Help! Oscillator starts then dies on Datron 4200 Calibrator - Page 1
Forum section: electronics / repair

author: essele
author_rank: Frequent Contributor
timestamp: April 14, 2019, 10:09:30 am
content: Hi,I've been working on fixing a Datron 4200 AC Calibrator, and sadly think I've introduced more problems than I've fixed (the original post is here:https://www.eevblog.com/forum/repair/datron-4200-ac-calibrator-fail-2-during-selftest/)I think I've narrowed the problem at hand down to the oscillator that starts, increases amplitude and frequency, but then damps quickly to zero (over a few cycles.)I think I've tested pretty much every input to the oscillator and can't see anything changing at around the time of the damping, so I'm really stuck now and would love to get some pointers ... I am way way way out of my depth in terms of understanding how all the control circuits work.I have attached the schematic, you can see it's a oscillator built with two integrators and an inverter, then there are some control circuits that handle frequency and amplitude control.I've also attached a couple of scope traces ... the first (ending 223) shows the tail end of the oscillations, and is setup as follows:CH1 - TP4CH2 - TP5CH3 - M23 pin 6 ... output of the amplitude controlMath - CH2 - CH3 (I think, I was trying to check that M15 was doing as expected comparing against CH1)The second (ending 236) shows the overall activity from power-on through to the oscillations stopping, and then the various signals moving to fixed positions. This one was setup the same.Other things I have tried:1. I've disconnected LK.C to disconnect the output, to ensure it wasn't causing the problem -- no change.2. I've moved LK.A to LK.F to use a test signal for frequency control -- no change, which presumably rules out the FREQ ERROR signal.3. I've removed the logic that drives the relays (RL1-RL8) to eliminate the switching in of different caps during the power on sequence.I haven't eliminated the switching in of resistance by the FET's ... could that be an issue? Dodgy FET?I haven't eliminated the possibility that the +/- Vsin and Vcos could be pulling things down?With the FET's I did consider that they might be all turning off, however my assumption is that would break the loop and hence you'd see an abrupt stop rather than a damping?Any clues or anywhere I can look next would be really appreciated!

author: essele
author_rank: Frequent Contributor
timestamp: April 16, 2019, 07:02:17 pm
content: So there is more activity a bit later on after power on ... this time I've got traces from TP5 (output of inverter), TP2 (output of first integrator) and TP4 (output of second integrator, which is then input to the inverter) as CH1,2,3 respectively.My understanding (having now researched a bit on dual integrator oscillators) is that each of the op-amps used will actually invert the signal, since we are using them in the inverting configuration, so that then leaves me with the strange dilemma that the second integrator appears to not be doing that. You can see run-up to the pulse in the picture where CH2 is at +4.4V yet CH3 is at 4.08V with no sign of movement.How is that possible?Is looks like it's pretty much just a mirror of CH2 with some slight amplitude differences ... I have swapped the op-amps with each other and it's didn't change anything.My understanding (and as checked with an online simulator) is that a positive input to the second integrator should actually start to cause the output to fall, but nothing is happening ... and then I have no idea what actually triggers the eventual pulse.

author: essele
author_rank: Frequent Contributor
timestamp: April 19, 2019, 11:47:21 am
content: Ok, I feel like I'm talking to myself, but (a) it helps me think, and (b) hopefully it will prove a useful reference...I'm now pretty sure that the strange behaviour of the oscillator (i.e. the non-conformance to the negative input) is actually because the resistors are switched out of the loop as part of the various stages of power on, so it leaves the loop unconnected, and therefore it's not running properly as an oscillator. So I think that was a red herring.So I have now systematically replaced all of the easily obtainable IC's on the Sine board with no change to the behaviour. There are only a few left that I'm still waiting for...M1, M5, and M9 - MC14569 ... I doubt these are at fault as I was seeing correct VCO and Synth O/P, plus they all compare similarly with a resistance check from pin to pin.M12 - 74C932N ... this could be an issue, no obvious shorts, need to wait for a replacement part to check.M15 - LH0032CG ... unlikely to be an issue as the inverter appears to be working properly.M38 - AD7527KN ... the signals from this part seems to be behaving, and I don't think it has an impact on the oscillator, so unlikely.M45 - uA714HC ... this is after the oscillator and I don't think it has a bearing (replacements on order anyway)M34, M40 - CA3083 ... these are used in the sin/cos squarer circuits and are directly related to the oscillator ...The CA3083 is a 5-NPN transistor array ... interestingly both parts check-out fine from a diode drop point of view, however base to emitter resistance for all-but-one of the transistors is about 90M, but for one transistor in one part it's 45M .. I really don't know if this is significant, both transistors seems to have similar characteristics when tested with a transistor tester, so doubt it's an issue (I could only find these on AliExpress, so it may be a while before I can try replacing them -- and then who knows what they will actually be!)There are still many more parts to test, but I'm starting to suspect that this may actually be a fault with the AC board, since this is where the AC AMPL ERROR signal comes from.Onwards and upwards...EDIT: just found the CA3083's on eBay (I clearly wasn't looking that hard) ... so hopefully will now have some replacements in a few days.

author: sorin
author_rank: Frequent Contributor
timestamp: April 27, 2019, 03:31:37 pm
content: Please can you share Datron 4200 service manual with us?Thanks!

author: essele
author_rank: Frequent Contributor
timestamp: April 27, 2019, 05:11:23 pm
content: Hi Sorin,I got the manual from someone else who paid handsomely for it and painstakingly scanned it ... I did chip in but I don't feel that I can just post them without his agreement. I'll drop him a PM.Just for completeness of this thread, the oscillator problem actually turned out to be a faulty resistor in an array (AN14) which messed up the 'summing amplifier' ... unfortunately I couldn't get a DIP package of similar tolerance so I ended up sticking in a surface mount version with an adapter and it all works fine.Lee.

author: essele
author_rank: Frequent Contributor
timestamp: April 28, 2019, 07:37:39 am
content: OK ... I've uploaded all the manuals to K04BB's web site (the last one is still uploading, I live at the end of a very long piece of string!) They are all in the recently uploaded section, and should hopefully be available to download in the next few days.The manuals are:- User Handbook- Calibration and Servicing Handbook Volume 1 (in three parts)- Calibration and Servicing Handbook Volume 2 (parts lists and schematics)The web site is here:http://www.ko4bb.com/getsimple/index.php?id=manuals