you get on. :cheers:
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Re: STe PSU repair (Mitsumi 68-4231A)

Postby simbo » Thu Jan 07, 2010 7:36 am

ok the burned diode is a 1n4746 18v 1.3w zener

it together with the resistor above it
clamp and rectify the 12v rail from any surges in LT and acts as a 'dummy' load if you disconnect the psu

d201 rectifies the +12v supply
so check its not shorted or leaky as i suspect it is
by lifting one leg and of it check it on 20 meg R range in both direction
one way should be open completely if it doesnt it leaky

if the psu refuses to start check R102 isnt open circuit its a 390K resistor beside the bridge and heatsink
this resistor supplies to the switching oscillator drivers and timing also the vadj stage its running voltage
if it fails the psu will do nothing {happens sometimes more than not happending if there is a problem down the line bad timing occurs and the psu will blow this resistor open circuit or to a very high value}
dont forget to replace the small 1uf @50v cap in this timing circuit
also worth checking D202 package
if its shorted then there will be poor voltage control and this can fry this stage
worth checking as i said above the 100R resistor its self

looking at the pics close
i dont see any other components that look dodge
so cold test the above and after fitting new bits
give it a test

so anyway beware that if the 390K startup resistor goes open circuit
and you plug the psu in the psu mains side stage will stay charge for a min or two
so wait five mins for any volts to beed off the main 350volt mains side cap
or youll get a surpise !!!

to calibrate the psu
connect it to the atari power on and check the +5v rail
set the pot for +5.2-5.6v {i recomend!!} .2-.6 v more makes a happy atari down the line

you can test the psu disconnnected from the atari {this type anyway }
i know i just checked one here
off the machine it shows 4.2v vcc and 11.2v 12v rail
on the machine it shows 5.4v and 12.6v

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Inks » Sun Jan 10, 2010 2:36 pm

OK, thanks for the help, all. I shall pick up a 1n4746 18v 1.3w zener diode and some PCB Cleaner fluid next time I'm in town. Should be said, though, I have exams on at the minute (I'm a Computer Science student) so it may be a few weeks before I get back to this properly- plus I've gone and left my soldering iron at home :? .

Cheers again, anyway :D . I'll let you know what I'm up to with it.

-Inks

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby MRAtari » Tue Jan 12, 2010 8:21 pm

not sure if i should ask this here or start another thread but it's the same subject and could possibly be of help to the person who started this thread....



today i have been trying to repair a Mitsumi PSU from an STE (the same model as above)

anyway i have found Q101 and Q102 on the primary side to be dead but PC 101 is ok and so is the bridge rectifier,R102 and caps

has anyone any idea what causes these 2 parts to go? anything else i should check? do you think i should just replace them and see if it works?

i've done this before on Atari branded PSU's and it can blow the new parts again....

any advice appreciated thanks :)

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Tue Jan 12, 2010 10:40 pm

MRAtari wrote:today i have been trying to repair a Mitsumi PSU from an STE (the same model as above)

anyway i have found Q101 and Q102 on the primary side to be dead but PC 101 is ok and so is the bridge rectifier,R102 and caps

has anyone any idea what causes these 2 parts to go? anything else i should check? do you think i should just replace them and see if it works?

A few more guesses:

Two likely conditions that would endanger the main transistor are:
-Transformer core saturation during a mains voltage surge.
-High current demand on the PSU output.
*These conditions could lead to the main transistor
being biased on continuously until it burns out.

The other transistor could be destroyed if R106 burns out
(measure it). R106 may burn out after the main transistor
burns out.

Furthermore, the zener diode on the output has proven it's
worth by burning out and scorching the board, and will
do it again. So put a 12 volt linear post regulator on the
12 volt output.
Check out the MIC2940A-12WT (micrel.com)
This regulator is not your average positive 12v regulator!

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby 1024MAK » Tue Jan 12, 2010 10:56 pm

If the transistors on the primary side destroy themselves, as a minimum you should check the values of all the resistors, replace all the electrolytic capacitors plus the 470pF (or equivalent) capacitor (C6 on the other design of PSU). I would also recommend that you replace the opto-isolator and the TL431C (IC201). If you are unable to replace at least check as much as you can.
Also carry out the test on the secondary side that I detailed earlier...
1024MAK wrote:If you have a variable power supply unit (even better if it has current limiting) you can do a basic test of the secondary side circuit. Do this test before and then after if you replace any part on the secondary side.
During this test do not connect the Atari PSU to the mains.

Connect the +V output of the variable power supply unit to the +5V output via a low value power resistor (a 10 ohms, 3 watt resistor is a good starting point). Connect the 0V / common / -V connection of the variable power supply unit to the 0V connection on the Atari PSU.
If your variable PSU has current limiting set it to about 200mA. Set the output to 6 volts.
Connect a multimeter set to voltage across the 10 ohms, 3 watt test resistor. Switch on the variable PSU, the meter will display the voltage, which should be less than 1.3V. If your variable PSU has a current meter, the reading should be less than 130mA.
If the voltage across the 10 ohms, 3 watt test resistor is higher than 1.3V, there is a short somewhere. If the voltage here is zero, there is a disconnection in the circuit somewhere.
Now disconnect your meter, then connect it across resistor R203. The voltage here depends on how much current is being used by IC201 to control the opto-isolator and therefore the transistors on the primary side of the power supply. As the "correct" value is not known, I cannot give you a value. But you should have a voltage.
Next check that you have about 4.3V on the +12V connection of the Atari PSU. This proves that current can flow from L202 / D202 / R204 connections to the +12V line via D201 and L201.

See also viewtopic.php?f=15&t=18180

A method of testing that sometimes enables you to find out what is going on is to use a normal light bulb (60W filament types only). Beware that this will be at mains voltage. Connect up as per the diagram below:-
PSU test circuit2.png


Do not connect the PSU output to the Atari computer.
This circuit limits the amount of power that the PSU can draw from the mains, therefore if there is a fault or problem with the PSU the lamp will glow brightly and the current will be limited. There is less likelihood that the PSU will suffer further damage. Test with a meter on the output connections. Unless you know what you are doing, don't even think about using the meter on the primary side, 1) it is at or above mains voltage, 2) unless you have another working PSU of the same type the values will not mean much.

Good luck :cheers:
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Re: STe PSU repair (Mitsumi 68-4231A)

Postby MRAtari » Thu Jan 14, 2010 2:56 pm

MRAtari wrote:not sure if i should ask this here or start another thread but it's the same subject and could possibly be of help to the person who started this thread....



today i have been trying to repair a Mitsumi PSU from an STE (the same model as above)

anyway i have found Q101 and Q102 on the primary side to be dead but PC 101 is ok and so is the bridge rectifier,R102 and caps

has anyone any idea what causes these 2 parts to go? anything else i should check? do you think i should just replace them and see if it works?

i've done this before on Atari branded PSU's and it can blow the new parts again....

any advice appreciated thanks :)



ok i've also found R 106 and R101 to be dead

R 101 gives no reading at all and R106 is high

i've tested all the other resistors and diodes and they are fine

would you say this is ok to test again now? or should i check other parts still? don't want to risk blowing up the new parts!

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Thu Jan 14, 2010 7:18 pm

MRAtari wrote:
MRAtari wrote:not sure if i should ask this here or start another thread but it's the same subject and could possibly be of help to the person who started this thread....



today i have been trying to repair a Mitsumi PSU from an STE (the same model as above)

anyway i have found Q101 and Q102 on the primary side to be dead but PC 101 is ok and so is the bridge rectifier,R102 and caps

has anyone any idea what causes these 2 parts to go? anything else i should check? do you think i should just replace them and see if it works?

i've done this before on Atari branded PSU's and it can blow the new parts again....

any advice appreciated thanks :)



ok i've also found R 106 and R101 to be dead

R 101 gives no reading at all and R106 is high

i've tested all the other resistors and diodes and they are fine

would you say this is ok to test again now? or should i check other parts still? don't want to risk blowing up the new parts!

In the least, replace the low voltage electrolytic capacitors on the primary side and PC101 in addition to the parts you found bad.
Also use an ohmmeter to check the four windings on the transformer. You should find that each individual winding has very low
resistance, whereas each winding is not shorted to another. For certain, the primary windings should be isolated from the
secondary windings (if you can measure the resistance, you need to 'hipot' test the psu).

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby MRAtari » Thu Jan 14, 2010 7:34 pm

Hippy Dave wrote:
MRAtari wrote:
MRAtari wrote:

In the least, replace the low voltage electrolytic capacitors on the primary side and PC101 in addition to the parts you found bad.
Also use an ohmmeter to check the four windings on the transformer. You should find that each individual winding has very low
resistance, whereas each winding is not shorted to another. For certain, the primary windings should be isolated from the
secondary windings (if you can measure the resistance, you need to 'hipot' test the psu).


i should have added that i have changed ALL the caps

can you explain how i test the 4 windings on the transformer?

thanks

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Re: STe PSU repair (Mitsumi 68-4231A)

Postby Hippy Dave » Thu Jan 14, 2010 8:34 pm

Q) can you explain how i test the 4 windings on the transformer?

Using a digital ohmmeter, so as not to forward bias semiconductors (*not on diode test range), probe the transformer pins on
the back side of the printed circuit board. Of the four pins on the primary side, two pins should have very low resistance
between them, furthermore, the other two pins should also have very low resistance between them, however, each 'pair' of
pins should have obviously higher resistance between them. The same should be true of the secondary. Finally, no pin on the
primary side should measure less than infinity ohms with any pin on the secondary.
* If you use an ohmmeter that forward biases components, simply be aware and account for it.
* Note that measuring resistance in both positive and negative directions can give different results depending on
semiconductors, capacitors, galvanic action, dirt, noise and stray currents from the meter and other sources.
You can average the two resistance readings if they are reasonably the same value.


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