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Old 3rd Jul 2020, 9:35 pm   #21
ms660
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Originally Posted by DonaldStott View Post
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?

Only you can tell us that

6.79V seems a long way from 1kV plus but I'm probably missing the point?
Yes, and so it should be, 1kV on the heaters doesn't mean to say there should be 1kV across them....Think of an output transformer in a valve radio, the primary will have say 20 volts across it but when the primary connections are measured WRT the chassis the voltage measured will be considerably higher....

Lawrence.

Last edited by ms660; 3rd Jul 2020 at 9:41 pm. Reason: Addition
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Old 3rd Jul 2020, 10:40 pm   #22
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Hi!

Can I recommend you get the Racal 9386 Manual?

This one has been scanned in much better quality with a white background with better quality text and circuit/layout diagrams!

The best way to check the CRT heaters is to view the back of the CRT of the 'scope with the lights off, where you'll be able to see the reddish–orange glow of the heaters at the rear of the neck or, by looking at the base, you should see an orange dot inside the centre of the base.

You really need something like an AVO 8 or a GEC "Selectest Super 50" to properly check the –1.6kV CRT cathode/grid supply safely – both these models of meter have a safe 2.5kV multiplier resistor incorporated, connected to a special separate terminal.

However, providing you switch the 'scope off first and allow 15 minutes (for safety) for the smoothing/e.h.t. doubler capacitors to discharge, it is possible to fault–find the circuits with a good DMM on the "ohms" ranges plus something like a TC1 Component Tester for the capacitors!

Let me know whether you can see the CRT heater alight in a darkened room, then I can advise from there!

Chris Williams
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Old 3rd Jul 2020, 11:15 pm   #23
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Thanks to Lawrence and Chris for their help and support.

I think I now understand the voltage 'enigma' with the CRT Heater.

I've managed to track down the Racal 9386 Manual which is indeed of a much higher quality.

Which TC1 Component Tester is recommended - I already have a DANIU MK-328 Tester which comes in very handy once you replace their flimsy plugs and leads! I still need to investigate the +20V, -20V, +110V supply rails and check the six 16uf 500V electrolytic capacitors. For the avoidance of doubt and any nasty jolts what's the best way to ensure that these big caps are discharged?

Meantime having spent some time with my Farnell 'scope in a darkened room I can confirm that I was able to observe the reddish–orange glow of the heaters at the rear of the CRT neck. Familiar smell as well and a warm glow when a hand was held in close proximity but with the lights on - but not too close!

The good news for all of you is that I won't have much time this weekend to do any testing as a behind schedule bathroom re-decoration project has a higher priority, apparently!
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Old 4th Jul 2020, 12:10 am   #24
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

A Farnell scope passed through my hands a few years ago, might even have been this one. It had a problem with the blanking circuit although that did show as flyback lines and was fixed by replacing an opto-coupler. It was difficult and dangerous to work on because of the very high voltages on the tube circuitry.
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Old 4th Jul 2020, 1:11 am   #25
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

The MK-328 will be fine for your capacitor measurements, the TC1 may be a better model.

I use the 328 and as you say the leads are not great.

As Chris recommends wait a minimum of 15 minutes before doing any work on the capacitors (also ensure the scope is unplugged and isolated from the mains supply before going inside). You can measure (carefully) the DC voltage across the capacitor to determine if it has discharged fully.

Note also many of the digital capacitor measuring meters have little protection against charged up capacitors, I blew a meter up where I thought the capacitor was fully discharged (it must have been close).

Some people will short out the capacitor to make sure it is discharged, this is not really recommended as it is not good for the capacitor and if any appreciable charge voltage present you can get quite a crack/spark which can make one jump. Better to short out through a resistor but you have to be very careful.

Tonight I have been testing my Solartron scope and I would estimate that some of the big capacitors were taking > 30 minutes to discharge fully, the discharge time probably extended due to the fact that I have most of the circuitry load disconnected.

Be careful and keep safe.

David
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Old 5th Jul 2020, 9:33 am   #26
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Originally Posted by DMcMahon View Post

Better to short out through a resistor but you have to be very careful.

David
Better description would have been "Better to discharge any residual charge by carefully/safely connecting a resistor across the capacitor, after first allowing at least 15 minutes for the capacitor (s) to naturally discharge after switching off power".

David
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Old 5th Jul 2020, 12:13 pm   #27
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Originally Posted by DonaldStott View Post
I think I now understand the voltage 'enigma' with the CRT Heater.
Hi Donald, there was no intention to complicate or distract your thread, I was just a bit concerned that there had been no previous mention by anyone that measuring around the CRT base/circuitry in oscilloscopes is potentially hazardous- "6.3V" sounds innocuous but it's associated with far higher voltages that can go somewhat beyond what most insulation on meter probes and leads is rated at. Even if you're aware and careful, it's a popular and public forum that someone could well find on a search who might be uninformed of the potential danger. Most people probably have some awareness that CRT TVs have high voltages inside them, but an oscilloscope seems less obviously dangerous- having had a near-miss with the mains EHT section of a CT436 'scope whose EHT cover was missing when I was much younger, I'm now kinda wary of them!

Another hazard with 'scopes is that a slipped probe around the high-voltage circuitry could ruin the CRT- not a good outcome.

Good luck with the quest, Colin.
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Old 5th Jul 2020, 5:12 pm   #28
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Hi!

As I remarked in a previous post, the large amounts of CRT grid/unblanking circuitry operated at –1500/–1600V derived directly off a mains transformer means these 'scopes are very hazardous to fault–find with the power on, so I recommend you proceed as follows, with the scope switched off, and ample time allowed for capacitors to discharge:–

1) Replace the following components with new or known tested good ones in the unblanking circuitry:–

a) R615, R616 and R617;
b) Opto OC501, TR612 and D627 & D628;

c) Using new VR25 or VR37 high–voltage rated metal–glaze resistors, replace the following with new:–

R601 to R606 (2M2) plus R607 (680k) and R610 (1M)

d) Check the track resistances of the intensity control RV601 and focus control RV602.

e) Using your MK328, make sure none of the 110V and 200V zener–diodes D619 to D624 have gone open– or short–circuit, – you will be able to read them in the forward direction, about 0.7V, but without a high–voltage supply you'll not be able to test the zener voltage – replace them with new if in doubt!

f) Replace C613 and 614 with new to ensure the auxiliary –12V voltage–doubled supply sitting "on top" of the –1100V cathode supply potential is in working order.

All the above can be done in safety with no power applied to the Oscilloscope, and if you carry out the work I enumerate in steps a) to f) above, you can be sure afterwards that all these parts of the circuit are as Scopex built them!

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Last edited by Chris55000; 5th Jul 2020 at 5:18 pm.
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Old 5th Jul 2020, 6:00 pm   #29
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Hi!

Further to the above, if you have any doubt about the +5kV p.d.a. supply, the diodes in the p.d.a. multiplier circuit are difficult to test without a high–voltage Curve Tracer, but you can get replacements of the "2CL77" type cheaply enough from the usual sources!

The voltage–multiplier capacitors C607 to C612 will probably be OK if your MK328 reads their value OK, but 22n high–voltage disc–ceramics are easy enough to find if you look around!

Chris Williams
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Old 8th Jul 2020, 8:05 pm   #30
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Originally Posted by PJL View Post
A Farnell scope passed through my hands a few years ago, might even have been this one. It had a problem with the blanking circuit although that did show as flyback lines and was fixed by replacing an opto-coupler. It was difficult and dangerous to work on because of the very high voltages on the tube circuitry.
Thanks PJL - as I don't have any display whatsoever I'll need to wait and see if I have a problem with the blanking circuit, visible as flyback lines?

Thanks also to DMcMahon (David) on the sensible advice relating to very high voltages and large capacitor discharge times. I do have a DIY lead with a 180MΩ 5W Ceramic Resistor and insulated croc clips that I use for this purpose.

The safety message was eloquently reinforced by turretslug (Colin) as sometimes we tend to forget that "Even if you're aware and careful, it's a popular and public forum that someone could well find on a search who might be uninformed of the potential danger." - so thanks for that reminder!

Of course Chris55000 (Chris Williams) has really gone the extra mile by providing a detailed procedure along with a list of components to test and replace where necessary - you really couldn't ask for much more, just hope I'm up to the job. I'll do the specified measuring over the next few days and order up parts where needed. I may have to revert back to the Forum for advice on where to source the relevant components? I treat all such activities as educational projects so it may take a little longer than expected as I try to understand the work I'm doing?

As a footnote I'm well prepared for the possible outcome that the CRT may be GUBBED, in the Scottish sense of the word i.e. " ...anything, or anyone, that has fallen into an irretrievable state of disrepair or damage". So I may be on the lookout for a replacement oscilloscope and might even contemplate going digital this time!
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Old 11th Jul 2020, 2:57 pm   #31
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Hold that result ...!

Having managed to remove the top and bottom covers I flipped the 'scope over to measure those large caps on the underside - all measurements in-circuit:-

C640: Spec 100uF Measured 120uF

C603-C606: Spec 16uF Measured 21-23uF

And then I noticed this:-

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That capacitor only has one leg, I said out loud, and hopefully it's not a special type only used in oscilloscopes - due to my relative inexperience with oscilloscopes I'm prepared for anything! Or could it be one of the components deleted in the Racal Amendments??

I'm looking at Page 15 of the Racal 9386 Manual showing the main board and components - what I've noticed is that all components fitted to the underside of the board are shown with a dashed outline, but not this capacitor?

Apologies but it's over to you guys for more advice and guidance?
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Old 12th Jul 2020, 10:26 am   #32
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Was it connected across C615?

If not which component(s) is the connected lead connected to?

The capacitor looks like a high voltage ceramic to me....0.02uF?

Lawrence.

Last edited by ms660; 12th Jul 2020 at 10:36 am. Reason: extra info
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Old 12th Jul 2020, 3:01 pm   #33
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Thanks Lawrence

As this capacitor will have to be replaced I simply disconnected the one remaining good leg which enabled me to stick a wire through the hole to the other side.

First annotated picture: -

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The picture above shows My Wire pushed through from the other side to show where the known cap leg connection was. It is fairly clear that there is a connection to a Brown Wire that runs to Pin 7 of the CRT and also to one leg of C615 (?) So at this stage I'm thinking that the cap with the missing leg must be C617 albeit mounted on the 'wrong' side of the board?

Section of board layout: -

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The board layout clearly shows C617 mounted on the same side of the board as most of the other components - for some reason it was moved to the other side?

Section of board schematic around the CRT: -

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Ignoring for the moment that there are two capacitors labelled C617 on this part of the schematic we can confirm that one leg (the known 'good' leg) of the 20nF C617 cap connects to Pin 7 of the CRT and is also connected to R614 'INTENSITY'. The other leg of C617 appears to connect to TR304 which sits alongside it on the board.

Second annotated picture: -

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The discolouration of the board to the right of TR304 tends to imply that C617 was indeed located here at some point i the past? Adding up all these bits of evidence I have indicated on the above picture my best guess at where the missing leg was connected?

And finally, the cap with the missing leg is labelled 0.02 which I am assuming is 0.02uF (20nF) - but what does the symbol above the numbers represent?

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Old 12th Jul 2020, 5:09 pm   #34
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Just a short addendum to show the connection from C617 to TR304 (BC212) on the schematic: -

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Assuming I'm going down the right track what would be a suitable replacement for C617 - I know it's 20nF and ceramic, but what about the voltage - it seems quite chunky!
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Old 12th Jul 2020, 6:03 pm   #35
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Looking at the latest photo's I would say yes, C615 is as annotated correctly on the component side of the board and the defective capacitor has been correctly annotated as C617.

I would replace it with a 0.022uF or 0.02uf (or 2 x 0.01uF in parallel) with a DC voltage rating of at least 2kV.

Lawrence.

Last edited by ms660; 12th Jul 2020 at 6:26 pm.
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Old 12th Jul 2020, 6:08 pm   #36
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Looking at the latest photo's I would say yes, C615 is as annotated correctly on the component side of the board and the defective capacitor has been correctly annotated as C617.

I would replace it with a 0.022uF (or 2 x 0.01uF in parallel) with a DC voltage rating of 2kV.
Thanks Lawrence - I'm of a mind to mount the new capacitor on the component side of the board along with all the others - any reason why it was located on the other side?

Would two of these in parallel be o.k. - I'm just a bit cautious because of the high voltages:-

https://www.cricklewoodelectronics.com/CKK10N.html

Or one of these: -

https://uk.rs-online.com/web/p/polyp...itors/1353496/
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Last edited by DonaldStott; 12th Jul 2020 at 6:17 pm.
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Old 12th Jul 2020, 6:35 pm   #37
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Any of those should do.

Not sure why it was on the other side of the board but if that's where it was originally as found then that's where I would fit the replacement.

Lawrence.
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Old 12th Jul 2020, 7:32 pm   #38
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

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Not sure why it was on the other side of the board but if that's where it was originally as found then that's where I would fit the replacement.
Of course I'm always interested in finding an explanation especially when you can see some discolouration on the board in my annotated pictures where C617 'could' have been at one time?

Maybe it ran too hot there as its directly under the CRT but surely that would have affected C615 as well?

Just curious but it won't hold up proceedings ...
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Old 14th Jul 2020, 10:25 pm   #39
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

My KEMET 22nF Polypropylene Capacitor PP 2kV DC arrived today and I wasted no time in getting it installed - on the 'wrong' side of the board of course!

The great news is that my dual-trace display is back, seemingly alive and well: -

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So far, so good but we're not out of the woods yet I feel!

Tomorrow I'll try connecting my Velleman Signal Injector as I know this has a fairly steady frequency of around 994Hz and also connect my Topward Function generator which will enable me to generate and view different wave forms - sine, saw-tooth and square.
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Old 15th Jul 2020, 4:22 pm   #40
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Default Re: Farnell 30-4D Oscilloscope - Help with Repair

Preliminary results of testing my 'repaired' Farnell scope using my Velleman signal injector and Heathkit signal generator (AF Out) to display a standard sine wave.

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As can be seen, although we have a nice clean sine wave we have a fainter signal superimposed coming from somewhere?

I then connected up my Topward function generator to display a sine wave, a saw-tooth wave and a square wave, all at 1kHz.

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In all instances we appear to have superimposed signals coming from somewhere?

Anyone got any ideas what I'm seeing - is it a grounding issue?
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