Heater thermsotat receiver unit

[kellogs]

Hello,

my central heating would not start / stop every time the transmitter thermostat would issue a start / stop command, but only some of the time. Took the receiver apart, found the film cap (yellow) next to the output relay to be off by a big factor (200 nF measured / 560 nF nominal) and proceeded to replacing it.
Put everything back together and... the receiver would not start at all. Switched back to its faulty yellow cap, still nothing. Of course, I don't know how I have managed to break it that badly. Measured its input at 227 V and its on-off switch to be still closing/opening the circuit.

I have noticed this gooey area in the middle of the PCB, feels wax-like and pretty dry; I don't think I have caused any component to spill that goo out but I would like a second opinion. A similar (if not same) substance is present in the two lower left corners - attached to that red wire and even inside the on-off switch. Could that be causing any issues ?

 

[Tha fios agaibh]

I don't see the relay or a yellow cap. ?

That gooey stuff just looks like glue used to hold the wire down to the board. It probably accidentally got in the switch. Not a problem.

But the area around Q1, Q2 is badly corroded and a may have been the original problem. Possibly the nearby cap leaked?

Are you sure about the values of the film cap? I have misread marked values many times.

 

[kellogs]

Sorry, they are both on the right side, big, rectangular; only their corners sticking out in the photos.
That is neither corrosion nor wax. The little red thingie with a screw on top, I believe it is its plastic casing melted down . Let me grab a photo of it from its side.
And yes to the film cap, it says "0.56 uF", not other weird notation. Anyway, that one is on the output stage, the receiver won't even turn on (has a green / red led for that matter). Actually the led never turns either red or green.

 

[kellogs]

Here it is. Sorry for the bad photo; The red screwed thing is not marked in any way (or not any more). Any chance I can find out what it initially was ?

 

[Harald Kapp]

Sorry @kellogs , your photos are far from crisp, rather hazy. Hard to recoginze anything useful with respect to your query.
We have some tips for taking good shots.

 

[Tha fios agaibh]

Look like it might be a variable Rf inductor, although I can't imagine why it would melt.

 

[bertus]

Hello,

I have tried to enhance the picture a bit:



As @Tha fios agaibh said, the red part looks indeed like an inductor.

Bertus

 

[Tha fios agaibh]

I'd check the diodes and then transistors for shorts.
Perhaps that tall electrolytic spewed it guts from its underside?

 

[kellogs]

Upon closer examination and @Tha fios agaibh's picture above it turns out the variable inductor is OK; it was just bad lighting making me think it has melted. Also, the tall cap as well as every other caps are OK, and no shorts on diodes or other 3 legged semiconductors. I guess I need to dig deeper.
Oh, and the dry goo is probably another spill of the same glue that holds that red wire down. Scrapped off bits and the underlying PCB is fine there.

 

[bertus]

Hello,

Could it be that the inductor was melted by the soldering?



Bertus

 

[kellogs]

Err, bad pictures again i think. It is not melted, it looks just as in that pic that Tha has posted earlier. And anyway, even without that inductor - same deal when i have put the receiver back into its place

 

[kellogs]

Ah, hijo del diablo!
I was installing it the wrong way: 230V mains on relay output and heater RT-in on the thermostat input
So, now with the new polyester cap fitted and correct wiring, it still behaves the same. I.e. "my central heating would not start / stop every time the transmitter thermostat would issue a start / stop command, but only some of the time". Back to investigations. What should I try next?...

 

[kellogs]

Hello,

I have done some live measurements and I have noticed that for [see second / third pic in the first post].

Q3, V_be = 0.7V, V_bc = 0V - when I touch the base or emitter of this transistor with one of the DMM probes then the relay toggles
Q4, V_be = 0.7V, V_bc = 0V - when I touch the base or emitter of this transistor with one of the DMM probes then the relay untoggles

Transistors have "1AM" marking, found this datasheet. The behaviour above happens for around 85% of the time. Maybe some capacitor connected to them went bad ? Thing is it does not matter whether or not the thermostat transmitter is commanding stop or start.

 

[Tha fios agaibh]

When I touch the base or emitter of this transistor with one of the DMM probes then the relay toggles.

The frequency is probably shifting with your interaction.

It could be a bad cap, but more likely to be a bad diode or something in the oscillator section.

Honestly, after looking for comonly failed parts I'd probably just pitch it and pony up the money for a new one, unless you've got an expensive scope and an abundance of time.

Sounds like Q3, Q4 are switching fine but your dealing with the rf portion (900mhz range?) that drives it.

 

[kellogs]

None of the 4 x 100 uF caps were anywhere close to 100 uF . The diodes looked fine.
Replaced them, did this and that, still nothing. Now it does not actuate the relay even with help from a probe. The cheapest thing to do right now would be to put together a new PCB and some components (got a cc1310 lying around) and make it act like the original receiver. Not hard, but time consuming. An on-off switch will do for now.

Thank you everyone!