Help trouble shooting my BK 3010 Function Geneator - Page 1
Forum section: electronics / beginners

author: nivek
author_rank: Contributor
timestamp: December 12, 2012, 02:39:16 am
content: Hi all,I am a software engineer by trade, but can't help but love playing with electronics. I recently purchased a "non functional" BK Precision 3010 function generator. After seeing that its all through hole construction, and no wacky custom IC's, I was confident I could fix it, given all schematics are available online. I figured it would be a great learning process.After receiving it, some quick tests showed it was at least "sort of" working. If the generator was set to Square Wave mode, the TTL output sends out the expected 5vpp signal, at the expected frequency. Looking at the actual output shows little more than spikes on the scope at roughly the correct frequency, nothing resembling the expected square wave. When switch to triangle or sin wave, both outputs show no activity other than the occasional blip where my scope (Tek 2213a) tries to trigger off something.After seeing this, I followed the EEVbog moto, and took it apart.What is nice for this, is that the schematics available on the net (http://bama.edebris.com/manuals/b&k/3010) are fully complete with expected wave forms and voltages, so I used that as my starting point for the trouble shooting process.After a few nights of troubleshooting, I am completely stuck as to how to properly decypher what could be the issue. I was hoping that one of you might be able to help push me in the right direction. The issue I'm having is as soon as I see something out of adjustment, I can't tell if its something downstream pulling a value out of spec, or if its an actual cause.Here is where I am now.I changed out both non tantalum electrolytic caps with new panasonics, I figured they might have gone south.Voltage rails all look within spec.+21V is reading 21.5V+5V -> 4.9V-5V -> -4.8V+15V -> 15.1V-15V -> -15.1VIf I set the function generator to the correct settings specified in the schematic, except in square wave mode, I can see the base triangle wave at point 1 (D5) and various other wave forms look correct as well. But as soon as I switch to sin or triangle mode, I get nothing.Working back from there, lead me to Q5 (a 2n5087 pnp). This is the first time I am seeing a voltage out of spec. In sin and triangle mode, the collector voltage is reading ~5.6V, where the schematic is stating it should be 1.8V. Switching to square wave mode pulls this voltage back down to the expected 1.8V. In either case, the base and emitter voltages are to spec. I am seeing the same thing for Q6 (Except for the voltages being negative).It is at this point that I am lost as to where to go from here. This seems like it is a sign of the larger issue, I am just not sure of what path to take from this "discovery". Do I chase down more voltages? I just don't know enough about the thought process for troubleshooting electronics like this. If it were a Java app... I would know just where to startAny assistance you could provide would help me greatly. I really want to get this generator back up and running, as it looks like a useful addition to my bench.Thanks so muchKevin

author: w2aew
author_rank: Super Contributor
timestamp: December 12, 2012, 03:02:38 am
content: Without digging into the schematic, an observation that I'll make is this. Getting 'blips' at the expected 'edge times' of a square waves makes me think you might have something as simple as a broken pcb trace, component or solder joint. These small breaks will look like a very small value series capacitor (open at DC, but will pass high frequency content) because physically it is simply a tiny air gap between two conductors. Of all of the functions/waveforms, the square wave has the highest frequency content. The faster the edges, the higher the frequency content. This would explain why you don't see remnants of any of the other waveforms, since they don't have the higher frequency harmonic content like the square wave does. Try tracing back from the output connector through the signal path to see if you locate a 'break.'

author: onlooker
author_rank: Frequent Contributor
timestamp: December 12, 2012, 03:10:04 am
content: QuoteIt looks like D5 is open and the 5.6V is from +5V power rail plus drop on D6.

author: nivek
author_rank: Contributor
timestamp: December 12, 2012, 01:18:07 pm
content: Quote from: w2aew on December 12, 2012, 03:02:38 amThanks for the tip, I will be sure to work my way through the output amp portion, I had been focusing mostly in the "triangle generator" portion thus far.Quote from: onlooker on December 12, 2012, 03:10:04 amonlooker, please forgive my beginnerness here. I am not seeing where D5 could be hooking to the 5V rail. Working from your suggestion however, should I further investigate whether this diode has gone bad? When the generator is in square wave mode, the diode appears to be chopping the triangle wave into the expected portions of a square wave (Working from the sample waveforms in the schematic)I should note that I did check continuity through the various switch contacts as one of my initial steps. I was hoping that it would be simple as a fouled contact, but that doesn't appear to be the case thusfar.Thank you both for your suggestions.

author: nivek
author_rank: Contributor
timestamp: December 12, 2012, 10:14:27 pm
content: A quick follow up, I finally came to my senses about what onlooker was speaking of about being connected to the 5V rail. A quick test of the both D5 and D6 shows they are only conducting in one direction (The proper one at that).When I get some time I will have to trace further, I just feel like I am flying blind without any idea as to where my focus should be. Am I working in the correct direction, or might it be better to work from the output stage backwards?

author: w2aew
author_rank: Super Contributor
timestamp: December 13, 2012, 12:00:59 am
content: You can work in either direction, but since you've got a node where (some of) the waveforms look correct, start there and trace their path towards the output to see where things go awry.

author: onlooker
author_rank: Frequent Contributor
timestamp: December 13, 2012, 01:17:20 am
content: QuoteOk, if D5 is good, then your earlier measurement that led you say,Quotecannot be correct since the voltage drop over D5 can't be greater than ~0.7V. You may want to recheck the voltage btw the collectors of Q5 and Q6. Maybe the traces connecting to D5 are broken.

author: lowimpedance
author_rank: Super Contributor
timestamp: December 13, 2012, 03:06:14 am
content: Have you got a triangle wave at point 3?, and a square wave at point 5 with any function selected?. If not what happens to the dc level at Q8 emitter when it disappears?. Looking at the cct there should be no interaction with the main VCF generator part by simply changing the function out select switch.

author: nivek
author_rank: Contributor
timestamp: December 13, 2012, 10:39:53 pm
content: Quote from: lowimpedance on December 13, 2012, 03:06:14 amThis would match my understanding, the main triangle generation part should not be impacted by the function.When the generator is set to Square mode, I get the expected Triangle at point 3, and the square wave at point 5.At this setup, the voltages all match what I should be seeing according to the schematic.Once I activate Sine or Triangle mode, both wave forms vanish.In Sine and Triangle mode:Q8: Emitter: 2.66V Collector: 4.95V Base: 3.38VIn Square mode:Q8: Emitter 0V Collector:4.95V Base: .69VWorking from there, In Sine and Triangle mode, the 2.6V is visible at pin 12 of IC5, and at the cathode side of D10 (-5V on the anode side). Looking at R45, I have the 2.66V on the "R31" side, and .36V on the IC7 side.The only place i can think of as to where it would come from is the amplitude control? Am I moving in the right direction?

author: nivek
author_rank: Contributor
timestamp: December 15, 2012, 09:20:48 pm
content: As an update,I did manage to get at least one part of the generator working again. I found that IC6 (A 7400 Quad Nand used as a "Square wave level shifter") was what was causing my main output to look like garbage when the square wave mode was active.I didn't have a 7400 but I did have a mc14011b which is also a quad dual input nand, and will operate within the range used by the 7400. I simply slid it in on top of the 7400 for an initial test, and once I did that, my 600ohm output was working in square wave mode, and the offset and amplitude controls appear to be working within their range.This didn't fix the sin or triangle modes, but I feel it is proving that the output amplifier is not the issue, nor is the main timing components. I am still not sure what might be causing the issue in the other 2 modes. I was originally thinking the sine wave shaper might be bad, but in square mode, probing the sine wave shaper shows the correct waves around the chip.Two questions.1) Is it worth getting an actual 7400 rather than soldering in the MC14011B? I'm not sure of the fine details between the two to know.2) Any body have any guesses as to where I should look for why the Sine and Triangle modes are stuff FUBAR?ThanksKevin

author: nivek
author_rank: Contributor
timestamp: December 16, 2012, 03:24:19 pm
content: Great news for those that are interested, replacing the 7400 has gotten all waveforms working, I did use the mc14011b for the time being , only down side I'm getting now is that the TTL output isn't working correctly (Given I didn't use a TTL part, this isn't exactly unexpected).Sine wave is a little out of "round" but I can play with the trim pots and get that back in line.Thanks all who helped