EXACT 628 Pulse function generator [fixed?] review - Page 1
Forum section: electronics / testgear

author: babysitter
author_rank: Frequent Contributor
timestamp: January 09, 2016, 04:34:17 pm
content: Does any of you gentlemen know these generators? I got one, would love repair assistance.Most of the time, the +5V test point sits at 0.5V, however, the device has managed to do a correct start a few times, but also showed buggy behaviour sometimes (scrambled waveforms).Is there any suspect I should check first?Some pictures below...

author: Vgkid
author_rank: Super Contributor
timestamp: January 09, 2016, 05:12:22 pm
content: Is that an lm399 I spy.

author: AF6LJ
author_rank: Unknown
timestamp: January 09, 2016, 05:16:25 pm
content: Start with the basics;Make sure you have clean supply voltages.

author: dom0
author_rank: Super Contributor
timestamp: January 09, 2016, 05:21:19 pm
content: Quote from: Vgkid on January 09, 2016, 05:12:22 pmIt certainly looks like the plastic cap of a LM399. But what would they need such a stable voltage reference for in a pulse generator for? I mean, they do generate the timing digitally, right? Right? No one in their right mind would go all the way and create stable, precise control voltages for what would essentially be an analog pulse modulator?!

author: babysitter
author_rank: Frequent Contributor
timestamp: January 09, 2016, 06:49:37 pm
content: dom0, Vgkid: Yes, I alspo consider this thing to be a LM399. Why? Because it has the cap.What might it be good for? I dont have a service manual (something I would really love to have), but maybe the heart of the generator is a varactor tuned oscillator and it is used there somehow, also, besides a frequency and a waveform, one characteristic of the output signal is a amplitude. it might be used to get the right level of amplitudyness.The truth might be told by someone who knows this device here.AF6LJ: This is what is going to happen, but sometimes you find people who can tell you which parts break first, its not necessarily the power supply.

author: babysitter
author_rank: Frequent Contributor
timestamp: January 09, 2016, 08:14:26 pm
content: Does anyone know that other mystery part, at picture 0160109_100057.jpg ? The big one, DNI 2130-0001?

author: AF6LJ
author_rank: Unknown
timestamp: January 10, 2016, 02:17:31 am
content: Quote from: babysitter on January 09, 2016, 06:49:37 pmYou are right; however you have to start somewhere, and it might as well be with the power supply.

author: babysitter
author_rank: Frequent Contributor
timestamp: January 19, 2016, 10:12:57 pm
content: Some progress: Voltages clean, havent seen the 0.5V problem in a while. Filter capacitors exchanged due to copious amounts of ripple, but ~1.4Vpp 100 Hz residual ripple still present at the positive leg of the 10000uF/16, still looks like a lot to me. The corresponding rectifier is also operating very hot, unpleasant to the touch.

author: bitslice
author_rank: Frequent Contributor
timestamp: January 20, 2016, 12:49:43 am
content: Quote from: babysitter on January 19, 2016, 10:12:57 pmMeasure the current.Then pop out all the ICs and measure the current again, it should be almost nothing.Refit the ICs in stages and see if the current increases more than you'd expect.

author: babysitter
author_rank: Frequent Contributor
timestamp: January 20, 2016, 07:59:18 am
content: I was no longer really awake when I wrote that.To isolate the problem if it is on rectifier side or load side, I disconnected the load by pulling the plug which goes to the regulators, it was immediately quiet at the cap, so load side. Lets see if Ifind some time to do further work today.There are two candidates running quite hot at the load side: A SN74Sxx logic chip and also that DNI "mystery device".EDIT: I am afraid this DNI means Dynatech Nevada Inc. Would be unobtainium then ?

author: babysitter
author_rank: Frequent Contributor
timestamp: January 25, 2016, 08:01:43 pm
content: Intermediate success: A cold solder joint was responsible for the intermittent operation, stable against hitting now.High temperature of rectifier and regulator still an issue.Also tried to hook up GPIB: I get a response when doing a serial poll on addr 5, LEDs show adressed+remote accordingly.Sending garbage showed first success, trying to figure out the command set.

author: babysitter
author_rank: Frequent Contributor
timestamp: January 25, 2016, 10:24:26 pm
content: I came to try some single and three-letter commands by hand, then wrote a little EZGPIB script to simplify scanning for valid commands. This list is far from complete as I aborted the search (Actually went from A** to F** and did a final scan at P** to find commands regarding Pulse operation today. Waiting for a EDIT.) The DIP switch doesn't select the GPIB address.Reverse engineered command chart for Exact 628 Function generatorFirst, use serial poll to find the device address. Mine is listening on address 5.numerical selection (ranking according front panel) marked with a #numerical values marked with a nCommands are not case sensitive. GPIB understands engineering notations, signed numbers and decimal point.CommandsAMPnSet amplitude in VBPRnSet Burst Period in sBCTnSet Burst countCALCal-LED flashesFRQnSet frequency Hz, when Frequency is shown step-up with SUP, step down with SDNJK#select front 1 or backside panel 0 mains output jackM#Trigger mode selectMEM#access memory number, followed by RCL (Recall) or STO (Store)MNV#orNV#inverted output on 1 or off 0OFF#Offset on 1 off 0OFSnOffset V, voltage with signPDLnPulse delay sPWDnPulse width sSW#Sweep mode select 0-5SWTnSweep timeW#orAW#Waveform select 1-7

author: tautech
author_rank: Super Contributor
timestamp: January 25, 2016, 11:18:19 pm
content: Quote from: babysitter on January 25, 2016, 08:01:43 pmYou would expect this to indicate a shorted polarised cap, tantalum most likely.As there is probably no HV present a finger test (temp) or a thermal image might help you find the culprit.