Galloni Cast Machine

470MΩ?

Good Morning PW,

Thanks for the reply - 470M + Ohm (symbol) is possible but it definitely was not an ohm symbol unless A or the delta symbol represents the "ohm" symbol in Italian.

I spent much of yesterday morning on the internet Googling all sorts of variations of the part number(s) and came up with zilch.

I will check back with Newark and see if this is a possible translation.

Thanks again.

I don't know when you sent your message to Galloni, but many Italian companies were on holiday last Thursday and Friday, so you might get a reply soon.

Holzfeller,

Ah, Holiday, very good point,

Thanks for the heads up!

KJK/USA

Have your brother-in-law disconnect one of the leads and measure with an ohmmeter.

The size would also be somewhat indicative of power capacity.

Mike,

The winding is cooked. I can not get a reading this is why I have to decipher the part number but thanks for the suggestion.

KJK/USA

Excellent Idea. I will give this a try.

Thank you for your help!

KJK/USA

Thanks WJMFire.

I was thinking along these lines but my biggest concern was damaging the machine as I am not that well versed on circuit boards at the component level.

Given it's age and lack of availability of replacement parts, I did not want to open a can of worms.

I will give this a shot.

Thanks again - KJK/USA

Seeing as this POT is fried, you may already have a different problem at the component level. The best you can do is limit the current as much as possible, which you will be doing with a very high resistance, and increase the current until you start getting your results back. The worst that will happen is you will burn up this POT too, in which case you are right back where you started, but with a different question.

Thanks for the info dcpppf.

KJK/USA

Could very well be an "A", but not in the sense of "mA", as depicted by the apparent space. The "A" would most likely be the "law" or the taper code of the potentiometer, i.e. Linear or Analog. Unfortunately the Law Codes vary by time and manufacturing area, plus they may have changed some time ago. So in the "old code" A meant Linear, C meant Log (or "audio") and F meant Antil-log. Then it changed, and A meant Audio / Log, B meant Linear and C meant Anti-log. So unfortunately if it were anything but an A you would have been home free, but alas, no such luck. I've seen references to the fact that European based law codes were always A = Audio or Positive Log taper, so that might be what you have.

Good luck.

Good Morning JRaef,

Thanks for the info you provided. I was not aware of the "law" or "codes".

This certainly is becoming a good learning experience for me.

Who said you can't teach an old dog new tricks

Thanks again.

KJK/USA

A few comments:

1. I highly doubt that the 470M means 470 Meg Ohms. A resistance that high can't carry a significant current without thousands of volts. The only place I've seen a resistance that high is in a high voltage probe used for measuring the anode voltage of CRT's (TV picture tubes). I don't believe I've ever seen a pot that high, and I've seen thousands of pots...

2. Capitol M is supposed to mean Mega-, but I have seen it used to mean milli- quite a few times. This could well be 470 milli-Ohms, otherwise known as half an ohm. If the pot is used to directly control a heater element, that could be a reasonable value.

3. I suspect that this is (or was) a wirewound pot. If even a tiny piece of the wire remains recognizable, the size (diameter) of the wire will be a clue to the original resistance. Larger wire means lower resistance, and vice versa)

4. It is highly unlikely that the entire resistance element was destroyed when the pot failed. If you can measure the resistance of a part of the element, multiply that value by the reciprocal of the fraction of the whole to get the entire resistance, (eg. if 1/5 of the element measured 2 ohms, multiply by 5 to get 10 ohms.)

5. If, as I suspect, it is a very low resistance pot, you may not be able to measure the partial resistance with an inexpensive Ohmmeter; it may indicate zero, or it may indicate the lead + contact resistance, rather than the desired value. If it indicates a fraction of an Ohm, don't forget to subtract the lead resistance.

6. Although several other posts have made mention, no one has specifically stated: The physical size of the pot is a good indication of the power it can handle. The replacement pot must be at least as large as the original, unless it is made of superior components (such as ceramic replacing fiber insulation).

7. In a device that old, there is a fairly good chance that it directly controlled the heater current (rather than controlling, say the trigger voltage of a triac). If so, it can't be more than a few ohms.

8. Strictly speaking, the device is only a potentiometer if it has three wires connected, one at each end and one at the sliding contact. If only two wires are connected, the same device is correctly called a rheostat.

9. If the device does indeed directly control the heater, then the device resistance must be similar to the heater resistance. A resistance equal to the heater resistance, in series with that heater will drop both the current and the voltage to half, so the power in the heater is dropped to one quarter of the full voltage value. This would cause a huge drop in the furnace temperature.

10. If it is directly controlling the heater, it may well be better to replace the pot/rheostat with a more modern system, similar to a heavy-duty light dimmer.

11. A picture is worth lots of words, and a circuit diagram is too! Give more information, and we may be of more help!

Please let us know what you find

Dick

Dick,

Thank you for the wealth of information.

I agree with you, I do not think it is 470 Meg Ohms either.

Here is more info and a picture:

It is the POT in the middle under the orange lamp. It does not look like anything special and has the diameter slightly smaller than a quarter.

It is about 3/8" thick. It has three solder terminals but only two of the terminals are being used - the two to the right when looking at the POT from the back.

I suspect it is wire wound (I suspect this because I had taken one very similar looking to this apart once).

The pencil mark in the picture just past "6" is the normal set point. Starting about a year ago, the machine would not heat up unless you moved the knob back and forth (above and below) the set point and then it would be good for a few weeks.

Unfortunately, there is no wiring diagram for the control head available. I have looked under all the panels and found nothing.

I will take the existing one apart and see what I can see.

Again, thank you for the wealth of info.

KJK/USA

Hi KJK, If the has fried you have probally got a secondary reason for the pot failure, pots don't usally cook up on their own accord. Something has caused the pot to cook. you need to look a bit further.

Are all three terminals being used on the pot or does it only use 2 terminals. If all three terminals are used and the pot is cooked evenly all over, I would suspect that overvoltage may have caused the problem, but if not cooked evenly then I would suspect that whatever is connected to the wiper of the pot is causing the problem. The symptom is not always the cause of the problem. You have to check the circuit for the real fault, the pot is only the weakest link.

I hope this is of help.

Cheers

Joe

Hi Joe,

Thanks for the info.

Only the right hand two terminals (looking at the back of the pot) of the three are being used.

One item that makes me suspect it is the POT only (hopefully) is that the machine is used in a very dusty, gritty (finishing room) environment.

The POT is not sealed and I suspect a lot of debris have accumulated over the years (again, HOPEFULLY). This is the only POT that the terminals are oriented up where the open case can catch debris. When I replace the POT, obviously, I will orientate the new one with the terminals down.

I will start at the weakest link, the POT, and try to work backwards.

Thanks again!

KJK/USA

First, I would like to thank you all for the help you gave me with this dilemma.

Second, here is the response I just received from Galloni;

Hallo Sir, the potentiometer's code is A128.006

Is a linear potentiometer of 470 K ohm ( 470.000 ohm) is available in any electronic shop but if you want to order from Galloni you can do it.

The lonely problem is : we have a minimum amount on the order; if you want some info about the spedition you can contact Giuliana

Best regards Luca

At least this takes some of the mystery out of the equation!

Have a great day all!

OK, we are getting somewhere. Mil means thousand in Spanish and Italian, so the M stands for thousand.

You are absolutely right about the contacts pointing up being a bad idea. It is conceivable that you just need to clean the contacts or bend the contact spring slightly, but since you know the value, and they are cheap, get a new one. Since there are only two wires, it does not make a difference which wire goes where, but it does make a difference which two terminals you use. Since you will be rotating the rheostat 180°, it will now be the left two terminals, so rotating the knob clockwise reduces the resistance and increases the current flowing to whatever device is next in the circuit.

Good luck!

Dick

Dick,

Go it!

Thanks again!

KJK/USA