741 series pass regulator wont start on unit switch on-How to modify circuit? - Page 1
Forum section: electronics / repair

author: djsb
author_rank: Super Contributor
timestamp: July 17, 2020, 02:20:38 pm
content: I have replaced the original 741 op amps on my PM5324 signal generator with new ones. Now the series pass regulator will not start on its own when the unit is switched on. The output at pin 6 of op amp 322 does not seem sufficient to turn on the Darlington pair. If I momentarily connect pin 6 to +V the circuit starts up and all the output voltages are as they should be (within a few 100mv or so).I'm guessing that the offset voltages of the newer op amps are more accurate hence the output does not swing sufficiently.I'm wondering how I can bootstrap this circuit into operation in a simple and elegant way. Any help gratefully received. Thanks.

author: djsb
author_rank: Super Contributor
timestamp: July 17, 2020, 05:45:31 pm
content: I'm guessing that a 1k or so resistor across the collector and emitter of each series pass transistor would work. I'll give that a try first.

author: duak
author_rank: Super Contributor
timestamp: July 17, 2020, 07:18:31 pm
content: I think it might be simpler to add a resistor between the zener diode cathode and the '+' power pin of U322. I would start with 100k.

author: bob91343
author_rank: Super Contributor
timestamp: July 17, 2020, 07:19:26 pm
content: A resistor from -12 to the opamp output in both sides might help.

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 17, 2020, 07:27:56 pm
content: Quote from: duak on July 17, 2020, 07:18:31 pmExactly. A bootstrap circuit.One question in everyone's mind, I'm sure: Why did you replace the original opamps? Were they faulty?

author: djsb
author_rank: Super Contributor
timestamp: July 17, 2020, 08:18:18 pm
content: Thanks for the suggestions. I'll give them a try.Just doing a substitution test. The original op amps are still working (at least in a crude test using a relaxation oscillator on a breadboard, and they should still work if I reinstall them)and now that I've installed sockets I can reinstall the originals easily.I've tested all the resistors and the Zener and transistors so the op amps where all that's left. I'm trying to trace some noise on the power supply that's causing instability in the main RF oscillator of this RF signal generator.

author: Labrat101
author_rank: Unknown
timestamp: July 17, 2020, 08:54:36 pm
content: Quote from: djsb on July 17, 2020, 08:18:18 pmTry adding a 0.1uf ceramic cap as close as possible to the op amp power terminals as per the manufacturesrecommendation . this usually solves problems with op amps

author: RoGeorge
author_rank: Super Contributor
timestamp: July 17, 2020, 09:17:26 pm
content: Quote from: djsb on July 17, 2020, 02:20:38 pmWhy?

author: David Hess
author_rank: Super Contributor
timestamp: July 18, 2020, 01:05:58 am
content: Quote from: djsb on July 17, 2020, 02:20:38 pmThat is a common problem in that type of bootstrapped regulator because startup depends on the operational amplifier having an input offset voltage in the correct direction.QuoteI have seen older correct designs where they deliberately added a couple millivolts of offset so that worse case offset in the wrong direction would still allow it to start. With the 741, this could also be done using the offset pins.My solution is usually to provide a little bit of current to the base of the power transistor which the operational amplifier can override. Or I add an offset voltage to the power pass element with a zener diode or similar.The best way might be to bias the zener diode from the input supply and include a diode so that the output then takes over.

author: floobydust
author_rank: Super Contributor
timestamp: July 18, 2020, 01:44:43 am
content: Why bother changing the op-amp, the LM741 is fine for the purpose. You'd didn't mention what part is plopped in place.Note this original circuit has no current-limiting resistor for base-drive to the Darlingtons, no capacitor across the zener for lower noise, and almost 42V for the -ve reg. op-amp. The LM741 good to 44VDC rails. This eliminates many alternate IC's.I've seen other 1970's PSU's same circuit like this but they powered the reference zener off the raw DC side. So I would say here the (new) +ve reg op-amp stays off unless there's enough differential voltage, with the common-mode voltage being zero at start-up. I thought the 741 needs a few volts of common-mode voltage to work properly, it doesn't like the -ve rail but datasheets are so old they don't say anything.

author: djsb
author_rank: Super Contributor
timestamp: July 18, 2020, 06:02:39 am
content: To clarify I am NOT changing the 741 for another DIFFERENT op amp. I AM putting in a NEW 741 op amp. The fact that the original 741 op amp works and that the new 741 needs a bit of tweaking to work properly proves that there is a difference. As mentioned I'm doing a substitution test NOT trying to upgrade the power supplies op amps with something "better". Faster op amps would result in disaster. The 741's are the only parts that should be used.PS Any suggestions for which value of capacitor to put across the Zener to bypass any (RF in particular) noise?

author: Labrat101
author_rank: Unknown
timestamp: July 18, 2020, 09:01:20 am
content: Quote from: djsb on July 18, 2020, 06:02:39 amThe 741 has been around for a long time and is 100% reliable .From what you have said it sounds like your replacement op amps ARE NOT originalsTry checking them off board in ether IC tester if you have and compare with the original ones.on a 741 can be tested using the Diode test on a good bench meter .If its a cheesiness fake the die print will be wrong and you canall day and get no joy.quick test safe on 741 . not 100% accurate but a fake will show large error.Diode TestPlace the Plus (red) prob to pin (-) 4 and check the reading on 2,3 & 6 then do the same on the original (inputs & out )you should get a reading of 0.76v aprox if they all read the exact same its ok if one reading is off by more than 0.02v its a die fake ..Only touch test don't hold the diode test on for more than needed to read the meter .I know some one here will shout but its a quick test that spots fakes 99% correct . IC tester is far better .BTW I would put the originals back and find the real problem.

author: RoGeorge
author_rank: Super Contributor
timestamp: July 18, 2020, 09:32:28 am
content: Quote from: djsb on July 18, 2020, 06:02:39 amThat's exactly what made me to ask why. AFAIK, integrated circuits does not wear out with time, like for example vacuum tubes, so why replacing the old 741 with new 741?Asking just out of curiosity, in the hope I might learn something I didn't know before.

author: Labrat101
author_rank: Unknown
timestamp: July 18, 2020, 09:38:00 am
content: Quote from: RoGeorge on July 18, 2020, 09:32:28 amI am sure you are right the originals would be good for ever .. there are no RF filtering in the circuit which suppises me . no ceramic10uf & 0.1uf on the main caps which could or maybe now have a hi ESR with mains ripple and other harmonics let loose .

author: djsb
author_rank: Super Contributor
timestamp: July 18, 2020, 10:32:50 am
content: I got the 741's from herehttps://www.ebay.co.uk/itm/LM393-LM324-LM358-LM339-LM741-TL071-TL072-TL074-TL081-TL082-TL084-NE555-DIP-IC/113815988168?ssPageName=STRK%3AMEBIDX%3AIT&var=414012315778&_trksid=p2057872.m2749.l2649Does this supplier have a reputation for supplying fakes?I'm going to build a Jeri Ellesworth inspired Wien Bridge Oscillator on breadboard to check them anyway when a few bulbs have arrived from China. Like I said earlier the new op amps work. They just need to be given a kick because of the backassward circuit they are used in. Not sure why Philips didn't use HF decoupling adjacent to the op amps. Maybe it's because of the positive earth system being used.Any suggestions for an analog IC tester I could buy or build?PS I've changed ALL of the old German Frako Electrolytic caps for Vishay ones. This thing was built in 1973.

author: Labrat101
author_rank: Unknown
timestamp: July 18, 2020, 11:22:13 am
content: Quote from: djsb on July 18, 2020, 10:32:50 amNow we are getting warmer .. There is 99.9% they are crap .. drop them in the Bin and use the originals .I never get chips off Ebay as most are NOT originals .Mix some IPA + Acetone and drop it on the label it will wash off .. the originals there Numbers are Lazar imprints .They are rubbish .There are 2 fake chips in the photo on that link.Update Fake chips can work but not as to the specs . so if you use them in replacement they will not work correctly .Question are you really what your caption says not being rude . because what you are doing does not make any logic senseUniversity Electronics Technician

author: djsb
author_rank: Super Contributor
timestamp: July 18, 2020, 11:41:24 am
content: They are ETCHED with the following markings(the markings do NOT rub off with Alcohol). I can feel this by gently running a small screwdriver over the marking with a jewellers' screwdriver under a microscope.St then (e3 in a small circle) CHNUA741CNK9E517and the other 1 same markings except forK8E517These are NOT fake and besides they work. Not wasting any more time arguing this point. Sorry.

author: djsb
author_rank: Super Contributor
timestamp: July 18, 2020, 03:56:29 pm
content: Quote from: Labrat101 on July 18, 2020, 11:22:13 amhttps://www.brunel.ac.uk/people/david-briscoe

author: Labrat101
author_rank: Unknown
timestamp: July 18, 2020, 05:15:11 pm
content: David .Pray Tell. let us Know what you are trying to achieve as you should really know the answer yourself.If you are trying to modify an old Philips / fluke signal gen from the 60s that was not meant to be used on modern day scopes That will show things that were not available then.Or that you really did know as you are a designer .that the 741 made by Texas instruments and the St version are not the same . due to copy right the internal circuit has few transistor difference.If you want help you have to give the whole story pictures full circuit diag etc . and what the real problem is.and not a test quiz that every one get a bit between the lines .@ 67 I don't do well with quiz's . I was at the Imperial college maybe 55yrs ago . and this sounds like one of oursig gens it was new then ...Have Fun Be well

author: djsb
author_rank: Super Contributor
timestamp: July 18, 2020, 05:24:19 pm
content: Here is more information on the reason why I started working on this instrumenthttps://www.eevblog.com/forum/testgear/philips-pm5324-rf-oscillator-instability/msg3102520/#msg3102520Yes, I could struggle along on my own. But I like to get a second opinion and I don't pretend to know all the answers. I'm still learning like everyone else.Anyway I'm giving it a rest for now. Thanks for all the contributions so far and have a nice rest of your weekend.

author: Labrat101
author_rank: Unknown
timestamp: July 18, 2020, 05:36:03 pm
content: No Problem I have the service manual for this box of tricks .

author: Labrat101
author_rank: Unknown
timestamp: July 18, 2020, 07:02:00 pm
content: Hi DavidThis old signal Gen is a classic and should be restored and not updated and those op amps must be returnedas they are the old SN72741 The new are (74 series) and this machine was calibrated to work along side the Tektronix 500 . I believe.I still have my TDS 340a which is a little newerYou won't be able to use it well on a new DSO it will be like comparing a wind up Gramophone, to Dolby stereo.Both do a good Job for there time .If its working still clean it up and enjoy it . Get an old Tektronix and your good to goHave a good weekend

author: floobydust
author_rank: Super Contributor
timestamp: July 18, 2020, 07:58:23 pm
content: I breadboarded the 9V reg and tried a half dozen 741's I have lying around, date codes from gold 1971 to 2000, Fairchild, Motorola, Intersil, TI.They all behave the same. If the input common-mode voltage is below around 2V, the output goes high. So this gives the start-up action. I did the test with the (+)(-) inputs shorted, so zero differential input voltage.I would say you need a real 741 and what you have got is a knockoff of something else. I also tried OP-27, LM358 half and they also worked. Sorry for getting confused and thinking you were trying a newer part.