Robonz wrote:HI Harry
Beware the BC548 and 558 can be very different. The will say 548A 548B and 548C This is the HFE which can imbalance things. You should really get some matched ones and test them to see what the HFE is matched or the deflection will be asymmetrical. You should also ask which one is supposed to be used.
Another option is to use a common audio amplifier chip starting with TDAxxxx, these are well known for deflection amplifiers.
Klaas Robers wrote:There is no point in the difference of the amplification factor of the transistors. The amplifier is so heavily feeded back, that this will eliminate all (relatively small) transistor differences. That is the strength of these amplifiers.
Also the coil resistance should do little to nothing to the static measurements, that is with the input (resistor of 6k8) floating or grounded. The only thing I can think of is that in one situation (coil of 1 ohm) the amplifier is wildly oscillating. You can see that with your oscilloscope. If that happens a small capacitor (1 nF) from the collector of the input transistor to ground will eliminate that. Make that capacitor as low as possible, just eliminating oscillations.
Any way, do these tests firstly with the preset potentiometer of 2k2 in the shorted position. The voltage between the bases of the 548 and 558 should be about 1 volt. Later you may preset that to a setting that will give the least distortion.
Klaas Robers wrote:What is the resistance of one of these windings? Is it 4 ohm?
If the 4 parallel windings was 1 ohm, I would say one winding = 4 ohm.
- All 4 in series will be 16 ohm,
- two of them in parallel will be 2 ohm
- these two parallel circuits in series will be 4 ohm.
- That will be suitable.
So 1 ohm, 4 ohm or 16 ohm. Make your choise!
Robonz wrote:I was not worried about the mismatched transistors not working. I have had it where the scan becomes asymmetrical with badly miss-matched hfe's.
You can also measure the BD135 and BD136 base to emitter. They should be around 0.7 volts with no signal running.
If you start off with 8 ohms it is safer as it will be less load until you get this thing running.
Harry, did you add the two 10uF caps?, It won't stop it working but its good to complete the circuit
Klaas Robers wrote:Harry I don't understand your measurements on the deflection yoke.
Now you have numbered the wires, after you desoldered the lugs that had each 4 wires connected, isn't it?
I assume wires 1, 2, 3 and 4 were at the left lug,
and wires 5, 6, 7 and 8 on the right lug, yes?
Or is it differently? How?
Because if wire 4, came from the left lug, measured to wire 5, which came from the right lug, shows a shortage, that is zero ohms, then I don't understand it any more...... Shorted will say: connected, as if it were a short wire.
This is opposed to "open", no connection at all.
I should assume, if wires 1, 2, 3 and 4 came from the left solder-lug and 5, 6, 7 and 8 from the right solder lug, that after desoldering them:
1 is not connected to 2, or to 3, or 4,
but e.g 1 is having 4 ohms to 5,
2 is having 2 ohms to 6
and the same for 3 to 7 and for 4 to 8
Can you still restore the original situation?
Or did you so many changes that you don't know any more how it all was?
Users browsing this forum: No registered users and 35 guests