Tektronix FG503 oscillations on peaks - Page 1
Forum section: electronics / repair

author: orbit257
author_rank: Newbie
timestamp: May 20, 2025, 10:01:23 pm
content: Hello guys, I bought for a few bucks old Tek FG503 (ser. number B036605) sold for parts or repair. Seller noted that when plugged into TM500 frame, lamp shows intermittent power. So I changed all tantalum caps for electrolytics, since I had them in my drawers, changed sockets for all ICs and Q569 on negative 20V rail since it was shorted. Everything started working as it should. However when I proceeded to calibration instructions, and then using notch filter, I noticed high frequency spikes on the signal. Same spikes are captured on the photos bellow. They are always on positive and negative peak of triangle or sine wave dominant on lower amplitudes. On higher frequencies they distort both sine and triangle waves on peaks.So my question would be: Is this normal, or something is dying?In case, that is not normal, I measured waveforms of power supplies and this kind of frequencies are on every one. What I can say is that, they are not from mainframe, since I tried powering it from Siglent SPD3303X-E and everything looks the same.Also I have done more measurements and eliminated output amplifier, since signal is coming to it with those oscillations, so the cause must be somewhere in loop generator. Square wave looks as it should, so there is no problem with that one. Also I measured all semiconductors and none shows any weird values. This was done with multimeter for now, maybe I will need to put them on the basic built in component tester in my Hameg 203-6.I got schematic and manual from tekwiki (https://w140.com/tekwiki/images/f/fa/070-1727-00.pdf). If anybody had some similar problem I would appreciate some tips & tricks for this. Thanks

author: ondraN
author_rank: Regular Contributor
timestamp: May 21, 2025, 09:26:34 am
content: The problem could be caused by replacing tantalum capacitors with electrolytic ones. Tantalum ones have very low ESR. I would replace tantalum capacitors with tantalum or gel capacitors or low ESR electrolytics plus parallel-connected ceramic capacitors.

author: orbit257
author_rank: Newbie
timestamp: May 21, 2025, 11:05:35 am
content: Yeah, you are making very good point. I also thought about that, but in the end after measuring every tantalum cap in the gen and every one being more of a resistor then a capacitor I did not pay enough mind to that. Anyway I have already whole set of caps in cart of my following order, so I will order them most likely today. I will post update when I change them.

author: ondraN
author_rank: Regular Contributor
timestamp: May 21, 2025, 11:36:48 am
content: Old Tektronix tantalum capacitors are terrible. They short, burn, and sometimes act like a zener diode or resistor. The worst are the ones in the square package (THT), I always change those.

author: orbit257
author_rank: Newbie
timestamp: May 29, 2025, 04:14:30 pm
content: So I managed to change all electrolytic caps for tantalum as original, but still no change. I guess it is just the way the generator loop works. On higher amplitudes the sine wave gets pointier then it should be, but on fft I got difference between first and third harmonic around 36dB, which is in spec (difference should be between 30 and 40 dB as I found or read somewhere else).

author: MadTux
author_rank: Frequent Contributor
timestamp: May 29, 2025, 11:00:07 pm
content: Look at the triangle generator, since everything in there starts with a triangle and gets converted to square or molested sinewave.Triangle generator is basically current source that charges a couple of timing capacitors.Charge current dependent on VCF input and freq setting.Maybe something fishy happening on that CR130/140.... rectifier bridge, since that spike looks about a diode junction drop in voltage?Hysteresis feedback with that 74S00 delay line and C226 might also be a good candidate for such things.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2025, 01:20:22 am
content: It is not unusual for a switching spike to be present in the output from the fast switching driving the integrator. I do not have an FG503, but my FG502 displays about the same switching spike when the output amplitude is so low. This may be common mode coupling to the output and sometimes the BNC connector for the output is isolated from the chassis to control this. On my FG502, the trigger output is isolated so that it does not get into the signal output.

author: orbit257
author_rank: Newbie
timestamp: May 30, 2025, 09:28:22 am
content: Quote from: MadTux on May 29, 2025, 11:00:07 pmHello, thanks for suggestions. Bridge has been checked with diode test and all diodes in circuit shows appropriate values. Also with scope I guess I could not find anything weird. Square wave looks good without any spikes, so I would say 74S00 circuitry is good. But yes, It must be something in triangle wave generator, since triangle comes out with those spike both to output and sine shaper

author: orbit257
author_rank: Newbie
timestamp: May 30, 2025, 09:34:23 am
content: Quote from: David Hess on May 30, 2025, 01:20:22 amAfter measuring mostly everything in the unit I came to same conclusion. Also I watched nearly every video about FG501/502/503 on youtube and some output waveforms looked better and some similar to mine so I let it be. Anyway I will try to isolate BNCs from the front panel since my unit is one of the older (old front panel design and brown pcb) and it has nothing isolated. At least I have a reason to look for some SG502 when if I want cleaner sine wave for audio testingIn the end SG505 would be the best but in Europe they are hard to come across and then there is price point. Here is most of the time everything at least twice as expensive as in the US. Another thing are import taxes when buying from outside EU ...

author: Vovk_Z
author_rank: Super Contributor
timestamp: May 30, 2025, 12:11:45 pm
content: Quote from: ondraN on May 21, 2025, 09:26:34 amI have newer seen a tantalum capacitor with a very low ESR in my life. All which I have seen had quite a large ESR, typically larger than LowESR electrolytes (at 100 kHz). The only thing which is really true that tantalum caps have very stable ESR. It almost doesn't change with temperature, comparing to any wet electrolyte type. That's why it has a wide use in aviation and possibly space.

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2025, 12:32:19 pm
content: Quote from: Vovk_Z on May 30, 2025, 12:11:45 pmTantalum capacitors have better high frequency response than aluminum electrolytic capacitors. To get the same impedance, which is really what matters, the aluminum electrolytic capacitor needs to be like 4 times the capacitance, and even that is not enough at higher frequencies where the capacitance of an aluminum electrolytic capacitor falls.

author: CaptDon
author_rank: Super Contributor
timestamp: May 30, 2025, 01:31:49 pm
content: I didn't think it was the 'capacitance' that was a problem at high frequency but the high series inductance of electrolytics?

author: David Hess
author_rank: Super Contributor
timestamp: May 30, 2025, 04:35:29 pm
content: Quote from: CaptDon on May 30, 2025, 01:31:49 pmIt could be, but the result is the same. On my variable frequency LCR meter it shows up as a decrease in capacitance at high frequencies, whereas the solid tantalum parts read the same.Update:Just to be clear, I think something else is going on rather than the series inductance, because the impedance can still be lowered with a larger value of capacitance. If it was the series inductance, wouldn't this be worse? A comparison using a network analyzer between various aluminum electrolytic and solid tantalum capacitors of different sizes would reveal what is going on.In practice it does not matter. For whatever reason at higher frequencies, a larger aluminum electrolytic capacitor is required to achieve the same impedance. About 4 times more capacitance is required.On a different subject, when I was messing with my FG502 for comparison purposes, I found that the sine wave output was missing. Tracking down the problem showed that the input 2N3906 differential pair for the sine shaping amplifier failed somehow. The individual transistors test good as diodes, for current gain, and leakage, so I am not sure what to think. Maybe there was a bad solder joint that I missed. The 82 ohm resistor got changed because it was in the way.Oh, and a 43 picofarad mica trimmer capacitor in the frequency generator was broken loose so the 1 MHz frequency band was off by 50%.