Crotech 3133 oscilloscope - UK Vintage Radio Repair and Restoration Discussion Forum

Grizm1000 · 21st Oct 2017, 10:44 pm

Recently purchased a Crotech 3133 dual trace 25MHz while it seems to work reasonably well, tested with a cheap signal gen on both channels. I am unable to focus the beam to a nice sharp fine line, it's a fat blob with the focus knob turned fully anticlock. I am without any technical details on this model, though somebody pointed me to a 3132 model schematic. So essentially if anybody knows where i can buy, beg, steal or borrow a manual and/or point me in the right direction for fixing my focus issues I would be very much grateful. Also how would I set about calibrating such a scope, I've never owned a scope before and would appreciate other finger pointing in the direction of suitable tutorial material. Many thanks in advance.

Ed_Dinning · 22nd Oct 2017, 8:46 pm

Hi have a look at the recent posts on this topic. I believe there is a downloadable book from Tektronix called "The x,y,z of oscilloscopes"
It is an ideal primer.

Ed

WME_bill · 22nd Oct 2017, 10:36 pm

Crotech 3133 25Mhz Oscilloscope.
I believe this model is an upgrade of the Crotech 3132 20Mhz, with the front panel re-arranged to a horizontal format.
I would expect therefore that the circuit of your 3133 will be very similar to the 3132, the manual for which is fairly readily available.
If you can't find it, post again and I'll see what I can find.
Can you post some pictures, and confirm the CRT size or type. This will help to confirm that it is indeed an upgrade as against an entirely new design.

Focus problems usually mean that one of the resistors in the eht focus chain across the eht supply has gone high. This should not be too difficult to find and replace.
Just looked at the circuit diagram for the 3132 in the manual. It is not the easiest to follow through, so you will need a pencil and paper to trace the circuit out.
wme_bill

RogerEvans · 23rd Oct 2017, 8:39 am

Have you tried adjusting the astigmatism? Focus and astigmatism interact to get best results, astigmatism should change the spot shape as well as sharpness. Having said that, problems wth focus resistor chains are not uncommon, but wait for the capacitors to discharge before trying to measure anything. If you do have to replace anything make sure that the new resistors have a suitable voltage rating, you may have to pay a little extra but cheap resistors will probably not last long.

Roger

ukcol · 23rd Oct 2017, 3:04 pm

Here is a scan I made of my Crotech 3131 manual, the CRT circuits may be similar and helpful.

The file is large, hence the dropbox link.

https://www.dropbox.com/s/5tfpqybkda...03131.pdf?dl=0

Diabolical Artificer · 24th Oct 2017, 7:34 am

If it isn't a series resistor could it be noise on the vertical amp ? Is it both beams or one?

Most scopes have a potential divider across the EHT composed of fixed resistors and pots with taps going off to the various grids of the CRT. The voltage on these grids deflect the beam from the cathode and move/focus the beam onto the phosphor on the front of the CRT tube. If a resistor is high/low then the voltage on that grid will be off spec and it won't focus.

A look at how a static CRT works will help you suss out any fault. See this circuit here - http://www.electronixandmore.com/projects/simplescope/ the potential divider is composed here of the 1M (ASTIG) at the top of the B+ (we call it HT here in UK) , the 470k and the 1M (focus) Also see - http://ecee.colorado.edu/~mcclurel/txyzscopes.pdf that Ed mentioned.

Andy.

ukcol · 26th Oct 2017, 7:26 pm

The 3131 manual is no longer in my dropbox (as post #5).

If anyone would like it in the future please contact me by PM.

Grizm1000 · 31st Oct 2017, 9:27 pm

Thanks for the replies guys, food for thought. The focussing problem is on both beams. Three pics of the front panel.

WME_bill · 31st Oct 2017, 11:09 pm

crotech 3133.
If your focus problem is on both beams, then it confirms quite nicely that it is the focus/brilliance chain to the CRtube, and nothing to do with the timebase or Y amplifier. wme_bill

Argus25 · 2nd Nov 2017, 1:24 am

I agree with WME Bill,
So the CRT's focus electrode is at the wrong potential it would appear, it will either be higher or lower than it should be to bring the tube into focus.

In most cases, normally as the brightness control is advanced there is a small defocus and the CRT's focus electrode requires a tad more voltage. Sometimes this is compensated for in the circuit design, but often not.

So if you look carefully at the defocus you have, you could check whether it increased or decreased with alterations in brightness.

If for example the focus seems to get worse as the brightness is increased, likely the the focus electrode is running too low a voltage. So that could mean for example a resistor in the top leg of the divider chain had gone high.

On the other hand if say the focus improved a tad at high brightness, that might mean that the focus electrode was running too high, and perhaps a resistor in the lower leg of the focus resistor chain had gone high.

(I assume the brightness control is working normally, if the CRT's grid-cathode voltage is way off, especially grid positive with respect to the cathode the focus will go way off)

Also, it's obviously possible for resistors to go low in value, but in most high voltage applications, they have a habit of going high.

21st Oct 2017, 10:44 pm	#1
Grizm1000 Retired Dormant Member Join Date: Oct 2017 Location: Rotherham, South Yorkshire, UK. Posts: 2	Crotech 3133 oscilloscope Recently purchased a Crotech 3133 dual trace 25MHz while it seems to work reasonably well, tested with a cheap signal gen on both channels. I am unable to focus the beam to a nice sharp fine line, it's a fat blob with the focus knob turned fully anticlock. I am without any technical details on this model, though somebody pointed me to a 3132 model schematic. So essentially if anybody knows where i can buy, beg, steal or borrow a manual and/or point me in the right direction for fixing my focus issues I would be very much grateful. Also how would I set about calibrating such a scope, I've never owned a scope before and would appreciate other finger pointing in the direction of suitable tutorial material. Many thanks in advance.

22nd Oct 2017, 8:46 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Crotech 3133 oscilloscope Hi have a look at the recent posts on this topic. I believe there is a downloadable book from Tektronix called "The x,y,z of oscilloscopes" It is an ideal primer. Ed

22nd Oct 2017, 10:36 pm	#3
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Crotech 3133 oscilloscope Crotech 3133 25Mhz Oscilloscope. I believe this model is an upgrade of the Crotech 3132 20Mhz, with the front panel re-arranged to a horizontal format. I would expect therefore that the circuit of your 3133 will be very similar to the 3132, the manual for which is fairly readily available. If you can't find it, post again and I'll see what I can find. Can you post some pictures, and confirm the CRT size or type. This will help to confirm that it is indeed an upgrade as against an entirely new design. Focus problems usually mean that one of the resistors in the eht focus chain across the eht supply has gone high. This should not be too difficult to find and replace. Just looked at the circuit diagram for the 3132 in the manual. It is not the easiest to follow through, so you will need a pencil and paper to trace the circuit out. wme_bill

23rd Oct 2017, 8:39 am	#4
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 381	Re: Crotech 3133 oscilloscope Have you tried adjusting the astigmatism? Focus and astigmatism interact to get best results, astigmatism should change the spot shape as well as sharpness. Having said that, problems wth focus resistor chains are not uncommon, but wait for the capacitors to discharge before trying to measure anything. If you do have to replace anything make sure that the new resistors have a suitable voltage rating, you may have to pay a little extra but cheap resistors will probably not last long. Roger

24th Oct 2017, 7:34 am	#6
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,657	Re: Crotech 3133 oscilloscope If it isn't a series resistor could it be noise on the vertical amp ? Is it both beams or one? Most scopes have a potential divider across the EHT composed of fixed resistors and pots with taps going off to the various grids of the CRT. The voltage on these grids deflect the beam from the cathode and move/focus the beam onto the phosphor on the front of the CRT tube. If a resistor is high/low then the voltage on that grid will be off spec and it won't focus. A look at how a static CRT works will help you suss out any fault. See this circuit here - http://www.electronixandmore.com/projects/simplescope/ the potential divider is composed here of the 1M (ASTIG) at the top of the B+ (we call it HT here in UK) , the 470k and the 1M (focus) Also see - http://ecee.colorado.edu/~mcclurel/txyzscopes.pdf that Ed mentioned. Andy. __________________ Curiosity hasn't killed this cat...so far.

23rd Oct 2017, 3:04 pm	#5
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Crotech 3133 oscilloscope Here is a scan I made of my Crotech 3131 manual, the CRT circuits may be similar and helpful. The file is large, hence the dropbox link. https://www.dropbox.com/s/5tfpqybkda...03131.pdf?dl=0

26th Oct 2017, 7:26 pm	#7
ukcol Rest in Peace Join Date: May 2003 Location: Harlaxton, Lincolnshire, UK. Posts: 3,944	Re: Crotech 3133 oscilloscope The 3131 manual is no longer in my dropbox (as post #5). If anyone would like it in the future please contact me by PM.

31st Oct 2017, 9:27 pm	#8
Grizm1000 Retired Dormant Member Join Date: Oct 2017 Location: Rotherham, South Yorkshire, UK. Posts: 2	Re: Crotech 3133 oscilloscope Thanks for the replies guys, food for thought. The focussing problem is on both beams. Three pics of the front panel. Attached Thumbnails

31st Oct 2017, 11:09 pm	#9
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Crotech 3133 oscilloscope crotech 3133. If your focus problem is on both beams, then it confirms quite nicely that it is the focus/brilliance chain to the CRtube, and nothing to do with the timebase or Y amplifier. wme_bill

2nd Nov 2017, 1:24 am	#10
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Crotech 3133 oscilloscope I agree with WME Bill, So the CRT's focus electrode is at the wrong potential it would appear, it will either be higher or lower than it should be to bring the tube into focus. In most cases, normally as the brightness control is advanced there is a small defocus and the CRT's focus electrode requires a tad more voltage. Sometimes this is compensated for in the circuit design, but often not. So if you look carefully at the defocus you have, you could check whether it increased or decreased with alterations in brightness. If for example the focus seems to get worse as the brightness is increased, likely the the focus electrode is running too low a voltage. So that could mean for example a resistor in the top leg of the divider chain had gone high. On the other hand if say the focus improved a tad at high brightness, that might mean that the focus electrode was running too high, and perhaps a resistor in the lower leg of the focus resistor chain had gone high. (I assume the brightness control is working normally, if the CRT's grid-cathode voltage is way off, especially grid positive with respect to the cathode the focus will go way off) Also, it's obviously possible for resistors to go low in value, but in most high voltage applications, they have a habit of going high.