Topics

Problomatic (broken) 464 storage scope.

mark@...
 

Ok,.. I am looking for some pointers if you would be so kind,... I have a broken 464 scope... the symptoms are as follows:-
1) The scope powers up,.. but the beam is not running correctly,.. ie as it was failing the beam was going back in time in a curly pattern,...
2) Beam finder does bring the beam in though
3) Fan does not make a continuous tone/noise... ie speed changes.
A) So far I have checked for magic smoke and burnt etc,.. so far nothing!
B) Have measure main voltages,.. And this is where things start going to pot:

5 volts DC measures 5.58v,.. with more ripple than I can measure
-8 Volts DC measures in at -8.4v again with loads of ripple.
+15 Volts DC measures in at 15.8v,. with loads of ripple
+65 Volts measures in at 68v and loads of ripple
140 Volts measures in at 134v with loads of ripple.

So clearly, ( I assume ) the power supply looks suspect,...

I sort of suspected failed input electrolytics.... but reading further forum stuff this seems unlikely as they are essentially Mil. grade components,.. so probably not the source of my problem.
I have also seen other utube video where the voltages are measured ,.. and mine are no where near close or as stable as these....
My other thoughts then go of to the Tant caps... are there are lots of them,.. the ones in the supply do not appear as short,.. as the is measurable resistance...(less than 10 ohms,.. is this normal?).

Now I'm thinking there may be issues on another board which is dragging the main supply down,.. causing the massive ripple and cycling,.. ( do not have a second scope,.. which makes life more difficult for sure ), But I think the supply may be in a cycle of dipping in and out,.. and failing to get to normal operation ,.. because of some down stream fault.

As the failing supply with also give the timebase scans trouble as there is nothing stable,.. I also noted prior to complete failure was that vertical calibration had gone to pot,.. simple DC signals of 5v did not show as 5v on the scope... so everything seemed to be in a cascade failure,.. but all leading be back to the power supply,.. but is this correct.

Before I start jumping in a stripping things apart,... isolating boards etc,... does anyone on have any pointers for me,.. as I am hoping someone may be able to offer some more informed paths to tread and try ,.. before I jump in with a soldering iron and spanners
many thanks

mark@...
 

Well folks,.. have done some investigating,.. and 'fixing',... and its would seem the series regulating transistor Q1734 had blown,.. and was no longer a transistor...

Having replaced this power transistor,.. my 65v supply now tunes in at 64.8Volts,.. which I take as pretty dam close to spec, ( is there a spec.,.. and all the other voltages all seem to fall into line,... as they are all referenced of the 65v supply,.. Except the 140v line,.. this still sits low at only 128v....

Looking at the circuit,.. the prime suspect (IMHO) is now focusing on C1722 which only has 64volts across it,.. whereas I would expect to see something closer to 90v... or maybe C1723 may also be rogue....
Also,.. as far as I can tell,. ( no spare scope to measure with ),.. the ripple appears to be bad,. as I cannot get a stable a.c. measurement on the dc rails.

Replacement of either or both caps seems to be a PITA, as I have read... does anyone have any suggestions,. or agreements that I need to just bite the bullet and change the large electrolytics in the 65v and 140v rails... ( where does the 140v rail get used,.. I have not found it as yet on my searches through the manual, other than applying some bias to the CRT tube ).
Many tx