Oscilloscope repair, Hameg 208, Focus fault.

[alanP]

hameg

Hi All

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

I already have the service manual and have done some basic 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. I do not have the scope in front of me or I would take some screen shots.

Both traces are present timebase is running, both diplays are totaly out of focus and about 20-25mm in height.

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

 

[E-design]

repair oscilloscope

I will check voltages and resistor values in the area indicated first. Focus problems are most often caused by high value resistors going open or much higher in value.

 

[alanP]

oscilloscope repair

I will check voltages and resistor values in the area indicated first. Focus problems are most often caused by high value resistors going open or much higher in value.

Hi

The values in the chain if you cannot read them are as follows, from top to bottom:-

R912=1.5MEG
R913=1.5MEG
R914=475K
R915=475K
R917=475k
R918=1.5MEG
VR902=470K POT
C909=10n @2kV

Thanks

alanP

 

[E-design]

scilloscope focus voltage

Measure all those resistors by unsoldering one end and make sure the focus pot does not have a cracked track and resistance adjust normally over its travel. You must make these measurements with 1 end lifted out of circuit for resistors and 2 connections lifted for the pot. Make sure the cap is not leaky or shorted.

 

[pranam77]

repair oscilloscope focus

Hai Alan
Usually screens going out of focus is due to below reasons and you may try remedies to solve.
1. The EHT cup behind the CRT is leaky and sparks jump out ( Clean it with IPA and Seal it with a sealant)
2. Carbon deposits on the back of the CRT. (Just wipe with clean cloth and try to keep the surface dry)
3. The CRT base is old (replace it)

I hope you shud fix the said problem with these remedies. Good luck.

 

[alanP]

faulty hameg

Hi All

Well, checked all resistors this morning, R915 was almost infinite at 18-22 Meg:Ω . Replaced this now all is OK with the focus. I will now go out and purchase a set of new resistors and capacitors for the power/HV board.

There are some other small problems, like noisy switches and potentiometers and what may be a storage problem at the fast end of the scope.

I need to sit and play with the unit now, so that I feel in control.

 

[E-design]

oscilloscope focus fix

Glad you got it fixed!