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Old 28th Sep 2017, 5:39 pm   #61
PJL
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Faulty TR662 or TR663 is the most likely. You could check the collector voltages are 30+ volts.

It's also worth checking the resistance from R663/C664 to chassis but it doesn't seem to be taking any current

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Old 28th Sep 2017, 9:00 pm   #62
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The collector voltage of the two transistors mentioned is 35.1V. I will test them with my multi-meter in the morning.

The resistance between R633 and the chassis is 278 ohms.
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Old 28th Sep 2017, 9:31 pm   #63
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

R663 has 0 volts across it
The current limiter TR664 should not be conducting
TR664 collector and TR662 base is at 3.2V
TR662 and TR663 collector are at 35V

In my thinking TR662/TR663 should be conducting.
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Old 29th Sep 2017, 4:13 pm   #64
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I tested TR662 and it was bad. I could not get a reading with my DMM on diode check. It was replaced with a BC337. I tested TR663 earlier in the thread and it tested good.

I've made some more voltage measurements:

TR662: C - 34.3V. B - 3.8V. E - 3.3V
TR663: C - 34.3V. B - 3.3V. E - 2.6V

2.6V on the positive side of C664 and -3.6V on the negative of C684.

Next I'll check the transistors involved in the -20V supply.
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Old 29th Sep 2017, 5:26 pm   #65
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The only transistor in the +20V supply that I have not tested was TR661. I tested it and it was bad so I replaced it with a BC337.

This has caused the voltages at C664 and C684 to increase slightly.

3.7V on the positive side of C664 and -4.5V on the negative of C684.
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Old 29th Sep 2017, 6:40 pm   #66
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

It seems to have made little difference. The -ve supply will not work without the positive.

TR663 emitter is showing as 2.6V so I think the over current protection may now be the problem. Can you measure the voltage across R663.
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Old 29th Sep 2017, 7:00 pm   #67
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The voltage across R663 is 83.3mV.
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Old 29th Sep 2017, 8:12 pm   #68
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

We need the e, b c voltages of TR661 to see if it is conducting. It's possible C665 is leaking or D663, the Zener, is faulty.
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Old 30th Sep 2017, 12:15 pm   #69
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I've measured the voltages on TR661:

E - 0.685V B - 1.2V C - 5.1V

I tested C665 (68uf, 16V) with my component tester:

Capacitance: 90uf. ESR: 0.31 ohms

D663 (6V2) tested bad using the diode check on my DMM. There was a voltage reading each way. On the Macintosh SE video board there are four 1N4150 zener diodes. Could I try these?
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Old 30th Sep 2017, 2:08 pm   #70
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I've looked into the 1N4150 and I don't think it is a zener diode. I subbed it in for a test and a green line appeared across the screen which could be manipulated with the scope's controls.

I turned it off after a minute or so as I started to smell something odd. It was not a burning smell, but the smell of heat; if you know what I mean? It could be the circuits warming up or something bad so I'll not take any risks.
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Old 30th Sep 2017, 2:41 pm   #71
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Don't run it with a normal diode as it will not be regulated and will overload everything. D663 should not be a 4150 and must be replaced with a 6v2 zener. Maplin QH09K is a BZX79C 6v2 which would be OK and is a stock item.

Either you are looking at the wrong part, someone has previously incorrectly replaced it with a 1N4150, or the circuit does not match the diagram.

It is a bit of a concern that so many parts appear to have failed! Maybe it was powered up with the wrong mains setting.

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Old 30th Sep 2017, 3:29 pm   #72
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

I've ordered the correct zener diode. It will arrive next week.

You are right about the amount of failed components. The original internal fuse was intact when I bought the scope so I suspect that some components may have failed during the time that it was stored. It probably spent a long time in a damp loft judging by the state of the carry case. Or it was taken out of service due to the fault in to 20V section.

I think it was used within a North Shields business.
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Old 3rd Oct 2017, 1:01 pm   #73
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The correct diode came today and I installed it. The photograph shows the trace. It is not in line with the graticule so the CRT will need to be adjusted.

Now I'll have to get it to produce a waveform. I've got a Heathkit signal generator (IG-102) that I could use.
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Old 3rd Oct 2017, 8:20 pm   #74
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Hi, not sure if this scope has a twist coil on the tube. If so there will be a small pot somewhere marked twist or tilt. This should sort out the trace. If not then the tube will need to be rotated (difficult if a rectangular tube)

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Old 3rd Oct 2017, 8:44 pm   #75
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

The CRT is round so it was easy to adjust. What would be the best way to test the scope? To produce the waveform pictured I grounded the clip and touched the probe to my finger.

I'll do some reading about scopes and how they can be used to help repair and align radios.

Thanks for all of the help everybody!
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Old 4th Oct 2017, 10:04 am   #76
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

That's looking like a very good result !

Now get yourself two X10 probes. Its possible to buy new probes for a very reasonable price from several sources - someone will be able to recommend a brand to look out for. The big brand names (like Tektronix or HP) are super expensive and not worth it for most applications.

dc
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Old 4th Oct 2017, 10:44 am   #77
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Hello Dave,

The scope came with a pair of X10 probes. They are marked 'P.E.T (EMI) Ltd. X10'. I think they will be good enough for what I indent to with them, which will mostly radio troubleshooting and alignment. When I learn how to do it, that is!
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Old 4th Oct 2017, 9:51 pm   #78
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Hi, glad to hear you have it going. See if you can locate the book by Tektronix on the web called the XYZ of oscilloscopes. It is amine of information. I may have a copy somewhere of it if you cannot get one.
I also have a little battery operated Tek gismo that showed how good their scopes were, failing that I have a sine/ square generator you are welcome to borrow.

Ed
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Old 5th Oct 2017, 6:04 pm   #79
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Thanks for the offer of equipment to borrow and recommendation of the Tektronix book, I found a PDF online.

I don't think this repair is over yet! After about two minutes the trace line disappears. The only thing that brings the trace back temporally is when I push the 'locate' button. Switching the scope off and turning it back on after a few minutes restores functionality for a bit.

I'll be making some more measurements soon. I quick poke about tells me that the trouble may lie in the +20V rail.
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Old 6th Oct 2017, 1:19 pm   #80
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Default Re: Scopex 4D-25 Oscilloscope Repair Help

Good progress and as you say, you should focus on the +20V and -20V supplies to make sure the regulators are working correctly. If you get the correct voltages, note down what happens to the voltages under failure condition. This will tell us if the fault is still in one of the regulators or if the regulator is entering the over-current protection state.

The first thing to get right on any equipment is the power supply voltages.
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