Tektronix 7844 problem

[amolas]

Hello !

I have a tektronix 7844 mainframe, and when I power it up, I have no
display, and the fan is not runing at all...
I only hear a "clack" aproximatively 2 times per seconds coming from
the psu

Is anyone have any idea of the source of the problem ? Is anyone have
already saw such a problem on a tektronix 7xxx series mainframe ?

Thank you for your answer

Have a nice day

Antoine

 

[Jim Yanik]

"amolas" <antoine.molas@gmail.com> wrote in
news:1155394984.244483.263930@m73g2000cwd.googlegroups.com:

Hello !

I have a tektronix 7844 mainframe, and when I power it up, I have no
display, and the fan is not runing at all...
I only hear a "clack" aproximatively 2 times per seconds coming from
the psu

Is anyone have any idea of the source of the problem ? Is anyone have
already saw such a problem on a tektronix 7xxx series mainframe ?

Thank you for your answer

Have a nice day

Antoine

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
220VAC,then KAPOWIE;fuses blow,xstrs short,diodes short;BAD news.)

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.

--
Jim Yanik
jyanik
at
kua.net

 

[amolas]

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 écrit :

"amolas" <antoine.molas@gmail.com> wrote in
news:1155394984.244483.263930@m73g2000cwd.googlegroups.com:

Hello !

I have a tektronix 7844 mainframe, and when I power it up, I have no
display, and the fan is not runing at all...
I only hear a "clack" aproximatively 2 times per seconds coming from
the psu

Is anyone have any idea of the source of the problem ? Is anyone have
already saw such a problem on a tektronix 7xxx series mainframe ?

Thank you for your answer

Have a nice day

Antoine



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
220VAC,then KAPOWIE;fuses blow,xstrs short,diodes short;BAD news.)

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.

--
Jim Yanik
jyanik
at
kua.net

 

[Jim Yanik]

"amolas" <antoine.molas@gmail.com> wrote in
news:1155458728.031463.267640@m79g2000cwm.googlegroups.com:

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...

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.

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


Thank you again !

Have a nice day

Antoine

--
Jim Yanik
jyanik
at
kua.net