Strange! As I said, it would be quite hard to design a voltmeter that couldn't cope with a reasonable range of frequencies - and that was as true decades ago as it is today. Ah, well
Kind Regards, John.
My volt meter melted when measuring btween exposed metalwork
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Kind Regards, John.
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Possibly some insulation or screening breakdown. Voltages induced with transformer action are proportional to frequency so at 27MHz there could be some pretty substantial voltage being induced.
Is there a manual about ?
Is there a manual about ?
Ah - in the context of the discussions about 50Hz, 100Hz and 1kHz, that's a very significant movement of the goalposts
I certainly wouldn't expect your average voltmeter to be able to measure that - but that obviously doesn't mean it couldn't be fried by it!I presume that the possibilities once there is a fault are almost endless! It sounds that even someone who is experienced/competent to work on the machine may have trouble finding a way of doing so safely when it's powered up!
Kind Regards, John.
Exactly why I won't be touching it. I am quite keen to get a "man who can" for the client, even if it is just so I can add the cost of my meter to his bill
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I assume you obviously looped tested at the wall isolator, and did a continuity check on the earth between the wall isolator and the machine?
There may be a fault in the machine, which due to a lack of earth, causes no fuses to blow.
There may be a fault in the machine, which due to a lack of earth, causes no fuses to blow.
Martin
You are right to walk away from this machine if you are not experienced / trained in the maintainance of 27MHz high power heating.
EDIT If the fault ( voltage that destroyed meter ) is only there when the 27MHz generator is on then the problem will be hard to diagnose without assistance from manufacturer.
The manufacturers based in India claim to have world wide representation so either you or the machine owner should contact them for a trained service engineer to assist.
My first experience of high power 27MHz heating was being sent to repair a machine that soldered the strands of tinned copper wires inside the insulation before the wire was cut and stripped. The wire was passed through a couple of turns of water cooled copper tube that formed the heating coil. 1/10 of a second was the normal "exposure" and that was sufficient to melt the tinning and solder the strands. The fright came with the demonstration of how the machine could melt the centre of a 6 inch nail before the ends became too hot to touch. Any metal close to the coil when the machine was running was at risk. I recall mention of an operator's wedding ring being heated by the coil if her hand was close to the machine.
You are right to walk away from this machine if you are not experienced / trained in the maintainance of 27MHz high power heating.
EDIT If the fault ( voltage that destroyed meter ) is only there when the 27MHz generator is on then the problem will be hard to diagnose without assistance from manufacturer.
The manufacturers based in India claim to have world wide representation so either you or the machine owner should contact them for a trained service engineer to assist.
My first experience of high power 27MHz heating was being sent to repair a machine that soldered the strands of tinned copper wires inside the insulation before the wire was cut and stripped. The wire was passed through a couple of turns of water cooled copper tube that formed the heating coil. 1/10 of a second was the normal "exposure" and that was sufficient to melt the tinning and solder the strands. The fright came with the demonstration of how the machine could melt the centre of a 6 inch nail before the ends became too hot to touch. Any metal close to the coil when the machine was running was at risk. I recall mention of an operator's wedding ring being heated by the coil if her hand was close to the machine.
I remember as a child, growing up in a house which was 1-2 miles as the EMI flies from a number of furniture factories which used RF gluing machines, how effective they also were as VHF TV jammers.
With high levels of 27 Mhz energy in the area an earth lead that shows near zero ohms on continuity with DC test current could have a significant impedance to energy at 27MHz if there are any bends in the cable. This can result in high voltage differences between the ends of the cable. Even running the cable metal to a ferrous object ( girder, iron pipe ) could create an inductive impedance.
Has the cabling to the machine been "tidied up" recently. Such as previously straight runs of cable being re-routed to then have a few bends in them.
Has the machine been moved recently ?
Was it ever intalled properly ?
There may also need to be an earth rod under the machine to disipate stray RF energy. Has that failed ?
I can beleive it, particular since, IIRC, the second harmonic of 27MHz would have been in the original TV Band I.
Kind Regards, John.
That's why I said that it would be difficult for even an experienced/competent person to work safely on this machine in its present state. Unless there is something visually obvious, diagnosis could be difficult without the 27MHz generator running, yet the whole thing sounds positively dangerous when it is running. Maybe someone should send for westie and his friens
Kind Regards, John.
Thats all very useful, but ignoring the HF RF for now, the two most obvious things to first check would be that there is an earth at the isolator, and that the machine is earthed, I am sure it is obviously meant to be earthed. If these things are sound, then move onto the next step, which is then quite specialist.
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I did work on HF welding machines in Convatec and there was a very strict procedure to working on them. They could produce some very high voltages in the order of 10kv and most meters will not handle that voltage. Earthing with HF flying around is not easy as one can get all sorts of currents building up and even a rusty bolt can act as a diode and produce some really odd effects. Seem to remember 7KW at 27MHz using a air cooled valve housed in a tunnel so you could not get anywhere near.
This is very specialist work and Convatec employed a specialist to work on these machines we were taught how to earth down to work on other parts of the machine but we would not be allowed to bond any part of the machine that was the specialists job.
I would not work on a machine like that even with my electronics and radio training and I would advise you back heal the job and refer it to a specialist. We all make mistakes but it takes a man to admit it. I also wrecked a meter working with high voltages. In my case a Spy metal detector and it burnt out the germanium diodes in my Avo Mk 8. I also thought I was working on a relative low voltage section before the final transformer but back EMF took it out even on 1000 volt range. Your not the first and I guess will not be the last.
This is very specialist work and Convatec employed a specialist to work on these machines we were taught how to earth down to work on other parts of the machine but we would not be allowed to bond any part of the machine that was the specialists job.
I would not work on a machine like that even with my electronics and radio training and I would advise you back heal the job and refer it to a specialist. We all make mistakes but it takes a man to admit it. I also wrecked a meter working with high voltages. In my case a Spy metal detector and it burnt out the germanium diodes in my Avo Mk 8. I also thought I was working on a relative low voltage section before the final transformer but back EMF took it out even on 1000 volt range. Your not the first and I guess will not be the last.
No tidying up has happened.
Not for 9 years
According to the owner yes; I will ask the operator tommorrow
Not installed.
I have found someone who has good experience in fixing these machines (abiet different makes); he cannot attend site for a week so has given me some checks and backround info to collect. Namely (with machine isolated having a look around at the internal bonds, getting a schematic, and getting the MI for install of the machine. Plus having a chat with the operators. So I shall try and tag it onto a busy day tm
Right, so for an update in case anyone is interested...
got an expert in, he said there is "stray RF everywhere" because it is an indian machine and the bonding is poor. He knew all the tolerences of the set up voltages on the machine and has extensive experience with them.
He wants me to do the bonding work for the customer and has given me instructions of what bonding he wants and where. So not a complete loss of work for me
So alls well that ends well. I might do some before and after photos, if any one is at all interested...?
got an expert in, he said there is "stray RF everywhere" because it is an indian machine and the bonding is poor. He knew all the tolerences of the set up voltages on the machine and has extensive experience with them.
He wants me to do the bonding work for the customer and has given me instructions of what bonding he wants and where. So not a complete loss of work for me
So alls well that ends well. I might do some before and after photos, if any one is at all interested...?
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