Stahl Soldering Station

[jobog]

I have a Stahl TCSS soldering station and the .33ohm, 3W resistor that goes from the gate of a BT136 triac to ground has burnt up.  The station uses a ZD 919-2 board.  I have checked everything on the board except the two ICs, which I have no way of checking, and everything checks OK.  How do these stations detect the temperature of the iron tip and know when to shut the heater off?  When I replace the .33 resistor the iron will heat up but never shuts off and in a short time the .33 resistor burns out.  The ICs are a 555 and an HA17358.  Could one of these ICs be causing the problem or could it be in the iron itself?  The iron measures about 5ohms across the heater.

[Paul Price]

If the .33 resistor burns out, my best guess without an exact schematic is that you have a shorted Triac and/or the Triac triggering circuit is itself shorted. If you can somehow remove the gate connection of the Triac to the circuit board, and the iron still gets hot, then you will know that the Triac is shorted.

[picandmix]

Your station looks to be a 936 clone so if you search on '936 schematic' you will find a number of circuits, one may match yours though suspect yours will be a slightly different and simpler version of the original hakko 936 circuit.

The iron contains both the heater and a sensor, if you look at the instruction manual shown on this ebay page it shows how to check the sensor with a meter.
http://www.ebay.co.uk/itm/312087-936-Digital-Electronic-Soldering-Rework-Station-60W-Temperature-Control-/291045808706?pt=UK_Home_Garden_PowerTools_SM&hash=item43c3ae9a42

[jobog]

Thanks for the link and the info.

Joe

[jobog]

Picandmix:

Finally got around to checking out the info you sent and I still have a question as to how this Stahl station detects the temperature.  Unlike the Hakko 936 the Stahl iron only has 3 leads going to it.  One is the earth ground that is connected to the iron tip for the ESD protection and the other 2 are connected to the heater.  The heater measures just short of 4 ohms so I'm assuming the heater is okay.  There is no sensor in the tip.

I was wrong about this .33 resistor that is burning out.  It is not connected to the gate but is connected to pin 1 of the triac.  The triac tests okay.

Can anyone tell me how this thing is sensing the temperature and what might possibly cause it not to shut off?

[picandmix]

Assume from the lack of a sensor in the iron then you have no true temperature control.

The lower form of 'controlled' stations just use what is basically a pot controlling the flow though the triac, like a light dimmer, which does produce a basic temperature control.

Have used one like that, but recently bought a 936 clone like the one shown eariler; think you will find they are a good step up.

Think you will find its a better move than messing around repairing your old one.

Just ensure you get a true 936 type which does have the two extra wires for the temp sensor in the iron.

[jobog]

Again, thanks for the reply.

I have purchased a new station.  I got the Weller WES51.  Now wish I had bought the Hakko for the same money.

Repairing the old station has now become a project without a good reason.  Just want to be able to say I fixed it and understand how it works.  The Stahl did hold a set temperature pretty well when it was working and the LED would cycle on and off with the heater.  Still don't know how it sensed a temperature to turn the heater off.  I'm assuming that in stations such as my new Weller, with a sensor, that the sensor is some type of varistor in series with the pot and when the resistance reaches a certain value the triac kicks the heater off.  Then, when the varistor cools down the total resistance changes and the heater comes back on.

Like I said, the Stahl station worked well until the resistor on the board burnt out.  Could there be a sensor in the same line as the heater buried somewhere in the ceramic heater?  Since the unit heats up but doesn't shut off any sensor in that line would have to be shorted so I don't know how I would be able to measure that.

I think I will keep plugging away off and on but it would be much simpler if I could find out if there is indeed a sensor and if not how it knows to turn the heater off and on.

Thanks again for the reply.

[Paul Price]

Could be that the heating element has a temperature coefficient and the circuit switches from powering the heater to measuring the resistance. Could be that the ground wire and one of the heater leads are the two leads of a thermistor.

If the power to the heating element is unisolated 120 or 240VAC then the ground lead is only a ground.

You can heat the old handle up with your new iron or even give it a treatment on the kitchen stove to find out the truth..let us all know.

It is also possible to use only two wires and a reversing voltage, so by using  steering diodes it is possible to switch between the thermistor temperature sensor(low current negative voltage) and powering the heating element(rectified full-wave positive voltage controlled by PWM or a phase triggered SCR/TRIAC).

[picandmix]

The best thing to do, is draw out the full circuit of that board, then you can see exactly how its intended to work, either as a basic dimmer or some form of feedback control.

Upload your diagram so we can have a look.


The WES51, whats wrong with it ? 

Would have liked to buy one of those myself at $100  but seems its a US only model; over here in the UK /EU the cheapest Weller Station is about $300+

[jobog]

It will take me awhile but I will draw out the circuit and post it here.

The Weller station works just fine but, as it turns out, I could have bought the Hakko with the digital readout for the same money.  I think I paid $119.00 for the Weller with 5 additional tips.  I'm finding the digital Hakko for around $90.00 with one tip.  The Hakko also has a ceramic heating element while the Weller is Nichrome wire.  All the reviews I'm reading now, after it's too late, prefer the Hakko.

[picandmix]

Weller has been the world #1 for so many years, don't think it will disappoint.

Digital displays are not going to make it perform any better, the more parts, the more to go wrong.

Reviews, are only as good as the reviewer and their opinion.
If you saw Davys review of the cheapo  $16 solder station ( aka 936)  you would never touch one , yet I recently got one and find it more than good enough, ok it may not last a lifetime but at the price ... and yes, I have used Wellers at work.

If you are having trouble with your circuit board layout, can you take a clear pic of both sides of the board and post them up ?

[jobog]

Here are pictures of the board.  I will keep plugging away on the schematic.

I know Weller was good and that's why I bought the WES51 but now I'm reading that since the assembly has moved out of the US the quality has really suffered.

[picandmix]

Should not spend your time worrying about the Weller, just use it, if it ever fails then your supplier or Weller direct should sort it out.

Nice clear photos, just a couple of points, is ic2  the 555 ?
The resistor below R4 , is that partially burnt as well ?
Does the station have a mains transformer, typically 24vac output , or its the board just fed straight from the mains voltage ?

You say the heating element shows 4 ohms, but with the iron disconnect from the station have you measured across the heating element and the irons metal body in case it is shorting to ground ?   You should not get a reading of less than 2-3 meg ohms if ok.

[picandmix]

Been able to do a very rough diagram , see attachment.

Need you to identify a few things to complete it and lay it out more logically.

First, details of how and what power wires are connected to the two sockets.

The positive /negative orientation of the Electrolytic caps.

The component number of some items and some values, like the Zener Voltage and transistor code where I have left a ? mark

A bit difficult to see all the tracks around the pot, but think I have worked out most of them apart from where R10 and C3 go diagonally down behind the pot ?

Find when working on transformerless designs like that you often get a chain reaction where one part blows the other etc etc or it damages another part only for the next item to fail when you power up.
Generally find its better and quicker to simply replace all the active parts, partic the Zener,. 
So a list of codes for the Triac, transistor, diodes , 2 ics (lm358 should do)   and the electrolytic caps would all be on my shopping list.
( on some very old  boards I will literally strip it and replace all parts to ensure its reliable long term, (commercial use) )

However whats the cost of those parts v a replacement ?

But don't forget to first check out the iron itself  for shorts !

[jobog]

Here are the answers to your questions.  If you need anything else just let me know.  I really appreciate all the time you are putting into this.

Thanks,
Joe

[picandmix]

Here are the answers to your questions.  If you need anything else just let me know.  I really appreciate all the time you are putting into this.

Thanks,
Joe

No problem, it gave me an excuse to escape the girly chatter for half a hour when friends came around. 

Will update the diagram and post it later, but just one more bit of info I need on the tracks surrounding the pot.
Can you check visually or with your meter these highlighted tracks around the pot.
Does the upper red track join the yellow track ?
Does the blue track just go as shown.
Do the lower red and purple track just go as shown.

J1 and J2, they are just jumpers, just pieces of wire with a ceramic body, also called zero ohm links, hence the single black band (0) ,expect its so the auto insertion machines could handle them like resistors.

[picandmix]

Revised diagram attached,  just needs you to confirm  those tracks mentioned in the last post.

[jobog]

Here are a couple pics with the pot removed.  It looks like they run just as you showed.

[picandmix]

Ok, have updated the diagram, so subject to your final check that its correct  and putting in the resistor values for future reference, my job done.

So, what route are you going to use to repair it , just the triac , res, and transistor or all the active parts ?

Also , short of the proper meters, do a visual check on the 4 electrolytic caps, are they secure, any leaking substances and are the tops of the tubes flat ?  if they are slightly bowing upwards thats a sign they are failing.

good luck

[Paul Price]

The original enigma was concerning how temperature control seemed to be working, from the schematic I see no feedback, so the temperature control is not temperature regulation an all you end up with is a cheap station with phase control of power using a triac.

I was hoping to see some feedback to create temperature regulation, phase control of a triac guarantees poor performance.

My conclusion then is to well,  Weller your way back to better joints, this Stahl iron is always only at the right temperature when not soldering, too cold when heat is needed or too hot if you stop soldering for a few seconds. Its a fiddle box.

[jobog]

I already have a triac and transistor along with the burnt out .33 resistor and the 200 ohm that's showing signs of heat.  I will replace these items and see what happens.  If it still doesn't shut off I will move on to the electrolytics and the ICs since I can get all of them from Newark for just a couple bucks.  I also have a Newark coupon for 20% off that expires the end of the month so an even better deal and I can see what else I need (want really) to make the shipping worthwhile.

Thanks for all the assistance.  I still don't understand how it knows when to shut off and will have to do some more studying till I do.

Joe

[picandmix]

You really do want to replace that Zener diode before replacing IC2.
If the Zener blows, the overvolts can damage IC2 then in turn damage IC1 etc  and so a chain reaction.


Ic2 A senses either side of the Triac and then passes its output to Ic2 B but notice cap C3, that will create a little time delay .
The other input of Ic2B is the control pot which it compares with the other signal and in turn triggers the 5 second  heat pulse from the 555.

Exactly how Ic2 A senses around the triac I'm not sure,  but think some other cleverer folk will be able to explain that for you.

[jobog]

Okay, I will replace the zener as you recommend.  Thanks again for all the work you have done on this.  I can't edit your schematic or I would add the values for the components myself.  If you are up for it here are all the missing values.

The board is a ZD 919-2 which seems to be a pretty common Chinese board for cheap soldering stations.

R1  52ohm, 5%, 3W
R2  300ohm 5%
R3  .33ohm 5% wirewound
R4  15K 1%
R5  13K 5%
R6  200K 1%
R7  3.3K 5%
R8  1.3K 5%
R9  51K 5%
R10  530K 1%
R11  100K 5%
R12  2K 1%
No R13
R14  130K 5%
R15  1.5K 1%
R16  5.1K 5%
No R17
R18  15K 5%
R19  10K 5%
R20  200ohm 5%
W2  75K 1%
Variable resistor  1.2K full range, linear
The 4 diodes are N4007's
The 2 small caps are .1uF
C4  47uF, 25V
C5  330uF, 25V

I think I've included everything here.  Thanks again.
Joe

[picandmix]

No problem, will do tomorrow, though will you check  the resistance of the Variable,  1.2k is not a common value for pot.

Are you measuring  it out of circuit and across the outer connections ?

There is often a stamp or lettering on the body stating its resistance.

The wattage for R3 might be hard to establish but its always better to get the biggest wattage that will fit in the space; plus it dissipate the heat better.

[jobog]

It's a 1K pot, just measures a little larger.  I'm replacing the .33 with a 3W as they are about .40c each.  Going up to a 4 or 5 watt raises the price to a couple dollars.