Weller WS51 / PU51 repair hints request

[jjimbo]

Hello,

I bought a used and untested Weller PU51 station without a soldering iron for cheap recently. When ON/OFF switch is "ON" the power connector blades measure approximately 37 ohms between them but without iron connected to the station the station LED does not turn on (which is normal for these stations I understand).

My idea was to buy a cheap chinese iron that works with Hakko T900 tips to test the station and if it is working then use it. Got insipiration from this video , bought this iron from AliExpress: https://www.aliexpress.com/item/1005005968150408.html (A2 type on that page).

Working out the PU51 station's soldering iron connector pinout was confusing but in the end I came to this version (when facing the fron end of station, see also attached connector picture):
PIN 1 heating element
PIN 2 heating element
PIN 3 sensor
PIN 4 sensor
PIN 5 ground
PIN 6 unused

This picture is taken from the WES51 manual, I could not find a WS51 / PU51 manual that would contain a picture of the connector or details on what pins to measure for the 24V AC.

The WS51 seems to be quite similar to WS81, they are contained in the same manual and although the WS 81 / PU81 has a 7 pin connector the actual pinout sequence for the first five pins seems to be the same (see pages 51 and 52 of the manual). I also attach the schematic of PU81 station that I guess is very similar to PU51 (the latter missing some components, including having just one opamp instead of two).

I rewired the chinese soldering iron to follow the above pinout. So pin 1 would go to  heating element + wire pad, pin 2 would go to heating element - wire pad, pin 3 would go to sensor wire pad, pin 4 to the other sensor wire pad and pin 5 to the irons grounding wire.

I am not sure if it would be important on the soldering iron PCB (there are just four pads on the PCB and wires go from the pads to the ceramic element / sensor; no other components on the PCB) which of connector's sensor pins is connected to which sensor wire pad. But I followed the sequence that PIN 3 connects to the sensor's first wire pad and PIN 4 to the sensor's second wire pad.

The heating element of the iron measures 1,3 ohms on sensor pins and 13,3 ohms on element pins. I do not know how this would compare to a normal Weller iron that could be used with the WS51/PU51. The default iron for WS51/PU51 would be LR21, but I could not find any documentation or other source that would name heating element and sensor resistance values for it.

When measuring AC voltage on the station without the chinese iron connected I get the following values:

PIN 1 to PIN 3: 26V when "ON", 0V when "OFF"
PIN 1 to PIN 4: 26V when "ON", 0V when "OFF"
PIN 1 to PIN 2: seems to be floating around 1,8V when "ON" and 3,6V when "OFF"
PIN 2 to PIN 3: seems to be floating around 4 V when switched "OFF" and 15V when switched "ON"
PIN 2 to PIN 4: seems to be floating around 4 V when switched "OFF" and 15V when switched "ON"
PIN 3 o PIN 4: 0V both "ON" and "OFF"

Please note that the soldering iron connectors spring pins on the station seem to be quite loose so I am not quite sure about these  PIN 2 values.

Now the problem - when I connect the station to power and switch it "ON" with the rewired chinese iron connected the LED on the station turns on and then the 315mA fuse at the power connector immediately blows. I do not think there is any other fuse on the controller board.

Any ideas on what could be the problem or what should be checked are welcome.


UPDATE: Apparently I had resoldered the iron wires to wrong pins. After correcting that today the station does not blow a fuse again.

But the temperature regulation does not seem to work, at least not correctly - with the minimum setting (150C) on the dial the station goes over 250C the station LED is constantly on and  there is no sign of the station stopping heating up (blinking led or temperature rise stopping).

I plan to swap the sensor wires in the iron as they have to with correct polarity in case of a thermocouple I understand, but I am doubtful this will help.

UPDATE #2: swapping the sensor wires had no effect - tip temperature over 400C with the minimal setting on the dial and the station LED constantly on, not blinking.

[m k]

Seems to be unsuitable temp sensor then.

Unplug the iron when it's hot and measure what the sensor value is.

Use a trimmer and check how Weller wants it.
If it doesn't work Weller can use three or four wire configuration.

[jjimbo]

Seems to be unsuitable temp sensor then.

Unplug the iron when it's hot and measure what the sensor value is.

Use a trimmer and check how Weller wants it.
If it doesn't work Weller can use three or four wire configuration.

Thank you for the suggestions.

First measure the resistance of the hot iron's sensor I understand. Or measure the voltage in mV?

Then I should use a potentiometer (or something similar) between the iron and the sensor input of the working station and see if the iron's output can be made acceptable for the station.

Or did you have in mind tweaking the two smaller potentiometers (or at least similar looking components) inside the station's case?

The comment about three or four wire configuration I did not understand - if the station's connector expects two pins for sensor and two for the heater I guess three pins should not be an option. But I am quite in the dark with all that.

[m k]

Resistance thermometers have different styles.

https://en.wikipedia.org/wiki/Resistance_thermometer

[jjimbo]

OK, I see now what you had in mind, thank you.

[inse]

I have repaired both Weller stations and irons already.
The iron has also a resistor parallel to the sensor on the connection PCB inside the handle.
Maybe to linearize the characteristic curve?
Cold resistance is around 25Ohms, hot (<400C) ~50Ohms

[jjimbo]

Thank you, now I know the approximate resistances of at least some Weller irons. And the information about the parallel resistor was also useful.

[inse]

There are also calibration pots on the station PCB.
You can change gain and offset of the regulation.
I don’t know (or better doubt) if you will be able to make it match to the printed scale.
Maybe you will have to overlay a new one matching your iron.
Let me search for any documents I might have kept somewhere

[jjimbo]

Needing a new overlay for the iron's scale would not be a very big problem. If I would get the station to be controllable by the dial on the station at all this would be quite good for starters, tweaking and a new overlay for temperature could then possibly follow.

I have not been able to try using the trimmer, as was suggested above, yet. But I plan to. Eventually.

Using the calibration pots on the control board inside the station is somewhat complicated because first I have no clear understanding what each of them does and second I also have no real understanding how this station is supposed to be opened without damaging it. The case has no screws, just plastic tabs, and I do not seem to find any instructional videos on opening that type of station. I tried to figure it out by looking at photos of replacement cases on digikey etc but this did not quite work out - I managed to get the station opened finally, but many of the plastic tabs broke off. I glued them back but I will certainly break them off when trying to open the case again.

[inse]

You have to start at the front panel, there are some rather tiny holes where you can unlock it.
I marked the original wiper positions with a small scratch and then tried to figure out what function they have.
In parallel I measured the tip temperature with a thermocouple inserted into a hole drilled into an old tip.
Maybe you find another way to make good thermal contact.

[jjimbo]

Thank you for the tips.