Component identification - oscilloscope psu repair - Page 1
Forum section: electronics / repair

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 03:26:41 am
content: HiI’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 onPeak amps DCTest 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

author: coromonadalix
author_rank: Super Contributor
timestamp: March 09, 2019, 03:40:01 am
content: what is the scope brand and model, maybe a schematics / manual with part number could be found ?

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 03:43:48 am
content: It’s a phillips pm3350aI’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!

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 03:45:35 am
content: Quote from: coromonadalix on March 09, 2019, 03:40:01 amPSU diagramSent from my iPhone using Tapatalk

author: Rerouter
author_rank: Super Contributor
timestamp: March 09, 2019, 03:46:09 am
content: It would be a Metal Oxide Varistor (MOV)https://www.jaycar.com.au/275vac-6500a-metal-oxide-varistor-mov/p/RN3404I 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

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 03:51:02 am
content: 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

author: Rerouter
author_rank: Super Contributor
timestamp: March 09, 2019, 03:56:18 am
content: 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/RN3430The guts of it really looked like a mov to me, Have not come across too many NTC's that split there sides,

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 03:59:13 am
content: 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

author: Rerouter
author_rank: Super Contributor
timestamp: March 09, 2019, 04:02:48 am
content: And if you measure its resistance cold in that state, what value do you get?

author: fzabkar
author_rank: Super Contributor
timestamp: March 09, 2019, 04:04:04 am
content: 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.

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 04:10:41 am
content: 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 stableColdHeating for 20 seconds with soldering iron @ 300 degreesSent from my iPhone using Tapatalk

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 04:16:29 am
content: 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 ohmsSent from my iPhone using Tapatalk

author: fzabkar
author_rank: Super Contributor
timestamp: March 09, 2019, 04:25:58 am
content: 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.pdfhttps://www.tdk-electronics.tdk.com/inf/50/db/icl_12/NTC_ICL_S153.pdfhttp://www.vishay.com/docs/29165/ptcel.pdfPerhaps the PTC device is intended for overcurrent protection:http://denethor.wlu.ca/cp316/sensors/KCxxxP-ND.pdf

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 07:26:03 am
content: 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

author: Rerouter
author_rank: Super Contributor
timestamp: March 09, 2019, 07:42:37 am
content: I would say yes, to try the 100 ohm one,

author: tautech
author_rank: Super Contributor
timestamp: March 09, 2019, 07:51:55 am
content: Quote from: charlesjudd on March 09, 2019, 07:26:03 amYou 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.

author: fzabkar
author_rank: Super Contributor
timestamp: March 09, 2019, 08:43:12 am
content: 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.

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 08:43:45 am
content: 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 resultsSent from my iPhone using Tapatalk

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 08:44:42 am
content: Quote from: fzabkar on March 09, 2019, 08:43:12 amCurrent draw in 0.4-0.6 amps DC @ ~300 voltsSent from my iPhone using Tapatalk

author: fzabkar
author_rank: Super Contributor
timestamp: March 09, 2019, 09:08:13 am
content: Quote from: charlesjudd on March 09, 2019, 08:44:42 amIs 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 whencold, 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

author: charlesjudd
author_rank: Contributor
timestamp: March 09, 2019, 11:40:11 pm
content: Found the schematic !X6014 is the AC power inputR6001 is the broken NTCNo values to work with though... ?

author: Chris56000
author_rank: Frequent Contributor
timestamp: March 10, 2019, 12:07:22 am
content: 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 WilliamsPS!http://markhindes.easywebstore.co.uk/VA1104_AKHTB.aspx

author: charlesjudd
author_rank: Contributor
timestamp: March 10, 2019, 01:04:22 am
content: 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

author: tautech
author_rank: Super Contributor
timestamp: March 10, 2019, 01:54:15 am
content: Quote from: charlesjudd on March 09, 2019, 11:40:11 pmGreat, 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.

author: charlesjudd
author_rank: Contributor
timestamp: March 10, 2019, 02:01:07 am
content: https://elektrotanya.com/philips_pm3350_pm3352_sm.pdf/download.htmlWait patiently for the download link to become available. Or PM me with an email address and I’ll share for google drive directly.Sent from my iPhone using Tapatalk

author: tautech
author_rank: Super Contributor
timestamp: March 10, 2019, 02:08:33 am
content: Quote from: charlesjudd on March 10, 2019, 02:01:07 amCool, thanks. Yep I know the elektrotanya routine.Grabbed and saved before I forget.