Function Generator help

[John McGivern]

Hi everyone, Just a novice I bought this generator to use for learning purposes unfortuneately its not working. The power led comes on and all voltages stated on circuit diagram appear ok. I am struggling to follow the circuit if anyone can breakdown the circuit to make it easier to understand I would be grateful. Any suggestions as to what is possibly wrong would also be of help. I do not have any sine,square, triangle waves I replaced all 3904 and 3906 trs. still the same.
I will attach drgs
Many thx, John.

 

[JimB]

OK, a few rambling points and questions.

What test equipment do you have?

Hi everyone, Just a novice I bought this generator to use for learning purposes

Good idea.

unfortuneately its not working.

How do you know it is not working?

The power led comes on and all voltages stated on circuit diagram appear ok.

OK, a good start.

I am struggling to follow the circuit if anyone can breakdown the circuit to make it easier to understand I would be grateful.

I will annotate the schematic and attach it to anothe post.

Any suggestions as to what is possibly wrong would also be of help. I do not have any sine,square, triangle waves

How did you determine that?
At an inspired guess, this thing is 10+ years old, one problem could be that the switches are "dirty".
What happens is the switch contact tarnish and dont make contact any more.
The usual cure is to spray them with switch cleaner and operate them several times, this breals the tarnish film and makes contact.
If you have contact cleaner use it, if not jsut operate every switch a few (ten?) times and see if it starts working.

I replaced all 3904 and 3906 trs. still the same.

Why, what made you suspect them rather than any other component?
Mass replacement is a poor fault finding technique, you risk introducing more faults.

JimB

 

[large_ghostman]

i cant find the link but ages ago i watched a good you tube vid on repairing things and it gave a list of the most common faults for things and like jim said its topped by dirty switches and then connectors being loose and things like that plus it dosnt cost a thing to unplug a connector clean it and replace you might also be lucky like i was with my scope that i thought was broke and turned out to be a fuse after all that one tip you will probally get told alot is look for bulging caps
good luck LG

 

[JimB]

Circuit sections annotated as per tha attachments.

How some of those circuits work has me scratching my head at the moment.
Others here may have more idea.

JimB

 

[John McGivern]

Hi everyone thanks for the replys.
JimB answers 1 Oscope and Dmm.
2 No amplitude on scope when FG connected double checked with Dmm.
3 Put scope on s9, s10, s11 no voltages.
4 No amplitude on FG so replaced output Trs.

All electrolytics have voltages.
Is C12 the timing cap? Incidently it has no voltage on it its a polystyrene cap cannot make out rating. Being just a beginner its quite confusing these drawings.
Hope this helps.
Best regards, John.

 

[JimB]

OK.

No, C12 is not the "timing cap".
C1, C2, and C4, in conjunction with R12, R17, R18, R19, R20, R21, R22 and R23 appear to determine the frequency.
All squashed up on the drawing as it is, I am struggling to understand how this oscillator works. It is a configuration that I am not familiar with.

As you ahve an oscilloscope, look at TP9 and see if there is anything there.
If not, move back through IC6 (IC6 is shown on the drawing as six individual transistors) and TR5.

If you find nothing at the gate of TR5, move back into the oscillator.
Check the voltages at TP7 (+15v) and TP8 (-15v).
Check IC1, IC2, IC3 and IC4.

Did you try exercising all the switches as I suggested before?

Have fun.

JimB

 

[KeepItSimpleStupid]

The #1 thing to do is "inspection"
The #2 thing to do is to check the power supplies; both values and ripple

With this sort of system you can use signal tracing or signal injection usually in a halfing fashion. (binary search). Is it present half-way. No what half. trace half of the half etc.

So, I would use something easy like 1 Khz and check for sine, square and ttl. Verify it doesn't work in all ranges. That's part of the verification.

Then check for a signal at the output of the oscillator.

 

[atferrari]

Divide and conquer

Have you an user manual? If so, read it first. Maybe you have to flip a switch to actually enable the output.

If the power supply seems OK, check if you have any input to R112 (or output from VR17). That will allow you to know where to look for next.

Sorry to tell but changing those transistors was pure nonsense. If by chance you create an additional fault, odds are against you.

Please tell the outcome.

 

[wkrug]

Ok, the Voltages are checked?!

I think the Oscillator only will work, when the Sqarewave Shaper is proper too.
Lock at the Line "A" the goes from the Output of the Squarewave Circuit to the Output of the Oscillator.
How that thing will work - I dont know exact.
But i think the Diodes D1...D4 are a Diode Switch.
IC1 and IC2 with the Transistors are a symetrical constant Current sources.
The Current can be choosen by the Pot VR1.

IC3 and IC4 and TR3, TR4 look's like a Constant Current Source too.
The whole "Oscillator" Stage is built symetrical.
I think that would be done, to make the Duty Cycle adjustment possible ( IC1, IC2).

But there is no foldback from the Timing Capacitators to the "Oscillator" Stage.
So i guess the Squarewave Circuit is needed for proper oscillation too.

The Output A of the "Squarewave" Section switches the Diodes in the rectifier pairwise on and of.
So the timing Capacitators will be charged and discharged by the 2 Constant Current sources IC3 and IC4, appending from the IC1 - IC2 settings. When the Switching point of the NE527 is reached the Output of that chip toggle's, the Rectfier Diodes swich over and the Cycle starts again.

What can you do.
First check all switches in the Oscillator section for proper contact by an Ohm Meter.
Check the Voltages on TP4, TP5 and TP6. They append on the Setting of the POT VR1
Next Check TP2 and TP3.
The Voltage there appends of VR1 and VR6, when the Duty switch is ON.
Be a little careful there, it is possible that there are suspect voltages, because when the switching diodes are on the opposite site of that current source there is no Load on the Output of the Transistors.
When measure some insufficient values, check the Voltages at the In- and Outputs of the According OP if they are plausible.

Next is checking the Squarewave Stage.
Get the Datasheet of the NE527.
Measure the Voltages at the Inputs 3 and 4.
The Voltage at Pin 4 should be around 0V to GND.
Does the Outputs have according Potentials?
Try to insert a little positive and negative Voltage at Pin 3 to test the switchover Function of that Chip.
The Switchover should be shown at the "A" Output.
Test the input Amp of the Squarewave Circuit for proper work ( TR5 and IC5 ) . Check the Voltages at all Transistors there.
Check all the rectifier Diodes by an Diode Tester ( Multimeter ).
When all that Parts are OK the Oscillator must work.

When measured that and there are some insufficient Values you can't explain, ask here again.

 

[John McGivern]

Hi JimB, Here are the results. Lubed all switches still the same TTL has 5Vdc. The results were in a table but when pasteing lost format but I think you will be able to understand the readings.

I was confused with Tr5 as it was split into 2 Trs on the circuit drawing then realised it was 3pins on each Trs on the circuit but only one actual 6pin transistor hope this helps JimB.
Thanks again, John.

Tr5 is a 6pin dual N-Channel-JFET
Gate 1 = 0Vdc Drain 1 = 450mVdc Source 1 = 15.33Vdc
Gate 2 = -15.34Vdc Drain 2 = -14.86Vdc Source 2 = -525mVdc



TP9 0Vdc
TP7 + 15Vdc
TP8 - 15Vdc

IC1 IC2 IC3 IC4
Pin 1 -15Vdc -15Vdc 0Vdc -23Vdc
Pin 2 0Vdc 0Vdc 10.87Vdc -10.89Vdc
Pin 3 0Vdc 0Vdc 10.87Vdc -10.89Vdc
Pin 4 -15Vdc -15Vdc 0Vdc -23.68Vdc
Pin 5 -15Vdc -15Vdc 0Vdc -23.56Vdc
Pin 6 - 2Vdc 1.979Vdc 10.22Vdc -10.25Vdc
Pin 7 +15Vdc +15Vdc 23.40Vdc 0Vdc
Pin 8 0Vdc 0Vdc 0Vdc 0Vdc

 

[John McGivern]

The #1 thing to do is "inspection"
The #2 thing to do is to check the power supplies; both values and ripple

With this sort of system you can use signal tracing or signal injection usually in a halfing fashion. (binary search). Is it present half-way. No what half. trace half of the half etc.

So, I would use something easy like 1 Khz and check for sine, square and ttl. Verify it doesn't work in all ranges. That's part of the verification.

Then check for a signal at the output of the oscillator.

Hi KeepItSimpleStupid, dont have a signal injector
Many thx for replying, John.

 

[John McGivern]

Hi Divide and conquer, thanks for replying will let you know the outcome, Cheers John.

 

[John McGivern]

Hi wkrug, thanks for replying I am just a beginner so some of the info you supplied is a bit above my understanding sorry for my ignorance but will post info here when I complete the readings. Many thx John.

TP2 = + 10.88Vdc
TP3 = - 10.88Vdc
TP4 = -1.372Vdc
TP5 = 0Vdc
TP6 = + 1.336Vdc
All diodes ok.

 

[JimB]

Sadly I have no more ideas as to what could be wrong, but I have had a look at the circuit and think that I understand how the oscillator works (maybe ).

IC1 and IC3 are a positive current generator.
Resistors R18, R19 and R20 set the current range.
VR1 adjusts the current.

IC2 and IC4 are a negative current generator.
Resistors R21, R22 and R23 set the current range.
VR1 adjusts the current.

The positive current generator charges one of the capacitors** C1, C2 C3 or C4 (depending on the frequency range).
The rise in voltage across the capacitor will be linear because it is a constant current charge.

Via TR5 and that cluster of transistors IC6 the voltage gets to IC7 which is a comparator.

When the voltage reaches some value, the comparator output changes state.
This switches* the current generator which is charging C1, C2 C3 or C4, to the negative current generator.
The voltage on the capacitor falls in a linear manner.

Again when the voltage at the comparator falls to some value, the comparator output will change state and we go back to putting positive current into C1, C2 C3 or C4 again.

So, at C1, C2 C3 or C4 we are generating a triangle wave.
The output of the comparator IC7 is a square wave.
And just how the sinewave is created, I dont know yet!

* The comparator switches the charging current via signal line "A" and the diode ring D1,D2,D3,D4.
** IC5 seems to do something strange on the low frequency range. It seems to act as a synthetic capacitor.

My brain now hurts, I must rest it!

JimB

 

[wkrug]

When I take a look at the Voltages you measured i think this Part of the "Oscillator" Circuit is OK.

Now lets take a look at the Point "A". There must be a Voltage about +9.5V or -9.5V.
Measure additional the Voltages at IC7.
Pin 3 and 4 must have about 0V. Pin 9 and 11 can have +10V or -10V, depending with Voltage is higher on Pin 3 and 4 of the NE527.

Test it, and Post the Results, please.

@Jim B
I think Your function description is near to mine. So I guess we are on the right Way.

I think the sine shaper works with the Transistor Array b.
It looks like a simple Current Mirror with exeption of the Resistor R54.
I think that Unit cuts the peaks of triangular Wave.

Edit!
I've found an Application Note at the TI Pages: https://www.electro-tech-online.com/custompdfs/2012/05/snoa665b.pdf
At Page 12 is a very similar circuit for sine shaping with description.

 

[KeepItSimpleStupid]

Question: Why are you guys concentration on DC voltages when a scope is available?

i think this whole "Oscillator" Circuit is OK.

With a scope, you would KNOW if the osc is OK, then use DC voltages to narrow down the part.

Besides using a DC voltmeter to check the power supplies FIRST, IMHO you should switch to scope troubleshooting.

John: I know you don't have a way to inject, but as a general trouble shooting method it was valid to mention. You do have a scope.

 

[wkrug]

Why are you guys concentration on DC voltages when a scope is available?

Because John McGivern said the Oscillator does not oscillating.
So the Part's of the Oscillator are in a static state.
To figure out which part causes the error we suggest to measure the Voltages around the Circuits.
Because the Voltage difference between Base and Emitter of a Transistor is hard to differ with an Oscilloscope.

i think this whole "Oscillator" Circuit is OK.

I meanth the current sources of the Oscillator that was named "oscillator" in a few posts before.

It's clear that the complete circuit between the Switching diodes TR5 and TR7- TR8 is also a Part of the Oscillator.

I think the failure is between D1...D4 and the Output A, because the Voltages of the costant Current sources Measurements looks good, but the Oscillator doesn't work.

When putting a Voltage with + and - 5V to the Pin 3 of IC7 via a Resistor ( 2k2 to 4k7 ), the Output A must toggle with that Voltage.

When that works it look's like the failure is in the Transistor AMP between T5 and R33.

@John McGivern
You're shure?! at the Output "A" is no oscillation visible?! ( Oscilloscope! )

 

[atferrari]

Question: Why are you guys concentration on DC voltages when a scope is available?

See my suggestion in previous post, to check for input to the amplifier at R112.

 

[John McGivern]

Hi again wkrug, There are some components on the circuit board not on the circuit diagram. If you look to the left of the light blue inductors there are three silver coloured components one is marked 930 FZ I would have to remove the others to try and identify them think they are polystyrene caps but as they are not on the drawing I cannot give you any info the reason I mention this is because the (930FZ) component is connected to c12/d2/d4 node. I think it could be a fuse. There are existing links one wire link and 2 using resistors I have tested these for continuity and ohms all seem ok.

NE527N pin9 linked via 100ohm resistor to DN74LS37 pin13.
NE527N pin voltages. Pin 1 = +10.27Vdc, Pins 2,3,4,5,7,10,12 all = 0Vdc, Pin6 = -10.32Vdc, Pins 8,13,14 all = 4.94Vdc, Pin9 = 1.83Vdc, Pin = leg been cut off IC 1.77Vdc on piece thats left. Hope this helps regards, John.

Check this out Lads

The way the triangle waves are made involves putting constant currents into a capacitor. First the capacitor is charged, and when it reaches a predetermined threshold, the polarity of the current is reversed, and the voltage heads down towards the negative threshold, at which point the current is once again flipped and the cycle repeats.

The comparator that determines when the positive and negative thresholds are reached outputs a square wave, which controls the current to the capacitor. This square wave is also available as an output.

To make a sine wave, the triangle wave is deliberately distorted, taking advantage of the exponential relationship of VBE and IC in a transistor.

Looking at the diagram, IC3 is the positive current source and IC4 is the negative. The op-amps before these control them, summing the frequency control and the external input. These currents are fed into a diode bridge, which is used to determine whether the positive or negative current reaches the timing capacitor.

You'll note that the frequency decade switches are able to change the capacitor being charged (C1 to 4), and also the value of the resistors in the current sources. By altering the currents, you need fewer capacitors (large ones get expensive, so reduce the charging current instead). IC5 is almost certainly being used as a capacitance multiplier instead of a 10u non-electrolytic device. You'll also note the duty-cycle control, that alters the balance between the currents...

The output from the capacitor goes to the FET (TR5). A high impedance is required here, as any current taken will distort the triangle wave. The following transistors in the CA3086 form a conventional amplifier, the output of which is on TP9. It goes to 3 places; the comparator (IC7), the sine converter (bottom right) and the waveform selector switch.

The comparator is a conventional enough circuit, with the output being amplified and buffered and fed back to the diode bridge (arrow A). The output from the chip is also fed to the TTL buffer and the wavechange switch.

The amplifier takes the "mixsig" from the switch and deals with it in a conventional manner, and has constant-impedance attenuators dealing with the output. Similarly, the PSU is entirely conventional, but perhaps not at glance if you're more familiar with 78xx regulators...
John got this from another forum, hope it helps. John. I cannot take it onboard not enough experience but will keep going over till it sinks in ha ha.

 

[wkrug]

Pin9 = 1.83Vdc

I think that's not OK.
First Check if there is an oscillation visible ( Oscilloscope! ). It could be a measurement error too.
The Output Stage Of IC7 is enabled by tieing up Pin 13 and Pin 8 to +5V.
That looks OK. Pin 14 is the supply of the digital stage of the IC, that looks OK too.
Supply at Pin 1 an 6 are present.

So the Output Stage of IC7 should make +5V or 0V at the Output 9 and 11.
But there could occur an overload from external components.

You have 1,83V there.
What Voltage is at the Emitter of TR6?
It should be around 1,2V.

There can be a failure of Tr6, Tr7, Tr8, D9, D10 or IC7.
Thinkable is also a failure at the circuit around Tr11.
Is there a Connection between Pin 9 of IC7 and the 100Ohm Resistor near VR11?
When yes solder out that resistor and measure again at Pin 9.
When that not change the Pin 9 Voltage solder out TR6 and measure again.

When than the Voltage increases then to +5V, put a Resistor with about 4,7kilo Ohm between Pin 9 and GND. When the Voltage break down again the Output Stage of IC7 has an error.
The IC has to be replaced than.

NE527N pin9 linked via 100ohm resistor to DN74LS37 pin11.

Really to Pin 11 of IC9 not to Pin 13??? Because Pin11 of IC9 is an Output.

Inform us, whats going on, please.