Identifying unknown components on heater thermostat board

[nipil]

Hello, i just repaired a thermostat (i replaced an electrolytic capacitor which was out of spec) and now i try to analyze how the rest of the board works, and i'mm halfway through making a schematic.

There are a few items i cannot understand, for which i kindly request your help :

1) R18 = The 5 pin yellow resistor network : 5X-6-182LF

this is a resistor network component, based on 1.8k resistors
but i spent 2 hours trying to find a datasheet or explanation about it
it is NOT a simple resistor network (not single type, not bus type)

as far as i have checked (there is nothing in parallel on the schematic) :
- from pin 1 to pin 2: 1.8k
- from pin 1 to pin 3: 3.6k
- from pin 1 to pin 4: 5.4k
- from pin 1 to pin 5: 7.2k
So the resistors are in series between each pin !

I cannot find the datasheet for any such resistor network, can you help me ?
I find a lot of components with 1 or 2 in the second part of the label, but never with a 6 in it !

PS: Actually that's quite great, because the commutator/slider above maps each commutator position to a different (cumulated) resistor value, which is output to the same pin on the microchip.

You can see a photo bellow (yellowish 5 pin component)



2) The microchip "60-MECO" or "60-MEC0"

It is an 18 pin DIP package. I guess this is a custom made chipset, second identification label : 12426MU (first heater) 1225P8H (second heater)

Can anyone help me find some documentation ?

You can see it in the second photo :





3) The ceramic oscillator "400(Cm)131"

My guess is that it is a ceramic oscillator (it took me 2 hours to discover that such components existed !)

As far as i can tell from what i have read, it should oscillate at around 4Mhz (i do not have a scope to verify it)

Am i right ? and how can i be sure ?

You can see it in the photo above


4) the 4 resistors with a red body

But there are a few other resistors with red bodies. These are on the part of the board which is hotter (discoloration), but are in series and simply dropping 80V and dissipate about 1/3W each to allow a led to be on when AC power is on.

For these 4, i wonder what the body color means (power dissipation capacity ? heat resistance capacity ?) please enligthen me... if it is heat, i don't know why R20 is red too, and not it's neighbours


You can see them (R20/R21/R22 and RP1) in the photo bellow :




5) There is a BIG BIG resistor (RP1) which is obviously some power capable resistor

I don't know (yet) what the big resistor is doing, not what its power requirement should be

Simple question, how do you usually find out the resistor spec (wattage) when analyzing a circuit ?

Same here, you can see it in the above photo



Thanks in advance for your help, i'm kind of new, but i'm having fun learning.

Regards,
nipil


PS: i cannot get attachement inserts to work, i'll edit the post later, sorry

[perieanuo]

R18 is something like r-2r network, those networks (resistors in series or series-parallel) are used generally to set threshholds for later comparators or setup values from digital microprocessor outputs (R/R ladder for example https://www.tek.com/blog/tutorial-digital-analog-conversion-r-2r-dac )
don't know about the micro
the ceramic oscillator is a 'classic' and I got a few ones burnt-up from oil spill over them. the ones I worked were generally 4MHz ones
the red body resistors are maybe 1W resistors, nothing special. maybe I got wrong the power
the big resistor is the one who 'makes' you Vcc supply voltage for the micro, if you dig a little bit he came in series with some rectifier diode and after the rectification you have some electrolityc for filtering and maybe a zener-type schematic for like V stabilised voltege for your micro
hope you got the main ideas

[srb1954]

the red body resistors are maybe 1W resistors, nothing special. maybe I got wrong the power

The red body resistors are probably power metal oxide types. The small ones would be rated about 1W while the big one would likely be 3W.

Check through component distributor's web sites for similar looking resistors and compare the physical sizes to determine the power ratings.

[srb1954]

2) The microchip "60-MECO" or "60-MEC0"

It is an 18 pin DIP package. I guess this is a custom made chipset, second identification label : 12426MU (first heater) 1225P8H (second heater)

Can anyone help me find some documentation ?

You can see it in the second photo :

This chip has the Microchip logo on it. You can possibly identify the family by looking through the Microchip catalog and comparing the pin-out  for fixed function pins like the power supply pins, oscillator pins etc

3) The ceramic oscillator "400(Cm)131"

My guess is that it is a ceramic oscillator (it took me 2 hours to discover that such components existed !)

As far as i can tell from what i have read, it should oscillate at around 4Mhz (i do not have a scope to verify it)

Am i right ? and how can i be sure ?

You can see it in the photo above

This is a ceramic resonator. It is just the frequency determining element for the oscillator in the microprocessor.

This is a Murata brand.  If it has 3 pins it will also incorporate several capacitors along with the ceramic resonator element. Together with the internal capacitors it will resonate at 4.00MHz.

5) There is a BIG BIG resistor (RP1) which is obviously some power capable resistor

I don't know (yet) what the big resistor is doing, not what its power requirement should be

Simple question, how do you usually find out the resistor spec (wattage) when analyzing a circuit ?

Looks like a 3W rated resistor.

If you are simply analyzing a PCB like this you can check though catalogs or component ditributors' websites to compare the physical size of resistors to get an approximate power rating. You need to make sure that you are comparing the same type or brand of resistor as carbon film, metal film, metal oxide or wire-wound resistors will often have different physical sizes for a given power rating. Also different brands of resistors may have different physical sizes even if they are using the same base resistor technology. The power rating of a resistor is not a hard and fast thing; it depends on how hot you want the resistor to get and what lifetime and resistance drift over that lifetime you are prepared to accept.

Lastly, you should check R10. It looks a little burnt.

[gamalot]

The yellow 5pin Bourns resistor network is in the configuration of 106 (series circuit).

https://dtsheet.com/doc/241112/bourns-4608h-803

The ceramic resonators is CSTLS4M00G56

http://www.farnell.com/datasheets/45562.pdf

[cheater]

Is there a reason why one wouldn't replace the resistor network with discrete resistors?

[perieanuo]

Is there a reason why one wouldn't replace the resistor network with discrete resistors?

those resistor networks have some simple purpose, simplify the pcb track management, minimizing board size and not to be forgotted, when you manufacture a network, values of that resistors are very close (same resisting material in production) and the precision is greater than when you put separate resistors.
of course you can replace in the end (and maybe understand why those are choosen ), but for be sure about precision, you should use greater precision standard for the separate resistors replacing the network, it's just math