Hi Dennis..in order to try help you..could you hand draw
the circuits and show us?.
At first glance any good HV transistor "must" be right if
excited by secondary winding , but if no being to
oscillate..perhaps some cap or resistor associated is out
Is any start up base current present?
debugging a very interesting one of a kind
engineering prototype that Tek made in 1970. No
schematics exist for it. It contains two small (2
½ cm x 6 ½ cm) CRTs. It does not work. I have
repaired most of it and have one major obstacle
left before it will spring to life. It uses a
single transistor blocking oscillator to generate
the CRT voltages. The CRT has all its voltages
connected via the rear socket so they do not
require a high anode voltage.
blocking oscillator circuit uses a single NPN
TO-220 medium power transistor. It runs off of a
40VDC supply. It was missing the HV fuse. I
guessed that the fuse was on the order of 0.5A
fast blow. So the transistor has to be able to
handle ½ Amp. Someone attempted to repair it once
before and, among other mistakes, they substituted
the wrong transistor.
guess that a blocking oscillator transistor has to
have a high Vceo and Vcbo on the order of a few
hundred volts because the HV transformer will
generate spikes as it turns off. Unfortunately
Beta goes down as Vceo and Vcbo go up. So I have
no idea if the circuit will work with a typical
High Voltage transistor’s Beta which is usually in
the 10 to 50 range. I thought I might need a
Darlington to get the Beta high enough but that
may not be the case if the HV Transformer’s base
drive winding is designed for driving a low Beta
transistor. What I am fairly sure of is that the
transistor has to handle +40V Vcc at 0.5A Ic. I
tried a HV transistor with a Beta of about 100and
it didn’t oscillate. I also tried another HV
Darlington Transistor (NTE2540) which didn’t work
output of the HV Transformer goes through a simple
2X multiplier. It appears to be OK and is not
shorted. There is a 0.6 Ohm resistor between the
emitter of the oscillator transistor and ground
for degeneration or to limit collector current.
hope to figure out what the right transistor might
have been is to find other single NPN TO-220
blocking oscillator schematics in use about the
same time (1970-1971) in Tek products that had a
small CRT in them. But I’m drawing a blank.
ANYONE SUGGEST OTHER TEK SCHEMATICS FROM 1970-1972
THAT WOULD SHOW A SINGLE TRANSISTOR BLOCKING
OSCILLATOR TO DRIVE SMALL CRTS?