Monday, December 31, 2012

New Year's Eve

Worked on the dead Leader o-scope last night.   The problem is in the high voltage circuit that provides bias for the trace of the CRT.

The circuit is shown below and copied from the service manual.  The 930 AC RMS volts on the power transformer secondary winding produces peak voltage of +/- 1315 volts (930 times 1.414 converts from RMS to peak volts).  On the positive cycle, V1 is +1315 and V2 is approx 0 volts because the diodes at V2 are forward biased.  So C1 is charged to 1315 volts at V1.  On the negative cycle V2 is -1315 volts.  Now current doesn't flow in the diodes at V2 because they are reverse biased. So the voltage at V2 is the voltage at V1 (-1315 volt) plus the voltage on C1 (another -1315 volts), so -2630 volts.  This voltage charges C2-C3 through the diodes between V2 and V3.  The voltage is filtered by C2-C3 & C4-C5 and the two 270 kohm resistors. Finally the two series 1000 volt zeners regulate the voltage  down to  2000 volts.

I found that C1 was dried out and no capacitance.  I replaced that capacitor and now the voltage comes up to -1600 volts or so.  There is a faint sound of arcing, so I suspect one of the other capacitors have dried out as well.  When I went to check the capacitance with my capacitance meter, I forgot to discharge the capacitors first, so there was still some charge on them.  So I blew the fuse on the capacitance meter.  I'll have to order a new fuse and order some more capacitors as well.  I can't get oil capacitors any more but have to order polypropylene capacitors, which are about $10 apiece.  So we're looking at a $50 to get it running, for a scope I paid $35 for.  If I had some patience and went to a few swap meets I could probably pick up the caps for a few pennies. 

However, if the scope runs, it should be worth at least $100 on Ebay, so it's still not a bad investment.  I may still be able to find some replacement caps from another source. 

For now, I've put the scope under the bench and will get to it after I get the parts.

Buying old equipment is kind of an adventure and a gamble.  Usually the equipment is simple to troubleshoot and easy to fix, but occasionally, it's impossible to get replacement parts, or the parts cost more than the cost of the equipment.  But that doesn't happen that often.

1 comment:

  1. Steve, keep track of the cost of the parts and I'll send you a check for them. I could actually follow your discription of the circuit. ha but just have to really get down to learning those programms for the circuit boards. I could blame it on old age but I'm still a genius in my own mind. ha