Scopex 4D-25 Oscilloscope Repair Help

[PJL]

I think you have the pins the wrong way around. Can you measure TR407 again with and without the fault.

It's of course possible it was a dry solder joint and re-soldering has fixed it!

[OldTechFan96]

I lifted one side of R426. When I pulled the trig. level knob out the trace did not disappear.

I measured the voltages on TR407:
E - -0.650V
B - 20.84V
C - 20.21V

The fault condition has not appeared since Friday or Saturday so I'll leave the scope on throughout the day and make sure that it is all right.

I think you are right about dry solder joints!

[PJL]

The emitter and collector voltages are reversed. This is a PNP transistor.

You should put R426 back if you think its OK now as I would not expect to see the fault condition with it lifted.

[OldTechFan96]

I've just checked the scope and the fault condition has appeared again. I'll measure the voltages on TR407 when the fault condition comes back.

Then I'll lift one side of R427 again and see if the fault condition appears.

[PJL]

I still think leakage in C408 is the most likely cause.

[OldTechFan96]

The scope has been on for two hours and the fault condition has still not triggered.

I found a 2.2uf 63V capacitor so I'll replace C408 and see what happens.

If I can get to scope to trigger the fault condition I'll remeasure the voltages on TR407.

[OldTechFan96]

I replaced C408 with a similar component that I had knocking around.

After two and a half hours turned on the fault conditions appeared. The voltages at TR403 stayed the same.

When I touch my finger to the tip of the probe I can make the scope show a waveform that slowly moves across the screen. I imagine that adjusting the trigger controls should cause the wavevorm to lock so that it is stable. No amount of adjusting gets the waveform to lock.

Tomorrow I will lift R426 and let the scope run for a few hours to see if the trace disappears.

[Station X]

One post moved to a new thread here:-

http://www.vintage-radio.net/forum/s...d.php?t=140421

[PJL]

We have come a long way but getting the trigger fixed will be very difficult without another scope. This is why I always have at least 2 scopes!

It uses tunnel diodes for the trigger which should work very well.

[Brian of Romsey]

There's a helpful video about tunnel diodes here https://www.youtube.com/watch?v=PuG8CCUbg58 with the part about how they are used in 'scope triggers starting at 15:03.

[OldTechFan96]

With the mention of tunnel diodes I thought that I should test the 1N3719 at position D407. Diode check showed it to have a forward voltage of 0.003V each way so I think it is bad. I reinstalled it and double checked to make sure it was the correct away around.

I turned on the scope and nothing came on the screen. I measured the voltages on the +/-20V and +110V rails. The positive rails were fine but the -20 rail only measured 1.6V at the negative side of C684.

I decided to measure the ECB voltages of TR682 (BD157):
E - -33V
C - 1.5V
B - -32V

The low collector voltage prompts me to investigate TR683, TR684 and TR681. Looks like that this problem could be similar to the one were the 20V rail was low.

There is never a dull moment with this scope!

[RogerEvans]

Testing tunnel diodes with the diode range of a DMM is not a good thing since the test current is very often larger than the diodes maximum forward current (typically 10 - 20mA) and you run a serious risk of destroying the diode. Also tunnel diodes conduct in the reverse direction. You can find direct replacements from time to time but they are not cheap. There are Russian tunnel diodes available from various sellers in Eastern Europe and I have had no trouble buying from them. The 1I305A is probably a decent match to the 1N3719, the Russian diodes have strip connections rather than wire ends. You need to take moderate care when soldering them since they are Germanium diodes and can be destroyed by overheating.

Roger

[OldTechFan96]

Thanks for the tip Roger. Hopefully I did not damage the diode with my DMM.

[dave cox]

A current overload could cause TR682 to turn off, check the voltage across R683. If TR684 turns ON it stops the drive to TR683 / TR682.

PS TR681 causes the '-20' rail to mirror the '+20' rail

dc

[OldTechFan96]

Hello Dave,

The voltage across R683 is 058.3mV.

[PJL]

It's not current limited so check B C E voltages of TR683 and base of TR682. Also measure the volts either side of R681 as failure of the 1.6KV supply could cause this.

[OldTechFan96]

The voltages on TR683:

C - -32V
E - 0.9V
B - 0.7V

The base of TR682: -32.8V

When I went to measure R681 I heard some crackling sounds and started to smell something burning. I switched off the scope and unplugged it.

I really would like to repair this scope but I think it needs to be in much more capable hands than mine!

[OldTechFan96]

I have a sneaking suspicion that the burning smell was coming from the mains transformer. I've been taking a bit of a break from the scope and will have a look at it next week.

[Diabolical Artificer]

Might be a good idea to measure the resistance to ground of your supply rails to see if you have a short before powering up again. Anything around 100 ohm's ish or lower is suspicious.

TR683 isn't turned on by the looks of it, it's a good idea to measure B - E directly, you should get 0.4v to 0.6v. It doesn't matter which way round your meter leads are.

Andy.

[OldTechFan96]

I've measured the resistance between the chassis ground and the output of the voltage rails.

+20V (on pos. of C664) - just under 1K ohms.
-20V (on neg. of C684) - 1.9K ohms.
+110V (on RV604) I can't remember which terminal has +110V on it so I measured both: 15K ohms and 1.5K ohms.

Next I'll disconnect the transformer secondaries, measure the resistance between them and then power the transformer.

If there is a short to ground, what would the resistance be?