Re: 475 high voltage circuit very low output

ehsjr
 

staffan_321 wrote:
Hello,
I have a 475 with a strange behaviour on the high voltage circuit. As I understand it, a transformer is driven by an NPN BJT, whose collector is coupled to the 'primary' winding and the base is connected to a feedback winding. In this way, it will oscillate at the self resonant frequency of the transformer and perform optimally. In series with the feedback winding is a regulator circuit that controls the DC level of the base current so that the resulting output level will be right.
There seems to be some kind of problem with the oscillation though. Using an oscilloscope to measure the base-emitter voltage of the driver transistor, this shows an oscillation with the right frequency (approximately 40kHz), but it is severely clipped at -0.2V and +0.7V. Average base-emitter voltage is +0.3V and apparently, one should expect something like -4V here.
Likewise for the collector voltage, there is a clipped sine at 3Vpp around +21V, where there should be a 40Vpp around +21V.
On the secondary side, there are a couple of outputs, they all show the same clipped behaviour and the resulting voltage is roughly what should be expected. Given the 3Vpp rather than 40Vpp there should be somewhat 1/10 of the expected voltage and that is what I get.
At one point the voltage is supposed to be -2450V and I measure -180V. Another point is supposed to be 300Vpp and I measure 25Vpp. I have not been able to probe the 5kV output of the secondary winding (going to a 5 to 15kV multiplier), but since all other windings show the same behaviour, can a secondary winding limit the complete circuit?
I have replaced the driver transistor with the same kind (2N3055) with no improvement and also tested the forward voltage of most diodes and bipolars in the regulator circuit and they all show approximately 0.6V. Difficult to test the capacitors though. Resistors seem to be OK also.
Measuring the voltage at given points in the schematic of the regulator circuit indicates that the regulator tries to increase bias of the driver transistor in order to increase the output voltage so it seems to be doing the right thing.
All supplies (-8, 5, 15, 50 and 110V) are OK.
Does anyone have a suggestion as to what can be wrong? Why is the Collector voltage and the base voltage clipped? All comments are most welcome. I very much would like to get this scope working!
Regards,
Staffan
Check the HV caps with an ohmmeter. Obviously that is not a
conclusive test, but you might find a shorted or partially
shorted cap. I got lucky with one of my 'scopes - I had a cap
reading ~5K on the ohmmeter, obviously bad.

Ed

Join TekScopes@groups.io to automatically receive all group messages.