HAMEG HM605 Oscilloscope

[Mr Scrofula]

Hi all - first post....

I have a HAMEG HM605 which I got slightly broken a few years ago which I bodged a fix to the output transistors on the trigger board. There were dry joints I could not access very well. I did some blind soldering and used a jumper wire on the top surface. This evening I thought I would fix properly, so I removed the top trigger board to access the underside and solder joints. It involved removing all the rods attached to the board and desoldering 2 wires, one yellow one labeled ZUP on the schematic and one orange one at the front which just connects to the external trigger. There were 2 ribon clips to remove as well. With the board able to be flipped over I went over all the joints by the 4 output transistors to CRT and any dry joints in that area, mainly the resistors adjacent to the transistors.

Reassembled, and turned on. Nothing on the screen - no flat line for either channel. Hooking up channel one to the inbuilt signal generator, I get the square wave, but it takes fairly precise adjustment of the trigger level, with a small window. When I add channel 2 I get the flat line for channel 2, but when I pull the probe from channel 1 out of the signal generator, both traces disappear.

For some reason I get nothing on the CRT unless a signal triggers, and channel 2 only works when channel one has triggered.

The area I was soldering in works ok, so I have not screwed anything up there, but for some reason I only get a trace when channel 1 triggers. I have the schematic, but don't quite know where to start regarding working out where the fault is I have introduced 

I gave the Y-Board a gentle clean of dust with a soft brush on the solder pin side, as I could see it with the TB lifted.

I am happy to give further details or try different things if that would help drill down to the problem area.

[Mr Scrofula]

I wonder if the trace is being blanked all the time except when triggering successfully ? I think that is the purpose of the ZUP wire I had to desolder on the timebase board. It was easy to desolder and resolder back on, and blanking works ok when triggering. So I am not sure. The TB board schematic is a bit tricky to understand.

[TaylorD93]

It sounds like you have pressed the push button for Auto/Normal trigger threshold.

Check the rod linkage is correctly set, the plastics can be a bit fragile with age.

Typically with it on "normal" trigger threshold you will only get a narrow window where the display will trigger and latch onto the signal.

if you are concerned about the sweep circuit, if you put the scope in XY mode it should show a solid beam (if Sine wave input) or two dots (if square wave input) this does depend on other settings ofcourse, but will eliminate the trigger circuit and give confidence the rest of the circuit is functioning.

another option is to try the Component Tester mode, this doesnt look at the normal Y inputs and injects into the trigger circuit. So if this works you can narrow down where any trigger issues are within your circuit.

edit: on the 605 model, the button i am refering to is the "knob" for the trigger level, it should pull in/out. if it does not pull out you have misaligned the linkage while reassembling the unit.

[Mr Scrofula]

Spot on TaylorD93 !

That was the problem. When removing the rod I had pulled out the switch, then replacing the rod the switch was pulled out on the pot, but the knob was in the pushed in position. I had picked the same problem up with the X-pos being in x10, but had recognised that when I first powered up and got the wrong frequency.

Not having ever used the switch on the level I did not and would never have realised what the problem was.

Sorted ! Once again, thank you very much.

[TaylorD93]

Spot on TaylorD93 !

That was the problem. When removing the rod I had pulled out the switch, then replacing the rod the switch was pulled out on the pot, but the knob was in the pushed in position. I had picked the same problem up with the X-pos being in x10, but had recognised that when I first powered up and got the wrong frequency.

Not having ever used the switch on the level I did not and would never have realised what the problem was.

Sorted ! Once again, thank you very much.

Glad to have helped

My 605 is a long term project where the HV circuit has failed multiple times, It was on ebay but i had little interest so have decided to have another go at fixing it.

Currently it shows a beam for a few seconds then disappears. Previously ive had the 741 and driver transistor for the HV PSU fail, as well as multiple resistors in the feedback network. Its been a pain, but i hope to finally get it fixed this year.