On Aug 21, 11:36 pm, Jim Yanik wrote:
Graham wrote roups.com:





On Aug 21, 7:36 pm, Jim Yanik wrote:
Graham wrote
roups.com:

Tektronix 2445, No EHT , oscillator board 'failed' common faults
?

You mean you have "no HV". It happens.

After many years of use, the scopes failed ..

Everything was fine, then lost the trace....On checking, the 750
mA fuse on the -15v un-reg line had blowen, turns out the A9 ,
eht oscillator board is drawing excess current .not oscillating ..
no signe of anything over heated

could be a bad multipler,or a bad CRT(anode loading problem)
Disconnect the xfmr drive to the multiplier,see if current draw is
too high.

.. scope powers up fine with A9 board 'out' looks to pass all self
tests

Q are there any 'common' fail's for this board , the power
transistor looks to be giving 'odd' reading's and ive ordered one
to replace it .. are there any other components that are know to
'fail' ?

Tnx

Graham ..

try unplugging the CRT anode lead,see if the scope powers up with it
disconnected. CRTs do fail and overload the multiplier.

--
Jim Yanik
jyanik
at
kua.net

Ok Jim,

Thanks for the reply, by what you say, i assume the switching
transistor is not the first item to suspect ?

I've seen more HV multipliers fail than HV oscillator xstrs.
(it's not a "switching" xstr,it does not turn hard OFF.)



With the board 'in' as soon as power is applied. the 750 ma fuse
blows , i dident see any oscillation , looks to be quite a load as
it pulls the -15v down to -12 (shows -19 with board 'out')

Too much of a load,and the oscillator never starts.But it does pull a lot
of current,enough to blow the fuse.Of course,shorted HV caps or bad diodes
can also be a problem. But it's easiest to pull the anode lead first,and
then to isolate the multiplier.



- Tube ?, I could try with the cable off

ANODE lead,the red one.Don't bother with the CRT socket at the back end.
If you pull the CRT anode lead and get a big spark when shorting it's plug
to chassis,then the multiplier and CRT are probably OK.

, but The oscillator is not
running and none of the tube voltages exist ... for that test would
need the ocillator running before you could 'load it' ?

Tnx

Graham .

Do you have a service manual with schematics???
The circuit descriptions are very valuable,2nd only to the schematics
themselves.

--
Jim Yanik
jyanik
at
kua.net- Hide quoted text -

- Show quoted text -

Some progress...

Found one fault, that was the soldered joint , center pin of the
switching transistor had failed , re made that and the oscillator
started running

Supply current 1.3 Amps , test wavefoem voltages all low , approx
50 % , sine waves clipped.

Tried disconnecting tube anode connector after running, tube produced
about 4 mm spark to earth.

Removed , two , connections from tags , rear of transformer to 'eht'
module ,

Supply current reduced to 450 mA . waveforms 'ok' re-connected
'upper' lead, (near qty 6 trim pots) slight increase supply current .
waveform's 'ok'

Re-connected 'lower' connection , supply current increrased to 1.3
Amps , wave forms low.

Not sure if lower connection is the anode or cathode (no pin numbers)
voltage source, either way, it looks like the 'eht block' has
failed ....

.... who would have one of those ?

Tnx

Graham ..