Experimenting Scan Coil for radar CRT(2014)

I have hook up with a china clone 5FP7 (13SS31D) and 7BP7 radar CRT , Anode is about 1.5KV It able to light up at so low voltage.. There is an old focus coil and yoke/scan coil i have, i realize the focus coil just wrap around the tube neck , focus coil and yoke/scan coil don’t have ferrite core and Normal Television Yoke/scan coil is come with ferrite core..

The focus coil seen like working in high voltage I hook up 300V power supply when i adjust around 150V i able to get a sharp focus spot for 5FP7 and 127V for 7BP7.

I wind my own Yoke /scan coil without ferrite core.. I use a square carton box and wind 200turn using 0.12mm CCA magnetic wire . .. I make 4 of them and put 90degree and parallel 2 of them.. I also wind 500 turn for focus coil.. The form is from toilet paper and soap with varnish to make it harder.

The scan coil work very well.. I able to swing whole screen with 20Vpp for full screen at anode voltage about 1.5KV

The disappointed focus coil I try to push up 27V 0.2A i can some focus but the spot reduce about 20% It seen like it need alot turn to make it work . I didn’t aspect Focus coil is difficult to make then scan coil.

I rewind the focus coil.. Using same wire CCA 0.12mm 5000turn resistance about 1.1K ohm.. I use 300V power supply adjust until about 56V 50mA the trace able to focus, The offset to right i suspect is due to the power transformer beside the CRT.. I ran out of power supply i using additional transformer for the heater and als create a simple sweep.. The vertical i simply use 5uF AC cap to make it like circle..

I really surprise the scan coil and focus coil i can simply make at home.. I have 3 of 5FP7 equivalent and 1 of 7BP7 equivalent and 5AXP4 , but i only have one scan coil so that is the reason i want to make myself.

I have test run longer the focus coil heat up warm.. I not sure is it normal? anyone have experience to use a real focus coil? it not very hot it stay warm.. I have just try for about 5 minutes. I need to adjust about 50V-70V 60-70mA .. the original coil it need to get 150V 20mA to get focus it seen correct for the power. Is that normal for the focus coil to heat up and stay warm?
Original Old scan coil and focus coil.. the focus coil able to focus by supply 200VDC

Above article i write in facebook during 2014 years. I hope one day i can make a TV using this radar CRT .

For the Scan coil ideal, the first i read it from british practical Television 1958 March magazine (backup) when i purchase vacuum tube from Uncle gerry I bought some of this magazine from him.. it show how to DIY that make me have confident i can do myself. I found there is page have all the collection of practical Television magazine.

Another Scan coil information can be found from 9JP1 CRT datasheets.

—————-Update 2020 years ————–
Recently I have a friend from china crystal radio forum Interest about my 13SS31D DIY Deflection coil and focus coil. So i continue experiment to find out more information about it. Myself i’m interested to use this CRT to make a round screen TV monitor which make it look like antique TV, i plan to start at build in Solid state before i go into Vacuum tube.

Above is the Schematic i using , the flyback transformer i wind myself using UYF-11 core and using mylar tape put at the core for air-gap. The deflection coil i series 100turn + 100turn as a pair for vertical and horizontal.

I build the high voltage flyback on breadboard.

Above photo show when i supply 100mA i draw the spot (without focus) and 150mA i draw another spot. I show the distance for 2CM when 100mA apply on deflection coil.

Above photo show 3.5cm when apply 150mA

The Deflection coil didnt make rectangular sweep, This is due to the 2 pair of vertical and horizontal coil didnt place exactly 90 degree..

After re-arrange the coil in 90 degree. I can get a rectangular sweep.

Above video setup According to the schematic. The Final anode voltage is 3KV , the First anodevoltage 250VDC , G1 is -24VDC to prevent burn screen , my tester only can provide maximum of -24V , although -24V it still very bright. The filament lamp is use to series the focus coil to increase the voltage and reduce my 0-300V 100mA internal transistor heat up. The focus coil need 80-90mA to get a good focus.. Since the focus coil is 5000turn , So the focus requre 400A-450A per turn.

I try to calculate if using 0.5mm copper wire 20mm wide of winding area with 1000turn, which require 400-450mA and i plan to run at 12V so i try to find out the approximate resistance which show in the calculator is 13.47Ohm , My supply maximum is 12V / 13.47 ohm which maximum current i can get is about 890mA , while 450mA is about 6V i need a buck converter to adjust the current require to get a good focus.

I found I have 1KG of 0.41mm copper wire , i try to calculate , if using 2000turn , the resistance will be 23.5 Ohm. and require current will be 400-450A/2000 =200mA-225mA , 12V /23.5 ohm =510mA maximum i can get. This seen like i can reduce my power for my focus coil.
—————-Update 28-OCT-2020—-
I add 5000turn more on focus coil which total 10,000 turn by using 0.12mm CCA wire. the total resistance about 2.9K ohm. To reduce the focus coil heat, now it require half of current to get good focus. I can get good focus when anode voltage is 2KV focus current 36.3mA, when 3KV focus coil current 45mA.

I rewind the deflection coil using 0.15mm CCA wire 150turn make 4 of it , each pair is 150turn parallel and total resistance is 27 ohm.

When 2KV , deflection coil current 100mA the spot move distance 13mm , when 150mA is 18mm.
When 3KV , deflection coil current 100mA, distance is 10mm, when 150mA distance 15mm.

The new focus coil 10,000turn with 0.12mm CCA wire total resistance 2.9K ohm.

The setup, the right my homemade tube tester use for supply Va1 =250V and Vg1 up to -24V and i series another 24V battery to make it able supply to -48V to Vg1.
Another small box on left of CRT is 0-300V 100mA supply for supply focus coil . Bread broad is flyback boost setup the voltage is from the left top adjustable power supply. i can adjust the voltage between 5-12V to get around 1.5K-4KV

The vertical coil connect to 12VAC , horizontal coil connect parallel with flyback primary coil.

Horizontal deflection coil connect to power amplifier. vertical connect to 12VAC

The coil is make as above, i using toilet paper troll having 40mm diameter as coil form. The focus coil is wind 10,000turn 0.12mm CCA (copper clab Aluminum) which give total resistance of 2.9K require focus current 30-50mA depand the Anode voltage.. the higher it require more current, also the position of focus coil when toward to screen require less current when toward to tube base it require more current as well. I using CCA wire is not because it is better just i have alot of it, The focus is work as current , so if you using copper wire it will have lower resistance to get same amount of current it require lower voltage of course the heat on coil will be lesser , sometime we need to design the coil to work close to our B+ working so that will be easy to match with our supply voltage. When using lower voltage, we can increase the current such as 10 times current 300mA-500mA and reduce the number of turn of 10times which is 1000turn.

For the Scan coil , i using a paper box as show in diagram i use to wind the coil on top of it , paper box is easy to remove the coil after wind. and the both pair of coil must place in correct direction both must have same winding direction when parallel pair, when series the coil the end of the top should go to start of bottom.

By experiment When Va2=3KV, Va1=250V , when coil 150turn is 100mA the spot move 10mm, to scan whole screen 100mm wide ( the total diameter is 110mm the edge is out of focus only useful area is about 100mm) it require 1A this is equivalent to 150A per turn. Let say we want to make it to work in 500mA full screen we require 150A/500mA =300turn , We can series a pair each have 150turn. in order to work in 12V, the total resistance must be lower then 12V/R > 500mA 24 ohm . When Using audio power amplifier to drive it. when Vpp is 12V , the resistance must be 24 ohm if the resistance is smaller then 24 ohm can series external resistance or reduce the Amplifier output voltage to get Ipeak 500mA . If using Square wave to make sawtooth current. the resistance must as small as possible in order to reduce linearity issue. let say a square wave pulse from TV flyback is total of 64uS and on Duty cycle is 56uS and voltage is 12V the Ipeak require 500mA, So we can use 12V x 56uS /500mA =approximate 1.3mH of total inductance is require , if the scan coil having less then 1.3mH inductance then series a inductance to make it total to 1.3mH by vary the inductance can vary the wide since it vary the Ipeak.

——————- Some chinese CRT datasheets ———-

18SS35D look similar to 7BP7 and 13SS31D or 13SS33D look similar to 5FP7.

—————–29-OCT-2020 ———-
I re design the deflection scan coil again.. This time i want to reduce the wire resistance. I have a spool of 0.41mm copper magnetic wire and instead of 150turn parallel , now i use series to make it 300turn which require 500mA to scan full screen assume 100mm screen wide.

I using the paper box to wind 150turn , i make a mark on start wire.

I make 4 of coil each 150turn.

The coil inductance is 4mH and 4.5mH is too high for 12V . for 4.5mH turn on duty cycle 56uS the Ipeak only 150mA is far below 500mA require. To get 500mA Ipeak the Voltage require 40V . I will keep this coil for vacuum tube version, 40V is just nice for Vacuum tube. I will plan to make another version , reduce to 75turn 0.41mm parallel for Horizontal each coil and a pair will have 75turn parallel inductance expected 200-300uH and Ipeak require 2A . for vertical , i will keep 150turn +150turn . since 4mH is good enough for vertical.

I make a new form and soap in varnish to make the form harder.

The second coil. the focus coil unable to fit, so i wind 1500turn DC resistance measure 36.4ohm , expect require current 200mA-300mA which require 7.28V -10.92V which very close to 12V , by using low drop adjustable regulator or Variable resistor about 20 ohm 5W and above.

The horizontal coil is 75turn parallel 2 of it to form 1 coil and a pair will parallel 4 of 74turn The DC resistance measure is 0.7 ohm , Inductance measure 150uH.

Vertical coil is same as previous 150turn one coil , and series 2 of 150turn coil to form a pair. DC resistance measure 10 Ohm, Inductance is 4.1mH

I Fire up the coil , the focus coil work at 10-13V depand the Anode voltage about 2KV-4KV. Finally i can get full Scan on horizontal. The scan shift to left, when careful position the Scan coil it can make it in middle. Also the sawtooth current is slightly offset up. maybe i have to find a magnet to make it on middle.

I realize the screen have Ion burn mark.

The horizontal current I series a 0.22 ohm resistor as shunt, so each div is 909mA ,Ipp is 2.27A, the frequency is slightly lower 13.38Khz .

The next i will re-design the flyback and make it work at 15V power supply, and output 3KV to CRT anode and also supply 250V for Va1 and Video amplifier. the focus coil work within10-12V , 15V supply will be good enough.

PAL 625 Line ,horizontal frequency is 15625Hz 64uS total of time , 4.7uS flyback time or MOSFET turn off, and 64uS-4.7uS = 59.3uS MOSFET turn on or the Scan time.

I’m using UYF-11 core as flyback transformer for power 13SS31D include 6.3V 0.6A heater , -100V for G1 , +250V for first anode and Video amplifier, 3KV for Final anode.

So my primary using 0.5mm magnetic wire wind 30turn .. Supply voltage 15V

PWM ratio =(59.3uS/64uS) = 0.926
flyback voltage or reflect voltage = 15V x (PWM ratio/(1-PWM ratio)) =189.2V
flyback voltage per turn = 189.2V/30turn = 6.3V
Forward voltage per turn = 15V/30turn x PWM ratio = 0.463V

To design -100V require Number of turn 100/6.3V =15.8turn round up 16turn
To design +250V , 250V/6.3V =39turn
To design 3KV , 3KV/6.3V =476 turn.
To design 6.3Vrms winding i try 4turn and calculate the rms value.
Forward Pulse at 59.3uS is 4turn x 0.463V =1.852V, Flyback pulse at 4.7uS 4turn x 6.3V =25.2V. To calculate rms voltage = sqrt (sq(1.852V) x(59.3uS/64uS)) +(sq(25.2V) x(4.7uS/64uS)) = 7V rms close to 6.3V

The core is add 1 kapton tape as air gap

Due to some mistake , my actual winding is not exactly same with calculation.
my actual winding
Primary = 30turn 0.5mm
Heater = 4turn 0.5mm
-100V = 16turn 0.15mm
+250V =37turn 0.15mm
+3KV = 337turn 0.15mm (actual is 337turn +37turn)

I modify the timing resistor , from 7.5K to 6.8K and 1.1K to 750R , Turn on time 55.4uS , Turn off time 5.6uS total time is 61uS . When i build in PCB i will add Trim pot to adjust it exactly the frequency require.

Actual measurement the Heater output measure by scope rms voltage is 6.1-6.3V. -100V pulse peak at -200V without filter capacitor. +250V winding the pulse 400V , and 3KV winding i get 4KV pulse. all the value measure is without capacitor filtering and load. except heater winding. the current only about 280mA at 15VDC

By measure from my media player video output . The output i connect 75 ohm resistor as Load and measure by scope. The dark level is 200mV and white level 700mV and below 200mV will be synchronous pulse .

Base on the oscilloscope signal i design a Video amplifier and Synchronous seperator circuit.. This circuit i havent build and test..

Today i build the circuit and test, I found that the bandwidth is terrible and 2SC1573 is difficult to bias, also the synchronous separator depend the 2SC1573 +150-200V . so i have modified and tested below circuit have best result. I increase the 1st stage of 2SC945 Ic current to make it have low impedance output to reduce the parasitic capacitor which it limit the bandwidth. I have change the synchronous separator instead of detect level , i use RC constant When have fast raise up pulse it will turn on the Synchronous Separator 2SC945 and output the 4.7uS pulse, this need to go through another Integrator circuit to further filter out Horizontal synchronous pulse and Vertical synchronous pulse.

The 470pF on 2SC1573 work as high frequency compensation. by adjust this value to make sure the 2SC1573 horizontal synchronous pulse look square or can use Sweep generator to get the frequency characteristic .

Above waveform green is from RCA video composite and Yellow is output from 2SC1573. the 2 stage of Amplifier having about 400 gain, which 50mV versus 20V about 52db. The color sub-carrier is able to amplifier , I dont remember my settle for my media player either NTSC or PAL, assume NTSC the subcarrier is 3.58Mhz , at least the amplifier having 3-4Mhz bandwidth Which is good enough for such old CRT.

Above show the TINY media player. i didnt play any video , just the player at interface menu. and black power supply for 12V , and the plastic power supply for 150V. Green trace is input signal directly from Media player, yellow i probe at 2SC1573 output and 2SC945 synchronous separator output.

By adding a Low pass filter to extract the Vertical Synchronous Pulse. The signal is not sufficient to drive 555 Timer Oscillator, if using 555 it need to add emitter follower with a strong pull down to drive 555 reset pin to synchronus 555 sawtooth. [15-NOV-2020 Updated the 2SC945 output for Vertical synchronous is in wrong polarity please refer to lastest schematic.]

I have re-design the contrast and bright control ..

Above is overall Plan schematic.. the next will try to build the PCB and test and tune the component value if necessary. [Update 16-NOV-2020 , this schematic is no longer working.]
Today i have tried to design Simplest sawtooth Oscillator and driver for electrostatic CRT . I using a NPN PNP negative resistance oscillator to generate a sawtooth, I try experiment and try to understand it, before i using 12K voltage divide it was so sensitive when i adjust slightly higher or lower the power supply voltage it stop oscillate. Later i found that this is cause by the latch up due to too high of gain for both transistor. In order to make the gain lower i change the divide resistor to 330R, so this time the NPN transistor require higher Ib in order to pull down 330R, so force the transistor go into more linear region. I add 10K on 2SC945 B-E for fast turn off.

Video above show the green trace is from video composite , the yellow trace is measure the 1nF which is sawtooth oscillator. This video i make before i add the high voltage amplifier.

After i fire up the circuit, i realize the Sync pulse and the oscillate is not sync exactly it should be it retract before the sync pulse start, later i realize due to the ouput from sync separator is go in negative pulse and this oscillator require positive pulse also is difficult to drive through 330R so i add a stage to invert the pulse. I also add a emitter follower buffer before go into high voltage transistor 2SC1573 it work pretty well. output amplitude from oscillator is 6Vpp and output from 2SC1573 is about 150Vpp.

I tried to Simplified to reduce 2 transistor. I found i can add 1.1K resistor in between 330R to 2SA1015.(the circuit write 2SC1015 suppose is 2SA1015) , i use 1.1K is what i have on hand 1K should work well.

The green waveform is video composite signal, the Yellow waveform is output from high voltage transistor.


I put everything into one PCB some circuit is modified base on Scope TV. The Horizontal Oscillator still using 555 timer and MOSFET 20N60 , The video amplifier and Synchronous separator and Vertical oscillator is base on Scope TV circuit. Vertical output is TDA2006 audio amplifier , audio output is LM386 . There is a constant curren for focus coil with OP-AMP LM358 and IRF3205 MOSFET.

Transformer coil put in Bee wax

Core add 10mm mylar tape as Air gap.

I power up with adjustable power supply and monitoring the current. the TDA2006 vertical output amp is no working, I guess is Fake one Temporary i using LM386 to drive the Vertical output. The Horizontal Sync pulse still no working very well, it modulate with 50Hz and make the flyback transformer very noisy. the vertical stil have linearity issue. Total consume 1.5A when focus coil is max and focus coil at mininum is 1A. The heater output is about 8V+ which is too high i use 4.7R 5W resistor to drop it back to 6.5V.

Lastest Schematic as below.

The vertical amplifier using TDA2006 everything work well, the output capacitor of TDA2006 increase from 1000uF to 2200uF the vertical linearity improved.

I using additional power supply to increase focus coil voltage from 15V to 24V current increase to 400mA the focus improved..Video have improve the Synchronous Separator stability was using 100nF with 47K now increase 47K to 82K.

The next i will try 50turn x 3 for each coil and 2 coil form a pair which is 50turn x 6 for horizontal will see will it increase the horizontal scan in full lenght.

I study and refer back the old transistor TV schematic . I realize the driving to Horizontal Output transistor is not drive to full scan , it only drive half of scan screen about 30-35uS this can reduce the power consumption. Another i miss out is the safety capacitor which limit the peak voltage during flyback. After understand about this immediately i modified the 555 resistor to make it turn on 30-35uS CS20N60 have build in diode across Drain and Source this diode can work as damper diode. After modify I series 0.22 resistor Scan coil as Shunt resistor and measure the peak current is approximately 1.1A and Ipp is 2.2A . During testing this flyback is another flyback i re-wind the output to anode only 2KV , lower the anode also increase the Sensitivity of deflector but the Focus quality is decrease, the edge of screen is unable to focus.

I modified Vertical amplifier TDA2006 add a output Negative feedback so it can auto correct the linearity.

I connect up 18SS35D which is 7inch Radar CRT they look like 7BP7 clone. 2KV anode is too low for 7inch CRT, surprise me is it have better sensitivity i though it need more power to sweep full screen.

I bought 2 13SS33D receive today 24-DEC-2020 .

Test unit 1, the yellow long persistant is dimmer then 13SS31D which make it more better for TV purpose.

I using 13SS31D CRT record KIKI delivery to make comparison ..

Today i received 23SS34D 23cm about 9inch Radar CRT . Photo below from left to right is 13SS31D 5inch , 18SS35D 7inch , 23SS34D 9inch.

It come with datasheets.