Philips oscilloscope

[markm6164]

Hi everyone

Sorry that this is my first post but i am new to electroncs and i need some help.

Yesterday i bought my first oscilloscope. Its a Philips PM3264 anolog scope. When i went to see it it was working perfect and all last night and this morning its was fine. I had it turned on on my bench and when i came to sit down the scope just powered down. I tried to turn it on but there was no power. I checked the fuse in the plug and the rear of the scope and both were blown. I replaced the fuses and as soon as i turn on it blows them. I opened the unit and located the main power pcb. I tried to isolate some circuits by disconnecting plugs between the power board and ajoining boards. The fuses still blow.

I don't know where to take it for repair and as i only paid £90 for it i think it will cost more to repair. I could do with the manual for this scope and some help in locating the fault if someone will help.

About me: I am just entering electronics. I am an auto electrician and have been for 11 years. I understand electrics as far as vehicles go and i wanted to expand into electronics hence buying the scope.

I hope i can stay with this forum and learn more about electronics but at the moment i am gutted i bought a scope yesterday and today i have this fault.

Any help will be greatly recieved.

 

[Resqueline]

Tough luck.. Was it properly set for the correct mains voltage? Unfortunately the only diagram I found on the web was very small.
Fortunately blown fuse faults are usually the easiest to fix. It's probably the mains rectifier or the main switching transistor that's shorted.
Just provide us with good pictures of the top (& bottom, if easy) of the power supply and we should be able to tell you where to measure.

 

[markm6164]

Thankyou very much for your reply. Yes it was set to 230 not 115. I will take some pictures and up load them.

 

[markm6164]

Here are the pictures as requested.

Power into the unit....

Through the mains filter to the power board....

Power board connection...

Power board both sides....

And main power connection onto board...

 

[markm6164]

Here are some testing results on the rectifier but the rectifier was still mounted to the board.

Pins 2 and 3 read as 0 both ways

Pins 1 and 4 red 503 and 0 other way round. (because its still mounted to board i suspect.

 

[Resqueline]

20m actually means 2 meters, but I guess you meant 20MΩ which means 20 Mega Ohms. It's not the best range to test diodes. Don't you have a diode range?
I don't see anything immediately wrong with the rectifier bridge though. Did you try to measure (or apply power to) the mains cord with the PSU disconnected?
It would be interesting to measure from 1 - 4 too. The pictures are not very bad but they could've been sharper. A resolution up to 840 x 630 is also ok.

 

[markm6164]

20m actually means 2 meters, but I guess you meant 20MΩ which means 20 Mega Ohms. It's not the best range to test diodes. Don't you have a diode range?
I don't see anything immediately wrong with the rectifier bridge though. Did you try to measure (or apply power to) the mains cord with the PSU disconnected?
It would be interesting to measure from 1 - 4 too. The pictures are not very bad but they could've been sharper. A resolution up to 840 x 630 is also ok.

Sorry i have modified my results above using the diode setting. The rectifier looks good.

 

[markm6164]

I was out of fuses today so i have ordered some more. I think next i should power up with psu disconnected like you said. If thats ok then with psu only connected. If it blows then its on that circuit board and if it doesn't i will connect each connector one at a time until the fuse blows. Does this sound ok??

 

[Resqueline]

Yes, it's worthwile to rule out the mains filter being shorted before carrying on with the PSU.
If the rectifier has to carry the short circuit current then it's likely to fail after a number of attempts. Better to measure for a short across + & - first.
Only very rarely is the main capacitor(s) shorted, it's more likely that the main switching transistor is shorted.

 

[markm6164]

Can i test the mains filter and main switching transistor now without powering up the circuit?

 

[Resqueline]

Yes, certainly. Just measure their ohms. The mains filter may have a breakdown at full voltage only though.
It may be difficult to distinguish between shorted main capacitors and a shorted main transistor - without disconnecting them.
The main transistor may appear to have a low b-e resistance, but this is due to the base drive circuit. C-e should always be high resistance.

 

[markm6164]

Ok well i will power the main filter with psu disconnected and see if it shorts. I will remove main transistor and test out of circuit. Thanks.

 

[markm6164]

Not had time to get some fuses yet but i did remove and test the main switching transistor. The transistor is a philips buy89 and all looks good. Here are the results with multimeter set to diode test...

Red probe to base and black probe to emitter i get 586Ω (other way round i get nothing)

Red probe to base and black probe to collector i get 537Ω (other way round i get nothing)

Collector to emitter i get nothing either way around.

So that all looks good to me.

 

[Resqueline]

Hm, ok, that's both good news and bad news. Where is that fault/short hiding? Did you measure aross the mains filter, and the rectifier + & - ?
One good news is that I found the PSU schematic hiding in here.
You'll have to put in pm3264 in a box and click 'zoeken'. Then you have to right-click on 'pm3264_power_supply_large.png' and select download.
The strange thing is that it calls for a BU208 as the main switching transistor, which has quite a bit higher spec's than the BUY89.
The power supply has a transformer base drive, and a current sense resistor, so there could hardly be any faults with the controller system or the secondary side.

 

[markm6164]

I wish i have some fuses to hand to confirm the short is on the psu and not a board off it. I want to totally isolate it. Anyways i did measure across + and - of the rectifier but only in circuit. With red probe to + and black to - i get nothing and other way round i get 508Ω. Here is the meaurements across the mains filter.

 

[Resqueline]

Well, there's your short right there, there's nothing more to test, try, or measure. Just replace the mains filter, or bypass it.

 

[markm6164]

Well, there's your short right there, there's nothing more to test, try, or measure. Just replace the mains filter, or bypass it.

Is it safe to bypass? I won't damage anything else? I have to find a replacement now.

 

[Resqueline]

Electrically it's safe to remove & bypass it, but from an EMC standpoint it may not be considered so ok as the 'scope may emit some more noise onto the mains.
There remains some filtering on the PSU itself though. You can replace it with just about any mains filter unit you can fit in there if you want to.

 

[markm6164]

wrote on the filter i have

250v ~ 1A KQF 3.38

0.12uF + 2x 2500pF XY +2 x 12mH

 

[markm6164]

Can i use either of these?

http://www.ebay.co.uk/itm/Seem-7761...al_Components_Supplies_ET&hash=item5ae2a7a429

http://www.ebay.co.uk/itm/Old-Unuse...al_Components_Supplies_ET&hash=item2a18aa22c5