PINTEK FG-30 function generator issue

[MetalHead]

Hi!

here is my issue, hope someone can help!

My function generator started adding parasitic "step" when signal is crossign 0 (changing polarity). That "step" can be seen on attached image and it distorts the sinewave. When frequency is increased the stronger is this parasitic "step". It appears regardless of the chosen function sine, triangle or sawtooth...  But when I add DC component to the signal and shift it up/down more than the amplitude of the signal itself, the "step" disappears - https://youtu.be/Y0fT9oYq1is

[Benta]

Hmm.
I've seen the same phenomenon in Class-B audio ampifiers when the output stage quiescent current is set too low.
That's where I'd look first.

How does it look when you load the FG output with 100 or 50 ohms?

[Manul]

Looks like crossover distortion to me. Output stage biasing problem? Hard to tell what exactly happened, I guess you need to poke arround the output stage and check things.

[Benta]

@Manul: we're on the same page

[Manul]

Roughly, here is idea of class AB output stage:

1. Steady state current is controlled by the bias voltage between two bases. More voltage equals more steady state current and less crossover distortion (it essentially controls how much "A" is in "AB"). I would start by measuring this bias voltage.
2. Common way of stabilizing bias are the diodes between bases. Could be 2, could be 3. Check them.
3. There could be a capacitor between bases, including small electrolytic. It's leakage current might lower the bias. Important to check that.

There could also be signal coupling electrolytic capacitors, might check them too.

All this is just general information, your device might differ.

[MetalHead]

Thanks for your suggestions!

Tried to attach a load 100 resistor - did't help, same problem. So I decided to look what's inside. At first glance noting wrong, nothing looks like burned... visually. But definitely something wrong.. So, here is how this FG looks from inside



What I have however noticed that on couple of BJTs inside on some legs are signals like that



(this is for square signal, same s# happens for other functions)

Is that OK at all?  Because to me it looks like there is some high frequency parasitic resonance in circuit.. The frequency selected on front panel was 150kHz

[Benta]

The ringing I'd leave out of the equation for the moment, it could be caused by the 'scope probes.

The output amplifier seems to be in the lower right corner of your picture (the transistors with the "star" heat sinks), and I'd start looking for bad solder joints in that area and perhaps even resolder the joints there (remove the old solder and use fresh eutectic).

Could you provide a zoomed, detailed photo of that area, please?

[MetalHead]

Thanks Benta!

here is the picture. Yes, that part of the circuit is responsible for signal amplification and bias. Also there is output resistance of 50 (even two)

Also there is BJT on picture that has white dot at the top - Q2. Its emitter is connected with emitter of adjacent transistor - Q3. The signal on base of transistor Q2 has proper form (sqare, sine or whatever I turn with the knob), but the signal on base of  transistor Q3 has those "damped oscillations". What makes me think that scope probe is not the cause of the ringing.

I will check the soldering from the bottom of PCB and write the result.  That's a good starting point!
Thanks again!

 

[MetalHead]

UPDATE!! Resoldering all components on PCB helped to solve "rinding" problem. Factory soldering was poor.

But the original problem remained - transition of the signal from positive to negative and two transistors with star heatsinks probably are responsible for that. As I mentioned before it's frequency dependent - higher frequency higher distortion..

Also on photo  there is blue variable resistor (or capacitor not sure) marked as "3Mhz"  and when I use screwdriver and turn it, "step" in signal transition turns into small "ringing".



Any idea?

[Uunoctium]

That circuit design looks very close to the 50yr old Wavetek 142.
I added an detailed description of output stage and schematic

[Benta]

Also on photo  there is blue variable resistor (or capacitor not sure) marked as "3Mhz"  and when I use screwdriver and turn it, "step" in signal transition turns into small "ringing".

That's a variable cap and the only one I would NEVER have touched. Sorry.

[MetalHead]

Ok guys, 

I checked all diodes and resistors in that corner of output amplifier and they all seems to be ok. I see noting wrong with them. I tweaked all variable resistors on PCB to make the form of signals as close to their ideal shape as I can and probably I will leave it as it is.

Thanks everybody for contribution and all help in solving this problem. At least it's better as it was before. Resoldering components by advice of Benta helped to solve the "ringing" issue and by turning small blue capacitor I found the condition where the picture looks acceptable for my needs

[MetalHead]

Not the end of this story! 

One of two BJT transistors with a star heatsinks, namely PNP had base-to-emitter open!! Also 0.5W resistor connected to emitter of that transistor was overheated so it had 430 instead of 450

Problem is now that I can't read a marking of that PNP transistor... 
What can be the complementary pair for BSX61 transistor to work in output stage amplifier circuit??

[Benta]

I doubt if that's the problem, I bet your complementary output transistors are OK. You 'scope pictures showed symmetrical output, which would not be the case if one is dead.
I would look at D4/D7, which seem to be the biasing diodes for the output stage.

430 ohms is well within +/-5% tolerance. I see no problem there.

[MikeK]

Some info if the OP doesn't already know it: A transistor needs about 0.7V from Base to Emitter in order to turn on.  This means that if you apply a signal to the base the output will be off until the input is greater than 0.7V.  This is crossover distortion...One transistor handles the positive signal, another handles the negative signal and there is a dead zone when neither transistor is on.  One design solution to this is to use diodes.  If the diode(s) are bad then you have the crossover distortion problem.  And this is probably why adding offset "cures" the problem.

[MetalHead]

Yes, but this transistor is not good for sure. I get 0.677V diode drop from base to collector and "OL" from base to emitter. Upper transistor (NPN) is good. Also all diodes in that corner of circuit board as well as small BJTs all have proper diode drops.

[Benta]

Forget those multimeter "diode testers", they're generally crap.
More interesting would be the DC voltages around the output stage.
And a photo of the lower side of the PCB in that area.

[MikeK]

Manual has a blanked out schematic.   

https://d17bck4wpaw2mg.cloudfront.net/att/a/0/1/l/a01lq6/fg30-32.pdf

[Uunoctium]

tl;dr


Pls measure the voltage drop across R96, which will gives the idling current.

Edit: read an wrong resistor value 

[Uunoctium]

i am wondering whole time, why in an actual model (FG30) an semiconductor with DC 11/95 is installed?

BSX61 is mentioned in Valvo Datebbook 67'.
Fast  transistor for switching applications eg. magnet core memory. No corresponding PNP complementary available.
That's not an ideal choice for an linear amplifier.

So my question, has Q13 been replaced during a previos repair?
2N2219/2905 much more suitable, also BC140/41 BC160/61

Found an very similar schematic (from GW Inst. 8020g)

[MetalHead]

Uunoctium,

That might be the case!!! Factory seal was broken when I bought this FG, but I didn't care as everything was working fine...

What I figured out this evening is that another transistor that stood in pair with BSX61 seems was 2N2905A. I managed to read what left from some letters -  "2 05  A". So your guess probably is right, instead of BSX61 there has to be 2N2219A NPN transistor with its complementary transistor 2N2905A 


Going to order the pair 2N2905A/2N2219A and put them in place. Will see what happens!

[Uunoctium]

 
if you are interested, here is the GW (= Good Will) SM with description of the output stage
https://elektrotanya.com/goodwill_instrument_gfg-8020g_2mhz_digital_function_generator_instr._sm.pdf/download.html

same source for the drawing
https://elektrotanya.com/gw_gfg-8020g_function_generator_sch.pdf/download.html#dl

[MetalHead]

PROBLEM FIXED! 

replaced 2N2905A and instead of BSX61 put 2N2219A, also replaced overheated resistor R202 as it loosed 20 ohms from 450. [ Attachment Invalid Or Does Not Exist ]

I think the reason of that parasitic "step" was that there was emitter-base junction open in right transistor and that might create capacitance that on hight frequencies was charging/discharging... making that distortion!

Thanks everybody for help!!! 
(Uunoctium  )