Hi,

Bad day at the office (shack) - two failure in one day ;-(

I recently turned on my trusty 7904A after perhaps 2 years of non-use...

It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and
then the traces dissapeared after about 10 seconds and the scope is
issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz
really. Now when I turn it on, I only get the buzz/tick.

Removing all the plugins makes no difference...

I'm not a HT person. Ideas? If it helps, I'm in the Uk.

Many thanks

Sounds like bad filter caps in pwr supply

"Mike Deblis" writes:

Hi,

Bad day at the office (shack) - two failure in one day ;-(

I recently turned on my trusty 7904A after perhaps 2 years of non-use...

It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and
then the traces dissapeared after about 10 seconds and the scope is
issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz
really. Now when I turn it on, I only get the buzz/tick.

Removing all the plugins makes no difference...

I'm not a HT person. Ideas? If it helps, I'm in the Uk.

Many thanks

The 7904 has an overcurrent protection circuit, which will cause the
ticking. This is a good thing.

First step, remove all the plug-ins. Still get the ticking? If yes, you
can isolate boards by removing connectors, one board at a time.

I had one of these fail that turned out to be a bad diode on the Z-axis
board.

Most likely problem: a bad cap on one of the boards. The tantalums go
bad and Tek used them by the truck load.

Good luck.

Steve.
--
Steven D. Swift, , http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA

"Mike Deblis" wrote in message ...
Hi,

Bad day at the office (shack) - two failure in one day ;-(

I recently turned on my trusty 7904A after perhaps 2 years of non-use...

It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and
then the traces disappeared after about 10 seconds and the scope is
issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz
really. Now when I turn it on, I only get the buzz/tick.

Removing all the plugins makes no difference...

I'm not a HT person. Ideas? If it helps, I'm in the Uk.

Many thanks

wrote in message oups.com...
Sounds like bad filter caps in pwr supply


It could also be that one of the high voltage power supplies arching to ground.

Check the HV wires for breaks in the insulation.

The post deflection accelerator is very high voltage so try not to get killed.

"Keyser Soze" wrote in message
om...
"Mike Deblis" wrote in message
...
Hi,

Bad day at the office (shack) - two failure in one day ;-(

I recently turned on my trusty 7904A after perhaps 2 years of non-use...

It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A), and
then the traces disappeared after about 10 seconds and the scope is
issuing a rapid "ticking" at about 200 Hz - more of a sharp-edged buzz
really. Now when I turn it on, I only get the buzz/tick.

Removing all the plugins makes no difference...

I'm not a HT person. Ideas? If it helps, I'm in the Uk.

Many thanks

wrote in message
oups.com...
Sounds like bad filter caps in pwr supply


It could also be that one of the high voltage power supplies arching to
ground.

Check the HV wires for breaks in the insulation.

The post deflection accelerator is very high voltage so try not to get
killed.

Is Jim Yanik (sp?) still around?

Thanks

"Mike Deblis" wrote in
:

"Keyser Soze" wrote in message
om...
"Mike Deblis" wrote in message
...
Hi,

Bad day at the office (shack) - two failure in one day ;-(

I recently turned on my trusty 7904A after perhaps 2 years of
non-use...

It briefly displayed both traces (linup is 2 x 7A26, 7B92A, 7B53A),
and then the traces disappeared after about 10 seconds and the
scope is issuing a rapid "ticking" at about 200 Hz - more of a
sharp-edged buzz really. Now when I turn it on, I only get the
buzz/tick.

Removing all the plugins makes no difference...

I'm not a HT person. Ideas? If it helps, I'm in the Uk.

Many thanks

wrote in message
oups.com...
Sounds like bad filter caps in pwr supply


It could also be that one of the high voltage power supplies arching
to
ground.

Check the HV wires for breaks in the insulation.

The post deflection accelerator is very high voltage so try not to
get
killed.

Is Jim Yanik (sp?) still around?

Thanks




The fast ticking,IIRC,indicates a failure in the PS itself,like a shorted
diode,bad electrolytic,open switcher xstr(s).Probably a primary-side
failure.
I strongly advise reading the serivce manual's circuit description on the
PS.
If you don't have a svc manual,then I would not attempt repair;it's a very
complex series-resonant switching supply.There are under/overvoltage
lockout circuits,several current and voltage feedback loops.

--
Jim Yanik
jyanik
at
kua.net