Author Topic: Hameg HM203-6 blanking glitch  (Read 942 times)

0 Members and 1 Guest are viewing this topic.

Offline shakalnokturnTopic starter

  • Super Contributor
  • ***
  • Posts: 2068
  • Country: fr
Hameg HM203-6 blanking glitch
« on: May 25, 2019, 01:41:23 pm »
I'm trying to get this old scope back to normal. (Normal should be enough, not new...)
When I picked it up it had no display due to lack of -12V (R761 burnt on "XY" board.)

Since replacing this resistor, scope is mostly working as intended, the only problem found so far is what looks like a small timing issue with the blanking control. (This model has no Z mod. input)

On lowest timebase the brief flash of the retrace can actually be seen on the CRT, followed by a bright spot on the left, the rest of the scan is normal.
On most of the timebase settings the traces appear with a bright spot to the left followed by a slightly dim start of scan, the remaining scan being normal.
On highest timebases the trace loses the bright spot to the left and the beginning of the scan, the trace fades-in on the first 1/3 of the CRT.

I've probed at it with another identical scope, looking at blanking control at anode (Pin 1) of optocoupler and ramp output at "TP" (R559 on timebase), on the fastest timebase I can make sense of what's probed and what's displayed on the sick scope. (Without understanding the cause however.)
On the lower speeds it just doesn't seem obvious... What am I missing? A couple of ns?

 

Offline shakalnokturnTopic starter

  • Super Contributor
  • ***
  • Posts: 2068
  • Country: fr
Re: Hameg HM203-6 blanking glitch
« Reply #1 on: August 27, 2019, 10:05:11 pm »
Got back to working on this one again yesterday.
After looking at the schematics long enough I decided to replace some decoupling capacitors in the timebase. The ones that seemed to make the most sense were C540, C541.
No luck, they were good enough nothing better after.
Then replaced C517 and C530, C530 was totally gone, nothing left to measure (over 40M \$\Omega\$ resistance and no capacitance), still no effect on my blanking problem.

Nothing related but I also stumbled across C750 in the final stage of the X amp that had never even been fitted.

My problem turned out to be T501 (BF199) in the blanking section. The transistor had more or less evaporated, all the AVR tester can make of it is a 30pF cap between B-E.

Happy end, thanks for all your input.  ??? :-X :clap:
 

Offline shakalnokturnTopic starter

  • Super Contributor
  • ***
  • Posts: 2068
  • Country: fr
Re: Hameg HM203-6 blanking glitch
« Reply #2 on: February 16, 2020, 11:27:26 pm »
As a follow-up to this repair:

This scope was sold end of last year as study equipment to a young "digital artist" teaching in a Swiss art school.
You can catch a couple of glimpses of it in their commercial demo video here:
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf