dropping AC voltage on secondary

[mariush]

The heating element of the soldering station may actually receive AC voltage, not DC.

On the classic Hakko 936, a triac with a zero-crossing detection optocoupler/driver is used to turn on and off the AC voltage going to the heating element.

In the pictures above, that BT137 on the small heatsink is a small triac with a gate that can be driver directly from a microcontroller ... in the case of that circuit board, directly from LM358 or that other transistor. Guess they don't care about turning on and off when the AC voltage is very close to 0, they cut the driver to reduce costs.

The LM358 uses very little current, and you only need enough current to power the opamp, the red led and current going to the base of the triac ... so you can get 5v from 24v+ AC using a plain diode (half wave rectification), a capacitor and probably using a zener diode (Ex. 5.6v zener)

For comparison, here's Hakko 936 schematic that uses a triac driver and triac : http://dalincom.ru/datasheet/HAKKO_936_schematic.pdf

and this slideshow also has schematics for Gordak 952 and other very similar stations, still using triacs to turn on and off the AC voltage. : https://www.slideshare.net/tarzan1a/hakko-936-gorsak-952-diy-analog-soldering-station-schematic

[floobydust]

OP, make an effort to find the schematic for your 936 clone controller board. Or just sketch it out. Guessing is not going to work well.
It looks just like the (cheaper build) Atten 936 here: https://www.eevblog.com/forum/reviews/atten-936b-another-hakko-clone/
But your smaller resistors are 1/8W not 1/4W. I found a schematic sorta like how it is for architecture- three zener diodes. Two make +/-7.5V for the op-amp power and a 4.3V for the reference voltage. I think some use thermocouple others use RTD as the sensor.

That circuit looks like it uses a floating 1/2 voltage node (thermistor/thermocouple return) which would get destroyed if there was ever a heater to temperature sensor fault.
If you have a short in the handle/heating element it would explain your smoke. I'd draw the schematic but no time and can't read all the parts.

[Sredni]

Not sure exactly what class transformer it is.

"Power transformer
Model: YB-E166*28F
Input: 110V/60Hz RED RED
OUTPUT: 24v.        BLU BLU"

Its a yihua 936 transformer.
The 24v (ac?) Feeds the heating element directly, or in my case 33v?.

I might be too naive, but...

24 Vrms are 33 Vpeak.

How did you measure 33V? What instrument?

[DavidAlfa]

Of course it's AC, don't you see the BT137 triac in your own pictures?
Sure, replace 7v5 zeners with 5v1 and figure out why it's not working...

[sahko123]

Is it 33 before or after rectification? 24vac converted to dc would be roughly 33v so you might already be getting what you want

[algorithm]

How do you not have access to a camera? They have been part of every mobile phone I've ever owned, including the very cheapest flip phone I was able to buy back in 2007. Cast off PCs are easy to find too, they get scrapped every day due to having such low value. Do you have a thrift store nearby? I have donated several digital cameras to those here and I see them on the shelves frequently.

Well it was just easier to say no camera than explain why I cant transfer the photos from the sd card to this tablet with no pc.

Ill be honest with ya, things are rough for me. I dont have a phone. Well a landline I do. I dont really go out into the world. But I do dream of going to thrift stores, I just never do. I also know I should get a pc but Im really poor.

All the things people work for in life I dont have. For 10 years ive just been isolated like a monk.

Im just a loser. But no big deal. Im not looking for handouts. although I admire your generosity and helping and your concern.

And who knows maybe things will improve someday.

[algorithm]

I might be too naive, but...

24 Vrms are 33 Vpeak.

How did you measure 33V? What instrument?

Fluke 12 dmm in ac range. 33v ac was what I read. But its working now so case is solved.

[algorithm]

OP, make an effort to find the schematic for your 936 clone controller board. Or just sketch it out. Guessing is not going to work well.
It looks just like the (cheaper build) Atten 936 here: https://www.eevblog.com/forum/reviews/atten-936b-another-hakko-clone/
But your smaller resistors are 1/8W not 1/4W. I found a schematic sorta like how it is for architecture- three zener diodes. Two make +/-7.5V for the op-amp power and a 4.3V for the reference voltage. I think some use thermocouple others use RTD as the sensor.

That circuit looks like it uses a floating 1/2 voltage node (thermistor/thermocouple return) which would get destroyed if there was ever a heater to temperature sensor fault.
If you have a short in the handle/heating element it would explain your smoke. I'd draw the schematic but no time and can't read all the parts.

And the winner of a gift from life or beyond life is floobydust. In india there is ancient belief that life has gifts from higher realms discoverable through life. I like to imagine floobydust earned that by presenting the gift of suggestion that made me think to try the simplest thing.
how ironic that for maybe years this sat for parts and all it needed was a new handle/thermistor. And I even tested the heating element resistance as mentioned earlier. Both identical handles but 1 element 19 ohms, the new spare 12. But the thermistor pins on the old handle tested wildly all over the place, jumping from 0.9 ohm to 10 and all in between from seemingly breathing in the same room. While new spare was 0.9 steady at idle room temp.

Noticed the old handles pcb pads looked crusty, like a white flux film was on some. Poor solder on 1 side on every through hole joint. But I fear If the thermistor is burnt I cant get it out of thd tube over the element. So even if I make sure the joints are better, it risks smoking r1 and r2.


By the way? Replacing all the parts I did wasnt needed. I burnt like 5 of both valus resistors trying to repair the board over the years. Wait, I do think I needed to replace the lm358. But everything else was good. Zeners were originally 7.5 im pretty sure. Well the reason I feel that way most is that the board started to smoke r1 and r2 after I put 5v zeners. Only not using that handle changed then. And now it works!
Tested voltage across resistor r1 (before trying new handlree, in the 4 secionds it took me to probe with dmm and power off a woft of smoke came off both resistors) I got 0.130 and if my math was right 0.1a would be the draw for 330ohm on 33v. But yeah new handle and the only semi hot parts are the 2 biggest resistors. R6 and r14. At 30c.

Thanks to everybody who took a crack and offered perspective.
This station is back in business after a long hiatus. Its thankful I bet.
Im thankful to not smell more burning resistors.

[Zero999]

Just a few points.

The LM358 will be powered from a zener voltage regulator. Even if the transformer were to output 24V, it would be at the absolute maximum rating.

The element is switched by a TRIAC and operates of AC, rather than DC.

The circuit should work find off a slightly higher transformer. The only part of the circuit which might need to be modified is the zener regulator. Increase the value of the resistor by 10% and use a ceramic power resistor and spacers, to stop the board from overheating, if practical.

[floobydust]

I'm glad you got it fixed, I could see no other reason for those resistors to burn up. I'll see if Ganesha or Shiva gave me the gift 
Mains voltage keeps getting higher and higher, it's a way to transmit more power on old utility equipment (wires, substation transformers etc). So "24VAC" is much higher than expected for that reason.

[tunk]

Im just a ....

You're not - I know the feeling of not living up to high goals you set for yourself.
Maybe you could try to look at the positive side, with a bit of help you did fix it.

[algorithm]

You're not - I know the feeling of not living up to high goals you set for yourself.
Maybe you could try to look at the positive side, with a bit of help you did fix it.

Thanks tunk. Youre right. And seems looking at the glass half carried over into another repair success.
Managed to wrap up a repair on a yammy receiver that took me a year+. Definitely improvement.   
Im looking forward to your next posts tunk. Always appreciate your perspective.