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Old 26th Aug 2017, 3:02 pm   #1
OldTechFan96
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Default Scopex 4D-25 Oscilloscope Repair Help

Hello all!

About six weeks ago I bought the aforementioned 'scope from a local junk shop complete with probes and a blue carry case. The 'scope is from a house clearance of somebody who probably was in the electrical repair trade. All sorts of test gear showed up at the same time.

The 'scope and carry bag look to have been stored in a damp environment giving it a damp smell and some rust damage to the case. Thankfully the inside of the 'scope was undamaged.

Before I switched the 'scope on it was thoroughly cleaned and inspected, which included cleaning all of the switches and potentiometers with Servisol contact cleaner. I also replaced the old mains cord. A 13A fuse was fitted in the original plug. The internal fuse tested good with my multimeter.

When I switched the 'scope on the power light came on and I heard the transformer hum for a few seconds. Suddenly the light when off. I checked the internal fuse at it had blown!

I don't quite know where to start with this repair. I've repaired transistorised radios vintages computers before so this will be new to me. I have a PDF of the service manual.

Where should I begin?

Thanks
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Old 26th Aug 2017, 5:19 pm   #2
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The usual suspects are the electrolytic capacitors, either aluminium or tantalum. Begin by checking the 'reservoir' capacitors immediately after each of the low voltage supply rectifiers. I found schematics for a 4D10 and that has +20V, -20V and +110V. If any of those capacitors are short circuit they will probably blow the mains fuse and may destroy the associated diodes. If the input to the low voltage regulators looks OK check for shorts to ground on the outputs of each regulator. Depending on the design of the regulator it might current limit or just blow the fuse.

Roger
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Old 26th Aug 2017, 7:06 pm   #3
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Thanks Rodger. I'll start by testing the two 22uf 450v capacitors on the EHT board along with all the diodes.
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Old 26th Aug 2017, 9:21 pm   #4
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The two capacitors on the EHT board test good on my MK-328 component tester.

All of the diodes test good on the EHT board apart from D601 - D604 which are 1N5399s.

The failed diodes showed 0.002V both ways on my multimeter (out of circuit) so are definitely bad.

Would replacing these diodes bring the 'scope back to life? Or should more testing be done?

I'll test the voltage regulators in the morning.

Thanks
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Old 26th Aug 2017, 9:38 pm   #5
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I don't have the circuit so I don't know where D601 - D604 belong. If they are part of the HVmultiplier it is likely that they failed because one or more of the capacitors in the multiplier failed first. I have found that the HV capacitors (above 1kV) really need a high voltage tester since they may show very limited leakage with a multimeter. I bought a Robin insulation tester for the purpose, a Megger would probably be fine. I would check the 'input' capacitirs on the lower voltage supplies as well before powering up again.

Roger
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Old 26th Aug 2017, 9:53 pm   #6
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Hello Rodger,

I'd attach the service manual that I'm using but it is too big to upload to the forum.
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Old 27th Aug 2017, 5:32 pm   #7
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Roger,
you can have a Scopex 4D-25 manual from here_hopfely its samme as OTF96 hase...
@ OTF96; you can measure resistance in the line of your failed diodes, if it isn`t short- I would close the cct and switch on the scope; maybe hasnt more problems.
Regards, Karl
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Old 27th Aug 2017, 7:18 pm   #8
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Scopex 4D-25.
While I am sure the advice on other postings is very relevant, it seems reading your description of the failure that the power supply has failed.
I would suggest you get that working first, before worrying about the tube EHT multiplier chain from C607 onwards.
As it blew the fuse, it suggests that the current taken by the +20v, or -20V or +110V has failed by a short circuit or the tube high voltage -1.6kv feed.
You say that some of the diodes D601 to D604 which are the Tube high voltage supply -1.6kv have failed.
The high voltage supply chain of capacitors, from C601 to C606, with diodes D601 to D604 is therefore area to be checked.
It sounds as if the chain of capacitors has failed, which has then affected the diodes.

Be careful working with this part of the circuit, as the capacitors will all hold (if sound) a very nasty charge. That is why each one has a 2Mohm resistor across it R601 to R606, intended to discharge them to zero in a couple of minutes.
As a start, I suggest disconnect EHT supply multiplier at C607. That stops the really uncomfortable 5kv voltages.

Now check the voltages for the four voltage rails +20/-20/ +110 /-1600v.
The -1.6kv sets the base for the -20/+20 rails, from the zener chain -200v D624. So until that is right, the other rails will be out.

I would suspect one of the capacitor in the chain has failed.
Replace it and all the others at the same time.
As a general policy, I suggest do not change anything unless it is definately faulty, as the unsoldering and resoldering will often encourage other faults to appear.
But in this case, do the lot C601 to C606 and associated diodes, almost any 1000v rated eg 1N4007 / BYX10.
Check C64, which just steadies the CRT heater, and C613, 20nf a high voltage type, probably ceramic Not likely to be in trouble..
Then check the voltages which appear across the zener diode chain D619-D624.
Once the 1.6kv is about right (+-10%), look at the other 3 lower voltage rails.
Let us know what you find.
wme_Bill m0wpn
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Old 27th Aug 2017, 9:17 pm   #9
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The 1.6kV supply is extremely dangerous in this scope.

You should try to reform the electrolytics C601-C606 before any attempt is made to switch on again. This could be done using a series resistor and a much lower AC voltage source connected as a replacement for the transformer or you could reforming them one by one. I would look for leakage of less than 2mA @ working voltage.
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Old 27th Aug 2017, 10:15 pm   #10
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I've been doing some component testing and this is what I've found:

What I believe to be the voltage regulators (1N5399) test good using the diode check function of my multimeter. Tested out of circuit.

The high voltage electrolytic capacitors C601 to C602 (22uf 450v Rubycon) and C603 to C606 (16uf 500v CCL) tested good with my component tester. Since I don't have the equipment to test for electrical leakage or reform capacitors I will order new ones.

Tomorrow I will test the low voltage side of the 'scope once I know how to do it properly and safely. Will removing C607 disable the 5kv EHT supply?

Hopefully powering up the 'scope with the EHT disabled will not cause the internal fuse to blow. Then I can make progress measuring voltages.

Thanks for the help so far!
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Old 29th Aug 2017, 12:21 pm   #11
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I spent some time yesterday working on the case of the 'scope. The two side panels were affected by rust and flaking paint so I sanded them down back to bare metal. The top and bottom half's are rust free externally and just need some more cleaning.

I'll try and do some voltage checks with the EHT disabled soon.
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Old 31st Aug 2017, 5:25 pm   #12
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Progress Report:

I have replaced electrolytic capacitors C601 to C606 and diodes D601 to D604. I also replaced a split 1k resistor.

I disconnected the EHT supply multiplier C607 then powered the 'scope up. Unfortunately the internal fuse blew straight away. It's a good thing that I bought a pack of ten fuses!

Where should I be looking next?
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Old 31st Aug 2017, 6:06 pm   #13
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Disconnect all secondaries to the transformer and check the transformer. It is definitely set to the right voltage?

If it's OK you can start putting them back!

Last edited by PJL; 31st Aug 2017 at 6:14 pm.
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Old 31st Aug 2017, 6:35 pm   #14
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

You need to check the +/-20V supplies and the +110V supplies, if the mains fuse keeps blowing one or more of these is probably short circuit.

You need to know the polarity of the ohms range on you multimeter, most usually it is opposite to the colour code, ie the black lead carries the positive voltage and the red the negative. You need to know this since the rectifying diodes will conduct back through the transformer secondary if you get it reversed.

Assuming you have the usual polarity, to check the +110V put the red lead to ground and the black to the +ve terminal of C640b or C640c. A short circuit in C640 will show something less than a hundred ohms or so. If C640 shows OK move the black lead to R643 on the output side of the regulator (I can't make sense of the comments in the manual that Karesz linked to, yours may be easier to understand). A low reading here has to be traced through the circuits that use the +110V.

For the +20V put the red lead to ground and the black lead to the positive terminal of C661 on the input side of the regulator and C664 on the output side. For -20V put the black lead to ground and the red lead to the negative side of C681 on the input to the regulator and and then to the negative side of C684 on the output of the regulator.

If any of these show a low resistance then to be sure you should try to isolate one side of the capacitor to check if the fault is the capacitor or elsewhere.

Regards,

Roger (without the d!)
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Old 1st Sep 2017, 11:33 am   #15
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Roger,

I now understand what you mean by 'checking the supplies', as in probing the relevant circuit with my multimeter on the ohms range to ensure that there is no short circuits.

I have printed off the schematics from the service manual (found here ). I will measure the resistance between the points mentioned in your last post and get back to you.

Does a very low resistance indicate a short circuit?

PJL,

I've had a look at the service manual and I am sure that the voltage is set for 230V. How do I go about testing the mains transformer?


Thanks for the continued support with this repair. I still have a lot to learn but I think that I'll pull through in the end.
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Old 1st Sep 2017, 6:36 pm   #16
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Before powering any bit of kit up, it is a good idea to check for low resistance on all supply rails. This is done by clipping your black lead of your DMM or AVO or whatever to the chassis/ground, then probing any caps or other points as indicated on a schematic with the red probe. This on the ohms/resistance setting - 1-2k range, with no power obviously.

As a general rule anything around a few kilo-ohms or upwards is ok. Any reading below 100 ohms, roughly speaking needs further investigation. Whilst probing watch the rate of change of your DMM. Again roughly speaking, if your reading goes up quickly from a few K up to 100's of Kohms, then that indicates a good cap. If the meter reading is slow to change or fix on a value, that can indicate or capacitor that needs looking at.

This is a good habit to get into before you even think about powering anything up - a good visual inspection and "buzzing" out (DMM on diode check) the power supply rails with reference to ground. It is also a very good idea to power any electronic device via a current limited mains supply. A simple lamp limiter is invaluable here and easy to knock up.

This saves doing any damage to other components. These scopes AFAIK use six legged matched (totally unobtainable) FET's on the front end/input. Whilst it's unlikely you'll damage these, it's best to safe than sorry.

To quickly test the mains transformer set your DMM meter to diode check. Probe where the wires from it connect to any diodes etc, black probe to chassis/ground. If your meter beeps and you get a reading of under a volt - typically 0.4v to 0.6v that shows a voltage drop of a diode. Switch probes, IE red probe to chassis/ground black to where the red probe was. If it beeps or shows SC then you have a potential problem.

For a proper test you need to unsolder/disconnect the leads from the tfmr. Identify the primary and secondary sides of the tfmr. Get a bit of paper or even better a notebook that you record your work in, draw a representation of said tfmr. Then check the resistance of the red wire to the black/pink or what ever. Note the findings on your drawing. As a rule the primary will read a few 100k ohms or there abouts. The secondary/s will read less, say a few tens of ohms. If however you get a short, say less than 1 ohm or a few ohms, then you could have a shorted winding. A better test is to check the insulation of primary to secondary using a Megga.

I'm talking in general terms here as regards value's. Use the schematic as a guide, you need to learn to read the circuit.

Lastly a simple capacitor reformer is easy to make. All you need is a transformer, a few caps and diodes to make a HV PSU using a voltage doubler, fed through a current limiting resistor. Disconnect any wires to said capacitor - reform. There are several threads on the forum about this subject as well as proven cap reformer design's. By building one you will learn something, and save money - big HV caps ain't cheap, especially if your scope has 20 of them like some valve scopes do.

Good luck, Andy.
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Last edited by Diabolical Artificer; 1st Sep 2017 at 6:43 pm.
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Old 4th Sep 2017, 4:42 pm   #17
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Thanks for the above posts! I now have a better understanding of how to test the power supply rails with a multimeter and how to find suitable test points using the schematic.

I probed the the low voltage points mentioned by Roger and here is what I found:

20.48K ohms on the positive side of C640.

I could not find R643 on the part placement sheet but I could see it on the power supply schematic.

10M ohms and falling on the positive side of C661.

1.1K ohms on the positive side of C664.

5.2K ohms on the negative side of C681

1.6K ohms on the negative side of C684.


I will use the schematic to more throughly probe all of the low voltage rails that I find. Then I will probe the 1.6KV and 5KV rails and report back. I'll also learn more about testing transformers.
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Old 4th Sep 2017, 6:43 pm   #18
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

If you have access to the mains transformer and can unsolder the connections, remove every secondary connection (there are 5) and then try plugging it in again. There is a possibility that the faulty capacitors have caused the mains transformer to overheat and the insulation has failed.

If it is OK then connect the earth (brown?) and try again
If it is OK then connect the 2 LT (violet?) ones and try again

No point in chasing around for faults after the transformer yet...
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Old 5th Sep 2017, 11:12 am   #19
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Those readings look ok, no obvious shorts to ground.

As suggested above test the tfmr. Once you know you have AC supply voltages you go on to test the rectifiers, caps etc. It's the old trick of breaking up a big problem into smaller parts.

Go carefully whilst testing the EHT, it can obviously ruin your day to put it mildly. Have you got a suitable test meter/probe?

Andy.
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Old 5th Sep 2017, 7:40 pm   #20
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I've had a look at the transformer and I'm trying to identify the five secondary connections.

I think that these are the connections to desolder:

Grey.
Yellow.
Green.
Brown.
Purple.

Are these the five secondary connectors? I've included some photographs of the transformer.

I don't have any suitable test gear to measure very high voltages. My DMM is good for 1000V and I have a Heathkit VVM that is good for about 1500V but it needs calibrating. I would like to eventually buy an Avometer 8. There is one in a local shop.

Thanks
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