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Old 5th Sep 2017, 8:43 pm   #21
Ed_Dinning
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Hi, if you are stuck you can give me a PM and bring it over, I can show you how to check it.

Basically you disconnect all wires except the mains ones shown in your picture (taking notes of what goes where) then you should be able to power the transform alone and see if it still blows fuses.
If it does it will mean a new transformer, or the original can be rewound.


Ed
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Old 10th Sep 2017, 11:29 am   #22
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I haven't had a chance this week to have a look at the mains transformer as the family has been joined by two kittens which I've been looking after.

Ed, If I get stuck I will take you up on your offer!
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Old 19th Sep 2017, 3:45 pm   #23
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Today I have been probing and testing the mains transformer and have made some good progress.

I desoldered the five secondary wires I identified from the schematic and measured the voltages:

Yellow - 320.3 VAC
Blue - 87.1 VAC
Pink 1 - 2.5 VAC
Black (ground) - N/A
Pink 2 - 162.7 VAC

I also metered out the transformer with my ohm meter and I think that it is in good order.

Next I will check for shorts to ground on the EHT lines.
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Old 19th Sep 2017, 9:36 pm   #24
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Hi , that looks quite healthy, try running it in this state for 1/2 hour and see if it gets hot. It should only get mildly warm.

Ed
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Old 19th Sep 2017, 11:38 pm   #25
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Something is not right and you should not be trying to measure the EHT with a standard DMM as the voltage may be too high for it.
All measurements taken from the black, I would expect:
1. The 2 pink wires will be the LT and should measure the same at 20-25V AC at a guess. Yours are completely different.
2. The blue wire I assume is the 110V HT. I would have expected around 100VAC but you are only measuring 87.1 which seems a little low.
3. The yellow wire would then be the EHT which based on a 6x=5KV and 2x=1.6KV, I would expect the peak to be about 900V=650VAC. You are only getting 320V.

These are very odd results and I wonder if you were measuring from chassis with the black earth wire disconnected and the secondary effectively floating? Can you take resistance measurements of the windings all taken from the yellow wire.

Last edited by PJL; 19th Sep 2017 at 11:48 pm.
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Old 20th Sep 2017, 9:03 am   #26
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

PJL,

This morning I have revised my voltage measurements. This time I used the transformer ground.

Yellow - 594.2 VAC
Blue - 147.2 VAC
Pink 1 - 26.4 VAC
Pink 2 - 26.4 VAC

When I measure resistance from the yellow wire to the other wires I get about 1.33K ohms.

My first set of measurements were taken using the chassis ground by mistake.

Thanks
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Old 20th Sep 2017, 4:50 pm   #27
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

That's better! Stay well away from that 600VAC!

Connect the black wire only back to chassis and switch on.

If that's OK then switch off and check the resistance to chassis of each of the landing points of the yellow/blue/pink/pink wire on the PCB. As there should be diodes in each circuit the resistance should be high.

If that's OK then we will use one of the low voltage pink wires to test the other circuits. Using a low voltage on the HT circuits will be safer and will help reform the capacitors.
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Old 20th Sep 2017, 5:58 pm   #28
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The complexity is that the +20V regulator won't work without the 110V supply and the -20V won't work without the -1.6KV and the +20V supply.

You can connect one of the pink wires to the 110V supply (normally blue) and check you are getting about 35V on C640a.

You can then connect the pink wire to the EHT (normally yellow) and check you are getting about -70V at the junction of D604/C603. You can also check the EHT by checking you are getting about 200V on the junction D616/R608.

If everything is still OK I would be inclined to connect the blue wire to the EHT (normally yellow) and leave it for a few hours to get the electrolytics reformed. Don't try to measure any voltages whilst doing this as the EHT will be 1.2KV!
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Old 21st Sep 2017, 2:05 pm   #29
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

PJL,

I reconnected the black wire and switched on the scope without a problem.

All of the landing points test fine with high resistance measurements apart from Pink 1 which gives a resistance both ways.

Does this suggest a failed diode in the low voltage part of the power supply?

(I had written a longer reply but it seems to have been lost.)
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Old 21st Sep 2017, 2:34 pm   #30
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

R403 is 680K but capacitor coupled (100Hz trigger) so shouldn't show any resistance.
R631 is 1K5 but connected to LED's so shouldn't show any resistance.

What value is it showing? I would guess the diodes are faulty but I am not sure if you are talking about the +ve or -ve rail?

It's a bit of a concern that these diodes have failed and need to be replaced. It may mean the capacitors C661/C6811 1000uF may need replacing. You can try reforming them by connecting one of the pink low voltage leads via a 2K2 resistor and monitoring the voltage on the capacitor. It should rise slowly.If it stops rising put another 2K2 in parallel with the other to increase the current.
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Old 22nd Sep 2017, 10:23 am   #31
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I should have better explained my previous post.

The black wire was connected to the scope without a problem.

I measured the resistance from the chassis to the landing point of the transformer wires on the PCB. I had the black probe to the chassis and the red probe to the landing point. Here are the results:

Yellow - O/L
Blue - 9.4M ohms
Pink 1 - 34.5M ohms
Pink 2 - O/L

I then swapped my probes around (red to chassis and black to the landing point) and measured the same points:

Yellow - 11.8M ohms
Blue - O/L
Pink 1 - 2.6M ohms
Pink 2 - 15M ohms

Ignore what I said about failed diodes.
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Old 22nd Sep 2017, 5:41 pm   #32
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Where was the split 1K resistor you replaced?

O/L I assume means overload or very high resistance.

In that case proceed with the suggestions in post #28.

Hopefully, you have already fixed the problems when you replaced the EHT diodes and capacitors but we should try to workout why the resistor failed.
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Old 22nd Sep 2017, 6:24 pm   #33
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The split resistor was R407.

I've had a look at the manual of my DMM. When measuring resistance O/L can mean open circuit or that the resistance is too hight for the DMM to measure.
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Old 22nd Sep 2017, 6:43 pm   #34
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

R407 is 10K in the circuit diagram?
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Old 22nd Sep 2017, 8:58 pm   #35
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

That's a discrepancy I noticed earlier. It could be a circuit revision as the scope is from 1973 and the service manual is dated 1977.
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Old 23rd Sep 2017, 9:50 pm   #36
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I have made the measurements suggested in post 28.

With the pink wire connected to the blue landing point: 34.6V on C640a.

Pink wire connected to EHT landing point: -12.8V on the of junction D604/C603 and 42.6V on the junction of R608/D616.

The last two voltages are way off the mark.
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Old 23rd Sep 2017, 11:57 pm   #37
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Just checking you do have the black connected to chassis?

It points to a problem with the -1.6KV supply but there isn't much to it. You already replaced the electrolytics C601-C606 and diodes D601-D604 so check you have them wired in correctly.

If they look OK, try measuring the resistance from D604/C603 junction to chassis.

You can confirm it is the -1.6KV by lifting one end of C607 and checking the -1.6KV voltage again.
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Old 25th Sep 2017, 1:28 pm   #38
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I've had another look at the scope and found that diodes D601 and D602 were the wrong way around! I corrected this and made some more measurements with the black lead connected to the chassis.

Junction D604/C603 - -71.3V.

Junction of R608/D616 - 38.8V

Resistance of D604/C603 to the chassis - about 6M ohms

I remeasured the voltage on D604/C603 junction with one leg of C607 lifted - -70.6V
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Old 25th Sep 2017, 2:59 pm   #39
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

That looks good now. Next try measuring the EHT voltage with the pink connected, you should get approx 200V on the junction D616/R608.

I am still a bit concerned about the failed 1K resistor. Was it burnt? Are you sure it was R407? The only way for that to be over driven would be a faulty TR401a.
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Old 25th Sep 2017, 4:38 pm   #40
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

With the pink (25V) wire connected to the EHT landing point I measured 38.7V on the junction of D616/R608.

The split resistor was defiantly R407. It was not burnt or otherwise distressed looking. I'll post a photograph when I find it.
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