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Hameg 208 Focus repair

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binbags

New Member
Hi All

Newbie/oldie here looking for some help on the above oscilloscope.:rolleyes:

I already have the service manual and have done some basic voltage and high/voltage checks with my fluKe meter and eht probe. All would seem to be in order, but both ch1 and ch2 traces are very out of focus. The high voltage diodes D941/2 appear to be in order as does the HV-cascade unit, this is giving me about 12Kv on the final anode. The other components (resistors etc) I have not yet got around to testing. The scope was purchased cheap with this fault, hoping that it was something simple to fix, maybe it still is. It's my old tired eye's that are letting me down.:confused:

I do not have the scope at home to do some screen shots, to show the problem. The two scan lines are about 20-25mm high with no image at all, just pure fuzz accross the screen.

There are three pdf files of the power supply to have a look at.

I look forward to any of you guys and gals replies.:)
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
Does the focus control do anything?.

Looks pretty simple to fix, with VERY few options.

Most likely (historically - not just on scopes) are the high value resistors either end of the focus pot, including the series row of them. Commonly high value resitors go high or O/C, particularly ones put in series like these. Next likely option (but a lot less likely) is the decoupling capacitor on the output of the pot, that could have gone short.

As you have an EHT probe, it should be simple to find out what the problem is.
 

binbags

New Member
Hameg 208

Does the focus control do anything?.

Looks pretty simple to fix, with VERY few options.

Most likely (historically - not just on scopes) are the high value resistors either end of the focus pot, including the series row of them. Commonly high value resitors go high or O/C, particularly ones put in series like these. Next likely option (but a lot less likely) is the decoupling capacitor on the output of the pot, that could have gone short.

As you have an EHT probe, it should be simple to find out what the problem is.
Hi Nigel

It's amazing how a new pair of eyes see from another angle. Yes, follow the chain until "0" volts is found. I will try at work where the scope is.:eek:

Thanks

binbags (Alan)
 

binbags

New Member
Hameg 208

It's been a common fault on TV's since Audioguru was a lad :D
Hi Nigel

Went away had a sleep, shower, went to work, found a very poorly resistor almost infinite resistance 18-25Meg. First one in the ground side of the focus pot, cured:):):):)

thanks for your wise thoughts.

There are some other issues with this scope, like noisy switches and pots, nothing that little squirt of switch cleaner won't cure. The main second issue is the memory display on CH2 at high speed. Before I do anything though, I need to have a play with it first and get use to it.

Thanks for now and am sure I will be in contact again.

alan
 

binbags

New Member
Congrats on the fix. Always good to see another scope saved.
Thanks for that;), I am now setting about the other scope a Kikusui COS5100, although finding the service manual for this is not easy. the scope is working OK, but a little noisey on CH2 input voltage selector. There is inside the unit on that channel 3x multiway wafer switches plus a pot, all of the components are inline on the input board.
This means taking the board out and removing the switch assembly to clean it. Not a quick or easy job. I like a challenge in winter:eek:
 

shimniok

Member
Whee :D I've sort of made a hobby of fixing the occasional scope, though all solid state up until I got this Heathkit IO-10 project... but that's a story for another thread ;)
 
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