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FeelTech FY6600 60MHz 2-Ch VCO Function Arbitrary Waveform Signal Generator

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soundtec:
I see in the THS3491 specific resistor combinations are recomended for  gain setting. What is the gain in the original fy6600's final op amp output? ,surely if we want the display to show a meaningfull output voltage  the right resistor combination needs to be used to set the gain the same as the original op amp set up ,simply leaving the originals in place will no doubt  means the gain wont be a whole number  .It seems setting the gain higher impacts on the distortion produced by the output stage in any case .Maybe someone can test the output stage gain on a machine with 3491's installed , I have a vague recollection the original setting on the unit is x3 gain for the 3002 op amp .

The spec sheet for the 3491 shows Rg and Rf to be 200 and 798 ohms for a gain of 5 ,2.1k and 2.1k gives a gain of 2 .
On the FY66/68 we have 499 and 1.5k in these positions . Tables are available for each op amp giving the optimum resistor values and gain .

soundtec:
THS3491:

Gain           rg            rf   
   2             2.1k      2.1k
   5             200       798 ohms
  10            78.7      704 ohms

THS3095:
Gain           rg            rf
   1              -             1.78k
   2            1.21k        1.21k
   5             249          1k     

THS3002
Gain          rg             rf
   1                            1k
   2                           680ohm
   5                           560ohm

So with 1.5k rf on the unit, a gain of less than 1 would be expected  ,3095 users would have a gain somwhere around 1.5 and 3491 would have a gain of around 3 .

So I setup the generators for 5.1volts 1khz sine,this should be 1.803  vrms

Fy6800(standard 3002 output stage)
Ch-1  1.7966 vrms
Ch-2  1.7815 vrms
Fy6600 (3095 x2,same Rf Rg)
Ch-1  1.7750 vrms
Ch-2  1.7735 vrms
at 0.51 volts (opa-686)
Ch-1  178.53mVrms
Ch-2  179.45mVrms

So not a big descrepency between the two opamps . I set the 6600 to .51volts output and used trimmers W3 and W5 to bring my output levels on both channels  to .1803 volts.
Now when I engage the 3095 at 5.1volts I measure 1.7935  on channel one and 1.7822 volts on the other channel so 10mV low on channel one and 21mV low on ch2 ,maybe its normal for the opamps to differ to that degree. I might check tomorrow  if the tollerances of the impedence/feedback setting components on the output play a part in the difference between channels .

soundtec:
SO a couple of ohms difference between the main 50 ohm output series resistor after the 3095 ,into 1Mohm meter input I cant see this making much or any difference, the output attenuation network is matched very precisely between channels  , so maybe theres either a difference between op amps or a slight variation on the feedback resistances between stages . Maybe a small gain control preset pot on the final opamp  output stage feedback network in order to match the gains exactly might be a good plan for the next revision of the machine from  Feeltech ,it would also facillitate setting up alternative op amps.It would certainly allow for much more precise calibration of the output amplitude at higher levels . As we stand all the amplitude decimal places below .00 are worthless ,an ability to calibrate the outputs would make lots of sense for the precision of the machine .The way things are you can calibrate either low or high range output precisely ,but once you change to the other range  its no longer calibrated

Thanks for the link to Robert Cordell JS ,bound to be some good stuff in there ,just downloaded and getting stuck in ,

DPA31:
Soundtech, If I may help :

 The gain of the output stage is 4
It is  G = 1+ rf/rg so G = 1 + R63/R45 on the original board gives 1 + 3

The difference you measure comes from resistors. But no need to adjust it alone as all the chain gain can adjusted by W05(CH1) and W03 (CH2).

I have replaced trimmers with Bourns SMD 1K 11turns 3224W-&-102E found on eBay
W26 : DC_AMPL_CH1_OFFSET
W05 : DC_AMPL_CH1_GAIN
W16 : DC_AMPL_CH2_OFFSET
W03 : DC_AMPL_CH2_GAIN
W01 : DC_OFFSET_CH2_CAL
W02 : DC_OFFSET_CH2_CAL

Dom.

soundtec:
Hi Dpa31,

If you look at the schematic the gain control pot is in the feedback loop of the OPA686 ,when I calibrate the output below 5 volts with W3 and W5 I can get it very close ,but as theres no fine tune of the final output IC's your either calibrated to high or low level signals ,but it cant be right at the same time on both .

The idea of using ten turn pots came to mind with me a good while ago ,Im glad someone did it ,I'd imagine its a lot less fiddly now to set things up accurately .Was there any trouble fitting the ten turn pots? ,I didnt know there were smd types of ten turn pot available . Are they pin compatible with the existing presets?



 

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