Hameg 303-6 trace issues

[paulbt]

Hello, guys !
I recently bought a Hameg scope with a few problems with trace stability, position and intensity.
I made two videos because it is hard to explain it's behaviour in detail.





So with intensity set at very low, everything seems ok until you move either ch1 or ch2 vertically. Near the top and bottom of the screen, the signal will get distorted in amplitude, rounded and bouncy. If I increase the intensity, the wave flashes and it's even more bouncy and the tube make some metallic buzzing when the intensity level it's towards maximum.

Things that seems to work well: focus, vertical amplification (V/div), timebase (sec/div), horizontal position, tigger, hold-off, ch1/2 GND coupling.

First I searched for bad electrolytic capacitors, all from the PSU seem to be ok with their ESR. I have also filmed all the boards with a thermo camera at my workplace and not even one capacitor goes more than room temp.
All PSU voltages are ok, I have also checked them with another scope from my workplace. None has bad ripple when the trace is going crazy or near top/bottom edges of the screen.
I checked on the scope the sweep's triangular waveform, it is ok, but when the intensity is high and the trace is flashing, the sweep goes blank (0V) then comes back to life.
With a multimeter I checked most of the ribbon cable connections and most of the components from X-Y final amplifier. They are all ok.
I cleaned and lubricated all the potentiometers and switches from the front panel with a special spray.
I also resoldered all the solder joints that looked bad to me, but I think none of them were loose/cold. I saw some repairing work done by the previous owner.
I have all the schematics for it, but not the layouts. I can upload them here if necessary.

So can you please give me a hint on which board the issue might be? Or how to use another scope to trace where the issue starts?

I really like this scope and I would be happy to have it fully working. I can afford a digital entry-level scope, but I like to challenge myself and work for something, not to get it for granted.

Thanks!

[kastell123]

Hello Friend
I can tell you from my long experience repairing TV. (over 40 years) Oscilloscopes power source and other circuits equal to those used in TV. Watch out!!! with the (ESR) of the electrolytic capacitors.
They can drive you crazy. Example I make in my workshop, 220 Mf / 25 volt electrolytic capacitor, with a perfect (ESR)
  and 80 Mf capacity, what do you think? when in doubt (if you do not solve). Better replace, by new.
Excuse my English, it is translated by Google
  Luck

[kastell123]

Si fuente de poder, no almacena suficiente energía, se manifiesta (el fallo) al solicitar más consumo, (más intensidad de brillo = más consumo)
Reemplaza electrolíticos primarios/secundario fuente de poder

[shakalnokturn]

The HM303-5 schematic is around I'm going by that and assuming they'll be very similar, I didn't find the HM303-6 service manual.

I'd be looking at the H.V. side of things, it could be defective HV regulation, the waves, bent traces and unstable intensity are hints towards that, why it would be dependent on position on screen I do not understand.


If you have a HV probe try checking how stable HV is, a few 10Mohm resistors are usually enough to do the job if you don't.
Other things to look at: Bad solders on CRT socket, open high value resistors in the HVPS section (mainly regulation feedback), HV regulation transistor, leaky HV ceramic capacitors.

[paulbt]

The HM303-5 schematic is around I'm going by that and assuming they'll be very similar, I didn't find the HM303-6 service manual.

I'd be looking at the H.V. side of things, it could be defective HV regulation, the waves, bent traces and unstable intensity are hints towards that, why it would be dependent on position on screen I do not understand.


If you have a HV probe try checking how stable HV is, a few 10Mohm resistors are usually enough to do the job if you don't.
Other things to look at: Bad solders on CRT socket, open high value resistors in the HVPS section (mainly regulation feedback), HV regulation transistor, leaky HV ceramic capacitors.

Thank you for your suggestions!
I do not own a HV probe and I do not have experience in probing HV stuff with regular probes on DMM/scope.
I had some thoughts that the -2KV voltage might be unstable or the WIMA polyester/polypropylene capacitors on that small board would be faulty, but didn't know how to investigate in this direction...
First I will try to check (even unsolder) most of the components from that CRT board and I hope I will be lucky.
One thing to note: the intensity is stable, I am adjusting it from the knob, only the trace on the screen is not stable anymore when I turn it harder.

You can download all the schematics of HM303-6 here:

http://mikroelectronica.ro/elforum_upload/HAMEG%20303-6/HAMEG%20303-6.zip

I hope the link works for you. The password is "elforum".
I asked the guy who shared these files to give me all the pages from the service manual, but he said that this is all he's got

[shakalnokturn]

Thanks for sharing what you have of the S.M., an incomplete one is still better than none at all.

After having a quick look at it I found out that your scope has dynamic focus, that would be an excellent link between spot position and instability, first suspect is C6015 (schematic_13-pag.25.pdf), still it is possible that the H.V. regulation is the cause (schematic_14-pag.27.pdf portion that says "DANGER HIGH VOLTAGE      ) and that the dynamic focus only causes it to show faster.

[paulbt]

FIXED !   

After a lot of effort in troubleshooting what might be faulty, I had an idea to check the only IC socketed on the TB board. I think it is IC3230A on page 12. It was almost stuck there because of corrosion on it's pins. I cleaned that as much as I could then I put it back. Now it works like new !

Dave Jones has an youtube video with a teardown of a Hameg scope and at some point he mentioned that if we buy old equipment and it has socketed ICs inside, we better take care of them because the contacts might be bad or loosen. So I should thank him for the hint !