Yet More Oscilloscope Questions

[bobheck]

Newbie here. 10000 pardons, forgive me and all that. I really don't know much about scopes, but I really want to learn. So I bought one I (thought) I could afford for around $100.

I have been reading the forum and I am astounded at the amount and depth of knowledge in here.

If I could take a few minutes of your time, please.

Just purchased a Hameg HM705 scope off of ebay. Seemed to work fine, clean lines, in good cosmetic shape. But I needed new probes.

The scope has a .2v/cm square calibration pad on it.

However, after finally getting a pair of probes, i noticed two things that bother me.

1. Trace 1, seems to show higher voltage than it should. This is even with the calibration (small knob on top of volts/cm knob) all the way counter clockwise. Same with 1x 10x 100x probes, consistent. I also tried with regulated DC voltage, same issue.

For this problem I assume something inside has to be replaced or adjusted. What do I look for? ( I know, loaded question). Is this something I can fix? I do know how to solder, desolder, etc...

2. Trace 2, seems to be OK with voltage shown. However, any small touch on the BNC connector makes image go haywire and even off screen. Seems to me something in or around the BNC connection needs resoldering. Does this sound familiar to anyone who can give me advice?


So, with what I have said, is this just $100 down the drain, or do you think this scope can be salvaged? Or, do you need more info? I would be happy to post pics.

I really hate to trash this scope, pretty good condition, really nice picture, 100MHz bandwidth.

Thanks in advance, I appreciate your patience with me.

Bob Heck

 

[audioguru]

Lots of intermittent and defective equipment is sold on E-Bay.
Get the service manual and fix it.
Or use only the channel that works properly.
Or sell it on E-Bay.

 

[mneary]

You should be able to salvage this one easily with some care. As audioguru said, the service manual is a big help. Or you could open it up and see if there's a pot marked something like CH-1 gain. The CH-2 BNC might just need to be tightened, or re-soldered. Once it's open this too should be easy to diagnose.

The users guide is a free pdf from the Hameg site, and it includes calibration instructions. Brush up on your German first.

 

[JimB]

1. Trace 1, seems to show higher voltage than it should. This is even with the calibration (small knob on top of volts/cm knob) all the way counter clockwise. Same with 1x 10x 100x probes, consistent. I also tried with regulated DC voltage, same issue.

How much over is the voltage reading?
Without numbers it is difficult to gauge.

For this problem I assume something inside has to be replaced or adjusted. What do I look for? ( I know, loaded question). Is this something I can fix?

There will almost certainly be a gain adjustment in the amplifier, but what needs to be done depends on how far off is the calibration.

2. Trace 2, seems to be OK with voltage shown. However, any small touch on the BNC connector makes image go haywire and even off screen. Seems to me something in or around the BNC connection needs resoldering. Does this sound familiar to anyone who can give me advice?

I assume that you mean touching the BNC connector on the scope probe when it is plugged into the scope input connector.
Have a good look at the connector omn the scope input, is it damaged? especially the centre pin.
Is the body of connector loose? It may just need tightening up a bit.
Or there is a poor solder connection, remove the covers off the scope and have a look.

So, with what I have said, is this just $100 down the drain, or do you think this scope can be salvaged?

It probably just needs a little bit of TLC.

 

[bobheck]

How much over is the voltage reading?
Without numbers it is difficult to gauge.

Ok, when v/cm = 0.2, and reading should be 1 cm (using test calib on scope, .2v sq wave), it is 1.2 cm (next tick up, 5 ticks per cm on grid) is closest I can get it. 1x setting on probe. Readings are proportionately incorrect also with 10x on probe, and when increasing v/cm, and also when using regulated dc output at diff voltages for testing.

There will almost certainly be a gain adjustment in the amplifier, but what needs to be done depends on how far off is the calibration.

Ok, 'in the amplifier' means pull the cover off and look for ch1 gain or something like a trimmer pot? Is that right?

I assume that you mean touching the BNC connector on the scope probe when it is plugged into the scope input connector.

Actually, yes that happens, but also touching the outer part of the bnc on trace 2 even when no probe is connected causes the straight line to jump and even disappear.

Have a good look at the connector omn the scope input, is it damaged? especially the centre pin.

Looks fine.

Is the body of connector loose? It may just need tightening up a bit.

Don't see a way to do that. I may post pics.

Or there is a poor solder connection, remove the covers off the scope and have a look.

I am betting this is it. When I pull cover will check.

It probably just needs a little bit of TLC.

I sure hope so.

Thanks so much for the quick reply and the help. I will post back my progress/findings thanks to everyone's suggestions.

 

[JimB]

Ok, when v/cm = 0.2, and reading should be 1 cm (using test calib on scope, .2v sq wave), it is 1.2 cm (next tick up, 5 ticks per cm on grid) is closest I can get it. 1x setting on probe. Readings are proportionately incorrect also with 10x on probe, and when increasing v/cm, and also when using regulated dc output at diff voltages for testing.

And when the gain is set to 50mV/cm, it should display 4cm, what does yours show?

Ok, 'in the amplifier' means pull the cover off and look for ch1 gain or something like a trimmer pot? Is that right?

Yes.

Actually, yes that happens, but also touching the outer part of the bnc on trace 2 even when no probe is connected causes the straight line to jump and even disappear.

Sounds like a broken connection.

Don't see a way to do that. I may post pics.

I am betting this is it. When I pull cover will check.

Pictures would be a good idea, unless there is someone lurking here who is intimately familiar with the insides of this particular scope.

JimB