Hameg HM303-6 fault - trace deflected off screen in vertical direction

[adriansmith31]

Hi have got a Hameg HM303-6 scope that was donated to me which supposedly worked however it was shipped in just a plastic bag with no packaging materials at all. Unsurprisingly it got damaged but it was just a few dents to the case and one of the BNC connectors is bent. The scope does power up but the trace is permanently deflected towards the bottom of the screen; if the position knobs are adjusted it can just be seen at the bottom. If you turn on XY mode it results in a faint vertical line that appears out of focus.

I found the service manual and schematics online however there are several revisions of the boards and some of the boards in mine (the PSU, CRT board, CRT itself and XY board) are different to the manual and schematic. I have determined that all power supply voltages are OK including the high voltage and the voltages to the Y plates are at 146V and 85V - both should be 85V when set to a particular setting. However I cannot find any fault on the XY board and the input to it from the signal board varies as the position knobs are adjusted making me believe that the signal board is OK.

Base drive voltages to both Y output transistors is OK, their collectors are connected to the plates with 146V and 85V on as mentioned above and the emitters have identical voltages as per the service manual - should be 7.8V but measuring 9V but both are identical so I think this is OK. The plate supply voltage from the PSU board is 146V.

It seems one of the transistors is not being switched on but both measure OK. It did, at one point work after poking around on the XY board but went back to the fault condition as soon as a signal was applied. I cannot get it to work again and there are no cracks on any of the boards, no bad solder joints or broken print. It's almost as if one the Y output transistors is open circuit but it's legs are not broken and it checks out OK.

Anyone had any experience with these scopes that could point me in the right direction? Spare parts are not available.

I've attached a schematic as per the manual. The final output stages are the same but in mine it's discrete transistors and in the schematic they are inside a custom transistor array IC.

Thanks

[TaylorD93]

Hi Adrian,

If its had a drop bad enough to bend the BNC connectors, i would be concerned that the CRT itself is now defective. It doesnt take that much to dislodge the metal plates inside the CRT and that cannot be repaired without replacing the CRT itself.

you could reach out to Michael at sky-messtechnik, I believe he used to be the Service Engineer for HAMEG and when R&S bought them out, he setup his business to support the legacy HAMEG kit. I was able to buy some new old stock HM604 panels from him which date back to the early 90s. So you may be suprised what he still stocks. certainly worth an Email.

If you want to investigate the electronics further, what i would do is this;

1) Set both inputs to GND, Put the Scope in XY Mode to eliminate the "sweep", that way all plate voltages are just DC (no ramping voltages etc)
2) try to align the controls to their "mid" points, just based on the pointers on the potentiometers. As it is an Electro-Static CRT, to "move" the beam, one plates voltage must increase and the other decrease. Typically if both plate voltages are equal then the beam is in the middle.
3) Move the beam fully to one side (X axis) you should find the Y plate voltages stay the same, but the X plates would have changed so that one plate is Vmax and the other is Vmin

e.g. say the X amp is supplied by a 100V rail (for ease of numbers), you would see something like this;

Beam Pos      X1     X2
Mid              50V    50V
Left               0V    100V
Right           100V    0V

4) move it back to the X axis mid position, repeat step 3 but for the Y axis, again with the beam in its "mid" point the Y1 and Y2 plate voltages should be equal, moving it up/down you will see one plate rise in voltage and the other drop

Try that out and see what you get, if each pair of plate voltages are varying and are kind of symetrical, then i would say your XY amplifiers are fine.

Im about to head to bed but can take a look at the schematics to give more accurate estimates to voltages etc tomorrow.

[adriansmith31]

Thanks for your suggestions I will try an email.

I tried your suggestion, the X plates are fine but the Y plate one voltage varies between 20 and 90V as the Y position is adjusted, the other remains at 146V regardless of settings. It's as if the output transistor is not being switched on or is open circuit (it checks fine with a multimeter though) so I've ordered a pair of final output stage transistors and will try swapping them out. The base drive voltages are identical on both Y amplifiers and the emitter voltages seem to be correct too.

It did work at one point making me believe there is a crack in the PCB somewhere or broken joint but I can't find anything. I also found if you unplug the component tester board both traces appear but cannot be adjusted beyond the middle of the screen. You can only adjust them up and down in the upper half of the screen.

Edit: I think I've found the problem. The collector lead has broken away at the transistor body and pressure from the multimeter probe was making it contact hence reading OK but when the probe is moved away the lead does not make contact with the body. I tried soldering it without success so Ill order those transistors.

[TaylorD93]

fantastic progress, well done!

as a test, could you swap over the output transistors and make sure that with a working transistor fitted, the previously faulty output/plate voltage now varies between 20 - 90V as you saw before.

If it does, then that proves it is the transistor at fault and the rest of the circuit works.

[adriansmith31]

Ill give that a go might not be today now though. Ill let you know how I get on.

[adriansmith31]

Just to let you know replacing the 2sc3502e transistor in the Y output stage fixed it.

Thanks