Scopex 4D-25 Oscilloscope Repair Help

[PJL]

The meter resistance may be an issue. What meter are you using?

You could now try connecting it back up properly and see if it still blows fuses. Don't forget the lethal voltages everywhere.

[OldTechFan96]

The resistance started at about 30M ohms and then fell to about 6M ohms.

The DMM I'm using is a Proster VC99. I would like to buy an Avo 8 eventually.

[PJL]

Input impedance is 10M so I would have expected the voltage to be a bit higher.

Try connecting the wires to their correct positions and power it up. Hopefully the fuse will not blow now.

[OldTechFan96]

Good news! I reconnected everything up and the scope fired up without a fault. Then I reconnected the CRT. After fiddling with the focus and intensity controls the scope produced a line.

How would I go about testing the scope? It's a 25MHz scope, what does this mean?

Thanks for all of the help so far!

[PJL]

Have you ever used a scope before?

Going back to R407 which you replaced with a 1K resistor. How did you know the faulty resistor was 1K as orange and red can be difficult to distinguish. Can you measure the resistance of R411 which should be the same value as R407.

Remember the capacitors in the scope will now retain dangerous voltages even after powering off.

[PJL]

It's now worth checking the stabalised voltages are correct:
C664 should have +20V on the +ve connection
C684 should have -20V on the -ve connection

The 110V supply has a comment about a circuit change so it would be best to measure it away from the PSU on R528.

If these are OK you will need to spend some time reading up on scopes so you understand the various adjustments.

[Ed_Dinning]

Hi, there are a few books around on the web that cover the use of basic test equipment including scopes. There is also the P Millett site that has downloads of dozens of tech books from the 30's onward that will cover what you need.

If you get really stuck you can bring it over and I'll give you a tutorial.

A 25MHz scope will basically display with reasonable accuracy a sine wave up to that frequency. I may not do so to other waveforms

Ed

[OldTechFan96]

I haven't used a scope before.

The resistance of R411 is 0.7K ohms.

The positive side of C664 - about half a volt.

The negative side of C684 - -1.7V

I could not find R528 so I measured the 110V supply from RV604 and R643 and measured 113V.

Thanks for the archive Ed! Lots of good reading there.

[PJL]

Looks like there are still some major faults to locate.

The regulators both have current limit protection at around 200mA.

1. First check you were measuring the correct voltages by measuring from chassis to either end of R663 and R683.
2. Check the unregulated voltages from chassis to C661+ and C681-
3. Measure the voltage ACROSS R663 so we can confirm the +ve regulator is in a current limit state. 0.6-0.7V would indicate it is current limited.

The -20v regulator won't work without the +20V supply so we should focus on +20V regulator first.

Back to R407, there is a possibility that your version of the scope does not use the same component designations as the service manual. Does the position of R407 correspond to the layout drawing in the manual?

[OldTechFan96]

From the chassis to the components mentioned:

R663 - 556mV
R683 - -1.77V

C661 - Could not find on parts placement diagram. It's a 1uf capacitor in the schematic.
Neg. side of C681 - -34.2

The voltage across R663 - 1.1mV

R607 corresponds to the correct place on the PCB. The service manual is a few years newer than the scope so there will properly be some changes.

[PJL]

OK, it is not taking any current so it looks like the fault is in the regulator so easier than I feared.

C661 is right next to C681 and both are 1000uF (1mF) and are pretty big so difficult to miss! We need to know the unregulated voltage as it could be the diodes D661 or D662 are faulty.

If it measures about +40V then measure the base voltage at the junction of R662 and TR662.

[OldTechFan96]

I found C661 and measured 35.3V on the positive side.

I could not find R662 on the placement diagram. I could find R661 and R663 though! I measered about half a volt on the base of TR662.

I noticed a heavily corroded pin on TR664. The pin broke with a light prod. I'll test it and reconnect it tomorrow.

[karesz*]

R407 (#48) hes coloring is for me clearly orange, also = 10K Ohm...
Regards-Karl

[OldTechFan96]

I've removed TR664. It is a BC184 NPN small signal transistor. I don't have a direct replacement but I do have some BC337-40 transistors knocking about. They are the only NPN transistors I have.

I've uploaded a photo of the resistors mentioned previously. It does look orange. I'll see if I have any replacements.

[PJL]

The BC337 should be OK.

Change R407 to 10K. It is safer to be higher than lower.

[OldTechFan96]

I have replaced R407 with a 10K resistor that I cannibalised out of a Macintosh SE video board.

TR664 has been replaced with a BC337.

[PJL]

and... what is the voltage on R663 now?

[OldTechFan96]

Now I have 0.715V on R663 and 0V across R663.

[PJL]

Still no output.

What is the voltage on the base of TR662 or collector of TR664?
If not 30+ volts then what is the voltage on either side of R661?
Double check you have the TR664 pins correct.

[OldTechFan96]

Base of TR662 - 3.1V

Across R661 - 110V

Left of R661 - 114.2V

Right of R661 - 3.2V

I've double checked the placement of TR662 and I'm absolutely sure that it is correct.