The output level on my BK Precision 4017A function generator is drifting.

For example, if I set the output level to 10.0V on a 2kHz sine wave, driving a variety of simple half-wave rectifier and diode clamp circuits, the function generator output level will hold fairly steady for perhaps 10 or 15 minutes then drift down to around 6 volts or up to around 15 volts over a period of a couple of minutes, then settle at the new level for a while.

Any suggestions what might be wrong please e.g. dodgy potentiometer, capacitor, or transistor, or are there just too many possibilities to diagnose? I have read the manuals, Googled FG trouble shooting (a lot on frequencies but nothing so far on output level) and emailed BK Precision support but no reply and I guess they are closed now for the weekend.

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    \$\begingroup\$ sounds like a drifting Vref of some kind ..... also, if a potentiometer has a bad connection, then wiggling the knob usually corrects the output .... try doing that to isolate the problem \$\endgroup\$ – jsotola Feb 2 '18 at 21:30
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    \$\begingroup\$ wiggle the knobs after the output drifts. see if it goes back to the correct output \$\endgroup\$ – jsotola Feb 2 '18 at 21:50
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    \$\begingroup\$ one thing that crossed my mind ... how do you know that the output has drifted? \$\endgroup\$ – jsotola Feb 2 '18 at 21:50
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    \$\begingroup\$ is the oscilloscope set correctly and working correctly? \$\endgroup\$ – jsotola Feb 2 '18 at 22:01
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    \$\begingroup\$ Look for a faulty LM399 chip It should be 90'C \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 2 '18 at 22:48

In case this is useful to others with similar problems in future, I opened up the function generator (power off and unplugged) and gently poked a few things, especially the output level potentiometer. Since then, the output level was dead stable for 4 hours yesterday and again for two hours this morning, so it seems to have fixed it, for now at least.

Link to other suggestions people made in addition to those above, with schematic and photo with the cover off.

As a follow up two weeks on...

BK Precision recommended first checking the output level potentiometer (spot on, it has been stable ever since) and have since dug around in their files and provided a component list with zoomed in sections of the schematic. It is a pretty ancient machine (schematic is from 1994), so all in all I say brilliant customer service from BK Precision!

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    \$\begingroup\$ To summarise the thread in case it goes down, anecdotally it seems the BK precision 4017a is known to exhibit several problems like this. I would recommend copying in any relevant information into your answer since stack exchange is supposed to stand independent. And make sure you accept your answer! \$\endgroup\$ – loudnoises Feb 4 '18 at 9:39

These are the other suggestions referred to above in case the link goes down (if the link is still there you would be better using that though, as I have summarised, possibly incorrectly, and you'll be able to contact the people who made these suggestions if you need clarification), and the photo. The schematic is a pdf and there doesn't seem to be any facility to attach that on this site, so screen shot below, but the original isn't much better.

  • Scope power supply voltage ripples from all the power supply outputs to identify which (if any) is faulty

  • Clean all contacts

  • Replace all faulty electrolytic capacitors

  • Such an ENORMOUS drift means something is seriously wrong (e.g. electrolytic or tantalum capacitor breakdown) but should be easy to measure

  • If there's an external power supply, check that

  • Post schematics so others can look and perhaps diagnose

  • Stick a scope on the Level adjusting potentiometer R300, both on the cursor and on the input and look at the output level, if the level stays fixed on the input, then focus on the cursor's level, if both are stable then the output amplifier is out of question, if the potentiometer input is stable, but cursor fluctuates, check the potentiometer.

  • If the input signal amplitude is unstable coming from the oscillator, there is a lot of work to identify the problem as there are lots of switches, potentiometers and stuff to clean and verify. Start by checking the +/- 22V power supplies and also +/-15V.

  • There is an RC oscillator, and it's not unheard of to have amplitude drifts if the power supplies are noisy or drifty. If the voltages seem OK use your scope on DC coupling and check the ripple of the power supply as, sometimes, due to component failure, when going from cold to warm, there can be drifts and noise.

  • Looks like it is based around the Maxim MAX038 waveform generator IC. Do you get the same problem with a resistive load of say 1k? A diode circuit can be a difficult load. If it is only varying with the diode circuit, then the explanation is purely in the output amplifier stage. If the problem is earlier, the output will vary with any load. If the problem was happening initially but is now not happening, then it could be something as simple as a poor contact of the wiper in the level control.

  • Measure the output of the DC offset amplifier which is pin 1 of U300A to see if it is drifting. Inspect the output resistors to see if they have overheated in the past due to an excessive load. It may be that the output transistors are running hot and are damaged. U301 may be damaged as well. It may be necessary to change the output resistors and transistors.

enter image description here


  • \$\begingroup\$ "These are the other suggestions referred to above", above this answer is the question. Answers will move around, perhaps you meant your other answer. \$\endgroup\$ – Harry Svensson Feb 4 '18 at 11:54
  • \$\begingroup\$ You can edit all of this into your first answer and make it better. This isn't so much a second answer an expansion of your first. This isn't a forum where you keep posting new replies. It a question and answer site. Your first post is the question. Your second is an answer. Answers (and questions) can be edited to improve them. Once all of this is in your first answer, you can delete this one. \$\endgroup\$ – JRE Feb 4 '18 at 13:32

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