05 December 2023

Troubleshooting an Hameg HM203-6 oscilloscope - no trace

Right after fixing the high voltage being too high (-2500V vs -1900V) on the Hameg HM203-6 oscilloscope, the trace disappeared. Note that I was able to see the trace at the restored -1900V cathode voltage and I was after the ineffective un/blanking control.

HV was there. Un/blanking signal was there. What could have possibly gone bad, now?

When operating in XY mode (or component tester mode if your scope has it), there is no retrace or blanking: you should always get at least a dot somewhere on the screen (be careful with phosphor burning!). I was not getting a beam, even in XY mode. So something was blocking the electron beam into the CRT: either missing acceleration or blocking grid at wrong voltage. That is where I concentrated my efforts.

I knew that BF199 was bad. Even with a new one nothing happened. BF440 was good as well as the optocoupler. At least they tested as expected on the DVM at low voltage.

Don't forget that components may (will) show a different behavior or value depending on the voltage applied. That's why I changed three 68 pF 2 kV capacitors that have been pinpointed as being prone to failure in online forums.

When all voltages were measured as expected I moved on to in-circuit component testing. The device MUST be powered off and unplugged! I take no responsibility for whatever damage to animated or inanimate beings you may cause.

Remove socketed components as they can be tested individually. Then proceed with this methodology:

  • resistors should exhibit the stamped value or lower, because they are in parallel with other resistors and alike: replace if value is too high or too low;
  • diodes should conduct in the A>K sense and exhibit an open-circuit or high impedance on reverse polarization: replace if forward and reverse voltages are too low for the diode type, or if it's open

I did not need to test capacitors as I found two open resistors and one shorted diode. I mark them in red in the schematic diagram excerpt below. See the thread on eevblog for the whole story.

A part of Hameg 203-6 oscilloscope circuit diagram.

So the Hameg HM203-6 symptoms were:

  1. trace too bright and not going dim/off with intensity control plus
  2. not un/blanking 

They were caused by:

  1. broken high voltage regulation, which probably lead to
  2. un/blanking failure and
  3. blocking grid control failure 

Components replaced:

  • one 741 op-amp IC
  • 3 high voltage 68 pF capacitors (perhaps not needed)
  • one BF199 transistor
  • two resistors
  • one diode 

Cost about 5€(2023)  and 10 man/hours. Troubleshooting value: invaluable.