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OT Hameg HM103 blanking


Jean-Paul
 

Bonjour a tous:

Have a Hameg HM103 scope found at a flea market in Paris for EU7, dead unblanking circuit but otherwise OK.

I have replaced the optocoupler, CQY80N, NPN, BF199 PNP BF440, and checked resistors and trimpot, all are fine.
Blanking logic input pulse OK, so quite a puzzle.

Last possibility: leaky coupling caps 68pF/2kV, I have seen many posts on EEVblog referring to their failure.
But I have no way to test caps here. I have only a variac, VOM and soldering iron!

I ask any Hameg veterans (perhaps in Germany or elsewhere in EU) to please inform me:

Could a failed cap just kill blanking, but scope works fine and traces is good otherwise?
How to diagnose if a cap has failed?
Will a failed 68 pF/2kV kill the semiconductors?
where else to look for the problem?

Your assistance is greatly appreciated!

Jon


Ray
 

Jean,Well ... I can attest that Hameg scopes and owners :) also exist in the US.As I used to own a 204 and still own a 507.Alas I have not run into the specific problem you discribe.By they way the button labeled CT activates a build in component tester. It puts pn junction curves as well as capacitor and res respomses onto the screen. (In circuit as well)One of the reasons I got the 204 & the 507 which also have this.This function alone is worth fixing your 103.Bon chance.RaySent from my T-Mobile 4G LTE device------ Original message------From: Jean-PaulDate: Sun, Oct 18, 2020 03:16To: TekScopes@groups.io;Cc: Subject:[TekScopes] OT Hameg HM103 blankingBonjour a tous:

Have a Hameg HM103 scope found at a flea market in Paris for EU7, dead unblanking circuit but otherwise OK.

I have replaced the optocoupler, CQY80N, NPN, BF199 PNP BF440, and checked resistors and trimpot, all are fine.
Blanking logic input pulse OK, so quite a puzzle.

Last possibility: leaky coupling caps 68pF/2kV, I have seen many posts on EEVblog referring to their failure.
But I have no way to test caps here. I have only a variac, VOM and soldering iron!

I ask any Hameg veterans (perhaps in Germany or elsewhere in EU) to please inform me:

Could a failed cap just kill blanking, but scope works fine and traces is good otherwise?
How to diagnose if a cap has failed?
Will a failed 68 pF/2kV kill the semiconductors?
where else to look for the problem?

Your assistance is greatly appreciated!

Jon


Jean-Paul
 

Hello again all!

Ray thanks for the PM,


I Have just posted schematic markup and trace image in album "HM103 Hameg"

I now suspect the 68 pF coupling cap C705 is leaky and has damaged the NPN transistor.

But am traveling and Limited in tools and parts to disassemble main board and replace the 68 pF 2kV caps.

Open to your kind advise!

Jon


petertech99h
 

Jean-Paul,
Is the 68pf 2Kv cap a ceramic disk cap? Sounds like it is.  I can't remember the last I had a leaky disk cap!
If there is evidence of dirt, oil, old flux right around the cap, I would clean it instead.
If it is cracked or chipped then replace for sure!

Pete

On Sunday, October 18, 2020, 11:53:45 a.m. EDT, Jean-Paul <jonpaul@...> wrote:

Hello again all!

Ray thanks for the PM,


I Have just posted schematic markup and trace image in album "HM103 Hameg"

I now suspect the 68 pF coupling cap C705 is leaky and has damaged the NPN transistor.

But am traveling and Limited in tools and parts to disassemble main board and replace the 68 pF 2kV caps.

Open to your kind advise!

Jon


Jean-Paul
 

Pete: Many thanks, Indeed a small ceramic cap, perhaps 6 mm diameter and 2 mm thick.

Three are used for peaking the optocoupler response, across the -1.8 kV HV to the CRT grid and cathode.
They look fine your point of cleaning is great, as they are connected to transistor bases and the PCB is somewhat exposed under cooling grille holes in the cabinet.

Other Hameg repairs have mentioned failed caps

I will try some isopropyl alchol later on.

Best Regards,

Jon