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Old 2nd Jul 2020, 12:51 pm   #1
DonaldStott
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Default Farnell 30-4D Oscilloscope - Help with Repair

I bought my Farnell 30-4D 'scope (a re-badged Scopex 4D-25) from a Forum member some time ago and it has served me well over the last few years but recently it decided to stop working!

It powers up ok and the red light comes on but there is nothing whatsoever on the screen? I'm fairly familiar with using the controls and settings but there is simply no display?

I have the User Manual and managed to remove the top and bottom covers but stopped there as I have no experience with repairing 'scopes so haven't a clue where to start! Those big 500V capacitors scared me!!!

So any help for a complete novice would be greatly appreciated.
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Old 2nd Jul 2020, 5:08 pm   #2
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Hello,

I had one of these for a short period a few years ago. It is what pushed me to join this forum! One of the problems was an open diode in the EHT section.

The blank screen could be due to a lack of EHT. I'd start checking the chain of diodes in the EHT multiplier with the diode check mode on your DMM. It is the board on the left.

I've had a mixed luck with testing diodes in circuit. Sometimes it may be better to lift a leg then test.
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Old 2nd Jul 2020, 7:32 pm   #3
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Thanks OTF96 - your Post above certainly gives me a great start as I had no idea where to begin testing. I'm assuming you mean the EHT Supply Board which appears to be in the most inaccessible location imaginable! How on earth am I going to get in there?

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In general if I have to lift one leg of a component to test it then it's usually worthwhile just replacing it? Be good to find out though if any of these diodes are in fact open?
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Old 2nd Jul 2020, 8:14 pm   #4
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Also worth just making sure that the low voltage supply rails are within spec as with most oscilloscope fault finding adventures.

Alan
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Old 2nd Jul 2020, 8:56 pm   #5
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Originally Posted by DonaldStott View Post
Thanks OTF96 - your Post above certainly gives me a great start as I had no idea where to begin testing. I'm assuming you mean the EHT Supply Board which appears to be in the most inaccessible location imaginable! How on earth am I going to get in there?

Attachment 210135

In general if I have to lift one leg of a component to test it then it's usually worthwhile just replacing it? Be good to find out though if any of these diodes are in fact open?
There should be two nylon nuts and bolts holding the EHT board to the chassis. Once these are removed the rear of the board is accessible. I remember it being a bit awkward, but it was doable.

I'd say check diodes in circuit first. If the diode tests as being open remove it from the circuit and test again to make sure you are not getting an erroneous reading.

Being diodes, they will test as open one way and 0.6V the other way using diode check on a DMM.

I'd advise against a wholesale replacement of diodes. 'Shotgunning' it is never a good idea.

Also check the other voltage rails. They are: +20V, -20V, 115V. Only check the EHT rails if you have suitably rated equipment.

Be careful as the final anode voltage is 5,000V!

EDIT: Also check the continuity of the CRT heater and test the voltage across the CRT heater.

Last edited by OldTechFan96; 2nd Jul 2020 at 9:03 pm. Reason: More info
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Old 3rd Jul 2020, 12:19 pm   #6
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Also worth just making sure that the low voltage supply rails are within spec as with most oscilloscope fault finding adventures.
Thanks Alan - I just need to find out where the low voltage supply rails are?

In hindsight I should have called this Thread 'My Farnell Oscilloscope Fault Finding Adventure!' I've done many different types of restoration but never a 'scope so this is an Educational Project for me as well, despite my age (now Retired.)

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Originally Posted by OldTechFan96 View Post
There should be two nylon nuts and bolts holding the EHT board to the chassis. Once these are removed the rear of the board is accessible. I remember it being a bit awkward, but it was doable.

I'd say check diodes in circuit first. If the diode tests as being open remove it from the circuit and test again to make sure you are not getting an erroneous reading.

Being diodes, they will test as open one way and 0.6V the other way using diode check on a DMM.
Thanks again OTF96 - I managed to work out how to access the EHT Board: -

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All diodes checked (in circuit) and they are measuring 0.6V or thereabouts one way and open the other way.

Also checked the six capacitors (in circuit) on the EHT Board and they are measuring around 16-17nF - the spec is 20nF so a little bit low?

All good so far ... ?

Quote:
Originally Posted by OldTechFan96 View Post
Also check the other voltage rails. They are: +20V, -20V, 115V. Only check the EHT rails if you have suitably rated equipment.

Be careful as the final anode voltage is 5,000V!

EDIT: Also check the continuity of the CRT heater and test the voltage across the CRT heater.
If I knew where the other voltage rails were I would check them. I have a DMM that goes up to 600V AC and DC so I don't think I'll be going anywhere near those very high voltages.

And finally, for now, where do I check the CRT continuity and heater?

Apologies for all the questions but any help is greatly appreciated.
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Old 3rd Jul 2020, 12:48 pm   #7
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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And finally, for now, where do I check the CRT continuity and heater?

Apologies for all the questions but any help is greatly appreciated.
According to a schematic I'm looking at, check the CRT heater continuity across pins 1 and 14 on the CRT, the voltage should be approx 6.3 VAC across the corresponding points on the CRT socket.

Lawrence.
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Old 3rd Jul 2020, 1:44 pm   #8
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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According to a schematic I'm looking at, check the CRT heater continuity across pins 1 and 14 on the CRT, the voltage should be approx 6.3 VAC across the corresponding points on the CRT socket.
Thanks Lawrence and noted.

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I don't see any way to measure this voltage without first gaining access to the CRT pins and sockets unless Pins 1 and 14 are on top? Looks to me as if you need to remove the CRT itself which is probably a step too far for me or can anyone tell me what colour the wires are that go to Pins 1 and 14? A bit Painting by Numbers I know!
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Old 3rd Jul 2020, 1:47 pm   #9
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Service Manual

Good news about the diodes. One of the 6 16uf 500V electrolytic capacitors could be open. Check them too. Also check the resistors on the EHT board.

Is the EHT fuse open? An open fuse would explain the fault.

Not having an EHT probe makes troubleshooting this area difficult. Maybe somebody local could lend you one?

We'll take a step back from the EHT tripler circuit and look at the low voltage rails.

Page 20 has some of the circuit for the PSU. Check everything that terminates in +20V, -20V, +110V. If one of these is missing it could explain the lack of trace.
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Old 3rd Jul 2020, 2:30 pm   #10
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Originally Posted by DonaldStott View Post
I don't see any way to measure this voltage without first gaining access to the CRT pins and sockets unless Pins 1 and 14 are on top? Looks to me as if you need to remove the CRT itself which is probably a step too far for me or can anyone tell me what colour the wires are that go to Pins 1 and 14? A bit Painting by Numbers I know!
I jumped in too soon (!) and should have had a closer look at the circuit diagram to ascertain where the 6.3V AC comes from: -

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All I need to do is find C613 and C615 on the main board and measure there - hopefully that's not too problematic for me?

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Old 3rd Jul 2020, 3:28 pm   #11
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

On the assumption that I am measuring in the correct places (see Post above) I seem to have CRT heater continuity across pins 1 and 14 on the CRT with a voltage of 6.79V AC.
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Old 3rd Jul 2020, 3:46 pm   #12
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

That's good. Proves that the CRT heater and 6.3VAC winding are good.

I'm curious about the EHT fuse. Have you checked it?
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Old 3rd Jul 2020, 4:43 pm   #13
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Don't wish to teach anyone to suck eggs or over-state the obvious, but the CRT 6.3V heater supply will be elevated to a very high negative voltage- a norm in mainstream oscilloscope circuitry, the sort of voltage that will be well beyond the ratings of typical meter leads/probes.

That snippet of circuit in post #10 looks to be a voltage doubler running from a 6.3VAC feed, maybe for part of the bright-up circuit associated with the tube cathode/grid arrangement.

Stay safe folks!

Colin
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Old 3rd Jul 2020, 6:57 pm   #14
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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I'm curious about the EHT fuse. Have you checked it?
EHT Fuse is good out of its holder according to my DMM Continuity setting?

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Don't wish to teach anyone to suck eggs or over-state the obvious, but the CRT 6.3V heater supply will be elevated to a very high negative voltage- a norm in mainstream oscilloscope circuitry, the sort of voltage that will be well beyond the ratings of typical meter leads/probes.

That snippet of circuit in post #10 looks to be a voltage doubler running from a 6.3VAC feed, maybe for part of the bright-up circuit associated with the tube cathode/grid arrangement.
Thanks Colin but your technical and very eloquent explanation is way beyond my level of experience or understanding?

Perhaps others can chip in here - as I said earlier this restoration is also an Educational Project for me.
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Old 3rd Jul 2020, 7:14 pm   #15
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Quote:
Originally Posted by OldTechFan96 View Post
I'm curious about the EHT fuse. Have you checked it?
EHT Fuse is good out of its holder according to my DMM Continuity setting?
I've been scratching my head regarding the 'EHT fuse' as the only fuse I've spotted on the schematics is F601 which is the fuse on the primary side of the mains transformer. Confusingly it is physically located on the EHT board. Perhaps I've missed something.

Alan
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Old 3rd Jul 2020, 7:29 pm   #16
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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I've been scratching my head regarding the 'EHT fuse' as the only fuse I've spotted on the schematics is F601 which is the fuse on the primary side of the mains transformer. Confusingly it is physically located on the EHT board. Perhaps I've missed something.
I just assumed that it was the Fuse located on the EHT board.

As you say Alan this is F601 and " ...is the fuse on the primary side of the mains transformer." Again I assumed that if this Fuse was blown then I would get no power whatsoever through the 'scope??
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Old 3rd Jul 2020, 7:38 pm   #17
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Again I assumed that if this Fuse was blown then I would get no power whatsoever through the 'scope??
Agreed. I don't think the EHT circuit is separately fused.

Alan
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Old 3rd Jul 2020, 8:27 pm   #18
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Quote:
Originally Posted by DonaldStott View Post
Again I assumed that if this Fuse was blown then I would get no power whatsoever through the 'scope??
Agreed. I don't think the EHT circuit is separately fused.

Alan
My mistake! I assumed it was an EHT fuse since it was on the EHT board.

If the fuse had blown the scope would be completely dead, including the power indicator LED.
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Old 3rd Jul 2020, 9:08 pm   #19
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Perhaps others can chip in here - as I said earlier this restoration is also an Educational Project for me.
The CRT heater is, in effect, tied to the cathode circuit, that reduces the voltage stress between the heater and the cathode and therefore reduces the voltage stress that the heater insulation would otherwise be subjected to. The heater, being tied to the cathode circuit, will be at a high -ve potential WRT the chassis as can be deduced from the schematic (1kV plus)

In effect, you can see similar in some radio receivers where one side of the HT rectifiers heater is connected to the rectifiers cathode to serve the same purpose.

Remember that in a CRT the cathode, grid, anode basically work just like they do in a radio valve, that's to say that the anode has to be positive WRT the cathode and if the grid to cathode voltage is less than the cut off voltage then current will flow from cathode to anode, and if the grid to cathode voltage is equal to or greater than the cut off voltage then no current will flow from cathode to anode also remember that all circuits are potential dividers, which can sometimes help with making sense of things.

Manufactures data for the CRT can be found here on PDF page 62 which also gives typical operating conditions:

https://www.rsp-italy.it/Electronics...%201973-11.pdf

Lawrence.
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Old 3rd Jul 2020, 9:21 pm   #20
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Thanks Lawrence - another digital document for my ever-expanding Library!

As I mentioned in Post #11 above "I seem to have CRT heater continuity across pins 1 and 14 on the CRT with a voltage of 6.79V AC." - have I measured in the right places (see Post #10) and do I have a working CRT heater?

6.79V seems a long way from 1kV plus but I'm probably missing the point?
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