Landis & Gyr Shunt motor assembly fix - Strange transformer PCB layout

[nixxon]

Hello

I am trying to fix parts of a 1980's hot water boiler that has failed in a house. It is the unit controlling the water mixing shunt actuator that has failed. The part is labeled "Landis & Gyr RVK 31.2". The servo actuator motor is controlled by the circuitry on the enclosed photo. I had to desolder the PCB mounted block transformer (220VAC/24VAC 1.5 VA, 63mA) that was hot and malfunctioning. (Transformer Output was 1 VAC rather than 24 VAC, Primary coil pins measure 2.7 kOhm, all three secondary coil pins show up as open circuit "OL").

According to all pin layouts I have seen online, these transformers have the primary coil connected to pins 1 and 5.
The secondary coil seems to be connected to pins 7 and 9 if only one secondary coil. If dual secondary coils, one coil seems to be connected to pins 6-7 and the other coil to pins 9-10.

The transformer I removed had only secondary pins 6, 9 and 10 present. On the PCB, pins 6 and 10 are connected to each other and one of the input pins of the full bridge rectifier. There is a trace on the PCB connecting Pins 7(pin missing from transformer) and 9 er to each other and the other input pin of the full bridge rectifier.

According to modern standards pins 7 and 9 should not be connected, no matter if there is one or two output coils.

Does anyone know if the PCB mounted transformers had a different pinout 30-40 years ago?

The output needs to be 20-24 VAC from the transformer to the full bridge rectifier.

I have ordered a 24VAC, single secondary coil safety transformer from Farnell (output pins 7 and 9 used):
https://no.farnell.com/myrra/44090/transformer-1-5va-24v/dp/1689052
Pinout:
http://www.farnell.com/datasheets/92169.pdf?_ga=2.50384361.1589188987.1538948474-1149254780.1537819090&_gac=1.90504552.1538948474.Cj0KCQjw3ebdBRC1ARIsAD8U0V4HijGH86wsJAEbVPoETdlRsRD3iYxybcOJV3iW_7SPSfpHzI_6vAUaAmN6EALw_wcB

My plan is to cut the trace on the PCB between pins 7 and 9. The transformer output pin 9 will then lead directly to the full bridge rectifier.
Then I will solder a wire between pin 6 and 7. The transformer output pin 7 will then lead to the full bridge rectifier via pin 6.

Any thoughts if this is a way to go?

[nixxon]

Pictures of the broken transformer.

It is labeled:

4500 4705.0
220/24 1,5 VA 83-10

Looks like it held up for 35 years this October. Not too bad

[nixxon]

The replacement transformer is this one
https://uk.farnell.com/myrra/44090/transformer-1-5va-24v/dp/1689052?st=44090

The unregulated voltage output is a little high at no load: 37 VAC. Maybe I should have opted for the 18V version https://uk.farnell.com/myrra/44089/transformer-1-5va-18v/dp/1689051

After removing the old transformer, I noticed a few lifted solder pads due to lots of heat from the broken transformer. The PCB itself was discolored brown by the heat as well.

I cut the PCB trace between pins 7 and 9 to avoid shorting the secondary and soldered the new transformer in place. The pin 6 solder pad was pretty busted, so I bodged in a wire directly to the full bridge rectifier input.

There is a 15V voltage regulator on the PCB that has a maximum input voltage of 35V. When I measured voltages on my desk after the repair, the voltage was ~28V while one relay was energized. I just hope the voltage doesn't rise above 35V when installed and running as the relay is energized only for short periods of time when operating the shunt motor. The voltage regulator UA7815UC is supposed to be pretty fail safe ("essentially indestructible") according to the specs: https://www.usbid.com/assets/datasheets/AC/ua7815uc.pdf

I sent the repaired unit to the owner yesterday. He reinstalled it in his heating system today. The repair seems to be a success as the shunt motor is operating again.

The owner (who was getting ready to pay up to $ 3000 for a new system) is joyfully sitting in his boiler room chair  watching the unit operating the shunt valve  

I hope my repair wil make his heating system work for many years to come.

[schmitt trigger]

Congrats on a succesful repair.

BTW, what a beautiful example of 1980's type technology! Including a hybrid circuit assembly.

[Jocke]

Thank you for a very inspiring reports nixxon.  I have EXACTLY the same circuit card with the same layout. I have corrosive problem with the pins on the same transformer as you describe - only 3 pins left in the corners of the transfomer and as I understand it there should be 5 - not 6 from the begining? I have also corrosive problem with the pins on the small black IC-circuit(?) very close to the relay 3 (REL 3) and the transformer. It has 2x2 pins and is connected to what you assign as + and -  on one side and to point 9 and 7 and to point 6 and 10 on the transfomer. - se your picture called "PCB Landis Gyr.png".  Do you know what this thing is? I had to replace that one too - but what is it? As I understand it from your text you call if "full rectifier bridge". It can be seen on your photo called: IMG_5851.jpg. Do you know where I can buy a new one? I have problem understand the text and specifications for the rectifier bridge.See my two pictures.
I searcehed the internet att it seems(?) a rectifier bridge is rather inexpensive and are only labeled by voltage and current like 400V 0.9A OR 250V 0.5A OR 500V 1A and most of them have 4 pins as can be seen on the PCB. If I understand selecting the rectifier bridge doesn't seem to be critical. I guess 1000V 1A should be OK as long as it can be fitted the the 4 holes on the PCB and knowing which side is the input?
I guess this one would suite well?
https://uk.farnell.com/vishay/df02m-e3-45/diode-bridge-rectifier/dp/1651041
Do you think so?

[Jocke]

BTW my transformer is labeld:

   4  5,00  4  705      ( there are some space around the digit 4)
 
   1,5 VA 68    07

I'm quite sure of the second line and a bit unsure of the third digt on the first line:
   4 5,?0  4  705

I have also the same voltage regulator.

[Jocke]

Hello nixxon!
Today I changed the transformer as you did. I ordered ,Farnell, 2 pieces the 18V and the 24V
I measured the primary coil - the one on the PCB was 2.8 kOhm and the new 24V was 3.1 kOhm and the 18V was 2.9 kOhm
and  the secondary coil was (original) 66 Ohm -  the 24V = 101 Ohm  anf the 18V=62 Ohm ( pin 7-9)
But I decided to use the 24V as you did but was a bit surprised as it's secondary coil was 101 Ohm - clearly more than the other two.
I have too little electrical knowledge to evalate this difference and what it actually means electrically.
Higher or lesser secondary voltage?

I my case I also had to change the bridge rectifier. I ordered 600V 1A DIL-4, DF06M.  # 30048111. I orderd 3 pieces in case I would broke a pin.

I put it in place about one hour ago and now IT WORKS!!!!! The LEDs is working and the shuntmotor moves back and forth as expected.
Many many thanks to you nixxon and your instructive report with many excellent photos with your yellow markings!
I think I also was lucky because the reason for malfunctioning in my case was due to corresive - the pins on the rectifier was broken and so also three pins on the transformer -  it was easy to see where the problem was.
many thank again.
/Jocke