Krohn-Hite 1200a, Sweep Function Generator - Page 1
Forum section: electronics / testgear

author: lukaq
author_rank: Regular Contributor
timestamp: December 11, 2015, 02:22:48 pm
content: Hello all,I have this FG, where I would like to know, if anybody has schematic or knows what each pot inside does. If nothing else, I will post pictures of it. It is pretty basic FG, but works great, I really like sweep function. Compared to DIY FG I made years back, this feels like I good step up for basic work

author: tautech
author_rank: Super Contributor
timestamp: December 11, 2015, 06:40:48 pm
content: Here's the operating manual:http://www.krohn-hite.com/htm/sources/PDF/1200A%20Manual.pdfQuick look around didn't find a Service manual.If it works fine why do you wish to know what the pots do?When a SM turns up, all will be revealed.

author: lukaq
author_rank: Regular Contributor
timestamp: December 14, 2015, 08:23:58 am
content: I've also looked through operating manual, nothing to special there.The reason would be to do calibration of some sort.Frequency compared to what is set on main dial is off on higher ranges, that would be nice to get is closer to what is real output.And then duty cycle of square could be closer to 50:50 (although there is symmetry adjustment on back side of the unit, that might change for all functions, haven't tried that, don't want to change sine)What would be best way to adjust sine (if even needed?Look at peaks? And have them half way for top of sine and half way for the bottom part?I don't know if there is any other adjustment possible, like how fast sine would rise and fall (ok at that point it would only look like sine). There are many pots, which I don't know what would do and where or when.Another thing: when would be good to change main capacitors?since they seem to be original, they also must be old.

author: tautech
author_rank: Super Contributor
timestamp: December 14, 2015, 08:41:04 am
content: Quote from: lukaq on December 14, 2015, 08:23:58 amIt would be unwise to adjust anything with out a Service manual.You don't say if it is a linear PSU, just guessing it is.The PSU caps condition can be checked in 2 ways: use a scope or a DMM set to AC mV and measure the power rails for ripple. After regulation you'd want to see less tha 10mV ripple.Or, remove them and check that measured values are no more than 10% from those on the label.And if you can do ESR tests, better still, linear caps need to be a low single digit Ohms value and SMPS caps well under 1 Ohm.There is the possibility that excessive ripple on the PSU rails is responsible for the problems you describe.Some good pics would help if you get stuck.

author: lukaq
author_rank: Regular Contributor
timestamp: December 14, 2015, 04:39:41 pm
content: I should post pictures... Supply is linearI agree, I won't touch anything without knowing what it does. But two adjustments are on outside of case, so user can adjust them.I mean, 49.6/50.4 or something in this range I wouldn't say it's bad, I'm sure this is not precision instrument in the first place, but I can check some stuff with scope and 34401a.I guess it is time for picture and some more detail "problems"

author: tautech
author_rank: Super Contributor
timestamp: December 14, 2015, 08:26:47 pm
content: Quote from: lukaq on December 14, 2015, 04:39:41 pmKrohn-Hite make nice stuff, a broken unit I have is almost entirely discrete, great build quality and easy to work on. You can keep these running for years IF you can get Service manuals.I have an old Philips FG also like yours with the frequency dial some way off calibration, does it matter for most use....no. Just connect it to a DSO and check the frequency setting is close enough for you needs and carry on.Distorted waveforms are another matter and as you might agree the PSU performance should be checked first in case it is responsible for the distortions. Quite likely IMO.

author: lowimpedance
author_rank: Super Contributor
timestamp: December 14, 2015, 10:41:26 pm
content: Another issue with old gear in general and FG with all their DC paths is ageing carbon comp. resistors (if used that is). These parts always drift up in value and can cause all manner of waveform issues due to circuit biasing changes.So a look to see if they are used, identify and measure to be sure.

author: Electro Fan
author_rank: Super Contributor
timestamp: December 14, 2015, 11:55:37 pm
content: http://www.krohn-hite.com/htm/forms/CustomerContact.htmIf you call them they are good folks who will tell you what documentation is available and what is long gone and give you a bit of time on the phone if you have questions about their older models.

author: lukaq
author_rank: Regular Contributor
timestamp: December 23, 2015, 06:22:52 am
content: Quote from: tautech on December 14, 2015, 08:41:04 amTwo main caps, a bit over 17Vdc, about 0.68Vac @100Hz ripple, after regulator: 13.55Vdc and 0.5mVac. This is not loaded, just power on. I think I will retest with 50R load

author: dom0
author_rank: Super Contributor
timestamp: December 23, 2015, 06:17:08 pm
content: The symmetry adjustment will most certainly affect all waveforms, since it will balance the run-up/run-down symmetry of the triangle wave generator. There are some ways to trim it accurately, e.g. exact duty cycle of triangle wave, or minimum distortion of the sine wave (FFT / SA / audio analyzer). Trimming it by square wave may not yield best results, because in some designs there is an inherent imbalance in the duty cycle of it.Usually these things have one extra trim for each multiplier and one trim to set the range of the dial.