Keyboard Shortcuts
ctrl + shift + ? :
Show all keyboard shortcuts
ctrl + g :
Navigate to a group
ctrl + shift + f :
Find
ctrl + / :
Quick actions
esc to dismiss
Likes
- TekScopes
- Messages
Search
|
Re: 453 trouble
stefan_trethan
Yes, for best frequency response the ground lead should be short and
toggle quoted message
Show quoted text
the area between it and the probe minimal. You may see ringing and other strange stuff with your separate ground lead. Probably OK for a rough check of supply voltages or slow signals, but definitely no use for ripple and noise measurements (in those cases sometimes even the supplied alligator leads are just too long). You need to think about what you are measuring and adapt accordingly. Using the absolute best setup all the time is also completely impractical. ST On Thu, Aug 16, 2018 at 6:05 AM, F4GNY <f4gny@...> wrote:
Question : sometime when troubleshooting radios, I use a ground lead |
|
Re: 453 trouble
Hello Folks,
toggle quoted message
Show quoted text
Thanks for all thoses interestings comments. As Ive reconnected the ground, and turned power ON : no differential main shut down occur, even when my finger played with chassis parts. I also mesured voltage between ground and chassis : close to zero -- few mV-.. The idea to use the same wall socket to avoid ground loop is fine. Question : sometime when troubleshooting radios, I use a ground lead between scope and radio to avoid using probe's aligator clip. What do you think about ? can I introduce false mesurements ? Regards Alain Le 15.08.2018 ¨¤ 20:40, Richard Knoppow a ¨¦crit?:
??? A hot chassis can also be caused by RF filter capacitors connected |
|
Re: Dead 7603
Well, after a week on vacation, I replaced the old 18,000 uF, 16 V C821
toggle quoted message
Show quoted text
with a new 22,000 uF, 25 V job that was the same diameter but less than half the length. I knew I was going to be out of town, so I ordered from Digi-Key just before I left, and the caps (I bought 2, just in case) arrived just before I got back. After removing the old cap by sawing off 2 of the leads right next to the edge of the rectifier board with a hacksaw blade and a utility knife, I was able to mop up the solder from the other 3 leads with generous use of rosin flux and large solder braid. The new cap's leads didn't quite match up with the holes, so I added a piece of 22 gauge wire from one. And... shorted it out on the underside of the board! D'oh! Tried again with the one lead with the wire on it cut down to size and covered with Kapton and electrical tape. Voila, that did it! 5 V rail is clean now. Still a bit of ripple on all supplies, particularly 15 V, but good enough for now. The 7603 is back to life and triggering on millivolt signals no problem. Thanks everybody for your help! Next project is replacing the backup battery (also purchased from D-K at the same time as the caps) in my HP 8566 spectrum analyzer, but that is a story for another group. And at some point I'll want to try to fix my botched job replacing a CMOS battery on my Tek TLA711 logic analyzer. If I don't get very far with that job, I'll ask here, unless there is a Tek logic analyzers group. Jim ------ Original Message ------
From: "Craig Sawyers" <c.sawyers@...> To: [email protected] Sent: 7/21/2018 11:43:55 PM Subject: Re: [TekScopes] Dead 7603 BTW, I did try a 7A22, a 7A26, and a 7B85 in the plug-in slots lastSince the 7B80/85 use +5V it is not surprising that they don't work --- This email has been checked for viruses by AVG. |
|
Re: Resurrecting a 475 - Request for Guidance
Roy Morgan
On Aug 15, 2018, at 7:35 PM, Tom Miller <tmiller11147@... <mailto:tmiller11147@...>> wrote:Our work benches vary quite a lot, and things found useful by some may not even be at others places: I have a relaltively common magnifying goose-neck lamp. The tubular fluorescent lamp surround a magnifying lens of about 3-1/3 or 4 inches diameter. This thing is very useful for some work, such as finding a tantalum cap with a hole in one end. Other folks use magnifying ¡°goggles¡± - I suspect these are more useful than the lamp with magnifier for some things. Roy Morgan k1lky68@... <mailto:k1lky68@...> |
|
Re: Resurrecting a 475 - Request for Guidance
tom jobe
Hi John,
toggle quoted message
Show quoted text
It's good to hear that you are making progress on your 475. Those large capacitors for the low voltage supplies in the aluminum cans often fail internally and do not show any bulging or other obvious failure. When you find a large ripple on one of the low voltage supplies and take that capacitor apart, you will often see one of the connections to the outside has corroded and completely disconnected the capacitor internally. Work on whatever seems to be the biggest problem you see and you will gradually get it all fixed. I have not worked on a 4xx scope in a very long time so have no idea about how things come apart. tom jobe... On 8/15/2018 3:25 PM, JR wrote:
It turns out that C965 (Board A9), a tantalum bypass capacitor on the +15 rail had gone bad; removing brought the +15 rail resistance to ground back to where it should be. Most of the capacitors look to be in pretty good condition; I have not come across any bulges, deformities, leaked electrolyte or burn marks on any of them, even the larger axial lead supply capacitors. |
|
Re: Resurrecting a 475 - Request for Guidance
On Wed, 15 Aug 2018 15:25:30 -0700, you wrote:
It turns out that C965 (Board A9), a tantalum bypass capacitor on the +15 rail had gone bad; removing brought the +15 rail resistance to ground back to where it should be. Most of the capacitors look to be in pretty good condition; I have not come across any bulges, deformities, leaked electrolyte or burn marks on any of them, even the larger axial lead supply capacitors.Look for the capacitors across the AC line in the power filter. They can fail, especially if you're in an environment that uses a 220 volt (nominal) line voltage. The value of nominal has been increasing over the years, and will bite you. Harvey
|
|
Re: Resurrecting a 475 - Request for Guidance
Look for a tantalum with a small hole in it.
toggle quoted message
Show quoted text
----- Original Message -----
From: "JR" <jswrussell@...> To: <[email protected]> Sent: Wednesday, August 15, 2018 6:25 PM Subject: Re: [TekScopes] Resurrecting a 475 - Request for Guidance It turns out that C965 (Board A9), a tantalum bypass capacitor on the +15 rail had gone bad; removing brought the +15 rail resistance to ground back to where it should be. Most of the capacitors look to be in pretty good condition; I have not come across any bulges, deformities, leaked electrolyte or burn marks on any of them, even the larger axial lead supply capacitors. I powered it on, and checked the supply voltages at the test points; all were within specification, although I haven't checked the ripple on each yet. I haven't checked any of the high voltage test points yet, as I'm currently lacking the equipment to do so safely. A couple minutes into testing the powered on scope (I couldn't find the test point for +105/160), some other component let go, with a loud pop, accompanied by a whistling sound and small quantity of smoke. So, the hunt is on for the culprit. I have removed boards A3, A7 and A8 so far, and have found no obviously failed components on any of them. I thought at first that something on A7 (timing board) failed, given the location where the smoke seemed to come from, but that doesn't seem to be the case. Is there any component in the recess along with the CRT known to fail in such a way? Interestingly, after the component failure, all the supply voltages were still within specification. Regarding the removal of the two attenuator boards/assemblies, is the best way to get at the nuts holding them to the front panel with a small spanner? I'm having a little difficulty getting them free. Thanks, John |
|
Re: Resurrecting a 475 - Request for Guidance
It turns out that C965 (Board A9), a tantalum bypass capacitor on the +15 rail had gone bad; removing brought the +15 rail resistance to ground back to where it should be. Most of the capacitors look to be in pretty good condition; I have not come across any bulges, deformities, leaked electrolyte or burn marks on any of them, even the larger axial lead supply capacitors.
I powered it on, and checked the supply voltages at the test points; all were within specification, although I haven't checked the ripple on each yet. I haven't checked any of the high voltage test points yet, as I'm currently lacking the equipment to do so safely. A couple minutes into testing the powered on scope (I couldn't find the test point for +105/160), some other component let go, with a loud pop, accompanied by a whistling sound and small quantity of smoke. So, the hunt is on for the culprit. I have removed boards A3, A7 and A8 so far, and have found no obviously failed components on any of them. I thought at first that something on A7 (timing board) failed, given the location where the smoke seemed to come from, but that doesn't seem to be the case. Is there any component in the recess along with the CRT known to fail in such a way? Interestingly, after the component failure, all the supply voltages were still within specification. Regarding the removal of the two attenuator boards/assemblies, is the best way to get at the nuts holding them to the front panel with a small spanner? I'm having a little difficulty getting them free. Thanks, John |
|
Re: 453 trouble
Craig Sawyers
A hot chassis can also be caused by RF filter capacitors connected from each side of the lineto the chassis. I would be much more suspicious of these if they exist in the 453 than of theelectrostatic shield in the power transformer.There are no such capacitors in the 453. Hence my comment regarding the primary capacitive coupling to the shield. I - er - looked at the schematic before I commented :-) Craig |
|
Re: 2465B Replace Aluminum Electrolytics with Tantalum
Tantalums are really good when used properly.
toggle quoted message
Show quoted text
1) ESR / ripple current rating MUST not be exceeded, creates internal self heating that will lead to internal shorts. 2) Use them for high frequency, low voltage switching regulators. 3) De-rate working voltage by 50% as a minimum. 4( Not much available at 35VDC and up MLCCs are a very reliable alternative to small tantalums On Mon, Aug 13, 2018 at 4:38 AM, Mark Wendt <wendt.mark@...> wrote:
LOL! LGBTQRC? ;-) |
|
Re: 453 trouble
As pointed out by others, there are times and places when (at least you think) you need to make a floating measurement (independent of the local ground.)
toggle quoted message
Show quoted text
I¡¯ve done this with mapping sonars that are powered and telemeter commands and data over a 10km coax. The instrument ground and the ¡°topside¡± ground are different. Bench testing has to be done carefully. When we connect between to two (on the bench) e.g. to trigger the scope, we transformer isolate the trigger signal¡.. The can be VERY dangerous - as someone said ¡°you can get seriously killed¡± or perhaps even worse, you can kill someone else. That said, it can be done safely with care and attention to detail and staying focused (among other things.) A safety observer is an excellent idea - they don¡¯t get wrapped up in the technical problem. A battery powered scope is a good approach but most of us don¡¯t have them and (often, sometimes, ?) their measurement capabilities aren¡¯t satisfactory. Clipping the ground inside the instrument (scope or any other) is an excellent way to set someone else up for injury or death in the future. Just say NO. A better way is to use (in the US) a ¡°two prong to three prong¡± adapter or a ¡°cheater plug¡± e.g. and external to the instrument device. You have to set it up, and it¡¯s quite visible - no surprises. Make the measurement(s) and take it out. -Dale On Aug 15, 2018, at 04:52 , Craig Sawyers <c.sawyers@...> wrote: |
|
Re: 453 trouble
A hot chassis can also be caused by RF filter capacitors connected from each side of the line to the chassis. I would be much more suspicious of these if they exist in the 453 than of the electrostatic shield in the power transformer. If there are such caps in the 453 lift them to see if the voltage on the chassis disappears. You might want to replace them. There are special capacitors for this service which fail open so they don't connect the line directly to the chassis.
toggle quoted message
Show quoted text
The voltage on the chassis will depend on the amount of capacitance and the impedance of the measuring device. You can always generate a long argument by discussing the differences between lightening protection grounds, power line safety grounds and grounds to eliminate common impedances in AF and RF circuits. On 8/15/2018 1:52 AM, Craig Sawyers wrote:
That would be typical if the chassis is ungrounded. Capacitance between the primary and shield (which --
Richard Knoppow dickburk@... WB6KBL |
|
Re: 453 trouble
Phillip Potter
Hi all,
toggle quoted message
Show quoted text
I acquired a 453 in the spring, which had the ground lug sawed off!? This must surely be a "thing"... I replaced the cord. Phil On 8/14/2018 10:01 PM, f4gny@... wrote:
Hello Folks, |
|
Re: 453 trouble
One time I picked up a pallet of 465s from an industrial repair service.
toggle quoted message
Show quoted text
All of them had the ground pin cut off. Paul On Wed, Aug 15, 2018 at 01:24:13PM +0100, Craig Sawyers wrote:
The way to get around it is to minimise the loop area - plug the two pieces of equipment into the same --
Paul Amaranth, GCIH | Rochester MI, USA Aurora Group, Inc. | Security, Systems & Software paul@... | Unix & Windows |
|
Re: 453 trouble
Craig Sawyers
Just done, yes its strange ?To get rid of interference loops. If two or more grounded instruments are connected together via a signal cable, the resulting loop picks up mains frequency interference (or double frequency - 100Hz or 120Hz depending on where the use lives). This is something that plagues audio - a so-called hum loop. The way to get around it is to minimise the loop area - plug the two pieces of equipment into the same power strip right next to each other, and cable tie the mains cables together. Minimum loop area gives minimum hum. The brutal and unsafe way is to disconnect the ground of everything except one instrument. Which is surprisingly common. Alas. Craig |
|
Re: 453 trouble
On Wed, Aug 15, 2018 at 03:04 AM, F4GNY wrote:
Must be coincidence, but this is the third 453 (including mine) I have read about here lately that the previous owner had floated. |
|
Re: 453 trouble
Hello Graig,
toggle quoted message
Show quoted text
Just done, yes its strange ?? Does something can explain such this action ? Thank Regards Alain Le 15.08.2018 ¨¤ 10:52, Craig Sawyers a ¨¦crit?:
That would be typical if the chassis is ungrounded. Capacitance between the primary and shield (which |
to navigate to use esc to dismiss