Old waveform generator EXACT electronics model 502 LCS & question. - Page 1
Forum section: electronics / general

author: israel
author_rank: Contributor
timestamp: March 24, 2012, 05:05:34 pm
content: Hi guys, has come to hand this signal generator. Its construction seems very good and high quality.This is a very old are the type of opamp LM741 (which existed at the time). I have in mind to modify it to improve their performance and have already begun to move; I removed all the LM741 opamps and installed sockets to replace them easily.The maximum working frequency is between 5.5 - 6.3 MHz I have checked with the oscilloscope that the 5 MHz is in of the LM741 opamp. And these are not the most suitable for those frequencies, and the noise is higher.The million dollar question is:Which is better?TL081TL071OPA134NE5534I think the last two are appropriate but I am undecided about whether to put the opa134 or NE5534.That you think about this?I wanted for the manual and schematic of this generator.I pay the copying and shipping charges with no problem. And if there is a download site would be fantastic also.Best regards,

author: israel
author_rank: Contributor
timestamp: March 24, 2012, 05:07:37 pm
content: More pics

author: T4P
author_rank: Super Contributor
timestamp: March 24, 2012, 05:27:32 pm
content: TL071 . Low noise .NE5534 . Low noise .OPA134 . Audio B*ll*cks .TL081 . Low Power .TL071 vs NE5534 = TL071 .Go for the TL071 .

author: israel
author_rank: Contributor
timestamp: March 24, 2012, 06:15:46 pm
content: QuoteThanks for your quick response. however observed in the datasheet of both the NE5534 opamp has better characteristics in high freq.TL071 = 3 MHzNE5534 = 10 MHzregards,

author: Lightages
author_rank: Unknown
timestamp: March 24, 2012, 07:18:42 pm
content: It is possible that the circuit was tweaked to be stable with the 741. If you change the op amp you might run into instability problems or distortion and ringing. Without seeing the schematic it is hard to say. Try the different op amps and see what happens. Without a distortion analyzer and careful analysis of the out put you will be shooting on the dark however. You cannot just replace one op amp with another sometimes and there are many factors to consider despite the simplified recommendation already provided.

author: T4P
author_rank: Super Contributor
timestamp: March 24, 2012, 10:24:31 pm
content: Quote from: israel on March 24, 2012, 06:15:46 pmDamn , i totally forgot the freq characteristics .

author: lowimpedance
author_rank: Super Contributor
timestamp: March 26, 2012, 02:29:59 am
content: Nice clean condition, pity you did not get the handbook.You probably will have trouble getting one too, they are not the most common generators to be found.!They are well built though and should provide good service, but watch for noisy wafer switches which couldcause the output to stop generating, making it look like a more serious problem !.I own a model 516 and luckily got the manual too, so was able to repair it after some idiot butchered it trying to find the problem!.The only problems I found with mine were the above mentioned switches which is the biggest pain/problem,some careful application of switch lube fixes for a while.Shorted tantalums (I see yours uses electrolytics) and one resistor drifting high in value which upset the biasing through thewave shaping to output stages due to all the path's being dc coupled giving distorted and offset waveforms.I can post a block diagram and some circuit for the main generator section only which you may find informative even if its for acompletely different model i suspect the principles will be similar. (the area you want to improve!)let me know.have funcheersJohn

author: MarkKeppinger
author_rank: Newbie
timestamp: May 22, 2012, 05:36:18 am
content: Actually, we made TWO of the Exact model 502 generators. The first was around 1970. The second, which you have, was after Jerry Foster sold Exact to Dynatech. They were then relocated from Tillamook, Oregon to Nevada, and went out of business a short time later.All analog Exact generators had the same basic circuitry; opposing current sources would charge and then discharge a timing capacitor. This would generate the triangle waveforem. The comparator with hysteresis would also create the square wave. The triangle wave was fed through an op-amp with three (four in our high end generators) diode bridges that would produce sine distortion as low as 0.3% at the lower frequencies.I'm not sure changing op-amps is going to gain you anything. From what I recall, most of the 741 op-amps were used in the current source circuitry, and not in the output stage, which was discrete. The 502 that you have came a little after my time, but the circuitry looks to be the same idea.I doubt I have a manual for yours, but feel free to e-mail me directly at mkepp@hevanet.com if you have any questions.Mark KeppingerExact Electronics, 1975 - 1978

author: saturation
author_rank: Super Contributor
timestamp: May 22, 2012, 12:39:12 pm
content: I concur too, without the schematic blindly making mods are simply hit and miss. Impressive to see one of the original designers post here! You can't get any better than that, and welcome Mark Keppinger.Quote from: Lightages on March 24, 2012, 07:18:42 pm

author: israel
author_rank: Contributor
timestamp: May 22, 2012, 04:59:32 pm
content: QuoteQuoteI am very glad to exchange views with you. Effectively as said in my tests did not find any difference when changing the original opamp LM741 for TL071 or LM7171.In fact one of the stages are heated in excess with LM7171, I guess it could be for the bias adjustment.This signal generator goes up to 7 MHz and what if I could check with the scope is that these HF signals are present in the legs 3 and 6, anyway is a very nice generator.Best regards