EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: fable on April 01, 2019, 08:43:42 pm
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Hello to everyone, I have two of these scopes with the same problem and the problem where I got stuck is that the trace is short. When I move it from left to right it stops one division before end. The only way i can make it longer is by raising +140v, it goes to a max 146v and even that is not enough.
I replaced all caps in PSU and all voltages are correct including - 1250v.
There was also few cold joints on both of them.
(https://uploads.tapatalk-cdn.com/20190401/6a1477eb22babd2cdaade261a74fccfb.jpg)(https://uploads.tapatalk-cdn.com/20190401/5781f22e25c5e9008bdda8263556a39a.jpg)(https://uploads.tapatalk-cdn.com/20190401/e265bbce7228d449e64126732ac612db.jpg)
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The problem could be in the last stage of the 'Final-X-Amplifier' circuit shown on the 'XY-/Cal-/TR-PCB' drawing.
I would measure the 'load line' resistors in the X output stage, R2106/R2107 and R2111/R2112, each should be 100K, but if they have aged, they could be higher, resulting in a short trace.
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Thank you for your time. On my schematic i cold not find those resistors but i assumed you were talking about R3109, 3110 and 3111,3112...they are 1k. Below is attached picture of x final amp. (https://uploads.tapatalk-cdn.com/20190402/86b66d907311f3131216577c63d8eedb.jpg)
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My mistake, I was looking at Hameg-HM604 drawings :-[
Try measuring R3114 and R3116, they should be 30.1K, a higher value would give a short trace.
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Similar scope and problem. https://www.eevblog.com/forum/repair/hameg-oscilloscope-horizontal-deflection-issue/ (https://www.eevblog.com/forum/repair/hameg-oscilloscope-horizontal-deflection-issue/)
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I cheked r3114 and 3116 and they are 30.2k also measured lot of other resistors and they are ok too.
Tomorrow I will post more detailed info of all measurements i did
@StillTrying
Tnx for help... I already read everything I could find on the net and this forum including this.
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With the beam stopped have you measured the D1 and D2 plate output voltages. If they match with the X beam position moved to the left and right edges of the screen I would think both output stages would be OK.
Does it overfill the screen with X set to X10.
I think VR3101 will be the X gain/amplitude, and VR3100 the gain/amplitude for X10.
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Bellow is the picure of screen when x is set to x10.
Adjusting the vr3100-3101-3102 does make change but within does two bright spots.
Trace is prettymuch the same on both scopes.(https://uploads.tapatalk-cdn.com/20190403/1f6bdd208d0b36eb9eaec57e89d315c0.jpg)
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Q1. Do you know if these faults developed over a short or long time, or are these two scopes a recent purchase, where you don’t know their history?
Q2. Can you confirm that VR3101 does change the (X1) width of the trace across the screen in a linear manner?
Q3. With VR3101 at minimum gain, how far does the trace go across the screen?
Your photo’s appear to indicate that the trace goes across approximately 85% of the screen width with VR3101 at maximum gain?
Q4. If the range of VR3101 was increased by 50% (half a turn), in your opinion, would that be sufficient to extend the X-amplifier gain, such that the trace would reach the full width of the CRT screen?
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I dont know any history of them,
I got them some time ago and they were totally dead...after replacing caps in psu and resoldeting some cold joints both of them started working but with this problem.
2. Yes, but only between does two bright spots and trace only expand-contract to left on the screen.
3. Right half of screen when it is at minimum. When turning gain up trace cannot pass does bright spots.
4. Thoes two bright spots are the end point of the trace.With vr3101 I can expand-contract trace length with x-pos i can move it left to right but all that only between does two spots.
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Red is connected on D1 and yellow D2 on crt.Make attention to the x Pos knob.
Thank you all for your help and sorry for my bad English(https://uploads.tapatalk-cdn.com/20190404/186a0cc87cfc8878685cc4fb4a935156.jpg)(https://uploads.tapatalk-cdn.com/20190404/5d0163f964f3d1dcfc822816e9057461.jpg)(https://uploads.tapatalk-cdn.com/20190404/858e03699ef7f27298544c68ec4b9f2e.jpg)(https://uploads.tapatalk-cdn.com/20190404/807e8118c3f2c947f7a522fe5c787636.jpg)
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I'm surprised even X10 doesn't get the trace to fill the width of the screen.
It looks like the D1 and D2 voltages are not going high enough - towards the +140V.
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We know that all the semiconductors within the X-amplifier are all OK (this would be obvious in a DC Coupled amplifier) as its almost behaving correctly. The +140V rail and the output load line resistors are both correct. The only problem that I can see is that the X-amplifiers gain is slightly below spec. Why this happens on two scopes, I don’t know. I was wondering if the X1 gain pot VR3101 needs a padding resistor placed in series with itself, to elevate the gain a little. VR3101 is 1K, so could we try adding a 470R padding resistor (half the value of VR3101) in series with VR3101 to elevate the gain a little? I’m sure that you will know which side of VR3101 the padding resistor needs to go.
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I was wondering if the X1 gain pot VR3101 needs a padding resistor placed in series with itself, to elevate the gain a little. VR3101 is 1K, so could we try adding a 470R padding resistor (half the value of VR3101) in series with VR3101 to elevate the gain a little?
I think lower resistance increases the gain, and that's near enough what the X10 switch does, but it doesn't help with the end to end amplitude, so I don't think it's that.
It's very similar to the previous linked thread, but with both outputs not pulling high enough towards the +140V, might be a very common fault.
The 250R VR3107 ? in the center of the emitters looks like it sets the quiescent current for the output transistors.
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Big thanks to all!
R3105 is totally open it measured 74k in circuit instead of 68k2 so I was suspicious and removed one leg and found that it is open. R3106 measured 74k so I replaced both of them and the scope is working like it should.
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Set the X1, then X10 gains/amplitudes with a known time signal while you're there.
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Set the X1, then X10 gains/amplitudes with a known time signal while you're there.
I was planing to do that. Btw on the other scope problem was the same...r3105 open and r3106 about 82k. They are both working now.