Author Topic: Landis & Gyr Shunt motor assembly fix - Strange transformer PCB layout  (Read 291 times)

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Offline nixxon

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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?
« Last Edit: October 09, 2018, 04:14:27 am by nixxon »
 

Offline nixxon

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Re: Landis & Gyr Shunt motor assembly fix - Strange transformer PCB layout
« Reply #1 on: October 12, 2018, 11:09:41 pm »
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
 

Offline nixxon

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Re: Landis & Gyr Shunt motor assembly fix - Strange transformer PCB layout
« Reply #2 on: October 12, 2018, 11:44:23 pm »
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 :popcorn: 

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


« Last Edit: October 13, 2018, 12:17:15 am by nixxon »
 

Online schmitt trigger

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Re: Landis & Gyr Shunt motor assembly fix - Strange transformer PCB layout
« Reply #3 on: October 13, 2018, 12:54:19 am »
Congrats on a succesful repair.

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