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Old 6th Oct 2011, 11:43 pm   #1
musonick
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Default Scopex 4D10B channel dead.

The second channel on my Scopex oscilloscope is dead alough the trace does appear when the 'locate' button is pressed. Worth fixing, easy fix or don't bother? Any suggestions welcome.

Nick.
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Old 7th Oct 2011, 10:13 am   #2
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Default Re: Scopex 4D10B channel dead.

Sounds as though it's working but the trace is being deflected way off screen. Usually all the trace locator does is bypasses any setting on the "Y" shift to centre the trace.


Perhaps a fault in the Y shift? Can you see if the trace is appearing on the top(or bottom) of the CRT.
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Old 7th Oct 2011, 10:32 am   #3
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Default Re: Scopex 4D10B channel dead.

Thanks Tim, the trace appears at the bottom when locate is pressed. Vertical shift has no affect on position and the trace is not activated by probe. However, the horizontal shift does affect the trace. I do have a schematic but have never ventured beyond the case of an oscilloscope! Channel A works perfectly well.
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Old 7th Oct 2011, 12:57 pm   #4
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Default Re: Scopex 4D10B channel dead.

As Tim suggests this is symptomatic of a shift fault. The Scopex is a fairly simple instrument so it shouldn't require more than the usual fault finding techniques you'd use on a radio or Television.

As a first step check the shift control and look for obvious faults, wires off, cracked tracks etc. and check for any obviously leaky capacitors. I'm at work and don't have access to a manual so I can't be more specific than that at the moment .
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Old 7th Oct 2011, 1:13 pm   #5
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Default Re: Scopex 4D10B channel dead.

OK, thanks all. I will have a look for the obvious and get back.
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Old 7th Oct 2011, 1:29 pm   #6
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Default Re: Scopex 4D10B channel dead.

Since this is not a true dual beam scope you can be sure that the Y output stage is OK. You have 2 identical (presumably) Y input channels so it's easy to compare voltage readings between them. This should narrow down the fault pretty quickly. The only tricky area might be the Y channel switch which may have some subtle problem.
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Old 7th Oct 2011, 1:56 pm   #7
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Default Re: Scopex 4D10B channel dead.

I think you're spot on, Jeffrey. On my Tek scopes, the beam switching is performed using a matrix of diodes. So, the fault is either in the switch, or the input amplifier on the duff channel. Most likely the Y amp.

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Old 7th Oct 2011, 3:13 pm   #8
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Default Re: Scopex 4D10B channel dead.

I am repairing my scopex at the moment. The fault is very similar, first it was one chanel and then very soon the second chanel went down as well. I currently suspect the dual FET in the input immediately after the i/p attenuator.
There is this thread which I discovered and is proving to be very interesting reading :-
http://www.electro-tech-online.com/g...-my-scope.html
It's been a while since I last looked at my scope (you know how it is - real life just keeps dragging you away from these thins!) but I intend to get back to it very soon. If I make any progress I will be sure to let you know

BTW I'm beginning to suspect that these instruments have a weakness and that the problem was caused by overvoltage to the input - I was measuring a small ac signal on top of about 70v DC when the trace disappeared so the scope was ac coupled at high sensitivity....

Cheers,
Steve.

PS I have a copy of the manual if you need it.
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Last edited by fetteler; 7th Oct 2011 at 3:14 pm. Reason: PS
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Old 7th Oct 2011, 6:32 pm   #9
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Default Re: Scopex 4D10B channel dead.

Having had a quick look at the manual, check the voltage at either end of the track of the vertical position control. Should be +600mV at one end and -600mV at the other. There is a preset 25K (RV52) connected across the front panel control (RV51) so this mat have gone O/C. Other components to check are C52 (I have known this to fail) and R54 (82k) which will cause a lost trace if it goes O/C.

The other likely cause is a failed TR52 (BFS28 which is a Darlington). I should have BFS28 in stock if you need one.

By the way, the component identifications are suffixed "A" for Channel A and "B" for Channel B.

These are one of the easiest 'scopes to work on so I'm sure you'll sort it out with little trouble.
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Old 7th Oct 2011, 6:58 pm   #10
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Default Re: Scopex 4D10B channel dead.

Maybe I'm misunderstanding something fundamental here but on my scope tr52 is a conventional npn transistor and is one of 5 a 14pin dil transistor array (CA3046)....
There is one of these arrays in each channel and some elements in each were found to be faulty in my case. Replacing the arrays did not correct the fault so there's more to be done....
I just noticed that this thread is concerning a 14D10B and my scope is a 14D10, perhaps there are differences?

Cheers,
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Old 7th Oct 2011, 7:53 pm   #11
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Default Re: Scopex 4D10B channel dead.

Oh, well that might have something to do with it, I'm looking at the 4D10 manual.

The 4D10 is entirely discrete, which is why it's so easy to work on.

Perhaps we should have a new thread for the 14D10.
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Old 7th Oct 2011, 11:32 pm   #12
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Default Re: Scopex 4D10B channel dead.

thanks all i will check through your sugested hotspots brian and get back to you

Thanks all. I will check through your suggested hotspots, Brian, and get back to you.
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Old 8th Oct 2011, 12:46 pm   #13
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Default Re: Scopex 4D10B channel dead.

Quote:
Originally Posted by Brian R Pateman View Post
Oh, well that might have something to do with it, I'm looking at the 4D10 manual.

The 4D10 is entirely discrete, which is why it's so easy to work on.

Perhaps we should have a new thread for the 14D10.
Brian,
My dyslexia/word blindness is in full swing!!!
It's only since reading your post and then carefully re- reading the title if this thread that I saw the difference between 4 and 14..... Crikey

Silly me.
Still, when I sort out my 14D10 I will post the results in success stories.

Sorry to have caused confusion,

Steve.
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Old 8th Oct 2011, 12:52 pm   #14
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Default Re: Scopex 4D10B channel dead.

Don't worry about it Steve - it happens to me all the time!

I'll look forward to seeing a thread in Success Stories. We don't often have test gear in there.
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Old 8th Oct 2011, 5:39 pm   #15
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Default Re: Scopex 4D10B channel dead.

This is not likely to be a tube fault.

When you say you've worked through the board, have you done voltage checks or component value checks? The only difference between Ch A and Ch B is the source voltage derivation for TR52B. This derives the voltage from the +30V supply and the suspect component is C53B.
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Old 8th Oct 2011, 6:11 pm   #16
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Default Re: Scopex 4D10B channel dead.

JUst one really silly point.

Give the ALT/CHOP switches a thorough clean. With both switches out Ch B is turned off.
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Old 11th Oct 2011, 4:38 pm   #17
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Default Re: Scopex 4D10B channel dead.

OK sorted and thanks for all your help. Should have picked it up first time but novice here. It was TR56B which is marked as 594 but no prefix. My schematic does not list a type or value. Any ideas or equivalents (preferably maplin) please?

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Old 11th Oct 2011, 6:23 pm   #18
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Default Re: Scopex 4D10B channel dead.

Well done!

TR56B is likely to be fairly simple to find a replacement for, try BF594 or 2N3983. I'll have a look and see if I have one later on. Time for dinner soon.
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Old 11th Oct 2011, 7:25 pm   #19
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Default Re: Scopex 4D10B channel dead.

thanks brian. i have to pop to maplin in the morning anyway and if they have no equivalent i shall venture to cricklewood elec. i did note that c53 is 200nf while the schematic gives it as 100nf. looks factory to me, possible later mod?
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Old 12th Oct 2011, 6:23 pm   #20
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Default Re: Scopex 4D10B channel dead.

thats all good and working now. it's the first non audio bit of kit i have tackled. thanks brian and others for the info.
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