Problem with signal generator

[caesparza]

The two C1815 read 0.6V between Base and Emitter. Fet reads 0.6V between Gate and Drain.

Helmholtz why the Gate of Q101 should sit at 0VDC? Is it not supposed to receive the signal from the Wein Bridge?

The range switch works, you can listen how the pitch on the output changes as soon as you switch to another frequency range.

[Helmholtz]

Helmholtz why the Gate of Q101 should sit at 0VDC? Is it not supposed to receive the signal from the Wein Bridge?

DC voltage at gate should be zero because it is referenced to ground by one of the resistors on S101B.
Signal is AC and should not produce a positive gate voltage.
But as said, some meters cannot correctly separate AC from DC.

[caesparza]

I have to correct myself, the range switch is not changing the velocity at all, in fact the output keeps coming unaltered with changes in the range switch, even though you can hear a difference in the sound.

What voltage should be expected at the Base of Q103?

[Chuck H]

Ok... Not to be cavalier about it, but...

I have that unit. Mine is a Leader model, but it's the same thing. I think they were made for a couple of decades under a few different brands. You can find them every day on (e)bay in good working order (age or how soon power supply caps may need service not withstanding) for about $120 USD or less. Just saying that unless this is personal, as in "I'm going to fix this thing just because." then it seems like a lot of time and effort for a very common and available piece of bench gear.

FWIW mine continues to work fine. Though I don't know it's actual age I don't expect it to last much longer without maintenance, but who knows. If it needs more than power supply caps or something easy to troubleshoot I'll just replace it. Especially if the matter became complicated at all or I thought an outside repair service was needed. Then I would certainly just replace it. Other things, like guitar amps, are just more gratifying to repair and service for me.

[caesparza]

Hi Chuck H,

I'm taking the unit as a learning experience, I have never worked with transistors before and the circuit seems simple enough to try to fix it. The unit worked in the past but after a supposed fix, it went back to the state it is. I really like the unit, the quality of the components and the fact that it is an old piece of equipment. I love to see those boxes in working condition. Just want to give it a last try (I already have a Tektronix CFG280), as a challenge, I guess you understand.

[Chuck H]

I guess you understand.

I do. And to be honest I've put time in on things less worthy of the effort than your audio generator. But when I read that you had a tech work on it I wondered if the bill wouldn't be almost as much as the cost of replacement. With a little patience and searching I got mine for $46 about ten years ago.

[Helmholtz]

What voltage should be expected at the Base of Q103?

Apart from the gate voltage all other transistor voltages look good.
It is essential that the output i.e. Q102 emitter roughly sits at 50% of supply voltage to allow for sufficient signal swing.

Q101/102/103 constitute a simple linear audio amplifier, Q101 being a high impedance input stage and Q102 and Q103 are a kind of push-pull (SRPP) output stage.
The NFB path is C105, VR102, TH101 (is the bulb good?) to Q101 source, which is the inverting input.
The parts to the left outside the black frame constitute the WIen bridge, which lies in the positive FB path feeding back to the gate.

If you disconnect the gate from the Wien bridge and wire a gate reference resistor of 1M from gate to ground, you can easily test the amplifier with a signal fed to gate and scope connected to the negative end of C106..

[caesparza]

Helmholtz I checked the resistance on Q101 Gate. It varies according to the selector. I don't get the 0VDC voltage on it, isn't it supposed to have some voltage drop on that resistance?. I measure the VDC at the Gate and get different values according to the range switch, the first two values are .2 and .5. I can't get a reading on the upper two ranges on my fluke 27.

I checked power supply with the scope and I'm getting 30VDC with 8mV Pk-Pk VAC, so that is ok. Checked the thermistor out of the circuit and works properly. Checked the caps for leakage and they tested ok.

Maybe a cold solder joint?

[Helmholtz]

..., isn't it supposed to have some voltage drop on that resistance?.

I don't see where the necessary positive DC current could come from, except if there's a leaky cap
Or is the gate voltage negative?
In any case carefully clean switch contacts sparingly using Dexoit D5 and blow out rotary capacitors with compressed air. Also Tuner spray may help here.

I can't get a reading on the upper two ranges on my fluke 27.

No idea what you mean with no reading.


Check the amplifier as suggested above.

[caesparza]

Helmholtz I just saw your last message, will try to test that FET the way you mention and inform the results, thanks for the clarification about the circuit and for your time.

[caesparza]

Helmholtz thanks for the SRPP circuit reference, I was not familiar with it and I had doubts about the way Q102 was connected, the amplifier seems to be alright. I attached three scope images form the following test points.

Q103 Base.
Q103 Collector
C106 negative side.

I checked High-Low Switch and volume control as well, they are working fine. I guess I could check the Saw Tooth part with this signal. So the problem seems to be on the Wein Bridge configuration?

[Helmholtz]

I attached three scope images form the following test points.
Q103 Base.
Q103 Collector
C106 negative side.

Looks good.

I guess I could check the Saw Tooth part with this signal. So the problem seems to be on the Wein Bridge?

What saw tooth? The rest of the circuit is a square wave converter (Schmitt-trigger). And yes, it should work with your signal.
As you say, the problem must be with the Wien bridge: Components or contacts.

Please not "Wein", sounds like wine to me.
The name of the German inventor was Wien (like the capital of Austria), pronounced "ween".

[caesparza]

Sorry the square wave, which is working too.

I decided to play with R109 (since that one didn't correspond to the schematic value) and connected a parallel decade box, strangely enough I started to get a more sine sort of wave when I decreased dramatically the value of the resistor. The images attached correspond to a R109 value of just 70 ohm.

So I think that the problem right now is the calibration of the Wien bridge (sorry about that, before).

[Helmholtz]

As the amplifier works fine with an external signal I see no sense in modifying it.
Both 3.3k and 5.6k should work fine for R109.
Lower values reduce Q101 gain and may cause distortion.
Total amp gain should be adjustable via VR102.
What is the resistance of TH101?

Did you restore the original ecap values?
If you reused old caps, are your sure they are not leaky?

[caesparza]

Thermistor reads around 14K. I tested outside the circuit and is working properly. I checked the caps C103, C104 for leakage with an IT-28 Cap Checker, installed 22uf 100V. C105, C106 are new Nichicon Caps.

The only thing I haven't tested is the VC104 40p variable cap.