Friday, March 23, 2018

Repairing a zapper

In the springtime, summer and fall, there is an appliance that works almost 24 hours a day, at least in my garage.
A zapper.

This one, a PestiTech, did work all the last summer and fall.
Really, in Arkansas is not an option to have a zapper to limit the "things" that can fly and bite you !
The garage is the main entry for these pests, so a good zapper is really the first protection.
Now it was time to restore it and ... of course dead !

Before to buy a new one, I did buy a new set of fluorescent lamps, typically is the "component" that is damaged first.
Anyway these lamps have a limited life anyway, typically about a year.

Change the lamps is quite trivial but expensive ... let consider that you can buy a zapper like that around 30/35$. The "original lamps" cost around 30$ !!!  Absurd !!
But anyway, found some equivalent lamps for just 19.95$ (sigh) and finally when they arrived ... nothing.  The new lamps were dead like the old ones, meaning that the electronic inside driving them was dead !

Before to trash it, I decided to give a try to repair it, but before to continue, let me say that loud and clear one very important thing :


In other words, if you are not an electronic engineer, don't even think to open the unit.
There is the potential to be killed.  If you are not an expert and want to play with that anyway, well, is your right to be included in the Darwin Award.
Good luck.

Said so ... I opened the unit and started with some visual explorations.
First of all, after disconnected the unit, discharged the high voltage grid and opened the unit, I disconnected the high voltage transformer.
The transformer is powering up the grid with about 2000 Volts.

So, while repairing the electronic ballast, the probable cause of the problem, I disconnected the power to the high voltage transformer.
I don't want to be zapped :)
Note that a ballast can be dangerous as well, so always proceed with caution when handling a ballast circuit.

A quick visual inspection didn't show any evident sign of problems, like exploded capacitors, fried resistors, etc.  But of course it doesn't mean nothing is broken.
I did power up my multi-meter and started to check the fuse (it was OK) and the semiconductors.
Lots of diodes first. All were OK, but always remember that unless you remove the component from the PCB, the measure can be faulty since other components are involved in the measurement.

Then I proceeded to measure also the transistors with the multimeter.
This ballast use two ST13003  high voltage fast switching power transistor, designed actually for ballast use.
The measurement of the component seemed ok, usually is one of the component that first fail in a high voltage circuit, this why I still was suspicious about them.
If one of this transistor in broken, one of the coils is not working and thus no way to have the lamps lit up.

This is NOT the schematic of my particular unit, but it can give an idea about what I'm dealing with (schematic found on the ResearchGate website) :

Since no components showed problems via multimeter I did run also some tests under current.
After installing the two new bulbs in the socket, using a non contact tester, I verified that some voltage was present to one side of the bulbs when the circuit is powered up.

Since I did not found an explicit component damaged I decided t give a try and ordered few transistors from DigiKey.  They are not very expensive, the main cost as usual is the shipment.

After the substitution of the transistors, the circuit behaved differently but still not working, meaning that the damage was more extensive than I originally thought.
Mainly for curiosity I did measure the old transistors removed from the circuit with a transistor tester and in fact both transistors were damaged.
One lost a junction and the other degraded completely the characteristics, sign that other components are damaged as well.
In order to destroy two transistors it means the circuit was probably very poorly designed.

A "new" normal transistor. Note the hFE of 77

One of the two damaged transistors. It is seen as resistor !

The other damaged transistor. Note the hFE of 19 !!!

So at this point continue to diagnose the circuit would become quite long and more expensive.

I saw three solutions :

  1. trash away everything and buy a new zapper
  2. change completely the ballast
  3. continue to debug the circuit
I opted for the second choice.
On Amazon I did look for a cheap ballast (around 15$ including the shipment) capable to drive two 10W fluorescent lamps.
When arrived I did cut all the wires from the old ballast and connected to the new one.

Wires connected to the old ballast

New ballast wiring

As soon as powered, the lamps become alive.
Few modifications to the wiring and the adding of a in line fuse holder on the wiring completed the repair of the zapper.

Maybe if I find some time, I will try to better diagnose the old ballast circuit.
I'll try to remove the capacitors and the diodes checking them "out of the circuit" to see which one is out of specifications to cause the circuit to don't work.
But for now, the old ballast goes in the "junk box" :)


It was worth to repair the zapper ?
Economically NOT AT ALL.
Buying the new lamps, the components, the new ballast and the time spent to track the problem and repair the unit, did cost TWICE if not more the price to buy a new one !
But morally I know I did the right thing and .. I had fun doing it ! 
Well, anybody has it's way to have fun :)

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