Scopex 4D-25 Oscilloscope Repair Help

[OldTechFan96]

I've been measuring the outputs +/- 20V rails and the +110V rails. They all measure as they should before and after the fault condition.

The voltages on the E,C and B of TR663 and TR682 respectively:
TR663: 20V, 28V and 21V
TR682:-30V, -20V, -29V

The ECB voltages on TR662 and TR664 range from 20V to 29V. TR661 is similar, with 6.2V on two pins and 20V on the other pins. These measurements were taken before and after the fault condition.

The voltages seem way too high. It's a shame that the service manual doesn't include the correct voltages on the transistors. The only two I can find are the same and are a few years newer than my scope.

I do wonder if something is overheating?

[PJL]

They look OK. 20V on TR663 emitter and -20V on TR682 collector.

Can you measure the voltages ACROSS R663 and R683 so we can see what current is being taken. Take initial readings and further readings after the fault condition has occurred.

Can you confirm that it still works OK on power up and then fails after a while?

[OldTechFan96]

The scope powers on fine and displays the trace line. After a few minutes the trace disappears. Pressing the locate button after the trace disappears causes the trace to come back, as long as the button is held in.

The following voltages were measured before and after the trace vanished.
Voltage across R663: Before: 559mV. After: 557mV.
Voltage across R683: Before: 444mV. After 446mV.

[PJL]

168mA and 135mA. No idea but probably OK but most importantly it does not change under the fault condition.

To be able to see the trace, the free run must be enabled and the scope must trigger. There is usually a lamp which flashes at each sweep. Is it on?

Fixing scope problems needs another scope so you may need to ask someone local for help.

[RogerEvans]

When you have to hold the beam finder in, does the trace appear above, below or to the right or left of centre? That might give a clue to the beam being deflected off screen by a fault in the X or Y amplifier.

Roger

[PJL]

Roger's suggestion is a good one but please also confirm the trigger lamp is lit. A vertical fault would be nice as this would be the easiest to locate.

[OldTechFan96]

Hello Roger and PJL,

I don't think that there is a trigger lamp on this scope. The only lamp is the on/off indicator. The trig. level is pushed in which sets it to auto. I've uploaded a photograph of the front controls.

With the locate button pushed in the trace does not move much from the centre line of the graticule. I've uploaded a photograph of the scope when the locate button is pushed in.

Thanks

[PJL]

Based on that I would say the trigger is not working. I will have a look at the circuit this evening.

[RogerEvans]

Are you sure that picture is after the trace had disappeared and then pressing locate?

Roger

[OldTechFan96]

I forgot to add which picture is which!

The first one is when I push the locate button. When the trace disappears it comes back for the time the locate button is pushed in.

The second one is a front shot of the scope's controls.

[PJL]

Attach your DMM to the sweep out. Set the timebase to 200ms. Watch the DMM value which should rise slowly and fall as it will be measuring a slow sawtooth. What happens when it fails?

[OldTechFan96]

With my DMM connected to the sweep out I see the pattern that you mention.

Once the fault condition appears the voltage stabilises at 2.31V.

[PJL]

Measure the voltages before and after failure on TR406 collector which should be +21V.

If it goes lower on failure then I would try replacing C408 which seems to be 2.5uF. Anything from 1uF to 5uF would be OK.

[OldTechFan96]

The voltage on the collector of TR406 is 19.7V before and after the fault condition.

I removed and tested TR604 (BC184). It tested bad using diode check on my DMM and my component tester. I'll replace it with BC337 after dinner.

[PJL]

I am not convinced your transistor testing is correct as you seem to find a lot of faulty ones.

Measure the b c and e voltages of TR407.

The 19.7 seems a little low as I expected it to be 20V + forward voltage of 2 diodes. To prove there is a problem in this area, can you lift one end of R426 which should stop free-run from being disabled.

[OldTechFan96]

To test transistors I used the following guide:

http://www.stickycarpet.com/pinx/test.html

The diode check function shows about 0.1V between B-E and B-C or it show OL.

The component tester shows 'damaged part' or gives a resistance reading. One transistor caused the self-test function to loop!

All of the transistors that have tested bad are BC184s. Because of this I tested TR406 and TR408. They both tested bad. Should I replace them or make the measurements you mentioned with the BC184s in place?

Thanks

[PJL]

Are you testing the transistors when still attached to the PCB? The test in the link would only work if they were removed.

Ignore the faulty transistors and take the readings.

[OldTechFan96]

I'm testing the transistors out of circuit.

[PJL]

If you have already removed them you might as well replace them. The red lead or +ve on the base should conduct to both the emitter and collector. The black on the base should not conduct. The leads must be in the correct coloured sockets on the DMM.

[OldTechFan96]

For the meantime I have decided to leave in the BC184s mentioned previously. I'm certain that I'm testing them correctly. I'll replace them at a later stage.

Voltages on TR407 (BC212):
E - -1.2V
C - 20.16V
B - 20.78V

These measurements were taken before the fault condition. I had the scope on for nearly an hour and the fault condition did not trigger.

I've noticed that when the trig. level knob is pulled the the trace line disappears. When the knob is rotated the trace flashes by, then disappears again. The trace appears again when the trig. knob is pushed in.

I'll lift one side of R426 tomorrow and report back. What will happen when 'free- run' is forced on? Is it like pulling out the trig. level knob?

Thanks