Weller PU81 slow (low) heating

Thread Starter

cvdabbeele

Joined Dec 28, 2020
23
Hi,
I'm looking for bit of advise here.
Many moons ago I bought this PU81. It has served me well for a long time.
Lately it has begun to heat up slower and slower.
Now I always have to set the manual dial to 450V to make it useful
(the unit has no real temperature meter)

Could anyone please advise on what the most common causes for this issue could be ?
(I'm still using the same tip as when I bought it)
- heating element?
- temperature sensor?
- switching transistor, triac or whatever they use?
I have used the unit once outdoors in the cold and I seem to have the feeling it is then when the issue started (and slowly got worse day by day)

Thank you for your time for reading this and sharing your valued advise
chrisV
 

MisterBill2

Joined Jan 23, 2018
18,535
Hi,
I'm looking for bit of advise here.
Many moons ago I bought this PU81. It has served me well for a long time.
Lately it has begun to heat up slower and slower.
Now I always have to set the manual dial to 450V to make it useful
(the unit has no real temperature meter)

Could anyone please advise on what the most common causes for this issue could be ?
(I'm still using the same tip as when I bought it)
- heating element?
- temperature sensor?
- switching transistor, triac or whatever they use?
I have used the unit once outdoors in the cold and I seem to have the feeling it is then when the issue started (and slowly got worse day by day)

Thank you for your time for reading this and sharing your valued advise
chrisV
The first thig tosuspect is a poor connection that has developed. and is limiting the avaiable power. That might be difficut to spot. It might be a faiing connection in the connecting cabe. or possiby a faiutre in the power supply. So those are areas to inspect.
 

MisterBill2

Joined Jan 23, 2018
18,535
It seems that a blown fuse should resut in NO heating at all, since usually fuses fail completely. I am aware of a partia fuse failure only one time.
 

Dodgydave

Joined Jun 22, 2012
11,303
Have you got a spare iron to plug in to compare , if not here is the circuit diagram , you can check voltages at the 5 pin socket, check for dirty contacts at the socket.

weller_pu81_schematic.pdf_1.png
 

Thread Starter

cvdabbeele

Joined Dec 28, 2020
23
thank you all for your suggestions,
The circuit diagram will be most handy. Thank you @Dodgydave
And the suggestion to try with a different iron to plug in is indeed great....if I had one

I was hoping someone else had encountered the (s)low heating problem, of even if it were a common problem.
Not sure where to start without a baseline (reference)
I guess I can measure some voltages and compare them to the circuit diagram
thanks again,
chrisV
 

Ya’akov

Joined Jan 27, 2019
9,168
I would check the iron itself and make sure, for example, the tip is in good contact and there isn’t corrosion or something else in the way of thermal transfer to it.

Also, heaters do sometimes fail so if you could confirm the resistance of the heating element, which is the 80W @ 24V is right, should be ~7.2Ω. If it is substantially higher that could indicate a problem.
 

MisterBill2

Joined Jan 23, 2018
18,535
One more possible cause would be if the photo-sensor for the standby function is party blocked, so that the "standby" mode is enabled. That should be simple to check. It is just a guess, though.
 

Delta Prime

Joined Nov 15, 2019
1,311
Is it stated as P2 which is P4 in schematic.
The reference temperature is being set normally with a 2k0 Potentiometer P2, which is connected to a 75k Ohm resistor to pin 6 of the LM 358 op amp. The 75k Ohm resistor is large enough to be overridden by another resistor for setting another temperature, which is in this case the standby temperature.

An optical switch detects when the soldering iron is being placed in its stand. In order to give the unit it's normal function when the optical switch is not connected to the unit, two pins break contact, and disable T103.
The resistor value fore the WSP80 soldering iron can be calculated with the following formula:

R=19.96+0.113*T

(R is in Ohm, T is in degrees Centigrade. Measurement had been done while T<200 degr. Cent.))
Higher temperature I could not measure. However, after some correspondence with Weller, I can confirm the resistor is linear to temperature, even for temperatures higher then 200 degr. Cent
1645971555034.png
 

Delta Prime

Joined Nov 15, 2019
1,311
A small PCB fitted all components for the temperature standby controller. The PCB fits behind the soldering iron connector, which is mounted on the main PCB.

Half of the board space is taken by three connectors:
- One for connections to the main PCB
- One for connections to the (optical) stand switch
- One for an auxiliary output, which controls a vacuum air pump.
1645971886429.png
 

Delta Prime

Joined Nov 15, 2019
1,311
Are you not seeing this?

Weller calls "Stop and Go". It could be made on an extra PCB and connections can be added to the normal PCB without removing any parts/traces.
5 connection have to be made:
- X2 (Vac)
- X3 (GND)
- P.6 LM358
- +5v stab.
- Anode LED (2 color led)
The only thing I changed on the original circuit board was the internal (green) led with a multicolor led, which has a common cathode.
 

Ya’akov

Joined Jan 27, 2019
9,168
Is it stated as P2 which is P4 in schematic.
The reference temperature is being set normally with a 2k0 Potentiometer P2, which is connected to a 75k Ohm resistor to pin 6 of the LM 358 op amp. The 75k Ohm resistor is large enough to be overridden by another resistor for setting another temperature, which is in this case the standby temperature.

An optical switch detects when the soldering iron is being placed in its stand. In order to give the unit it's normal function when the optical switch is not connected to the unit, two pins break contact, and disable T103.
The resistor value fore the WSP80 soldering iron can be calculated with the following formula:

R=19.96+0.113*T

(R is in Ohm, T is in degrees Centigrade. Measurement had been done while T<200 degr. Cent.))
Higher temperature I could not measure. However, after some correspondence with Weller, I can confirm the resistor is linear to temperature, even for temperatures higher then 200 degr. Cent
View attachment 261698
You said:

Another individual did have the same problem and found a solution here you go kiddo.
But no "problem" seems to be stated in the post you linked to. What am I missing?

And the TS said:

Many moons ago I bought this PU81. It has served me well for a long time.
Lately it has begun to heat up slower and slower.
Which doesn't seem to be addressed by that post. I can't tell which part of what you quoted is relevant. Could you clarify?
 

Thread Starter

cvdabbeele

Joined Dec 28, 2020
23
Good inputs
@Yaakov Good advice ,
I opened the soldering iron and sanded the back of the tip and the top of heating element (the bar) in order to ensure a better contact. It feels a little better, but that could be wishful thinking.
Would I dare to add some heat-conducting paste to it? Or would that paste burn and make things worse?

@Delta Prime and @MisterBill2

The stand-by switch seems to work fine. When I remove the iron from the holder, the LED on the unit (see attached picture) starts blinking, indicating it is heating up. When I put it back, the LED goes off.

@Delta Prime Thank you for sharing that chart.
That is great information. At room temperature, the resistance of my heating element is about 20 Ohm (assuming I am checking the right pins of the connector). However at 350C (give or take), I measure 40 Ohms. According to the given formula (R=19.96+0.113*T), it should be closer to 60 Ohm. Also, I have a WSP81 and the graph is for a WSP80.
Is that a "known thing"? Do heating elements get "old" ? If they do, then the temperature control should start to work harder to compensate for it.... so, not sure what to think about this.

Schematics (from above)
I understand the purpose of the P4 and the OpAmp in the schematic diagram, however I don't understand why there is another OpAmp preceding that one. And what is the use of P1 and P3... seems all unnecessary to me.

Test:
When I put the pin of the soldering element in some (cold) water, I see the LED changes from "blinking" (=iron is at desired temp) to "fully on" (=unit is heating). So this part also works.
However, with the unit set to 350C, by turning the potentiometer on the front panel to that value, see picture; (without any real, measured, indication of the temperature), solder is not melting. I use the same solder as before. It has been "good" in the past.
(I also have a bad roll from AliEpress. With that one, it's like soldering with mud)
With the Dial set at 400C I can pretty much use it. It seems a bit slow in heating up but it melts solder.

So far my feeling is that the heating element or the temp sensor might have shifted/drifted/worn-out.
Is that a "known thing"? Do they do that?
Maybe time to invest in good temperature meter...

chrisV
 

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Ya’akov

Joined Jan 27, 2019
9,168
Good inputs
@Yaakov Good advice ,
I opened the soldering iron and sanded the back of the tip and the top of heating element (the bar) in order to ensure a better contact. It feels a little better, but that could be wishful thinking.
Would I dare to add some heat-conducting paste to it? Or would that paste burn and make things worse?
I would not use thermal paste, it is not intended for that application.
@Delta Prime Thank you for sharing that chart.
That is great information. At room temperature, the resistance of my heating element is about 20 Ohm (assuming I am checking the right pins of the connector). However at 350C (give or take), I measure 40 Ohms. According to the given formula (R=19.96+0.113*T), it should be closer to 60 Ohm. Also, I have a WSP81 and the graph is for a WSP80.
Is that a "known thing"? Do heating elements get "old" ? If they do, then the temperature control should start to work harder to compensate for it.... so, not sure what to think about this.
My reading of that post has the resistor in the chart as the standby temperature setting resistor, not the element.

But, a naïve calculation considering the 80W and 24V would lead me to expect single digit resistance for the heating element so even 20Ω is high by that reckoning. I don’t know if that is correct, though. I couldn’t easily find any reference to the expected value.

[EDIT: fixing up mangled QUOTE tags]
 
Last edited:

Thread Starter

cvdabbeele

Joined Dec 28, 2020
23
My reading of that post has the resistor in the chart as the standby temperature setting resistor, not the element.
But, a naïve calculation considering the 80W and 24V would lead me to expect single digit resistance for the heating element so even 20Ω is high by that reckoning. I don’t know if that is correct, though. I couldn’t easily find any reference to the expected value.
Indeed, Vcc is only 24V so with a resistance of abt 50 Ohm, this would give only 11,5 Watt
This could explain why it is so slowly heating.
 

Thread Starter

cvdabbeele

Joined Dec 28, 2020
23
And a bit more progress again:
I was able to disassemble the soldering iron itself. This video (in German)
showed me that I had to bend some kind of a lock-pin first to allow the soldering iron to be unscrewed.
Inside I found 4 wires, of which the 2 brown ones were a bit thicker, so I guess those are for the heating element.

The resistance between the two brown wires is abt 9 Ohm..which on 24v, gives us 64 watt... (assuming the value does not change with the temperature... it probably does)
The resistance between the two others is abt 27 Ohm... I assume that is the temperature sensor

Photos attached, in case we get to the bottom of the problem and some one else run into the same issue

I'm slowly getting somewhere.
Shifting my focus to the electronics now.

Correction:
the back of my PU81 says: PU81 + WS81. I assumed WS81 was the model number of my soldering iron.
Now that I have everything disassembled, I noticed an inscription in the plastic molding of the soldering iron saying "wsp80" (see picture). I now assume WS81 is the model number of the soldering holder with standby switch when the iron is at rest.

Thank you all again for your guidance so far,
/chrisV
 

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