----- Original Message -----

From: jeromequelin

To: TekScopes@...

Sent: Wednesday, October 10, 2012 10:25 PM

Subject: [TekScopes] 468 triggering issue

?

Hello everyone.

I just got this wonderful 468 that is in great physical shape, in and out (the boards are gorgeous!), but sadly I believe that it has a triggering issue. I am rather new to electronics and this is my first scope so bear with me.

Simply put, I can't seem to trigger on any signal and adjusting the trigger level does not seem to do anything. Let's pretend for a minute that I want to display a stable cal. signal (300mV, 1kHz square wave). Here are my settings :

Non store mode.

Vertical

- Volts/div: .1V

- Vert mode: Ch1

- Coupling: AC

Horizontal

- Horiz Display: A

- Delay time position: fully counterclockwise

- Time/div: .1ms

-X10 mag: OFF

Trigger

Trig mode: auto

A trigger source: norm

Trigger slope: positive

A trigger holdoff: norm

No mater how I adjust the trigger level, what I get on the screen are two solid traces, shaking ever so slightly, which allows me to guess that there's a waveform somewhere in there. The trigger LED is off. The really bizarre thing happens when I switch to "norm" trigger mode and fidget with the slope switch: every time I change the trigger slope from + to - and vice versa, the scope triggers for a very brief instant, the trigger LED goes on and immediately turns off, and the ghost of the square wave appears on the screen. There is one way that I can somewhat stabilize the signal: by adjusting the trigger holdoff level until the trace drifts slowly from one side of the screen to the other (it sometimes goes back and forth a few times a minute), at which point I'm able to clearly see a square wave (which by the way seems way out of spec: I estimated 1.6kHz).

As far as troubleshooting is concerned, the only thing I was able to do with my minimal test equipment is check the DC voltages, especially the +55V rail, and they all appear to be correct. I haven't checked the digital supply voltages but I assume this wouldn't have an effect on the analogue end.

I am sorry for this long wall of dry text, with no pictures but I am currently away from the scope.

Thanks in advance for your help!

Jerome.

Feed the test signal into the Ch1 and the Ext trigger input. Select Ext on the trigger source. What happens?

Tom

Alright I realized I could couple the cal. signal into the ext. trigger
bnc with my second scope probe... silly me! Unfortunately it didn't
change anything, meaning that I still have my untriggered signal no
matter how much I turn that trigger level knob. I have been patiently
browsing the service manual (though I have not read it religiously cover
to cover quite yet) and I haven't found a good chart for troubleshooting
the triggering section yet. For all I know the problem could be more
general and not be caused by faulty circuitry on the trigger board at
all!
Thanks a lot Tom for your input.
Jerome.


--- In TekScopes@..., "jeromequelin" <jerome.quelin@>
wrote:

It looks like I'm going to need some more coax for this :(
But if I go into "A Trig View" mode, change source to ext. and press

the beam finder button, a straight but slightly distorted trace appears
beneath the signal and here again adjusting the trigger level doesn't
change anything.

Next thing on my to-buy list : bnc to bnc coax and a T adapter.

--- In TekScopes@..., "Tom Miller" tmiller11147@ wrote:

Feed the test signal into the Ch1 and the Ext trigger input. Select

Ext on the trigger source. What happens?

Tom

Another suggestion: Set the SOURCE switch to LINE triggering, and with the vertical sensitivity cranked up pretty high (i.e., like 100 mV) on the channel that your probe's connected to, touch the probe tip to your finger. You should see a triggered 60 Hz signal (assuming your house current is 60 Hz), assuming that your body is not grounded. Alternately, use an old AC adapter ("wall wart") that has an AC output, but turn the channel's vertical sensitivity down (i.e., like 5V).

--- In TekScopes@..., "jeromequelin" <jerome.quelin@> wrote:

Alright I realized I could couple the cal. signal into the ext. trigger
bnc with my second scope probe... silly me! Unfortunately it didn't
change anything, meaning that I still have my untriggered signal no
matter how much I turn that trigger level knob. I have been patiently
browsing the service manual (though I have not read it religiously cover
to cover quite yet) and I haven't found a good chart for troubleshooting
the triggering section yet. For all I know the problem could be more
general and not be caused by faulty circuitry on the trigger board at
all!
Thanks a lot Tom for your input.
Jerome.


--- In TekScopes@..., "jeromequelin" <jerome.quelin@>
wrote:

It looks like I'm going to need some more coax for this :(
But if I go into "A Trig View" mode, change source to ext. and press

the beam finder button, a straight but slightly distorted trace appears
beneath the signal and here again adjusting the trigger level doesn't
change anything.

Next thing on my to-buy list : bnc to bnc coax and a T adapter.

--- In TekScopes@..., "Tom Miller" tmiller11147@ wrote:

Feed the test signal into the Ch1 and the Ext trigger input. Select

Ext on the trigger source. What happens?

Tom

Thanks for your suggestions jtjewell83. Sadly I was not able to trigger on a line signal (either picking it up from my finger or from a 24V AC signal). I have checked the trigger level pot and there doesn't appear to be any short or open circuit. The only thing that "works" about this pot seems to be the slope switch, which enables me to trigger on a signal for a fleeting moment in NORM trig mode, as I have described in a previous post (I have just now noticed that when I exercise this switch from + to - and vice versa in AUTO trig mode, the trace drifts ever so slightly faster for a fleeting instant. It doesn't briefly trigger as it does in NORM though). I am going to have to make a little video to give you all a better picture of what is going on. I might be doing something completely stupid! I am also in the process of reading the "Theory of Operation" section from the service manual and looking at the block diagrams and troubleshooting charts. There could be something wrong with the signal picked up from the CH1 preamp board that prevents the trigger circuitry to generate a trigger signal (if that makes any sense whatsoever!)

I'll keep you posted this weekend, and thanks again for the support. I am learning so much already, I do not regret an instant my purchasing this scope. Now if only I could buy another functional one to troubleshoot the Tek :)

Cheers,
Jerome.

--- In TekScopes@..., "jtjewell83" <jtjewell83@> wrote:

Another suggestion: Set the SOURCE switch to LINE triggering, and with the vertical sensitivity cranked up pretty high (i.e., like 100 mV) on the channel that your probe's connected to, touch the probe tip to your finger. You should see a triggered 60 Hz signal (assuming your house current is 60 Hz), assuming that your body is not grounded. Alternately, use an old AC adapter ("wall wart") that has an AC output, but turn the channel's vertical sensitivity down (i.e., like 5V).

--- In TekScopes@..., "jeromequelin" <jerome.quelin@> wrote:

Alright I realized I could couple the cal. signal into the ext. trigger
bnc with my second scope probe... silly me! Unfortunately it didn't
change anything, meaning that I still have my untriggered signal no
matter how much I turn that trigger level knob. I have been patiently
browsing the service manual (though I have not read it religiously cover
to cover quite yet) and I haven't found a good chart for troubleshooting
the triggering section yet. For all I know the problem could be more
general and not be caused by faulty circuitry on the trigger board at
all!
Thanks a lot Tom for your input.
Jerome.


--- In TekScopes@..., "jeromequelin" <jerome.quelin@>
wrote:

It looks like I'm going to need some more coax for this :(
But if I go into "A Trig View" mode, change source to ext. and press

the beam finder button, a straight but slightly distorted trace appears
beneath the signal and here again adjusting the trigger level doesn't
change anything.

Next thing on my to-buy list : bnc to bnc coax and a T adapter.

--- In TekScopes@..., "Tom Miller" tmiller11147@ wrote:

Feed the test signal into the Ch1 and the Ext trigger input. Select

Ext on the trigger source. What happens?

Tom

So here's a new episode in my troubleshooting adventures. While following one of the diagrams (troubleshooting diagram number 3, the non-storage portion), I encountered a note recommending that I disable the storage section of the scope. It took me a while but I eventually found directions on how to do this. But these are kind of crude and I am not sure I did everything correctly:

1. Disconnect P3008 from the A12 Trigger Generator Sweep Logic board (see Figure 8-17)
2. Disconnect P483 from the A23 circuit board and connect a jumper between the two square-pin connectors on the board. I found this to be a bit confusing: is it asking to short the two pins of conector P483 with a jumper (which I did by soldering a piece of wire) or should I short more than one connector, and if so which one?
3. Move P307 (at the rear of A14 Vertical Mode Switch board) to the Test Mode position (see figure 8-15). But what is this Test Mode position? I am sure it is referenced somewhere in the 600 pages but after a couple hours of skimming through both volumes I can't seem to find it. There's no mention of a test mode connector on A14's schematic. The only thing I could see is this other connector, P206, that is sitting on top of P307. I switched 307 with 206 it but the scope wouldn't display anything and the fan was not turning anymore.

So I must have gone wrong somewhere and If anyone knows how to disable the storage unit, any guidance would be greatly appreciated. Thanks for your patience with my lack of experience, I'm really enjoying this repair so far!
Jerome.

So here's a new episode in my troubleshooting adventures. While following one of the diagrams (troubleshooting diagram number 3, the non-storage portion), I encountered a note recommending that I disable the storage section of the scope. It took me a while but I eventually found directions on how to do this. But these are kind of crude and I am not sure I did everything correctly:

1. Disconnect P3008 from the A12 Trigger Generator Sweep Logic board (see Figure 8-17)
2. Disconnect P483 from the A23 circuit board and connect a jumper between the two square-pin connectors on the board. I found this to be a bit confusing: is it asking to short the two pins of conector P483 with a jumper (which I did by soldering a piece of wire) or should I short more than one connector, and if so which one?
3. Move P307 (at the rear of A14 Vertical Mode Switch board) to the Test Mode position (see figure 8-15). But what is this Test Mode position? I am sure it is referenced somewhere in the 600 pages but after a couple hours of skimming through both volumes I can't seem to find it. There's no mention of a test mode connector on A14's schematic. The only thing I could see is this other connector, P206, that is sitting on top of P307. I switched 307 with 206 it but the scope wouldn't display anything and the fan was not turning anymore.

So I must have gone wrong somewhere and If anyone knows how to disable the storage unit, any guidance would be greatly appreciated. Thanks for your patience with my lack of experience, I'm really enjoying this repair so far!
Jerome.

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

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

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

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

----- Original Message -----

From: jeromequelin

To: TekScopes@...

Sent: Thursday, November 01, 2012 10:25 PM

Subject: [TekScopes] Re: 468 triggering issue

?

Hi everyone.

I haven't had much time this week to work on my trigger issues but ?I have been doing various DC measurements and have found some serious discrepancies that I had not noticed before. First of all on the trigger assembly (A12) I have been probing mostly around U445 and found the following:

• the +8V and -8V rails for U445 sit respectively at +9V and -7.71V
• pin 7, trigger input #5, (which should be at 0V) is at -7.25
• voltage between pins 10 and 11 (which should be +1V) read 0V
• pin 8, slope select, (which should be -1.8V) is at -1.7V

I did wiggle the caps a bit (that probably wasn't too smart) and when I turned my scope off and back on nothing came up on the screen and all LEDs on the front panel were out. I rechecked all the main power supply voltages (once again just the DC component) and sure enough the +5V rail now sits at 0.694V... (all the others are within tolerance) Looking at the schematic, the 5V rail is?dependent?on one of the 5000uF caps that I "inspected" (C935). My "magic" touch may have killed it...

So it looks like right now my biggest priority is to bring the various power rails back to their original vigor (or is it?), which leads me to my next and possibly final question for now. I went ahead and removed the trigger board to gain better access to the leaky capacitors and inspected the damages up close. It looks like at least 3 of them are dead (I do not have an ESR meter but the blackened goo under seems to indicate leakage) and I hope the electrolytes have not corroded the traces. I have done some research and if I want to replace these with good quality caps (Cornell Dublier) it'll cost me $20 to $30 per cap (which could bring me to $150 if I wanted to replace all 5, to be safe). Now that's quite an investment for a scope I paid $80 and while I am determined to go through with this repair, if I cannot find a cheaper source for equivalently speced electrolytics I'm going to have to put this project on stand by. I only searched on the usual sites (mouser, digikey) and on the Cornell Dublier website, but has anyone got a good recommendation for me?

Thanks in advance for the precious insight.

Jerome.

--- In TekScopes@..., "jtjewell83" wrote:
>
> 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 http://www.simpson260.com/downloads/downloads.htm.
>
> 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
>
> --- 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" 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" > > > >
> > > >
> > > >
> > > > 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" 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:
> > > > > http://www.sillanumsoft.org/ . 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"
> > > > > 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
> > > > > > >
> > > > > >
> > > > >
> > > >
> > >
> >
>