HP 1222A scope generates sawtooth instead of flat baseline

Jan 20, 2004 3 Replies
J

Hi,



Yet another scope in trouble ...



HP 1222A. Has always been working fine, but recently (after a longer pause) it suddenly has developed a "sawtooth" baseline.


- periodicity ~9 ms, int'y ~1 graticule



- both channels are equally concerned



- A/B gain setting or grounding does not change amplitude



- both channels can be moved up/down w/o problems



- input signal overlays the "sawtooth"



- sawtooth reacts on trigger (or vice versa ;-)



- sawtooth becomes "steeper" if moved beam is moved to the lower part of the CRT



Power supplies:



+95 V adjusted, OK
+210 V reads +219.5 V
+12 V reads +14.20 V, -12 V reads -14.25 V (at least symmetrical ;-)

I suspect that the input channels are OK and that the problem is probably related to the vertical deflection (or sweep?).



Before diving into the circuit ... does that ring a bell ... anybody?



Please feel free to contact me either via the newsgroup, or per direct mail.



Thanks in advance for any help, and greetings from Lausanne!


- Joerg


[ xpost to s.e.equipment & s.e.repair, f'up to s.e.e. ]
joerg dot hau at swissonline dot ch * Lausanne, Switzerland http://www.mysunrise.ch/users/joerg.hau/ "All standard disclaimers apply". remove "nospam." from my address to reply (this became necessary due to increasing SPAM)
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J

Joerg Hau wrote in news: snipped-for-privacy@54693.user.dfncis.de:

Did you check for ripple on those DC supplies? Are the regulated supplies actually regulating when you vary mains voltage with a Variac?

Also,do you have a service manual or schematics?

Jim Yanik jyanik-at-kua.net
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J

Hi Jim

Admittedly I did not even think of this before (the values were "approximatively correct" and steady) ... and indeed, there is a fat ripple of about 2.2 Vss on everything except the +95 V supply. Now I've really got something to start with :-)

BTW, I've put a picture of the patient showing its symtoms at . The different slope of the two traces is only is due to the different position on the screen.

I don't have a Variac :-(

Yes, but I thought I'd ask here first ... I'm pretty well used to work _with_ a scope, but much less _inside_ of it ;-)

Thanks!

- Joerg

joerg dot hau at swissonline dot ch * Lausanne, Switzerland CHANGED URL: http://homepage.sunrise.ch/mysunrise/joerg.hau/ "All standard disclaimers apply". remove "nospam." from my address to reply (this became necessary due to increasing SPAM)
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C

Sounds like Ripple on one of the power supplys. Check with another scope, or with your meter on AC mode. Check the supplys for ripple. Probably a bad filter cap in a power supply.

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