Hi guys,

A couple of simple questions for you.

In the attached picture, the ceramic cap on the left is marked .1uf, now in my mind this is a standard 103 cap, however the 103's I have are all 1/4 of the size, now this unit is maybe 20 years old so is this cap just this big due to older manufacturing processes or is this larger because of a higher voltage rating?
The circuit in question is 12v

2nd question, the cap on the right, is this a polyester type? A capcalc site suggests this is a 3.4nf, would a ceramic cap be an appropriate replacement or can someone suggest a suitable replacement?

TIA

 

The ceramic disk marked 0.1μF would be a "104" and yes, modern MLCC's (multilayer ceramic capacitor) are better and smaller than the old disks and can still handle 50V. The other cap does appear to be a polyester film type and can be replaced with a 3.3nF polyester or polypropylene film type.

 

Why would you want to replace these capacitors? These should be very stable.

 

Thanks for the replies, I'm replacing as these are the last components on the board which haven't been replaced and the board does not function correctly.

I know these caps should be pretty bomb proof but as the last original bits on the board it's the last resort before replacing the whole lot with a PIC.

 

Can you tell us what the board is supposed to do?
If you do not have a circuit diagram, give us the part numbers of the ICs.
Looks like a 555 timer and a monostable.

If you can provide well focused photos of the top and bottom of the board I am sure we can figure it out.

 

I've took the pics, joined and mirrored, so pcb should be correct.

Pressing push button bottom right (as you look at picture) resets the unit. pressing button top right triggers relay to change state for duration the the pot is set to. once tripped unit can't be tripped again until reset is pushed.

The LEDs follow the button pushes, so when unit is reset the bottom led is on, when tripped the top LED should come on and the bottom go off.

What happens is the bottom LED just dims and the trip can be pushed and repushed without resetting.

This unit didn't work at all until IC's and Transistors were replaced.
It looks like someone has had a go before me, as at least one trace has been over soldered to allow additional current flow. I have successfully repaired the same trace fault and replace the 2n222a on another one.

On this one I have changed the trannies, diodes, chips and electrolytic caps.

IC1 555
IC2 4011
Small trannie is 2n222A
Big trannie is BC148

Resistors are as yet unchanged but are the next to be tested. I decided however right/wrong the ceramic caps where more likely to have failed rather than the resistors?

 

It's called an adjustable, resettable off delay timer since the load relay is triggered on immediately and has an adjustable off delay.

The small transistor would be a 2N2222 (another 2).

Have you seen a difference in circuit performance with different pot settings? I ask because there's obvious environmental damage to the pot housing. If the switches are original and have been exposed to the same conditions, I'd consider them suspect as well. With corrosive environments working on a board you have to be concerned about little things that you normally wouldn't, like the condition of the IC sockets, solder, circuit traces, even conductive layers of mineral deposits bridging things, especially under components.

 

No the pot seems to function fine the time it holds open for is consistent.

But I'll go desolder the switches to check anyway.

The larger trannies are actually BC142's

The IC sockets are new, as with a lot of the joints which I've been over.

These units aren't external but may have been stored in not a great environment. I've checked most of the traces since taking this picture, I've bridged a few with cable and it hasn't made much difference.

 

Nope the switches made no difference, without the switches still the same problem where the Reset LED stays dim and allows trip to be tripped again and again.

 

Is there any possibility that one of the transistors is installed the wrong way?

 

The transistors are installed in the same configuration as another unit which works ok.

The BC142 is a direct replacement for what was already installed.
The 2n2222A replaces a 5449

 

There is a good possibility that the 2N2222 is installed the wrong way.
This transistor is backwards from most others. (Interchange C with E).

Edit: Looks ok according to the photos.

 

Without a schematic or the ability to make measurements, the only other thing I can suggest is to double check polarities of capacitors, diodes and ICs and the values of passive components.

 

The transistor between the LEDs is installed the wrong way.
The emitter should be towards the TRIGGER push button and the collector towards the RESET button. The base is in the middle.

This transistor is used to drive the RESET LED only and doesn't need to be high current. A 2N2222 will do.

The other transistor closer to the relay needs to sink higher current in order to energise the relay. I wonder if the transistors are in the wrong locations.

 

Here is your schematic:

 

less than $2 solution

Even if I acknowledge admiration of older technology, it's not always worth the efforts. It's OK for hobby/for learning or research, but a professional would not put too much effort into such a circuit, if it could be done in 2 hours, using a small inexpensive controller.

If you totally understand how the circuit works, try to rebuild it using different parts, maybe more transistors, maybe PNP transistors, etc.

Or move the functionality into a small microcontroller.

 

less than $2 solution

Even if I acknowledge admiration of older technology, it's not always worth the efforts. It's OK for hobby/for learning or research, but a professional would not put too much effort into such a circuit, if it could be done in 2 hours, using a small inexpensive controller.

If you totally understand how the circuit works, try to rebuild it using different parts, maybe more transistors, maybe PNP transistors, etc.

Or move the functionality into a small microcontroller.

Nothing personal here but it raises a point:

Why paint a picture when you can take a photograph?
Why use the stairs when there's an escalator?
Why ride a push-bike when you can take the car?
etc.

And this is why sooo many consumer items cannot be repaired and have to be thrown away nowadays and no-one understands how things work.
The cost of 'progress' is measured in more than hard cash, but hard cash rules.


Mr Chips: nice work!

 

Thanks.

It is cheaper and more efficient to throw something away and build a new one with new materials.

Or is it? When we don't factor in the cost of cheap energy, priceless resources and long term environmental destruction, the price we now pay is greatly understated.

Our expansionist monetary system relies on ever increasing levels of consumerism and consumption. Almost everything we do today is non-sustainable. Sooner or later we will face the consequences.

 

Nice job with the schematic MrChips. I notice you didn't show C3 as electrolytic.

 

That is because C3 is a timing element and an electrolytic would be a poor choice.