Jerome,
One way to measure ripple on a DC supply line without a scope is to cap-couple into your Fluke with like a 1 uF or so cap. The cap blocks the DC from the meter, so the meter will only measure the ripple (AC) component. Just put the cap in series with the + meter lead. Observe polarity and remember to flip it for a negative supply (or just flip the meter leads and keep cap on + lead). Set the Fluke to AC volts. The 87 III reads true-RMS, not peak-to-peak. The cap's voltage rating should be higher than the voltage of the DC supply line that you're measuring!!! And be careful!!! Oh, you might want to slightly load the meter side with like a 1M ohm or so resistor so the cap's leakage current doesn't affect the readings. The meter should be about a 10M ohm load. Make a table of measured Vrms for all supplies, then convert to Vp-p (assume a sinewave). Someone in the group please chime in here if I'm advising Jerome incorrectly, or if I'm missing something. (It's been a long time since I've done this, since I have so many Tek scopes. LOL.)
The old Simpson 260 analog meters had a 0.1 uF cap (inside meter) at the meter's OUTPUT terminal. This terminal is mainly for measuring audio frequency levels on a DC bias, not 60 or 120 Hz ripple levels. But you can get relative measurements, even at 60 Hz. See the schematics and operator manuals at .
I'm no expert on replacing the large caps. I used solderwick and 2 soldering irons for my 468 repair job. 2 were bad/open, but I went on and replaced all 4 (or is it 5??) I do not have a vacumm desoldering iron. I just removed as much solder as possible with the wick, and heated 2 joints simultaneously then VERY gentley rocked/pushed the cap sideways a little, removed heat, repeated on other joints and in opposite direction, repeat, repeat, repeat, until it was removed. Not easy with just 2 hands - 2 people is better. I did lift a few of the doughnut pads, but if I remember correctly, they were the un-connected ones, so that really didn't matter. Remember, a few jumper's are needed, since the metal 3/4-terminal cans do that, but 2-leaded caps don't. I don't think any desoldering iron that Radio Shack sells will have a tip with a large enough hole, but I'm not sure on that. I took some before and after pics. Maybe I'll post them, but they're others in the photos section of Tek Scopes.
If you get back to troubleshooting the trigger circuit, don't worry so much about the signal level. I beleive the 468's calibrator is 300 mVp-p, not 500. You're mainly looking for some kind of a signal at the pin 7s.
Glad to help and hang in there,
Jimmy
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--- In TekScopes@..., "jeromequelin" <jerome.quelin@...> wrote:
Thanks Jimmy, I am always very excited to hear from you (and from everyone else of course!)
Since my rudimentary Global Specialties 2001 function generator seems to be quite out of spec (according to my Fluke 87 III, recently calibrated) I decided I'd apply the 1kHz signal from the calibrator (It's not 500mV as specified in the manual but I thought it might stille give me something). Maybe I can somehow try getting a 500 mV 1kHz square wave from my SG and try again? As for the voltages I did check them for ripple and they all appeared to be fine (I did that with my Fluke) but a second thorough check sure is in order now. I'll do this tonight as well as checking the DC voltages around U445. It looks like the trigger board is going to have to go and I'll give the coupling and source switches a good cleaning.
As for the replacement of the electros do you recommend I get a vacuum desoldering gun (of the like of the Hakko 808) or would my low tech solder pump and whick would do the trick on a multilayer board? Somewhow I doubt it. I have seen that Radio Shack sells a cheapo soldering iron/solderpump combo that might just do the trick. Maybe I can try with this?
I'm going to have a nice chunk of time this weekend for experiments so I'll follow closely all your recommendations as usual and will get back to you with some (hopefully) positive results this time. Thanks again for all your help, I'm having so much fun :)
Cheers everyone,
Jerome.
--- In TekScopes@..., "jtjewell83" <jtjewell83@> wrote:
Jerome,
Did you apply the 1 kHz square wave to both Ext. Trigger inputs, like the manual says? If you did and you don't have a signal at pin 7 of each IC, I'd say you have a problem in one or more of the SOURCE or COUPLING switches (possible water damage maybe????). Oh, have you checked the +8 and -8 volt supplies at least at Q430A/R433 (+8V) and Q430B/R???-can't read schematic (-8V)?
Also, check the DC voltages around U445, if you haven't already. Some are on schematic (in the ovals).
I thought you had already check ripple on all the supplies, but I cannot remember for sure.
I had to replace the big electrolytic caps in one of my 468s. It's a job, but if done carefully, you'll get good results. Maybe about 8 hours total for me, including time spent to order the new caps. You do not have to remove the Interface Board to it, just the Trigger Board. Be carefull and try not to damage the pads/traces too much. And you'll need to add a few jumpers for the negative/can side of several of the caps. Plenty on that in the archives for 465, 475, 468. Make sure you get polarity right on each cap too!!! Post back if you're not sure about this.
Jimmy
--- In TekScopes@..., "jeromequelin" <jerome.quelin@> wrote:
Hello Jimmy and everyone else,
I finally have some time to give you a little summary of what I did. I followed Jimmy's advice and put together a sound card oscilloscope probe (even did an attenuator circuit for voltages above 1V) and probed a few test points on the A12 trigger board. Sadly on pin 7 of U240 and U445 I cannot seem to get a signal that makes any sense (nothing that the sound card scope can trigger on) and this lackluster signal doesn't seem to be a result of my very precarious probe because I could detect and trigger quite nicely on the 1kHz calibrator signal with it. I did set up the scope according to the manual (ext. trigger on both trigger A and B, AC coupled, external triggers...) but I could not make sense of what I observed on these two pins.
I did make two new observations:
1. whenever I probed U445 the scope triggered briefly, just like when I twiddled the slope switch. It happened every time I made contact with my sound card probe and every time I removed the probe.
2. This is a grim discovery : it looks like a couple of the large electro caps (C926 and C935 by the looks of it) from the main power supply have been releasing their corrosive guts and left a black trail around them... this is not good as I think I can't easily replace these guys... (and removing the A15 interface board on which they dwell seems like quite a production!)
All in all I think the sound card scope option just isn't going to cut the mustard. I am going to put this project on the side for maybe a month or until I can finally get hold of a functioning scope. In the meantime I'm going to start giving this little guy a good cleaning and maybe work on replacing these electros if at all possible!
Thanks so much for all your willingness to help me with my meager experience and scarce test equipment (I know this kind of long distance troubleshooting can be very frustrating for those helping!).
Jerome.
PS: I am located in the USA, near Princeton, NJ.
--- In TekScopes@..., "jtjewell83" <jtjewell83@> wrote:
Jerome,
I'm thinking you're going to need a scope to trace signals (unless you
get really lucky with your DMM!!!).
For a free PC sound card based scope, try this one:
<> . Make up a
simple cable that plugs into your sound card's line or mic input on one
end, and like a DMM probe tip (signal) and alligator clip (ground) on
the other. Then play with this with like some AA batteries, a 9-volt
battery, AC adapter with an AC output, etc., etc., to get comfortable
with this s/w. I'm thinking all or most sound cards have AC-coupled
inputs, so DC sources may not shown an indication when connected, except
maybe a quick spike. (I've never used this s/w much, just with the
internal mic in my laptop.) Hopefully, 9 volts DC won't damage the
average sound card's line or mic input. Maybe someone else in the group
knows of a better free PC sound card scope. I know there're others in
the TekScope archives.
Once you get that going and understand approximate peak-to-peak signal
levels, then start looking at what kind of signals you've got at pin 7
of U240 and U445 on the Trigger Gen. and Sweep Logic Board. Sheet 6,
A&B Tigger Generator. It looks like Q230A/B and Q430A/B have plenty of
drive capability (assuming they are good), so the sound card's Line
Input impedance shouldn't be a problem at these test points (44 and 41
respectively). Mainly concetration on U445, since that's for the A
sweep. The waveforms are just before the schematic. Let us know if you
do something like this, and we'll go from there.
If someone can think of something better for this situation, please feel
free to chime in.
Oh, what part of the world are you located? Maybe there's a group
member nearby that could help you.
Jimmy
--- In TekScopes@..., "jeromequelin" <jerome.quelin@>
wrote:
Yes I have but it won't trigger either. It looks like other than the
triggering issue the storage mode is fully functioning (the markers
seem
to work, the various display modes, norm, enveloppe, avg, seem to
behave
normally). I didn't have much time to work on it today but I read the
theory of operation some more and decided to start checking
transistors.
I checked Q119 (it's alright) whose emitter is used to generate the
trigger signal for ch1. Would you recommend that I start checking
every
transistor on the triggering board?
PS: I "fixed" the problem with ch1 and 2 traces being always on, a
problem I had created by mis-plugging a connector (P307)
Jerome.
--- In TekScopes@..., "jtjewell83" jtjewell83@ wrote:
Jerome,
Have you tried triggering while in storage mode?
Jimmy
