Micro-wave Daewoo KOC-972T TRANSFORMER problems

[Les Jones]

Hi spec,
I don't think power rating will be any problem wth the transformer supplying the 3 volts. I think I would prefer using resistors to drop the voltage in this case as calculating the RMS voltage of the odd shapped waveform that would be produced using diode droppers would be difficult. (We need consider the RMS value as it is used to heat the filament.) The 35 volt winding will only be supplying about 25 mA. (The votage on the reservoir capacitor will be close to 50 volts on peaks and there ia 1K limiting the current to a 24 volt zener. I notice that this capacitor is only rated at 50 volts which puts things close to the limit. I think that the 13 volt winding will need to supply at least 500 mA as it drives a number of relays. I was thinking if we used a toroidal transformer for the 13 /12 volts then we could wind on an extra winding for the 3 volts. I would guess it would be about 6 turns per volt so we would only need about 18 turns. We would have to be sure the hole in the middle of the toroid was not filled with resin for mounting it.

Les.

 

[spec]

Hi Les

I don't think power rating will be any problem wth the transformer supplying the 3 volts. I think I would prefer using resistors to drop the voltage in this case as calculating the RMS voltage of the odd shapped waveform that would be produced using diode droppers would be difficult. (We need consider the RMS value as it is used to heat the filament.)

Yes, fully agree that resistors will be ideal for the 3V filament supply. I didn't mean to imply using diode drops for the heater supply.

The 35 volt winding will only be supplying about 25 mA. (The voltage on the reservoir capacitor will be close to 50 volts on peaks and there ia 1K limiting the current to a 24 volt zener. I notice that this capacitor is only rated at 50 volts which puts things close to the limit.

This is where I think a couple of diode drops will reduce the voltage sufficiently. (for a drop of more than 1.8V a couple of series Zeners can be used).

I think that the 13 volt winding will need to supply at least 500 mA as it drives a number of relays. I was thinking if we used a toroidal transformer for the 13 /12 volts then we could wind on an extra winding for the 3 volts. I would guess it would be about 6 turns per volt so we would only need about 18 turns. We would have to be sure the hole in the middle of the toroid was not filled with resin for mounting it.

The difference is, 13V=18.32V peak and 12V=16.96V peak. subtract 1V for the rectifier diode drop gives, 17.32V and 15.96V respectively, which is only an 8% drop. This should be fine: relays have pretty wide operating voltage ranges. If push comes to shove, we can always fit a 3 amp, say, Schottky diode in place of, or in parallel with, the original rectifier diode.

spec

 

[JohnMcC]

Hi Les & Spec,

Les wrote "It's worth doing the same test on the microwave. I suspect that these are displays made just for those products so you would probably not be able to obtain spares. I wouldexpect a resistance reading around 70 ohms. One other thought. Look on ebay for anyone selling these products on ebay. Even if they are faulty there is a chance that the transformers may be OK. (Unless they are being sold by someone that lives very close to you.)"


Cannot find this equipment 2nd hand on ebay or any other sites that I have tried............have set up alerts on ebay if anything is put online by a seller. Ebay does not work at all well in France

I know I have buggered up this transformer...........but I have taken the readings anyway.

With the transformer removed from the board :

Primary: Terminals 1-2 = infinite ohms
Primary: Terminals 4-5 = 18 ohms
Primary: tried all other combinations...........nothing, infinite ohms.

Secondary: Terminals 2-3 = 12 ohms
Secondary: tried all other combinations...........nothing, infinite ohms.

ON THE BOARD ITSELF on the secondary terminals with the transformer removed:
Terminals 1 & 4...........reads 40 on the x10k setting.
Terminals 3 & 5...........reads 30 on the x1 setting.

Hope this is helpful

Thank you,
John

 

[Les Jones]

Hi John,
How do the numbers that you are using for the pins on the transformer relate to the numbering system on the schematic on page 39 of the manual ? I would like to know the resistance between points 8 and 10 where the transformer was connected. These connect to pins 1 and 25 of the display so you could measure the resistance there if it is easier. This is to find out if the filament of the display is still intact.

Les.

 

[JohnMcC]

Hi Les,
I am having a hard job trying to read the manual............so I am not sure !

Have just been and measured the resistances between points 8 and 10 on the circuit board (without the transformer in place) , and unfortunately it is open circuit

Thank you,
John

 

[JohnMcC]

The attached photo shows how I have been numbering them.

What I wrote earlier.............is not the same numbering system so I will try again
ON THE BOARD ITSELF on the secondary terminals with the transformer removed:
Terminals 1 & 4...........reads 40 on the x10k setting. This is 4 and 8 in the photo (albeit on the transformer)
Terminals 3 & 5...........reads 30 on the x1 setting. This is 7 and 10 in the photo (albeit on the transformer)

In black ink above I was just counting the terminal 1, 2, 3, 4 , 5 as they appear on the secondary side.

Hope this is clearer, otherwise I will try again.

Note that on the board..............4 and 5.............and 8 and 10 ..........are both open circuit

John

 

[Les Jones]

Hi John,
I think your numbering of the transformer pins is wrong.
This picture is from the manual. I am assuming it is from the etch side. (Pictures of the board from both sides would be useful.)

From your picture of the transformer.
starting from top row left hand end.
Correct number 1 your number 2
Correct number 2 your number blank
Correct number (Would be pin 3 but cut off as it is not used.
Correct number 4 your number blank
Correct number 5 your number 1

Continuing from bottom row right
Correct number 6 your number 4
Correct number 7 your number 5
Correct number 8 your number 7
Correct number 9 your number 8
Correct number 10 your number 10

We need to know the resistance reading between points 8 and 10 on the board to see if the filament on the display is OK. This will give us a better feeling as to the filaments on the hiFi.

Les.

 

[JohnMcC]

Hi Les and Spec,
Am home early from my weekend away.............dreadful weather !!

Thank you for going to so much trouble analysing the schematics.
Sorry if I have got the numbering system incorrect. I will try again.
I must admit I am struggling with the Daewoo microwave (and the Pioneer Hifi) because very little is written on the circuit board and transformer, and I find the schematics hard to read.

Pictures of the board from both sides would be useful.)

Please see attached. By the way I put all photos that I take in the original "onedrive" link : **broken link removed**
should you wish to review all the photos in the same place........(and do the same for the Pioneer HIFI).
If you require other photos (or a better photo), I will take more.

We need to know the resistance reading between points 8 and 10 on the board to see if the filament on the display is OK. This will give us a better feeling as to the filaments on the hiFi.

The resistance is 30 ohms between points 8 and 10.

Thank you again,
John

 

[JohnMcC]

Photos are in the link, but I will attach them here as well

 

[Les Jones]

Hi John,
Thank sfor the very clear pictures of the board. The good news is that 30 ohms seems about right for the resistance of the filaments in the display so I think it will be OK. I suggest getting the microwave working first then looking further into the HiFi problem. I had a thought about the freezer. I think you are having trouble getting the higher rating varistors suggested by spec. As an alternative you could connect a number of the original type in parallel. Sorry you had bad weather for your weekend away. Did you go far away from home ?

Les.

 

[JohnMcC]

Hi Les & Spec,
That is great news that the resistance of the filaments seems to be about correct.
Let us concentrate on the microwave first as per your suggestion.
With regards the freezer...........everything is on order (albeit from China !!! Sorry, it is so much cheaper !!).
I have ordered the higher rated varistor and the fuses.

My weekend away was to see my wife's parents, so my father and mother in law. They live in the Loiret...... 3 hours drive. They have two recent additions to their family on their pond.........two swans.......which was lovely to see........see photo attached.
I came home in treacherous conditions last night..........fog and rain.......wish I had waited until this morning !

Apparently you are enjoying more clement weather in the UK

Kind Regards,
John