varioprop 12s repair

[dr pepper]

I see Nige, isnt that a switch, or is it more of a selector.
Sounds like they are building a frame, with pwm pulses from each channel being generated by switching or selecting through its relevent control pot to the multivib, presumably theres some sync pulse or delay somewhere on the way.
Going to add the circuit to my scrapbook, I wouldnt use the ttl chip, but maybe the rf stages will be handy with a pic or arduino.
Not sure what you mean by that vari21, couldnt find anything on this page relevant.

 

[Nigel Goodwin]

excuse me what type of capacitor should i use ,how to make that please do you have a diagram or something ,sorry my electronic level is low.

You don't need to 'use' a capacitor, they are already in the circuit (one for each channel).

 

[vari21]

so how can i replace that ic chip , is there any ic equivalent or any other solution like puting transistors on each chanel instead.

 

[Nigel Goodwin]

so how can i replace that ic chip , is there any ic equivalent or any other solution like puting transistors on each chanel instead.

Why do you want to replace it?, what makes you think it's faulty?.

 

[Les Jones]

As the supply voltage to the IC is only 3.9 volts I think finding out why that is the case would be the best way to proceed. I suggest measuring the voltage across the 6.2 volt zener (D103) and measuring the voltage across R122 to get an idea if the circuit is drawing too much current or if there is no voltage across it then the emitter follower T110 is faulty with an open circuit base collector junction. I had difficulty at first trying to understand how it worked as my brain was fixed on the idea of a positive pulse traveling through the shift register but I now think it is a negative pulse so that only the selected pot is controlling the capacitor discharge . (As all the other outputs need to be high so that only one diode is conducting. ) I have not yet worked out the rest of the way it functions.

Les.

 

[Diver300]

No, not as such - it's similar to reading a pot on a PIC without an ADC - you charge a capacitor and measure how long it takes to do so. In this case you're actually wanting timed pulses, so no 'measuring' involved, just the outputting of each pulse in turn. I recall a similar chip been used a number of decades ago in a magazine project for an RC system.

So it's essentially a simple ADC, created by waiting for a capacitor to charge to the switching threshold of the chip - so using LS or HC versions 'might' not work, as they probably have different threshold voltages?.

It's not an ADC, it doesn't digitise anything. It is a resistance to pulse position converter. Neither the resistance nor the pulse timing is digital.

Also, I don't think it is using the switching threshold of the IC. The threshold is provided by T107 as far as I can tell.

The variable resistances are connected to pins 3,4,5,6,10 and 11, via diodes. On a 74164 all those pin (plus pins 12 and 13 that aren't used here) all outputs. The 74164 is being used to select which of the variable resistors is active at any one time, so just one variable resistor controls the pulse timing.

I haven't followed the circuit exactly but I assume that each resistor is used in turn to create a string of pulses where each pulse spacing depends on just one resistor.

I saw another radio control transmitting circuit that used a CD4017 in much the same way. Here are examples:- https://norcim-rc.club/Radio6.htm

I don't know why the design didn't use a CD4017, which would seem to be the more obvious choice.

 

[vari21]

hi nigel and les i ve made some measurements with my digital multimeter set on DC
with black probe touching the ground of the radio.
d103 cathode 04.58 anode 00.01
t110 base 04.57 emiter 11.37 colector 03.94
resistor122 11.34 11.61 i ve measured it both sides.

 

[Nigel Goodwin]

hi nigel and les i ve made some measurements with my digital multimeter set on DC
with black probe touching the ground of the radio.
d103 cathode 04.58 anode 00.01
t110 base 04.57 emiter 11.37 colector 03.94

You've got collector and emitter wrong way round.

resistor122 11.34 11.61 i ve measured it both sides.

That's only 3mA through the resistor, so it doesn't look like there's too much current been drawn through the regulator transistor. There's also a fairly 'decent' 0.7V between base and emitter of the transistor, which suggests the transistor is OK - and makes me suspect the zener diode (d103) might be leaky?

 

[vari21]

sorry i did nt mention that i ve already changed the diode d103 i put 6.2v instead, i also changed the t1110 with bc238b t102 with bc238b t103 with bc238b also t104 with bd400 why i ve change them because i suspected them ,i did nt mention that the r108 gets very hot before changing t104 sorry about that.
i will post some picture of the pcb

 

[vari21]

here are some pictures of my pcb as we can see original t104 bd400 that i changed with bd137 , a picture of the original d103 and the new 6.2v diode soldered with a picture of the transistyor i ve changed t103 t102 i replaced them with bc238b
t101 i replaced it with 2n2222 i could did something wrong.

 

[Les Jones]

It looks like you have multiple faults. r108 getting hot would have been caused by t104 having a collector emitter short so you ave fixed one fault. (I is possible that it may have also damaged Dr101 in its collector circuit as it would have been passing about 250 mA.) Back to 5.5 volt power supply problem. I assume that when you say you replaced t1110 you mean t110. Can you clarify what you mean when you said "i put 6.2v instead, " (According to the schematic that is what it should be.) As you have replaced D103 and t110 the readings do not make sense. Can you post pictures of that part of the board from both the component side and the etch side so we can check that you have fitted t110 correctly. (D103 must be the correct way round otherwise the voltage across it would have only been about 0.7 volts.)

Edit. I have just seen the picture s that you have just posted.T110 does look like it is fitted correctly. (Unless you used a BC238L which would have a different pinout if it exists.) Can you disconnect the base lead of t110 and re measure the voltage on the top of D103 with respect to ground and also measure the voltages with respect to ground on both ends of r122.

Les.

 

[vari21]

sorry i made a mistake when i said t1110 i mean i replaced t101.

 

[vari21]

d103 04.57 with t110base lead disconnected(desoldered).
r122 11.62 11.62 on both sides with t110 base lead disconnected (desoldered
i attached a picture of the t110 .

 

[Les Jones]

It looks like the zener diode is faulty. Try replacing it again or if you do not have another spare you could try connecting two forward biased small silicon diodes in series with it. The cathodes (End with the band.) towards the zener. Almost any silicon diodes you have in stock should work. (1N4148, 1N914 or even any of the 1N400x series of diodes.) Do not reconnect the base of t110 at first. Between ground and the anode of the top diode you should measure about 5.8 volts. If you get that reading you can re connect the base of t110 and check that the reading is still about the same. Then test the voltage between ground and the emitter of t110. It should be about 5.2 volts.

Les.

 

[dr pepper]

I get it now, the 4 pots are scanned generating pulses, then theres a 4 pulse space of blanks, no pulses.
The rx has a r/c timeout circuit that resets the decoder via a diode during the 4 pulse blank period, always keeping the tx an rx in sync.
Elegant.

 

[vari21]

i used this one DIODE ZENER 1,3W 6,2V VISHAY tolérance 5%
the cathode soldered to t110 base, anode to the ground.
i have another brand new, i will solder it and take measurements. if it is the right diode.

 

[Les Jones]

As the current through the zener diode is only about 1.5 mA a 1.3 Watt zener diode may not give the correct voltage. You would need to give the exact part number so we could look at the data sheet for it. As you have one try it first without t110 base connected and see if you get close to 6.2 volts.

Les.

 

[vari21]

This is the original d103 , i took the picture from my another working varioprop12s is written on it ITT 700 or something but im not sure. i dont know the part number of the new diode bought it here.

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[vari21]

this bis the diode im going to solder.

 

[vari21]

hi les jones i took off the diode which was on, and replaced it with a bran new one result is
i ve got 05.99 on the d103 cathode with respect to the ground. recconected the t110 base, test with multimeter set on DC black probe touching ground
t110 emiter 05.33
t110 collector 11.27
t110 base 05;99