Tektronix 7844 problem

"amolas" wrote in news: snipped-for-privacy@m73g2000cwd.googlegroups.com:

First be certain the line selector is set for the correct input line voltage.If someone's set it to 220VAC and you try to run it on 120VAC,it's not going to run;undervoltage lockout. (If set for 120VAC and 'run' on

The clicking is the PS trying to start up(it's a series-resonant switcher)and either encountering an excessive load on the secondary,problems in the primary switching circuit,or problems in the mainframe outside of the PS,then shutting down. Common problems are shorted diodes and bad electrolytic caps(ESR,dried out).There's also a decoupling PCB with tantalum caps piggybacked on the interface board in the mainframe;those caps like to fail.The PS also monitors the HV multiplier module current,so HV problems can also cause it to click.

chances are high that there are no "quick fixes".

Its a very complex power supply,with multiple sensing loops,and you absolutely need a service manual to troubleshoot it.And the input line filter caps store a very healthy charge(>300VDC),not healthy if you short them or get across them.I use a 1K 2W resistor glued to a stick to discharge each line cap before working on the PS circuit.

Also,you want to run the scope with at least two plug-ins in the slots.

also,ICs with a 155-xxxx-xx marking are TEK-made only;out of production,and no longer available new from TEK.

Google "sphere bc";they have some TEK parts and cross-references.

Thank you for your answer !

I have found a tantalum caps short circuited in the low voltage psu. I replaced it, but now, there's another problem...

The unit starts, but although illumination knob are at 0, the light on the crt is very strong, and the unit stop after about 30 seconds of operation...

I will look deeper in the service manual to find the solution...

Thank you again !

Have a nice day

Antoine

Jim Yanik a =E9crit :

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"amolas" wrote in news: snipped-for-privacy@m79g2000cwm.googlegroups.com:

Could be a problem with the DC restorer diodes in the CRT circuit;they break down under HV(or short),and then the CRT beam is on full,then the PS detects the excessive current draw and shuts down the PS.