Hameg 512 Oscilloscope not working

[blauerscharik]

Hi guys,

I bought an Oscilloscope at the flea market, which is not fully functional. 

The problem is, I have this fat line as you can see on the picture. When I turn the focus knob, nothing happens. So I opened it up and checked the poti on the schematic, it says 500K lin. And I do get that value. What else could it be? And do you recommend that I replace ALL the caps, which I read on the web somewhere before should be done.

Since it's a german product I could only find a german manual.

Thanks a lot

[jonpaul]

The old Hamegs are fine, I have 1980s HM103, 204-5 still going strong.

DO NOT blindly replace caps.

DL the schematic thats all you need.


First,  check all DC PS voltages with a VOM.

Open up, look for obvious corrosion, leaking lytics, damages.

Buy a good cpmtrol cleanrer spray (tuner cleaner/lube)

Clean and luber EVERY control and switch, and exercise.

The CRT/HV  controls are be recessed on the main PCB.

Every high R resistor in the CRT circuit must be checked as high meg R often increase in R over the decades.


Bon Chance


Jon

[Zenith]

I agree with Jon Paul.

Check for anything burned, previously repaired, or obviously not right, such as loose connectors.

Check the power supply voltages are correct with no excessive ripple. I've found on old Hameg scopes that the main reservoir capacitors often need replacing. There are smaller electrolytics after the regulator which can fail and cause ripple at a few MHz. That might explain what you are seeing. Apart from those I've not found that electrolytics in Hameg scopes cause problems. I wouldn't replace all the electrolytics in the scope without a good reason.

A lot of problems with old scopes are caused by dirty switch contacts. They are often closed units, but if you take the board out, so you can access them properly, there's usually some hole through which you can squirt switch cleaner and then work the switch 20 or 30 times.

As for your focus problem:

It could be caused by the resistors around the focus pot being open circuit or their values have changed. If you replace them make sure to use resistors with the correct rated voltage. It looks as if one end of that chain of resistors is at around -1300V. There could be some other problem with that part of the circuit, such as a bad soldered joint.

It could be caused by the astigmatism control being totally out, either because  it's been adjusted wrongly, or because of resistors as before. The wiper on that is supposed to supply about 115V.

It could be the high voltage supply to the CRT is wrong.

It could be something else, such as an internal connection in the CRT being broken. Rare, but it can happen.

You just have to tackle it methodically. Make sure the power supply lines are correct first. Neglecting that can waste a lot of time.

[blauerscharik]

Thanks for your help so far.

I did an overall visual inspection and couldn't find anything burnt or out of the ordinary. There are some inductors that have a weird coating which looks like they are burnt but they all look like that so I guess that's fine. I did break two of those scraping it with a toothpick 

As for the resistors around the poti. There was indeed one 1.5M resistor open. So I guess I have to order a couple things first now and then try again. 

[Zenith]

Thanks for your help so far.

I did an overall visual inspection and couldn't find anything burnt or out of the ordinary. There are some inductors that have a weird coating which looks like they are burnt but they all look like that so I guess that's fine. I did break two of those scraping it with a toothpick 

Toothpick as offered in some restaurants, or the cruel dental pick which dentists use to loosen perfectly good fillings and inflate their bill, and cause pain which ejects you from your seat?

I wouldn't have messed with those at this stage, because although they might need to be spot on to assure the unit is in the condition it was in when it left the factory, they are unlikely to account for what you are seeing.

The overall inspection with eye and nose is important with equipment of this age. There could easily be an obvious fault, or an unfinished or incompetent repair.

Next make sure the supply voltages are what they should be and stay that way.

As for the resistors around the poti. There was indeed one 1.5M resistor open. So I guess I have to order a couple things first now and then try again.

Buy ones with sensible voltage rating, which are still available and not expensive. Such as

https://cpc.farnell.com/w/c/electronic-electrical-components/resistors/resistors-fixed-value/through-hole-resistors?resistance=1.5mohm&voltage-rating=3.5kv

I'm not particularly recommending them and your preferred supplier may be better, but I'd avoid ebay for anything critical.

I suspected it would be a dead resistor. High voltage resistors were problematic and expensive in years past and I've fixed a few vintage scopes which had suffered heroic but futile attempts to repair them, with whatever was in the junk box by way of HV resistors.

I'm sure you will get it working, but anyway, you have now embarked on a fascinating voyage of discovery.

[blauerscharik]

Toothpick as offered in some restaurants, or the cruel dental pick which dentists use to loosen perfectly good fillings and inflate their bill, and cause pain which ejects you from your seat?

The ones you get at the restaurant 

I wouldn't have messed with those at this stage, because although they might need to be spot on to assure the unit is in the condition it was in when it left the factory, they are unlikely to account for what you are seeing.

Yeah I know, I wasn't sure what that coating is, I thought they are burnt so poked them a little and they kinda crumbled right away, so I guess there were not any good to begin with. Now I am looking for new ones. They are the 680u ones on the same board. Do you know what I should use instead?

Next make sure the supply voltages are what they should be and stay that way.

I will check that once I replaced the components for sure.

avoid ebay for anything critical.

I try to avoid ebay whenever possible. Especially since I had a bad experience with components before.

[Zenith]

Yeah I know, I wasn't sure what that coating is, I thought they are burnt so poked them a little and they kinda crumbled right away, so I guess there were not any good to begin with. Now I am looking for new ones. They are the 680u ones on the same board. Do you know what I should use instead?

I could only see one inductor on the circuit diagram I looked at. It looked as if it was to block noise on a power supply line. It's possible I'm looking at the wrong diagram as there were several HM512 scopes.

Most of the inductors I've seen in old scopes have a plastic former and base and a tuning slug. I don't see that sort around much these days. I suppose you could rewind them as I don't think they will have that many turns. If they are fixed then through hole fixed inductors are available. Suppliers of vintage parts may have them. As a last resort there's always ebay. Not all components sold on ebay are rubbish, it's just that there's a good chance they are.

[blauerscharik]

I could only see one inductor on the circuit diagram I looked at. It looked as if it was to block noise on a power supply line. It's possible I'm looking at the wrong diagram as there were several HM512 scopes.

Most of the inductors I've seen in old scopes have a plastic former and base and a tuning slug. I don't see that sort around much these days. I suppose you could rewind them as I don't think they will have that many turns. If they are fixed then through hole fixed inductors are available. Suppliers of vintage parts may have them. As a last resort there's always ebay. Not all components sold on ebay are rubbish, it's just that there's a good chance they are.

I attached some pictures and marked them. As a matter of fact the third one is broken too. But that one didn't get the toothpick treatment 
Yes you are right, they have a plastic base. Wouldn't just a regular inductor work? Something like this:
https://www.reichelt.com/de/en/vertical-inductor-09hcp-ferrite-680-h-l-09hcp-680--p138666.html?&trstct=pos_0&nbc=1
or this:
https://www.reichelt.com/de/en/stationary-inductor-09p-680-h-09p-680--p1131.html?&trstct=pos_0&nbc=1

[Zenith]

I can't see why those inductors should not work. It doesn't look as if the originals were variable and tuned to some particular value. They are cheap enough, so try them and if they are not suitable, try something else.

[blauerscharik]

I can't see why those inductors should not work. It doesn't look as if the originals were variable and tuned to some particular value. They are cheap enough, so try them and if they are not suitable, try something else.

Thanks. I ordered everything, and will post my results once I soldered all the components in.

[KrudyZ]

Not helping here, just an anecdote.
I had a Hameg 512 some 40 years ago. It was my first oscilloscope and I begged from my parents and close relatives for contributions and saved all my money for over a year to buy it.
I always turned down the brightness to a minimum between uses, since that's what you were supposed to do to not burn in the phosphor.
One evening I forgot to turn it off so it was left with minimum brightness and it died over night.
Apparently, the design burns off the extra beam power in a power resistor and it fried.
However, Hameg was really nice about it and fixed it without charge.
Overall, it was a great 20 MHz dual channel scope and was super useful to debug my home made TRS80 clone and used on many other adventures.

[blauerscharik]

Not helping here, just an anecdote.
I had a Hameg 512 some 40 years ago. It was my first oscilloscope and I begged from my parents and close relatives for contributions and saved all my money for over a year to buy it.
I always turned down the brightness to a minimum between uses, since that's what you were supposed to do to not burn in the phosphor.
One evening I forgot to turn it off so it was left with minimum brightness and it died over night.
Apparently, the design burns off the extra beam power in a power resistor and it fried.
However, Hameg was really nice about it and fixed it without charge.
Overall, it was a great 20 MHz dual channel scope and was super useful to debug my home made TRS80 clone and used on many other adventures.

That's what you get for taking care of your equipment 
I assume you don't have it anymore.

[elecdonia]

I looked at the schematic. Very clever circuit design. Relatively simple but should be very functional. The high tension voltages for operating the CRT are derived from a 1255V winding on the power transformer. One end of this winding is grounded. The other end of the winding supplies 1255V AC with a tap at 1150V. The 1150V tap is sent through a voltage doubler to provide +3kV DC for the second anode of the CRT. The 1255V AC from the power transformer is half-wave rectified, filtered, and then regulated by a string of series-connected zener diodes to provide -1320V and -1420V DC for the CRT cathode circuit. Using both negative and positive supply voltages provides a sharp bright display on the CRT due to having a combined total of 3.4kV from CRT cathode to second anode.

The blurry out-of-focus condition and the inoperative focus control is most likely a result of one or both of the two 1M5 series resistors located in the focus control circuit opening with age. Resistors of 1M and up definitely have limits for maximum voltage which can be applied. Their failure rate increases with applied voltage even when it remains well under the maximum rated voltage.

In this case each of the 1M5 series resistors must be rated for 500V or higher. One replacement option is to connect several ordinary resistors in series to achieve the desired total of 3M ohms. Ordinary 1/4W or 1/2W resistors are typically rated at 100V to 150V max. Placing 11 270K resistors in series gives 2.97M ohms. And the voltage per resistor is <100V.

I doubt there is any fault with the factory-original 500K ohm variable resistor for the focus control, or with the 330K ohm series resistor. However these should be checked by ohmmeter.

The negative CRT power supply voltages are -1320V and -1420V. Both exceed the maximum safe input voltage for most multimeters, so take care when measuring. Also do not attempt to measure the +3kV second anode voltage directly. However, if you have a “high-voltage probe” (left over from the CRT television and computer monitor era) this can be used to measure both the -1420V and +3kV. These high voltage probes divide the input voltage by 100 when used with a multimeter having 10M ohm input resistance. So -1420V would measure as -14.2V.

I recommend getting the focus circuit working first so that a well-focused display can be obtained. After that the remaining circuitry can be evaluated to identify if any electrolytic capacitors require replacement. Contrary to popular opinion it isn’t always necessary to replace every capacitor in vintage gear.

[blauerscharik]

I looked at the schematic.......

As always thanks for the great answer.

I ordered a 1M and a 560K, 2W each and put them in series. The focus now works fine.
I broke some of the inductors and ordered new ones which I also soldered in.

I don't know much about scopes but I have the feeling it still isn't working properly. I can't really put it in words what the issue is, but I'll try. The first thing that doesn't work is the function generator. The square wave doesn't look right and the saw tooth wave doesn't really work at all. I get a diagonal line across the street and just can move it around but nothing happens.

I tried to measure some volts on the boards which I could identify on the schematic and one thing was suspicious (see picture).

I really don't know where to start looking for issues. And like I said I don't know much about scopes yet.

[blauerscharik]

Does anybody have any ideas?

Thanks

[KrudyZ]

Were you looking at the square wave and saw tooth with the scope itself or did you use a second known working one for that?
Do you have any other signal sources available, even just an Arduino that you could program to output a toggling bit?
Also, do you have any scope probes? How are you making your connections?
WRT the voltages you measured, there are both external control knobs and internal potentiometers that would affect the adjustment range. Sometimes the wiper contact opens up and wiggling it around can restore the contact, especially on old equipment that has been sitting around.
The two voltages you measured (70 V and 195 V) are about the same distance away from the nominal 130V, so this might simply mean that your horizontal position is turned all the way in one direction. Does the trace only go partially across the screen?

[Zenith]

There seem to be a number of versions of this scope and it's hard to say whether the manual I've seen corresponds exactly with yours.

From the circuit diagram you've shown the square wave generator looks like the calibrator and consists of an astable (T57 and T59) with the output through an emitter follower (T59). The circuit looks quite simple. The purpose of this is to present a square wave at around 1kHz and an amplitude of somewhere around 0.5V ( in this case 0.2V) to compensate a 10X scope probe properly. You connect the probe to the calibrator output and adjust a preset capacitor on the probe to display a square wave without overshoot or undershoot. In other words a square wave with a 90 degree angle, which looks like a square wave from a text book.

If you use a probe on its 1X setting or use something like a BNC lead with crocodile clips, you ought to see a square wave.

Usually with basic analogue scopes like this, you can adjust the amplitude of the square wave by adjusting a preset potentiometer, but the frequency is fixed and is only approximately 1kHz.

If you don't have another square wave source, you can make one up using a 555 timer or a relaxation oscillator using TTL. Those ebay function generators using an 8038 or XR2206, are rather limited but look good for their low price.

Unfortunately with oscilloscopes you soon get to a stage where you need another oscilloscope to fix them easily.

The sawtooth looks as if it's supposed to be 5V peak to peak. I can't see it on the circuit diagram I've seen. I can't imagine it's very useful and if the rest of the scope works, I wouldn't worry about it at this stage.

As for the voltages on  the output of the X amplifier, if you can centre the display properly, don't worry about them. That's to say, when it displays a line with the trigger on auto, you ought to be able to use the X-shift control to move the line to the right, so the left end is visible and to the left so the right end is visible. There's usually an adjustment so that when the X-shift is in the centre position, the centre of the line matches the centre Y line, but if that isn't quite right it isn't normally a problem.

[blauerscharik]

Were you looking at the square wave and saw tooth with the scope itself or did you use a second known working one for that?

Yes with the scope.

Do you have any other signal sources available, even just an Arduino that you could program to output a toggling bit?

No I don't have anything else. But I tried AC voltage from the receptacle.

Also, do you have any scope probes? How are you making your connections?

Yes I had probes laying around from before I had the scope not sure where they are from or what "quality"
Might even be cheaper ebay stuff. I have two probes and they behave differently when I plug them in. Even though Im not changing anything on the scope.

The two voltages you measured (70 V and 195 V) are about the same distance away from the nominal 130V, so this might simply mean that your horizontal position is turned all the way in one direction. Does the trace only go partially across the screen?

I can move it and center it. It does flicker though and the lines move from left to right or right to left. I barely can make it stop.
I added some pictures

[blauerscharik]

There seem to be a number of versions of this scope and it's hard to say whether the manual I've seen corresponds exactly with yours.

I added the manual in my first post.

From the circuit diagram you've shown the square wave generator looks like the calibrator and consists of an astable (T57 and T59) with the output through an emitter follower (T59). The circuit looks quite simple. The purpose of this is to present a square wave at around 1kHz and an amplitude of somewhere around 0.5V ( in this case 0.2V) to compensate a 10X scope probe properly. You connect the probe to the calibrator output and adjust a preset capacitor on the probe to display a square wave without overshoot or undershoot. In other words a square wave with a 90 degree angle, which looks like a square wave from a text book.

Understood. But I don't get a square wave I added some pictures to the previous post.

The sawtooth looks as if it's supposed to be 5V peak to peak. I can't see it on the circuit diagram I've seen. I can't imagine it's very useful and if the rest of the scope works, I wouldn't worry about it at this stage.

Yeah I kinda thought so too. Just wasn't sure if the rest works "fine".

As for the voltages on  the output of the X amplifier, if you can centre the display properly, don't worry about them. That's to say, when it displays a line with the trigger on auto, you ought to be able to use the X-shift control to move the line to the right, so the left end is visible and to the left so the right end is visible. There's usually an adjustment so that when the X-shift is in the centre position, the centre of the line matches the centre Y line, but if that isn't quite right it isn't normally a problem.

Understood. Thanks

[KrudyZ]

Assuming the square wave trace was taken with your scope probe at 10:1 attenuation, then the overshoots would be from the probe not being compensated properly.
There should be an adjustment on the probe itself, usually for a flat screw driver that when you turn it will change the edge transitions of the square wave.
What you have is undercompensated, resulting in the overshoot, proper compensation would be a nice square wave and undercompensated would have a rounded corner.
Note that the probe has a defined range it can compensate for which might not overlap with what is needed for your scope.

The scopes input impedance is 1 MOhm || 30 pF
If you look at a random scope probe like this:
https://www.amazon.com/Sumnacon%C2%AE-P6100-Sensitivity-Oscilloscope-Probe/dp/B00XJH2M02
You see it has a compensation range of 10 - 35 pF, so that one should work.

[Zenith]

So you have two probes. What sort are they and do they switch between 1x and 10X, or are they only 10X? If they are 10X, or on the 10X setting, you would get a distorted square wave like that if they were not compensated for the scope and for the channel of the scope you are using them with. Have you tried adjusting the compensation screw. to correct the overshoot? There are proper adjustment tools for the probes made of plastic or ceramic or with a very small metal blade, but you could get away with a jeweller's screwdriver. Probes are normally sold with the adjustment tooIs but they usually are lost over time. Is the same on both channels? What if you switch the scope from AC to DC coupling? I'd expect two probes which haven't been compensated for a particular scope to show different results.

It could be that one, the other, or both channels of the scope are faulty, the probes are badly adjusted or just the wrong ones for that scope, but the scope should have no problems with most probes. One or both probes could be faulty. You can see that sort of waveform when a scope probe has a bad internal connection.

What do you mean by "AC from the receptacle"? If it's the mains (line) socket at 220V, it isn't a good idea to put a scope probe on it directly.  The mains waveform is usually a distorted sine wave because of appliances using thyristors and putting noise on the mains.etc.

The picture shows part of a sawtooth that isn't very linear, but it is a sawtooth. Decrease the timebase speed and decrease the Y sensitivity of the channel you are using and you should see several ramps and drops and be able to make the height comfortable to view. With a sawtooth with a height of say, 4 divisions, you ought to be able to set the triggering to the positive slope so that the trace moves slightly as you alter the trigger level.

[blauerscharik]

There should be an adjustment on the probe itself.

The adjustment screw doesn't change anything. So the probe might not be good enough then.

[blauerscharik]

Have you tried adjusting the compensation screw. to correct the overshoot?

Yes I have a 1-10x probe. The compensation screw doesn't change much...

What if you switch the scope from AC to DC coupling?

There is also not really a change.

It could be that one, the other, or both channels of the scope are faulty, the probes are badly adjusted or just the wrong ones for that scope, but the scope should have no problems with most probes. One or both probes could be faulty. You can see that sort of waveform when a scope probe has a bad internal connection.

I think the probes are not good enough. I'm gonna try to get new ones some time. Thanks for the help

[p.larner]

ive not read all posts but check power rails dor ripple,had a tek 465 doing similar things,it was a bad cao on one of the rails,changed the cap and all good.

[Zenith]

Have you tried adjusting the compensation screw. to correct the overshoot?

Yes I have a 1-10x probe. The compensation screw doesn't change much...

You can usually see a difference, even though it doesn't produce the nice 90 degree edge you want. The difference might be between extreme overshoot and moderate overshoot but it should be noticeable.

A bit of wire, or preferably a piece of co-ax with crocodile clips. should produce a fairly good square wave at 1kHz, which is about what most calibrators operate at. That's assuming the calibrator is producing a good square wave.

What if you switch the scope from AC to DC coupling?

There is also not really a change.

And both channels the same?

It could be that one, the other, or both channels of the scope are faulty, the probes are badly adjusted or just the wrong ones for that scope, but the scope should have no problems with most probes. One or both probes could be faulty. You can see that sort of waveform when a scope probe has a bad internal connection.

I think the probes are not good enough. I'm gonna try to get new ones some time. Thanks for the help

You have to try to narrow these things down. It helps if you have a known good signal source, known good probes and a known good scope. You only have this scope.

A time back I had a 50MHz Hitachi scope which worked very well on channel 2, but it was impossible to adjust a probe for channel 1. Channel 1 didn't work very well with a 1x probe either. It was very distorted. I have several scopes, probes and signal sources. It turned out to be a destroyed resistor before the attenuator in channel 1. How it came to be destroyed is a mystery because it's hard to see how it could be made to dissipate so much power as to burn out in that circuit, but it was. When it was replaced, the scope was perfectly fine after minor adjustment.

I try to help. I don't always succeed.