Philips PM3215 Oscilloscope problem

[vosperd]

Hi
This scope was working perfectly last week but on switching on this morning it was dead. no led or graticule.
Turns out the fuse F101 mounted on the mains transformer has blown.
This is a special Philips fuse, wire ended clear plastic about 1cm square.
The label says to only use the correct Philips fuse but are they obtainable anywhere?
Don m5aky

[barrymagrec]

If memory serves this is a thermal fuse - I may have some but I won`t be able to find out until Friday / Saturday.

[vosperd]

Hi barrymagrec
It would make sense that it was a thermal fuse as it is slotted in against the transformer.
If you have any to spare it would be great. Happy to reimburse of course. If not then it would be useful to know if they are available anywhere.
I have downloaded a manual from the internet archive but for some reason it seems to work extremely slowly in Acrobat and I can't print from it.
Don m5aky

[barrymagrec]

Are they anything like RS 176-9508? (discontinued)

It`s just that I have some that I brought back from my late dad`s house the other day and I can`t think why else he would have had them.

I am slowly bringing stuff back from the house and he had quite a lot of Philips spares of that era.

If not, as I say, I will look on Friday / Saturday.

[vosperd]

I've just found the section in the manual that refers to this thermal fuse.
The instructions are to remove the fuse, dismantle it and replace the fusewire.
I'll have another look at it tomorrow and remove it to see what it's like and post a photo.
I've not found it in the parts list yet.

Don

[MrBungle]

If it goes again then it’s possible the transformer is shot. It does happen with those scopes unfortunately. All is not lost as it’ll run off a 24v meanwell DC power supply fine. I converted one here. The only cost is the loss of line trigger.

Make sure you replace the RIFA cap under the little fibreglass shield if you haven’t already as well. They like to explode.

[Jac]

If the fuse is not damaged, only open, you can repair it with hot air.
Keep the 2 wires together on the end en allow hot air to let the special solder (very low melting point) melt again.

Hope this helps.

Jac

[vosperd]

Hi,
Thanks for the replies and the advice.
I've removed the thermal fuse and it was blown.
Unfortunately one of the connecting wires has broken off so although I can repair the fuse wire it's not useable.
I've replaced it with a length of fuse wire to continuue the investigations.
With the transformer disconnected from the psu board I get 24V out from the secondary so that looks ok.
However, when I connected it up and momentarily applied mains without switching on the scope I got a loud hum. Needless to add I switched it off pretty quick!
No short between pins 3 and 4 on the pcb mounted plug but pin 3 is reading a dead short to ground.
Looks like I have a faulty bridge rectifier, V201, or C202.
The thermal fuse is a Philips item and has a number 55171.
Investigations continue tomorrow.
Cheers
Don m5aky

[MrBungle]

Probably the main filter cap. That's what killed the one above I did. I had one before that which had shorted the secondary of the switching supply so worth checking for shorts on those caps as well. The blue philips ones tend to go short.

Good luck. Hope you get it working.

[vosperd]

Removed the psu board last night and it turns out that the BY250 200 has a short. Can't see anything else faulty so far.
I found a BY250 amongst my spares but it was a 250/100. I found a supplier that has the correct part but it's probably not going to get posted until after Easter now.
Don m5aky

[vosperd]

That should be BY225, of course!

[barrymagrec]

I`ve had a reasonably thorough look through my Dad`s Philips bits but surprisingly didn`t find any thermal fuses - sorry.

It`s quite possible they`ll still turn up - I`ll pm you if so.

[vosperd]

Hi. Thanks for looking.
Just out of interest I've attached a photo of the fuse in question.
I see that there are thermal fuses available that will fit but it's a question of getting the right spec.
It seems that, looking at previous posts, Philips were keen on fitting these to their kit whereas many other manufactures didn't bother.
It will work without it if needs be.
Looking at the circuit diagram, power is applied the the transformer and power supply as soon as the scope is plugged into the mains even when switched off at the front panel
Don m5aky

[factory]

The Jan 1978 parts catalogue only has two thermal fuses listed, neither have the number quoted in the post #8.


David

[ortek_service]

CPC used to sell packs of 5 of (2A?) small black plastic rectangular radial thermal-fuses, that you could embed inside small transformers, in a range of cut-out temperatures. And I thought could be used, with some sleeving on the bare leads, for a bit extra safety. Probably easier than the metal axial-leaded 10A heating element protector ones, that need to all be sleeved with high-temperature material.

I can't seem to find these radial ones on the CPC website or on the catalogue page for axial ones: https://www.easyflip.co.uk/CPC_Catalogue/?page=1509

But I have found these motor-protector devices - also available in several temperature trip values:
https://cpc.farnell.com/microtherm/t...ermal%20cutout and also some other on the catalogue page: https://www.easyflip.co.uk/CPC_Catalogue/?page=1526
These are self-resetting (so less hassle of having to resolder sprung contacts with LMP solder that Philips also did on some old TV's)

[vosperd]

Thanks for the info David and Ortek-service,
The number I quoted is stamped on the insert of the device. I'm not sure of its relevance.
The spec of the top fuse looks right. At first I thought that the current rating was a bit low but the scope ony consumes 28W so 300mA looks right.
The chances of finding the original item is low. I will see what happens when I replace the bridge rectifier.
As the rectifier had a dead short I suspect the fuse blew from excess current rather that temperature in this case.
I don't know yet if it is just a random component failure or there is a problem further on yet to be discovered.
Don

[barrymagrec]

The fuse in the PM 3212, which I think is pretty similar PSU wise to the PM3215 is the 4822 252 20007.

[ortek_service]

Note: If the original Philips 'thermal' fuse, is also a current limit fuse (being as it seems they had fuse wire in it), then if replaced by a thermal trip etc. device, that isn't designed to trip on excess current, then you'd probably also need an additional fuse of appropriate rating in series if there isn't already one elsewhere on the mains-supply input.

[Chris55000]

Hi!

A lot of old Philips Hi Fi units, CD players, etc., e.g. 22GF653, N4302, etc., monochrome portables, in fact most of their small pieces of gear using 50 Hz mains transformers are fitted with this fuse 4822 252 20007, so it's worth keeping an eye on eBay for old Philips mains transformers or Philips audio units going cheap for parts, or ask Members on here perhaps if they've any Philips remains that have a transformer they don't need!

A look through many (too many to list!) Philips audio s.m's turns this fuse up so it's not as difficult to get as it seems!

Chris Williams

[barrymagrec]

@ vosperd

I had another look and found some - PM me your address and I`ll get one off to you.