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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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2nd Jul 2020, 12:51 pm | #1 |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
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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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2nd Jul 2020, 5:08 pm | #2 |
Octode
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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. |
2nd Jul 2020, 7:32 pm | #3 |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
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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?
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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2nd Jul 2020, 8:14 pm | #4 |
Nonode
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Location: Halifax, West Yorkshire, UK.
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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 |
2nd Jul 2020, 8:56 pm | #5 | |
Octode
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Location: Newcastle Upon Tyne, Tyne & Wear, UK.
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
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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3rd Jul 2020, 12:19 pm | #6 | |||
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
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.) Quote:
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:
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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3rd Jul 2020, 12:48 pm | #7 | |
Dekatron
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
Lawrence. |
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3rd Jul 2020, 1:44 pm | #8 | |
Octode
Join Date: Aug 2015
Location: Glasgow, UK.
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
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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3rd Jul 2020, 1:47 pm | #9 |
Octode
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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. |
3rd Jul 2020, 2:30 pm | #10 | |
Octode
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
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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3rd Jul 2020, 3:28 pm | #11 |
Octode
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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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3rd Jul 2020, 3:46 pm | #12 |
Octode
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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? |
3rd Jul 2020, 4:43 pm | #13 |
Dekatron
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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 |
3rd Jul 2020, 6:57 pm | #14 | |
Octode
Join Date: Aug 2015
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Re: Farnell 30-4D Oscilloscope - Help with Repair
EHT Fuse is good out of its holder according to my DMM Continuity setting?
Quote:
Perhaps others can chip in here - as I said earlier this restoration is also an Educational Project for me.
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3rd Jul 2020, 7:14 pm | #15 | |
Nonode
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
Alan |
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3rd Jul 2020, 7:29 pm | #16 | |
Octode
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
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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3rd Jul 2020, 7:38 pm | #17 |
Nonode
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Re: Farnell 30-4D Oscilloscope - Help with Repair
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3rd Jul 2020, 8:27 pm | #18 | |
Octode
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Location: Newcastle Upon Tyne, Tyne & Wear, UK.
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
If the fuse had blown the scope would be completely dead, including the power indicator LED. |
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3rd Jul 2020, 9:08 pm | #19 | |
Dekatron
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Re: Farnell 30-4D Oscilloscope - Help with Repair
Quote:
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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3rd Jul 2020, 9:21 pm | #20 |
Octode
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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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