Component identification - oscilloscope psu repair

[charlesjudd]

Hi
I’ve identified a faulty component in my oscilloscope. Large round grey disc. One of the legs has become unsolder from it.
I have no idea what the component is.



I appears to be after the AC DC conversion.
The component is inline on a positive DC Rail.
Peak inrush voltage when the oscilloscope is powered on is +220 volts DC.
After power on the voltage settles down to 152 volts DC. However, display does not turn on so I’m unsure if this part of circuit is under full load.

Peak volts on power on


Peak amps DC


Test leads connected to problem component area (find large blue cap closest to multi meter, just to the right of that - not a NTC terminator inplace of the big grey disc)


I picked up some 1k 10k and 100k ohm varistors from jaycar today to try and see if this is the component type... they all pop when power applied.

I’ve tried soldering legs back into the original grey disc. But solder just beads and runs off. Cleaned and re-cleaned. Tried flux, flux solder, and flux less solder (best results but not strong connection).

If I can’t replace the part (it also seems to be functional, except for no legs)... does anyone have advice on what prep is needed on the disc to allow it to accept solder ?

Any ideas where I can get another one? Or what a modern replacement is?


Sent from my iPhone using Tapatalk

[coromonadalix]

what is the scope brand and model, maybe a schematics / manual with part number could be found ?

[charlesjudd]

It’s a phillips pm3350a
I’ve got the original hard copy service manual. But that does not have schematic for PSU. Just the smarts, logic, and adjustments.

I’ve found online a schematic of the PSU, but It is a logical depiction of functions, as apposed to a proper schematic with parts and values.

If someone had a full  PSU  schematic, that would be great!

[charlesjudd]

what is the scope brand and model, maybe a schematics / manual with part number could be found ?

PSU diagram



Sent from my iPhone using Tapatalk

[Rerouter]

It would be a Metal Oxide Varistor (MOV)

https://www.jaycar.com.au/275vac-6500a-metal-oxide-varistor-mov/p/RN3404

I Cant say the components you where using where MOV's because it sounds like you where fitting NTC Thermistors,

Edit: As to the image, that is the block level diagram, see if pages later on in the manual have component values and things like resistors / capacitors

[charlesjudd]

The MOV you sent a link to... I tried as an experiment last night. So I have 5 of them here to play with.  Nothing powers on with a MOV fitted.

I’ll refit one now actually as I’ve not metered voltage between the legs when fitted.

The “non-success” lead me to the “maybe it’s a varistor” though.  But they seem to explode when power is applied.


Prior test outcomes:
MOV fitted = nothing powers on.

NTC Varistor fitted = mini NTC explosion, everything seems to power on (fan & lcd from display, backlight for screen graph c/y matrix) except tube display. 

So suspect this mysterious grey disc has something todo with high voltage DC circuit for the tube display.




Sent from my iPhone using Tapatalk

[Rerouter]

Ok, so it must then be an NTC, and something is pulling a lot of power through it indicating a fault elsewhere, I would suspect its something with a lower resistance than a higher one,

e.g. https://www.jaycar.com.au/15-epoxy-dip-ntc-thermistor/p/RN3430

The guts of it really looked like a mov to me, Have not come across too many NTC's that split there sides,

[charlesjudd]

Cleaning the disc with isopropyl and flux remover allows (bear with me) plumber solder to be stuck to the grey disc.

So I have had a chance to put the grey disc back in circuit temporarily. And all is good in the world. Everything turns on.
It’s just not a permanent fix due to reliability/strength of solder connection to the disc.




Sent from my iPhone using Tapatalk

[Rerouter]

And if you measure its resistance cold in that state, what value do you get?

[fzabkar]

Assuming that the device is in series with the supply, then it must be an NTC resistor intended for inrush current limiting. If you warm it up with your iron or hair dryer, its resistance should drop significantly. I would expect something of the order of 10 ohms or less when cold, depending on the operating current of the circuit.

[charlesjudd]

About 120ohms at the moment.
Interesting... when hot last night I was getting k ohms and close to 2M at one point.

Quite stable now. Tried heating with soldering iron and it remained quite stable

Cold


Heating for 20 seconds with soldering iron @ 300 degrees



Sent from my iPhone using Tapatalk

[charlesjudd]

I changed from alligator clips to regular fluke pointed test probes.
Put probe tip on the front and back and measured again... as this format mimics how the solder legs were connected.

It’s about 500k ohms


Sent from my iPhone using Tapatalk

[fzabkar]

Your measurements would suggest that the device is a PTC resistor. These are commonly used to automatically degauss a CRT at every power-up. I don't understand where they would be used in a scope, though.

Just FYI, here are NTC inrush current limiting resistors - a completely different animal:

http://www.mouser.com/ds/2/168/920-325D-LR-245176.pdf
https://www.tdk-electronics.tdk.com/inf/50/db/icl_12/NTC_ICL_S153.pdf
http://www.vishay.com/docs/29165/ptcel.pdf

Perhaps the PTC device is intended for overcurrent protection:

http://denethor.wlu.ca/cp316/sensors/KCxxxP-ND.pdf

[charlesjudd]

I’ve used a piece of wire (leg off a capacitor) and bridged the connection where the mysterious grey disc was.

I’ve had the oscilloscope running for the past hour without issue.

Temp is good. Voltage is good (although DV voltage has risen 2-3 volts as the oscilloscope has got warmer)

Now I’m happy that the only faulty part is the mysterious grey disk.  What do I need? Can I get it from jaycar tomorrow? Or is it a more specialised part?

I’m concerned the 1k & 10k thermistors have all gone bang when tested so far. 

I note none of them are voltage rated?  Nor rated for AC or DC ?

Should I try a 100ohm one and see if the high resistance was causing the problem?




Sent from my iPhone using Tapatalk

[charlesjudd]

[Rerouter]

I would say yes, to try the 100 ohm one, 

[tautech]

Now I’m happy that the only faulty part is the mysterious grey disk.  What do I need? Can I get it from jaycar tomorrow? Or is it a more specialised part?

I’m concerned the 1k & 10k thermistors have all gone bang when tested so far. 

I note none of them are voltage rated?  Nor rated for AC or DC ?

Should I try a 100ohm one and see if the high resistance was causing the problem?

You need a power rated one suitable to pass the 45W that these Phillips use.

Essentially they're to soft start the SMPS.

If your junk pile has any PC PSU's, they usually have one near the fuse and mains input, normally ~16mm dia green disc and while these are for several 100W it should still take some bite off the inrush to the scopes PSU.

[fzabkar]

ISTM that soft starting would require an NTC rather than a PTC. I would measure the current drawn by the circuit and use that to calculate the required wattage rating.

[charlesjudd]

I don’t have any old PC psu’s floating around. Kicking myself I threw out an old UPS last month salvaging only the fan and connectors.

I’ve ordered 2 types of higher power NTCs to try... I’ll update when they’re fitted with results



Sent from my iPhone using Tapatalk

[charlesjudd]

ISTM that soft starting would require an NTC rather than a PTC. I would measure the current drawn by the circuit and use that to calculate the required wattage rating.

Current draw in 0.4-0.6 amps DC @ ~300 volts



Sent from my iPhone using Tapatalk

[fzabkar]

ISTM that soft starting would require an NTC rather than a PTC. I would measure the current drawn by the circuit and use that to calculate the required wattage rating.

Current draw in 0.4-0.6 amps DC @ ~300 volts

Is that the current draw for the whole unit? IIUC, you stated that the faulty "disc" affects the HV supply, but all other sections of the circuit are operational.

If you do the arithmetic, the power dissipation at 120 ohms would be approximately 20W - 45W, as already stated. And these are the figures when cold, so they would only get worse for a PTC resistor. Something isn't right.

Edit: Your measured power consumption is 120W - 180W. The following document specifies a power rating of 70W for the PM3350A.

http://www.testequipmentdatasheets.com/index_cards/P/Philips/PM3375,_PM3355,_PM3365A,_PM3350A.pdf

[charlesjudd]

Found the schematic !
 X6014 is the AC power input
R6001 is the broken NTC



No values to work with though... ?

[Chris56000]

Hi!

There is – it's marked '82E' – in other words 82 ohms cold at room temperature!

All you need is an NTC thermistor about 70–80 ohms cold of about the same diameter as the original – in fact if I'm not mistaken, the original device would have been a Mullard/Philips VA1104, which was used extensively in early 1970s colour TV power supplies using thyristor H.T. regulation, and this device used to part company with it's legs in exactly the way you've shown in your scope!

Go to the Internet Archive, you can download a Mullard Passive Components Data book from the 1970s that covers these thermistors, both NTC and PTC types, and you might still be able to get a VA1104 n.o.s!

Chris Williams

PS!
http://markhindes.easywebstore.co.uk/VA1104_AKHTB.aspx

[charlesjudd]

Thanks Chris,
Here are the specs of the NTC if anyone needs them.
If anyone needs the schematic for their project, let me know. About 250 pages of a we’ll written manual.






Sent from my iPhone using Tapatalk

[tautech]

Found the schematic !

Great, can you post a link to it please.
I had a look in the docs for a PM3070 I have and there wasn't any PSU schematics.