Voltcraft 2080 oscilloscope troubleshooting

[Einkleinermensch]

Hello guys! I've got by occasion an old Voltcraft 2080 oscilloscope and trying to check it's operation as described in it's manual (I've made some photos of it). Until step 20 everything works good but there is a second "ghost" signal image on the screen. It is much darker than main oscillation image, but still visible (see photo 5). When I do steps 20-24 it seems to be working, I can magnify a portion of signal (photo 6). At step 25 when I set TIME/DIV to 10 us/DIV, I get no image of edges even by adjusting DELAY TIME POSITION drive. Actually, the signal line becomes pretty short and not adjustable instead of showing me a signal edge (photo 7). Is it a problem with a sweep generator or with one of timebase cirquits? What can couse this problem? Thank you!

(I'm sorry for my bad english  )

[Armadillo]

I can't visualize clearly, but I think is a retrace line problem.
You need to check and adjust the retrace line blanking control.

Edit: Have you recently done anything to the oscilloscope?

Edit: Adjust VR540 until ghost disappeared.
If this solved your problem, I would advise you to do total re-alignment, adjustment and calibration especially on all the power supply voltages and bias control.

Unfortunately the service manual is in foreign language, can't read it any further.   

[Le_Bassiste]

Hello guys! I've got by occasion an old Voltcraft 2080 oscilloscope and trying to check it's operation as described in it's manual (I've made some photos of it). Until step 20 everything works good but there is a second "ghost" signal image on the screen. It is much darker than main oscillation image, but still visible (see photo 5).

you may want to check the DELAYED TIMEBASE setting. did you make sure that it is switched to something like "MAIN ONLY" or "OFF"?

[StillTrying]

"At step 25 when I set TIME/DIV to 10 us/DIV, I get no image of edges even by adjusting DELAY TIME POSITION drive. Actually, the signal line becomes pretty short and not adjustable instead of showing me a signal edge (photo 7)."

When step 25 says set TIME/DIV to 10 us/DIV. it must mean the DELAYED TB speed, it doesn't make much sense the have the MAIN on 10us/Div and the DELAYED on the 100us/Div.

Only the 1st photo looks wrong to me, sometimes there's a switch setting to make the MAIN or DELAYED part of the trace the brightest.

[Einkleinermensch]

Armadillo, thank you, when I opened the oscilloscope, I've found that VR540 seems to be broken by previous owner: its cap was just separated from its body. I'll buy a new one tomorrow and try to adjust.

>Have you recently done anything to the oscilloscope?
No, just opened its housing.

you may want to check the DELAYED TIMEBASE setting. did you make sure that it is switched to something like "MAIN ONLY" or "OFF"?

Hmm, how can I do this? If you mean the MAIN/MIX/DELAY switch, I've used it as described in operator's manual.

UPD:

When step 25 says set TIME/DIV to 10 us/DIV. it must mean the DELAYED TB speed, it doesn't make much sense the have the MAIN on 10us/Div and the DELAYED on the 100us/Div.

Only the 1st photo looks wrong to me, sometimes there's a switch setting to make the MAIN or DELAYED part of the trace the brightest.

Thank you, I understand now. I've tried to switch DELAY to 10 us, now it shows something that supposed to be the edge, but it is not as showed in manual, see the attachment. Even at 2 us Delay I'm unable to make it longer.

[StillTrying]

"I've tried to switch DELAY to 10 us, now it shows something that supposed to be the edge, but it is not as showed in manual,"

The manual's Fig 4-3 showing a 20us risetime edge is just a diagram, your 1kHz's actual rise and fall times will be only ~0.2us, which is 100X faster than the diagram. So your last photo looks about right to me.

Delayed timesbase's are always fiddly to use, there might be nothing much wrong with it. Most of the time I think you're better off just using the X-POS, PULL X10 instead!

[carl_lab]

The scope looks similar to an older HAMEG scope (HM203, HM303...?).
Don't know if that could help you...

[Kleinstein]

The ghost image could be from the "mix" mode of the delay switch. It is there to indicate the part that you will see with delay active. I have the slower version (2020 of that scope). If I remember right it uses two intensities. The delayed mode is not used very often - many simple scopes don't have it at all. I have not used it the last 15 years or so.

A double image could also be caused by bad triggering, sometimes triggering an a wrong edge. However at least the 2020 is rather good in triggering - better than the similar Hameg ones (e.g 203).  Especially the extra hold off knob helps if the repeat frequency is just at the edge. The instructions should include a full schematics - so no problem there.

[Einkleinermensch]

Thank you guys! I've changed the broken resistor, but it's adjusting seems to do nothing. A ghost image is still on the screen in the main mode 

[Armadillo]

Thank you guys! I've changed the broken resistor, but it's adjusting seems to do nothing. A ghost image is still on the screen in the main mode 

The VR540 cannot be doing nothing to the screen, it plays a distinct role on the screen. There are few prime suspect components there, the optoisolator, transistors, the 2KV capacitors etc.... You should start measuring the voltages around the circuit attached.

Edit: oh please be safe, and don't blame me on your safety and I cannot be responsible for it. 

[Armadillo]

Measure the voltages based on 2 conditions;

1: turn the VR540 to the left around left 45 degree, measure all the voltages shown by the Red arrows with respect to the ground

2: turn the VR540 to the right around right 45 degree, measure all the voltages shown by the Red arrows with respect to the ground.

Please tell us your measurement.

[Einkleinermensch]

So here are the measurements:
VR540 45 degrees to the left:
          D541 (Anode): -1258V
          On Q541's collector: -1258V
          Between R548 and R551: 12.00V
          U541 (1,4,5): 0.92V, -1258V, -1258V
VR540 45 degrees to the right:
          D541 (Anode): -1258V
          On Q541's collector: -1258V
          Between R548 and R551: 12.00V
          U541 (1,4,5): 1.01V, -1258V, -1258V

[Armadillo]

Is that a minus voltage I am looking at? Can you re-measure it for confirmation.

Edit: Turn off the power supply. Use multimeter on diode Mode, check Pin 1 and Pin 2 of U541, forward and reverse. The LED seems like not drawing any current.

Edit: It is optoisolated, so maybe different ground reference potential. But the optoisolator seem like not working. Please change U541.

Except Pin 1 and Pin 2, Please measure the voltages of the rest of the measurement points with respect to emitter of Q541.  [The result should be the same, indicating that the optoisolator is not conducting but at least we seeing positive voltage with the correct reference ground now.]

Edit: Order 4N25 and change a new one.

[Armadillo]

After you installed the new U541, remember to return VR540 to Mid Position before turn On.
After turn on, then slowing adjust until ghost image disappeared.

[Einkleinermensch]

Thank you, I've ordered a new optocoupler!

I've measured all voltages with respect to Q541's emitter: It is all around 1V in every point except between R548 and R551, in this Point it's more then -1800V (my multimeter can't measure such high voltages).
I've checked also U541's pin 1 and 2 at switched off power supply: it's 1.6V by direct switching and 1V in reverse, seems like really dead diode.

[Armadillo]

Thank you, I've ordered a new optocoupler!

I've measured all voltages with respect to Q541's emitter: It is all around 1V in every point except between R548 and R551, in this Point it's more then -1800V (my multimeter can't measure such high voltages).
I've checked also U541's pin 1 and 2 at switched off power supply: it's 1.6V by direct switching and 1V in reverse, seems like really dead diode.

At ~31volt transformer, you cannot be measure 1V. So can you please re-measure as per attached markup sketch. Yes do change the U541.

[Einkleinermensch]

At ~31volt transformer, you cannot be measure 1V. So can you please re-measure as per attached markup sketch. Yes do change the U541.

It is 0V on both capacitors and 0.5V on the Zener. Maybe one of capacitors is dead?

[Armadillo]

At ~31volt transformer, you cannot be measure 1V. So can you please re-measure as per attached markup sketch. Yes do change the U541.

It is 0V on both capacitors and 0.5V on the Zener. Maybe one of capacitors is dead?

Take out the Electrolytic and check it. Check the diode also. Measure the AC 31 voltages whether it is there. Check why in case not there. You found the defective circuit.

[Einkleinermensch]

Thank you for your help! I'll find the problem

[Einkleinermensch]

Well, I've changed the optocoupler, tested all diodes in that cirquit and gave a look to an electrolytic. Everything seems fine. I've tried to disconnect that 3-pin connector "~31V, ~6.3V, Ref" and measured first 2 voltages in respect to ref. They both are around -3V to the Ref pin. Can it be possible that something happened to the transformer?

[Armadillo]

If you can't even get close to ~31V unplugged, then likely the transformer is faulty.     

[Einkleinermensch]

Maybe the transformer is unified so I can look up for the same or similar

[Armadillo]

Trace for dry joints at connectors, broken traces etc until you trace the wires back to the transformer.

[Einkleinermensch]

The 31V wire comes directly from the transformer. The resistance between ref wire and 31V is way more than 10 MOhm, so it's either somehow broken winding or just a bad contact between wire and transformer's pad. I'll give it a closer look tomorrow. If the problem is in the winding, I'll just make a small AC-DC converter for 31*sqrt2=44V and take some power from 6.3V bus. The 31V supply needs around 0.125W, and 6.3V bus' max power rating is 0.95 W, so I think there's enough power to drive both buses.