I've just had to go inside my venerable Hameg HM604-2. These old analogue scopes are a dying breed, but they offer some advantages including very low noise performance as some (this one) still use linear PSUs.
The fault was that the couplings to the two vernier amplitude controls, which are concentic to the amplitude switches, had both let go (one was so loose that when I tilted the scope to move it, it slid out of the switch centre
As this scope has been so reliable, I hadn't needed to go inside previously, but when I saw where and how the preamplifiers were situated, it looked nigh on impossible to get inside them without everything coming out of the chassis first!
The Hameg service manual, although it contains the all essential circuit diagrams, has no procedures to guide you in a tear down. I searched on the net, but couldn't find much to help. I did find one thread where someone had a similar problem, but I couldn't find whether or how it was resolved.
Removing just 2 rear panel screws allows the cover to slide off. I could now see the 2 preamplifiers in their steel cans. Take any actuating rods which are in the way and set them aside. Removing all the knobs etc and then the front panel via the 6 self tappers into the front panel plastic moulding exposes the nuts that hold the amplitude switches to the metal sub-front panel. Also there may be a wire soldered to the front panel inner metal layer which needs to be desoldered before you can lift away the plastic and metal composite front panel.
Then the 2 metal strips which come from the sub-front panel which are soldered to the bottom section of the preamplifier can are desoldered, you might as well at this stage desolder the bottom section of the preamplifier can too, and remove it from the main preamplifier can, you need it off later anyway. Be aware you need a lot of heat and maybe a 100W iron would be better for the job. My puny 50W Ungar really struggled! (
See photo 1).
Next desolder the short flexible wire from the input BNC card from the preamplifier pcb (
the blue wire in photo 2).
Press down on the white multiway connector at the end of the preamplifier to remove the ribbon and Channel 2 can be jiggled up and out of the chassis. The main preamplifier can has 4 indents which are soldered to the pcb. Desolder these (
photo 3) and release the nut holding the switch to the can, and the preamplifier pcb can be removed.
Now I could see the issue, as if I hadn't guessed: the plastic coupling had split. You can take the bracket that holds the potentiomer off the preamplifier pcb, it's just 2 nuts and bolts and you need to desolder the resistors on the pot too.
(
photo 4) I took some time in glueing and refitting the coupling to ensure that the inner shaft was now tight enough.
Channel 1 got the same treatment but there was also, on mine, a very thick wire connecting the Channel 1 main preamplifier can to the chassis below, and it needs that big boy iron again! (
photo 5) shows channel 1 refitted and channel 2 will be next.
Refitting the main preamplifier cans by resoldering the 4 tabs, and the smaller bottom can which needs resoldering to the main can gets the preamplifiers ready to be refitted. I couldn't decide if fitting the bottom can after installation might be better, as at least you can get to the input wires to resolder them to the preamplifiers
.
(
photo 6).
The last hurdle is the refitting the front panel with all those knobs and switches to align - what a pain
, but take your time and don't force it.
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