DC offset on b&k 3020 sweep / function generator

[leydenjar]

I've been using this unit for the last three years as my only troubleshooting function generator. I've grown accustomed to it to the point of buying another recently. The problem I see with it is that I can't dial in the amplitude/dc offset gracefully at all. That is to say I can't be exact with this circuit.

Is it possible to install another pot or alter the circuit so that I can dial in the exact amplitude and dc offset that I want?

MAnual attached.bk3020_owners.pdfBK 3020 Schematic.pdf

[Jazz P Bass]

What are you testing that you need a dc offset?

[soundguruman]

what he is saying is that he wants to set a zero offset....
The internal controls can be extended, to pots with knobs. What you need is the schematic.
There is already DC offset controls and amplitude adjust on the inside of the unit.

[drewl]

Should be calibration pots inside for setting those correctly.
I've calibrated just about any test equipment you can think of way back at an old job.

[soundguruman]

yes, there is cal pots inside, about 20 of them.

[Jazz P Bass]

I guess the question is 'what is the question?'
There is a amplitude adjust pot R201.
There is a DC offset pot R210.
Do they not function?

[Enzo]

Seriously, I'd be interested to know where you'd need the DC offset. I understand what it is, but I can't think of a time ever that I needed that. The only place I can recall that signal offset mattered was in video waveforms.

[leydenjar]

I don't need dc offset that I know of unless I might do an experiment in the future?

I would however like to accurately set the output of these function generators.

As they are it's almost impossible to set onE to say 4.5 ac v exactly.

The closest I'll get is something like 4.612. Not very accurate.

I'm most i nterested i a mod to make this very accurate in terms of setting the output voltage.

Thanks for the suggestions.

[Jazz P Bass]

"I'm most i nterested i a mod"
How about repairing it?
For a start, you could check the DC voltages that are marked on the circuit.
There may be a clue there.
Have you tried 'cleaning' the db switches?

[Enzo]

Um, are you saying the control is too coarse? I mean if you try to turn it down from 4.612 to 4.5 are you saying the control is so touchy you overshoot and hit 4.9? I hope you are not looking at panel markings and expecting them to be accurate. I guess I am unclear on why you cannot set the control to any voltage you want. In what way does it fight you?

[km6xz]

I am guessing he means the front panel dc offset, not calibration trimmers inside. That 10k pot is not going to be very fine resolution in settings. I would either put a switch to remove it when not needed or use a 10 turn precision pot. But more likely I would not use such a function generator in audio work, way too much distortion and lack of symmetry in waveform, poor frequency stability and accuracy of frequency and level.

[soundguruman]

I don't need dc offset that I know of unless I might do an experiment in the future?

I would however like to accurately set the output of these function generators.

As they are it's almost impossible to set onE to say 4.5 ac v exactly.

The closest I'll get is something like 4.612. Not very accurate.

I'm most i nterested i a mod to make this very accurate in terms of setting the output voltage.

Thanks for the suggestions.

what you do is replace them with 5 turn or 10 turn pots.
Then you can dial it exactly.
Move them to the front panel if you want it accessible...

[Jazz P Bass]

Kind of makes me wonder if anyone (The OP & SGM) has bothered to read the manual.
Front Panel controls 14 (amplitude) & 16 (DC offset).

[Jazz P Bass]

Additional informtion is available in the manual.
Such as Theory of Operation & a full Calibration procedure.
There is also info on actually using the DC offset function.
Curiously the Specifications list a + - 5% accuracy of the output reading.

[soundguruman]

Kind of makes me wonder if anyone (The OP & SGM) has bothered to read the manual.
Front Panel controls 14 (amplitude) & 16 (DC offset).

Bothered to read the manual? I had one of these units for 32 years.
The POINT being that the front panel controls are too ROUGH. AND you can't adjust a precision finite amount of level or offset.
I would THINK you would have figured that out for yourself....by "reading" the posts.