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DavidBelgium
Posted: Sat Sep 12, 2015 8:45 pm Reply with quote
Joined: 15 Nov 2014 Posts: 9
Andy,

I removed ALL diodes at the secondary and checked them each with a lamp and a PSU in forward and reverse direction. None of them was broken.

I also injected the respective output voltages by means of an external lab powersupply and measured at the same time the current consumed and the charge/discharge cycle of every capacitor. All currents are normal (what is wasted in the discharge resistors) and the caps are all fine. It should be as they are all new.

I'm now back looking at the primary side and that's for me unknown terrain given that the datasheets of regulator KA1M0680 are very little detailed on the functioning.

I checked the VCC circuit resistors after having them soldered out (R1/R3) Both are fine and perfectly in spec.

When I measure at the cathode of D2 (VCC), I see voltages between 10,4 and 14.8V. Ramp up from 10.4 to 14.8 in 136ms and ramp down in 17.6ms.
So total period = 153.6ms = 6.5 Hz, which is well below the oscillating frequency listed in the datasheet (+- 65Khz)

But as the voltage on D2/VCC/C9 + is sawtooth like, I presume the thing wants to start but fails to (overload/overtemp/no feedback/...)
The coil seems to be energized for a brief period hence the output voltages at the secondary are also considerably lower than expected ; about 1/3 to 1/4th of what you would expect.
On the anode of D1, I see that the voltage is 312, slowly ramping up to 320V and then peaks to 356V for a duration of 15-20ms. This happens every 150ms in line what I measure on D2. So the FET in U1 seems to be on for 130ms and then is off for 20ms.

The heatsink is cold, so overheating can be excluded
C7/C9/C24 were also new elcos, so I will exclude that too
I also checked C6 and C8 with an LCR meter. Their capacity is ok and the resistance not measurable.


The startup treshold for a KA1M0680RB is between 14 and 16V an typically 15V. So the top of 14.8V is just or just not enough to start the PSU. Maybe it browns out all the time.

Leaves me:
-Check the transformer, but how to do that in a correct way
-Check the shunt regulator U2
-Check the optocoupler
-Change the U1 component

I just don't understand how the chip U1 works and I don't have anything to compare it with.
All switched PSU's I have laying around use other components for the oscillator and other feedback systems.

I might try to order a new U1 chip, but they seem to be made of unobtainium and are considered obsolete.

Any hints/comparable measurements are welcome.
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alan_m
Posted: Sat Sep 12, 2015 11:08 pm Reply with quote
Frequent contributor Joined: 18 Oct 2006 Posts: 3428
DavidBelgium wrote:


I'm now back looking at the primary side and that's for me unknown terrain given that the datasheets of regulator KA1M0680 are very little detailed on the functioning.

.



Application note
ftp://radiozona.myftp.org/radiotehnika/spravochnye_materialy/Mikroshemy/PDF/an-4104.pdf

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DavidBelgium
Posted: Sun Sep 13, 2015 8:21 pm Reply with quote
Joined: 15 Nov 2014 Posts: 9
Thanks Alan

The link was dead however I found another one thanks to the name of the file

https://www.usbid.com/datasheets/usbid/2001/2001-q1/an-4104.pdf
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DavidBelgium
Posted: Fri Sep 18, 2015 7:30 pm Reply with quote
Joined: 15 Nov 2014 Posts: 9
[quote="alan_m"]
DavidBelgium wrote:


I'm now back looking at the primary side and that's for me unknown terrain given that the datasheets of regulator KA1M0680 are very little detailed on the functioning.

.




Looked at the old U1 that I replaced when I changed the caps. There was a short between Fbk and VCC and as result of that, the internal voltage sources are broken.
Then replaced optocoupler, the snubber cap C6, Diode D1 and C8 too.

Soldered out the new U1 and when applying VCC at 15V, I get a nice 7.5V on pen 4(Feedback) and a nice 5V on pen 5(soft start) and a the current is approx 0.01A. When I drop VCC around 9.8 it stops consuming current, so I think it shuts down more or less as indicated in the datasheet. So it seems fine imho.

So except for the transformer, inductors, some resistors and the shunt regulator U2, I almost replaced or checked the entire psu.

I could not imagine that T1 would be gone.
Somebody a spare lying around ?
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andyfras
Posted: Fri Sep 18, 2015 7:53 pm Reply with quote
Frequent contributor Joined: 25 Jul 2005 Posts: 3393
DavidBelgium wrote:
Somebody a spare lying around ?

I can offer an exchange PSU - I like a challenge.

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DavidBelgium
Posted: Mon Sep 21, 2015 6:39 pm Reply with quote
Joined: 15 Nov 2014 Posts: 9
andyfras wrote:
DavidBelgium wrote:
Somebody a spare lying around ?

I can offer an exchange PSU - I like a challenge.


Andy,

Can you give me a quote P&P included.
It's not mine, so I have to check with the owner if he is willing to spend any money on "old" hardware
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andyfras
Posted: Wed Jan 18, 2017 10:22 am Reply with quote
Frequent contributor Joined: 25 Jul 2005 Posts: 3393
I have recently repaired an Icecrypt T2400 PSU for a forum member. It uses the poor quality Sam Young capacitors as used in the Toppy.

If anyone is interested, I could provide a kit.

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