Electrostatic filter with bad PSU?

[pentium]

Since I currently am residing in southern ontario on my grandparents home I have to do the dirty work and keep everything running.
Today I had to tackle the filter. UIt's one of those Sears electrostatic filters that installs between the furnace intake and the fan and is composed of two filter cassettes and the power supply which also acts as the service cover.
For a few years it has not worked. I had a live outlet and a good cleaning of contacts revealed nothing. I then opened up the power supply and saw part of my answer. I first saw that a small baord close to top had a transistor blown apart and a resistor had chipped. whatever it's use, it used to be connected between the power switch and the transformer board and was now disconnected and bypassed. I then looked at the main PCB. I really saw nothing until I removed the board and on the underside I instantly saw that the traces for both the white and black 120V leads were gone.
I have a crude shop here but after some work I was able to put new traces in and expected everything to be fine and dandy now. Wrong.
As soon as I plugged it in, a different trace blew out. This time between two diodes and a 200v, 47u philips cap. Seeing how one diode leads to the white lead and the other diode goes the the black lead (remember, white and black were 120V leads) I think I might have an open diode but I lack the tools to test.
I really wish I could post photos but I am currently away from my camera.

Next oddity.
If I cut my trace between the white lead and one of the diodes in question while at the same time I disconnect the H/V lead, the unit comes on but if the H/V lead is connected, the system won't come on because of a ground short.
What's strange here is that the only likely place for that short, where the H/V lead connects to the first cassette is insulated and even with my meter set to .0001 ohms I still can't find that ground leak
I even further insulated the connector and it STILL shorted (the front LED didn't light)

What's going on?

[starfury1]

Re: Electrostatic filter with bad PSU?

well mate got no real idea on these things
sure a few here probably have

To me if something is blowing PCB copper traces off the face of the earth it sound like a big short of some kind.

So were's the protection on this, like a fuse?

photos would help.

maybe this link might give you and idea as to what maybe happening

Just remember if there is ten ways to skin a cat manufactures will used every one of them and the variations
So you circuit could be way different.

Page link

Schematic

its using SCR's
not to say your circuit is but possibly,
these are sort of like a "one shot switched, rectifier diode"
.
Be careful in there, there are some nasty Voltages

Consider the stresses on some of these components I'd be incline to check every one of them before throwing the juice switch

So yeah a multimeter and knowledge of how to use it to test the components would be a great help here.


I'll have to leave this for the more experience with these things

Hope its of some help anyway

Cheers

[pentium]

Re: Electrostatic filter with bad PSU?

Okay, I'm really limited on resources but I went and scanned both sides of the board and after waiting for my photos to upload (over a 56k connection ) Here's what we got.

The back side of the board.

The top side of the PCB.

Don't bitch about image problems. It takes me too long to redo the photos and the glossy look on both sides of the board is a coating of rubber (or something to the likes) that came from Sears. I hope you can read my markings.

[starfury1]

Re: Electrostatic filter with bad PSU?

well it looks as if its nothing really like the circuit I found above, so thats not of a great deal of help

You did a good job with the photos and yes they are of help

From a quick look it appears to be a bridge rectifier circuit made up of 4 diodes (I would have to redraw it to be sure) one side of which goes via that large resistor with the 2 red bands (can't make out the colour of the third brown or black, which is the multiplier) its possibly 22 ohm or 220 ohm...more likely 22 ohm would be my guess but like I said no idea on these things

The capacitor appear to be a filter cap and probably has a 200V odd rating

The diodes appear to be 1 A type rectifier diodes I would replace all four of them to start with plus the transistor type semiconductor (whatever that part is)
(they actually look a little smaller then 1A but hard to tell)

not sure what type of diodes cause I cant see the numbers and I think the USA may use different numbering ...if it was me I would use 1N4007 these have a 1 amp rating with a PIV of 1000V

So should be fine for mains rectification

As I stated above a multimeter and knowledge of how to check components is going to be needed.

The bigger question is what killed these components.
So replacing them may not solve the problem...in other words you maybe fixing the symptoms not the fault.

(maybe the HV transformer is shot?)

but you wont know till you do replace them.

I would advise putting a fuse (2A or so) in line for testing, cheaper to replace a fuse then a hand full of components...maybe the old TV lt bulb trick would be the go.

There is a component marked VR1 hiding under the heat sink, I don't know what that is
since the the heat sinked transistor went boom you will need to check around that area too for other faulty components

you could rebuild the primary side of that pcb for not that much money.

At a guess the primary side is an oscillator of some kind

Anyway at this point replace the diodes and the transistor device...check as much as you can.

like I said I dont fix these so no real experience with them.

Hope this is of some help...and becareful!

Cheers

[kc8adu]

Re: Electrostatic filter with bad PSU?

first suspects are those diodes.

[pentium]

Re: Electrostatic filter with bad PSU?

Well, the first thing that comes to mind was a lightning strike a few years back.
While this house is equipped with lightning rods we still had the lightning jump from the ground rod to a drain pipe and the antenna stack.
I think that was the second time the house was struck by lightning. It can get pretty nasty here.
Anyways. I assume that it was that lightning strike that obliterated the PCB.
The voltage regulator is undamaged and it looks like the diodes held off the surge before the traces blew out. I can only see the need to replace four diodes.

[starfury1]

Re: Electrostatic filter with bad PSU?

lighting strikes are alway good for killing electronic components.

hopefully it will fire up with the bridge replaced
(it is as described above)

let us know how you go

cheers

[pentium]

Re: Electrostatic filter with bad PSU?

We will have to wait until the next time I go into Stratford.
I hope it's common stock because I'm gone next weekend.

[pentium]

Re: Electrostatic filter with bad PSU?

Wow.
Even two good checks through in the yellow pages found nothing close to what I need. We are off to Stratford tomorrow and I have a small list of places but if I don't find the part, I have to reassemble the dead system and the house won't have any kind of air filter installed.

[pentium]

Re: Electrostatic filter with bad PSU?

Well after an hour of searching I finally found four of he Diodes I needed in an "assorted pack" at the source. Every other shop that would sell what I need is one or two counties away. Nobody makes their own electronics anymore.

Anyways, I installed them, made positive they were in the right orientation and after checking to be positive my trace repairs were good I reassembled the unit and plugged it in.

Nothing. Not even a "pop" of another trace blowing out.

Knowing what has happened in the past, I disconnected the H/V lead and plugging it in again the system came to life.

Well I knew the H/V lead was not damaged so all signs now point to a short to ground somewhere else. Nothing found.
I don't get it. It's like any Insulator I use (even glass) is ignoring physics.

[starfury1]

Re: Electrostatic filter with bad PSU?

Ideally mate no traces should blow....

I take it, it looks like it working with the HV lead removed
but once you plug that in and turn it on the whole thing appears to be shut down.

The most prone side to failure would be the HV side of things under normal circumstances but if it was a lightning strike that killed it anything possible.

From what I can see their appears to be 3 HV diodes if the markings are anything to follow. CR 9,10,11.

From the link I posted, it appears that this thing is going to operate at close to 5KV (5000 volts)

Without tracing out the circuit for a better idea of how it might work.

The basic flow is direct rectify the mains to DC then chop to AC, use the transformer to step the voltage up to HV AC then rectify it to HV DC
Apply this to plates to create an electrostatic field.
Use the electrostatic fields to filter out dust and crap from the air.

Seems if there is no load it maybe happy but once a load is on it it dies
Now how and what type of load the plates presents I don't know
As I said no idea on these really.

So you have three basic areas it could be failing in
The primary side, the secondary HV side or the load (as in plate setup)

Looks like there may be some "voltage multiplier" on the HV side too
see here and here the Wiki here

If a component is failing under the HV thats going to be hard to tell I guess from any static testing (but not the guru here)
how you are going to go about testing this circuit I have no real idea either

maybe adopt some EHT type testing but still you will need equipment that can handle HV

I have reversed the PCB side in the photo below so the components can be more easily over laid in the mind to get an idea of what the circuit might be.

At this point I'll have to leave it for someone that knows a lot more about HV circuits then me.

Just remember its potentially a DEADLY circuit so be very careful

wish I could be of more help

Photo 500kb approx.... so yeah bit big but left it for the definition

[pentium]

Re: Electrostatic filter with bad PSU?

Wll I guess I'm going to have to get Real help on this one.
There should be a Searvice facility nearby.