Re: Single Transistor Blocking Oscillator Question

Don Black <donald_black@...>

Or transformer, doubler faulty? Try disconnecting the doubler from the transformer and see if it oscillates.

Don Black.

On 26-Sep-13 5:59 AM, iglesia_cristiana_arpas_eternas@... wrote:

Hi 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 of value.

Is any start up  base current present?


---In tekscopes@..., wrote:

I am 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.


The 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.


I can 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 either.


The 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.


My best 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.





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