Wavetek 166 50MHz FG fix: factory error, design flaw or you tell me - Page 1
Forum section: electronics / projects

author: onlooker
author_rank: Frequent Contributor
timestamp: August 24, 2013, 08:53:33 pm
content: I hope the title is not too much on the sensationalism side. What I really want is to compare notes about the "Wavetek 166 50MHz Pulse/Function Generator". I hope to get comments about the problems I am facing and about better fixes.Long story short: I have been eying on an old function generator with similar specs as HP 8116a (with sweep opt) for a while. Then, I ended up of getting this Wavetek 166 on Ebay BIN for $95 (shipping included).The pictures had the 166 with cover taken off by me and with some mini clips inside for diagnosing problems.Like the HP 8116a, the service manual of 166 with schematics and construction details are freely available online. On the other hand, Wavetek 166 had an all thru hole construction, and used no customary parts. This means it would be much easier to maintain. Wavetek 166 lacks the digital ctrl and remote/programmable features of HP 8116a. But from my understanding, both share the same relatively simple analog approach in the core.In fact, on paper, some of the specs of 166 are better than 8116a. A particular spec of interest is the amplitude stability. The 166 is speced as with in 0.05%(10 mins); while amplitude stability or similar terms could not be found in 8116a's manual (or, for that matter, most other FG manuals I had a chance to glance).With the above said, I am new to both 166 and 8116a. I hope someone knows both better can make comparisons between the two for us, especially in regarding to the analog implementations.As for the 166 I got, I liked the fact that it has many knobs, BNCs and switches. It was listed as in working condition. Accuracy checks aside, all my initial tests went well and the results were better than I expected for an equipment of 40+ years old.Then, problems started to show up when testing at the extremes of the specs. So far, I have found four reproducible problems and I mostly fixed two of them. The more puzzling part about the two fixed problems is that they looked like some factory error and design flaw. For these problems, maybe someone knows better can provide better explanations or suggest better fixes. If you also own an 166, you may help to show how your 166 behave under the same conditions.---------------------------1). Problem#1 (?factory error?, fixed):Independent of the other settings, the x0.001 range showed only half waveform (time-wise) for each period.It turns out the problem was caused by two missing wire connections on switch#1 as shown,Both the schematics and the wiring diagram showed the two connections. It also can be argued that:a). The involved tabs had never been soldered. This ruled out the possibility that the connection wires were removed from previous repairs.and,b). The switch was the factory original since the silk screen letters are on top of the solder in the locations where the switch legs were soldered to the PCB. Any out factory replacement would have destroyed the letter printings.For these reasons, I felt problem#1 was a factory error. Then, the fix was also easy: just soldering in the missing wires.----------------------------------I will update the descriptions of other problems later.

author: lowimpedance
author_rank: Super Contributor
timestamp: August 26, 2013, 01:42:28 am
content: I had a peek inside an old 166 at work and the two connections on the switches were indeed there. However I also noticed that your unit has an extra wafer at the rear of the switch bank (closest to the pot at the rear, near your test clips!). The unit here only has one wafer by itself. Clearly different revisions.Since the unit is not mine I don't have time to do any tests on its actual behaviour.Nice old generators though, enjoy.LowZ

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 26, 2013, 01:22:20 pm
content: I'm liking itStaying tuned for the next "episode".

author: onlooker
author_rank: Frequent Contributor
timestamp: September 03, 2013, 02:52:28 am
content: Thanks, to all for showing interests and to lowZ for providing helpful hints.I had noticed the extra wafer on switch#1 and later forgot all about it. Now, I just did a little tracking: the only function of the wafer is to parallel, for the x10M range, a 15k resistor to the -12V half of the the FREQ VERNIER as shown in the red circle in the drawing.Presumably, it was used to correct some linearity issue(?) with the FREQ VERNIER. Since this dial is shared by all ranges, the linearity problem should have being caused by something that is particular to the x10M range. It may also be relevant to my another problem (#3) that is also related to this range.Now Let me describe the next problem I am facing.2). Problem#2 (?design flaw?, partially fixed)I attached the relevant schematics (with my notes there), so those who are detail inclined may better understand where I stand now and possibly offer better insights for diagnosing/fixing the problem,schematics 1 (at the end of this post)schematics 2 (at the end of this post)(For the full service manual, please go to,Wavetek_166_manual)The problem occurs only under the (single) TRIG and DBL TRIG modes in the two high frequency ranges x1M and x10M. Other settings are irrelevant.The TRIG and DBL TRIG modes output one or two periods of a waveform after each external (e.g. manual) triggering event. The problem shows itself as some severe distortion in the waveform, in which the 1st quadrant is time-wise elongated and amplitude-wise "chopped" (see pic below). The green trace in the picture is a hand drawing to show what a correct waveform should look like.After a careful look, one would find that the waveform is not really chopped, but, instead, the distortion is due to a switching to a slower dV/dt slope at around the mid point of the positive voltage span. After some scoping and metering, it becomes apparent that the problem started in the TRIANGLE AMP part.The TRIANGLE AMP is for generating triangle waveforms by alternatively charging and discharging the (per range) integration capacitors with a pair of constant current sources (I+,I-). The FREQ VERNIER varies the current values of I+ and I-. The charge/discharge alternation is triggered by the voltage on the range capacitors reaching a fixed value (about +-1.25V by the manual and +-1.35V for my case).Except for range x10M (the highest) that has the range capacitors non-switched, the range capacitors for other ranges are switched in or out with switch#1. These range capacitors are sit between the GND and the current source side connection point (point "A" in the next picture). Three different means are used to do this capacitance switching:a). Low freq ranges: using capacitor multiplier circuitry.b). Mid freq ranges: mechanically switching on or off the capacitor connection on the current source side.c). High freq ranges (x100k and x1M): using diode switch to add capacitors by switching the connections from the GND side; while the current source side are permanently connected.I guess c) is supposed to be a clever design for reducing the paracitics from the switch mechanics that may affect these higher freq ranges(?). Then, from what I can see, it also caused my problem #2 (unless CR14 and the capacitors have 0 leakage, then the problem becomes intermittent).The next picture shows a simplified version of what was happening as I understood it now.In short, the bad waveform with range x1M is caused by a diode (CR14) of the x100k range wrongly conducting that added the capacitors of range x100k to range x1M. The next picture shows the probed waveforms on the "A" point (same as TP6) and the anode of CR14.Such cross-talks also exist between the ranges x100k and x1M, and other ranges. But, for lower freq ranges, the capacitance introduced by the cross-talks are small compare to the range capacitance, that is, there will be not much distortion resulted. Therefore, the problem is only prominent for the range pairs (x100k, x1M), (x100k,x10M) and (x1M,x10M).With the reasoning above, the problem should have existed in all new 166s with the same design. That is, it implies a "design flaw". On the other hand, facing a product from a reputable company, it could be more reasonable to assume a reasoning flaw here than a design flaw there. For example, the real cause of problem#2 may have been some defect yet to be identified; and it was developed after years of normal use.At this stage, I am really interested to know how a non-defective 166 behaves under those same conditions. All helps are appreciated. But, two tests can be particularly helpful here:a). The simple test: Set the 166 with frequency=1x1M, GEN_MODE=TRIG, WAVEFORM=TRIANGLE. Connect the FUNC_OUT (or TP6) to a DSO under the single shot mode. Now, flip the MAN_TRIG switch on the 166. If the waveform shows no error like in my picture, flip it several times more to make sure the problem is not hidden from intermittency.b). More involved test: Do the same test as in a) using TP6 and with x10 probes. In addition, hook the 2nd channel probe of the scope to the anode of CR14 (see picture).If you could, please do the tests and compare the waveforms with those in the picture below:----------------------Despite some doubts, I went on to try a temporary fix just to see what can be corrected.Remember the theory is that: The bad waveform with range x1M is caused by diode CR14 of the x100k range wrongly conducting that added the range capacitors for range x100k to range x1M. The same also happening to CR15 of the x1M range that is causing problem to the x10M range. But I will just use CR14 for the convenience of explanation.Again, in theory, the solution looks obvious: To stop the cross talk between the two ranges, one needs only to raise the forward threshold voltage of the GND connected diode CR14 to a value above 1.25V or 1.35V (the triangle waveform Vp). At the 1st look, this may be done by using a zener (or other volt regs) or serial in more diodes. But, these solutions do not work well since they introduce too much parasitics (capacitance and resistance) comparing to the original design and they show up as new visible waveform distortions.To be more precise, CR14 was having about 2~3pF capacitance when off which is seen by the x1M range and about 5R resistance when on which is seen by the x100k range. Any fix should have parasitics not too much worse than these numbers.For example, zeners did not work well since the ones in my possession all have a junction capacitance >50pF. Some internet forum has discussions about using transistor (e.g. 2N222A) b-e breakdown as a low capacitance and low dynamic resistance zener. But, I have yet to try it.Another thought is to serial a few fast or Schottky diodes. It actually worked better especially for problem caused by CR15. But for CR14, the combined dynamic resistance had caused voltage dips (>0.15V) at the peaks of the waveform for the x100k range. Nevertheless, the interesting thing is that Schottky worked better than the fast diodes I have.Anyway, after some experiments, the temporary fix I ended up to use is showing in the next schematics. Changes are made in three places.Here are some waveform pictures after the fix, some are just to show general capabilities of the 166.

author: mtdoc
author_rank: Super Contributor
timestamp: September 09, 2013, 07:11:44 am
content: I picked up one of these for a good price on ebay. Good old school analog dial porn...Quick check shows it does not have your problem #1 but it does have problem #2. Looks like you have it right. #1 = factory error. #2 design flaw - though I have to think they would have picked this issue up - lots of these out there - ? why was it allowed to persist?I haven't done it yet but when I have a chance I'll try your more involved test.In any case - no biggie to me since I don't suspect it will impact my use of it. If it does I'll try your fix which looks like the ticket. Thanks!

author: NEE8C
author_rank: Newbie
timestamp: September 29, 2013, 04:27:13 pm
content: I also own a Wavetek 166 and have been troubleshooting a problem similar to your #3 that you mentioned near the end of your blog. This distortion problem that seems to start at about 15 MHz gets more severe as 50 MHz is approached and is most evident with the square wave shape selected. Mine didn't do this when I first got it a few months ago, so something has obviously gone belly up. Hopefully we can share some successes here as we progress to a solution(s).Nice job in documenting your troubleshooting and fixes to date! Seems to be a great generator for its age.

author: acourtois
author_rank: Contributor
timestamp: October 24, 2022, 10:51:00 pm
content: Just got a Wavetek 166 at an hamfest last week. This weekend my friend and I calibrated the beast. Not super-easy.- Problem #1 is not there, as the wiring is correct on the gang-switch.- Problem #2 absolutely present.

author: texasluva
author_rank: Newbie
timestamp: September 19, 2025, 03:31:23 am
content: Hello and just saw this posted today about a Wavetek model 166 50Mhz Function generator. I've repaired many of these when I worked for Wavetek service and Fluke Electronics service center in Carrollton, TX. I have knowledge of most every model starting from the 100 series in the early 1960s. This specification reads the following for triggered wave forms.Triggered vs. Gated mode: The Model 166 offers both triggered and gated modes. In triggered mode, it outputs one complete waveform cycle for each trigger signal it receives. As long as your external trigger source is 10 MHz or less, you can trigger a 10 MHz waveform.This means much above 10 Mhz it will not. So the that is the design for this model. You were chasing down something that is not bad. Of course the problem with the soldering for the lowest or low freq. range is something that got past QC and was not soldered. The waveform is so slow that it is most likely not checked in QA. In repair we would check and and find that failure to solder and have a complete functional waveform. I hope this helps in this #2 matter those that thought it was bad but it's not. I can assure you this.

author: texasluva
author_rank: Newbie
timestamp: September 19, 2025, 03:35:58 am
content: Let me tell you about the triggered waveforms above 10Mhz.Hello and just saw this posted today about a Wavetek model 166 50Mhz Function generator. I've repaired many of these when I worked for Wavetek service and Fluke Electronics service center in Carrollton, TX. I have knowledge of most every model starting from the 100 series in the early 1960s. This specification reads the following for triggered wave forms.Triggered vs. Gated mode: The Model 166 offers both triggered and gated modes. In triggered mode, it outputs one complete waveform cycle for each trigger signal it receives. As long as your external trigger source is 10 MHz or less, you can trigger a 10 MHz waveform.This means much above 10 Mhz it will not. So the that is the design for this model. You were chasing down something that is not bad. Of course the problem with the soldering for the lowest or low freq. range is something that got past QC and was not soldered. The waveform is so slow that it is most likely not checked in QA. In repair we would check and and find that failure to solder and have a complete functional waveform. I hope this helps in this #2 matter those that thought it was bad but it's not. I can assure you this.

author: texasluva
author_rank: Newbie
timestamp: September 19, 2025, 03:43:21 am
content: Hello and just saw this posted today about a Wavetek model 166 50Mhz Function generator. I've repaired many of these when I worked for Wavetek service and Fluke Electronics service center in Carrollton, TX. I have knowledge of most every model starting from the 100 series in the early 1960s. This specification reads the following for triggered wave forms.Triggered vs. Gated mode: The Model 166 offers both triggered and gated modes. In triggered mode, it outputs one complete waveform cycle for each trigger signal it receives. As long as your external trigger source is 10 MHz or less, you can trigger a 10 MHz waveform.This means much above 10 MHz it will not. So the that is the design for this model. You were chasing down something that is not bad. Of course the problem with the soldering for the lowest or low freq. range is something that got past QC and was not soldered. The waveform is so slow that it is most likely not checked in QA. In repair we would check and and find that failure to solder and have a complete functional waveform. I hope this helps in this #2 matter those that thought it was bad but it's not. I can assure you this. It was allowed to persist because it is only specified to 10Mhz and below. I remember this from over 20 years ago when I may have questioned it. Sorry to have taken all this time since I just found this and did some searching. Hagd.