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453 trouble
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Hello Folks,
Ive trouble on my 453, I discover that AC voltage is running on chassis +/- 75V ......... I opened the main plug and the ground pin was unconnected. Does anyone experienced the same trouble ? is it normal when ungrounded or internal PSU show a defective componement ? Thank for your help. Regards Alain |
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Craig Sawyers
That would be typical if the chassis is ungrounded. Capacitance between the primary and shield (which
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is connected to the chassis) in the mains transformer will cause this. First thing to do is connect the ground in the mains plug - the previous user did a dangerous thing by disconnecting the ground. There may be other problems to solve, of course - but make the scope safe first. Craig -----Original Message----- |
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Hello Graig,
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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 |
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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. |
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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 |
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One time I picked up a pallet of 465s from an industrial repair service.
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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 |
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Phillip Potter
Hi all,
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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, |
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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.
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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 |
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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.)
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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: |
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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 |
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Hello Folks,
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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 |
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stefan_trethan
Yes, for best frequency response the ground lead should be short and
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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 |
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On Thu, 16 Aug 2018 06:05:57 +0200, you wrote:
Hello Folks,Yes and no. The ground lead for the probe, when properly attached, minimizes ringing on the signal. The long lead from chassis to chassis will not. For low frequency signals (especially sine waves), you're not likely to see much of a difference. Go play digital, and you will. The length of the probe's ground lead and where it is placed also have an effect, especially if you've got a different ground potential (by millivolts) across a PC board or a chassis. Harvey
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Hello Guys,
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Many thanks for technicals comments, thats great ! I must keep your advice in a corner of my brain when I work on any stuff with digital signal. Regards Alain Le 16.08.2018 ¨¤ 15:35, Harvey White a ¨¦crit?:
On Thu, 16 Aug 2018 06:05:57 +0200, you wrote:Hello Folks,Yes and no. The ground lead for the probe, when properly attached, |
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