I think my Amiga (and modem?) is hyper-sensitive to E/M noise! What to do?

[AmigaMance]

  Hi.
 I think my Amiga 1200 is ultra sensitive to external electromagnetic noise! Here is why:

 Symptom 1: If i switch the room fan ON and OFF a few times, while i write something to my HD, the whole system _WILL_ hang with the HD led being permanently ON. WTH! :nervous:
 This can also happen with the room light, but more rarely.
 What can i do about this? Except of placing the fan away from my Amiga or... not touch switches while my HD is working? :-(
 If it matters, the fan is located more closely to my Amiga (desctop) than to my HDs which are hosted to a pc tower.
 Also, the E/M shield of my Amiga has been removed, but i think its purpose is to reduce the emission of E/M radiation FROM the Amiga, not to protect it from external one. Or am i wrong about this?


 Symptom 2: If i turn the room fan ON or OFF while i download something my modem stops for half second and then continues to download again. Same thing happens with the room light.
 If i flick the light/fan switch quickly, the modem will disconnect.. It will also disconnect when certain types of motorbikes are passing across the street. :roll:
 I don't know if i should attribute this to the phone cable, modem, serial port (silversurfer) or to my Amiga itself. So perhaps we should focus at the first symptom which is more worrying anyway.

[Chain]

did you tried to plug both things in another power plug each? maybe its some power spike transferred thru power cables and not thru air

[DrDekker]

The light and fan switching on/off can certainly cause spikes, especially if the light and fan use a fair bit of current and the switch contacts are dirty/burnt.

As for the interference from motorbikes, that's usually down to the metal supressor casing being removed from the sparkplug caps (to reduce electrical shorts in wet weather), although I'd have thought that this type of interference would only affect aerial/satellite based reception/transmission.  Perhaps someone more knowledgeable can shed more light on that issue.

It could just be that you've got some dodgy cable shielding.  

[AmigaMance]

did you tried to plug both things in another power plug each? maybe its some power spike transferred thru power cables and not thru air

 Yes, i have plugged then fan and the PSU in the same plug, thus what you suggested sounds logical. An electrical negative spike instead of an E/M pulse. So, i decided to experiment a little with it:

a) Kept the fan at its original place (near the desctop) but plug it in a different plug: (which is located in the same room though. i don't now how much it matters) No results. The HD hangs after a few switch clicks. :-(

b) Re-plug the fan where it was but move it about 2 meters away from the Amiga. No results either. :-(

 Atm, it seems like an E/M thing, IMHO, but to be sure i will plug an extension cable, move the fan to another room and fire it up from there. That should exclude it or confirm it...

As for the interference from motorbikes, that's usually down to the metal supressor casing being removed from the sparkplug caps (to reduce electrical shorts in wet weather)

 Yep.

although I'd have thought that this type of interference would only affect aerial/satellite based reception/transmission. Perhaps someone more knowledgeable can shed more light on that issue.

 Well, i can't exclude a coincidence but i think it affects my connection. The modem either disconnects or lowers the connection speed.

It could just be that you've got some dodgy cable shielding.

 Although the HD problem is what bugs me right now, how can i tell if the phone cable is shielded in the first place? It looks like a very simple one.

[alexh]

Get a surge protecting power supply.

Put the metal shielding back on your A1200.

Ensure that ground is wired up (if possible).

[DrDekker]

Are you using the original Amiga PSU?  If so, try a different PSU, or better still, adapt an AT (200W-ish) PSU.

Interference from motorbikes aside, it maybe that a.c. is getting through to your mobo - perhaps part of the rectification has broken down in the PSU?

I had this once on one of my A600's.  It caused all sorts of weird effects - HD errors, graphical glitches, sound distortion/oscillations, to name but a few.  A 'new' AT supply sorted it out.  

[Matt_H]

More likely you have interference in your electric lines or your circuit is overloaded. Make sure you're using a surge protector (never plug Amigas straight into the wall) and that your ground/earth wire isn't being bypassed.

a UPS would probably also solve things.

[Zac67]

The caps in the PSU and/or onboard may have gone bad. They age and lose capacitance, thus lose their power to protect the logic from mains ripple.

I'd try another PSU to see if that changes anything. If it doesn't (or not sufficiently), check (bulging/leakage)and replace all bad and all large capacitors on the mainboard. A pain but these things do age.

[mrad]

Interference can be of 3 types:

Conducted emissions can come through the power lines.  It seems unlikely to me that conducted emissions would affect both your modem & your Amiga though.

Radiated emissions are the result of electromagnetic fields.  The amiga shield is, indeed, primarily intended to save every radio and TV in the area from your Amiga, but, as any owner of the poorly shielded TRS-80 Model 1 knew, the interference can go both ways.

A ground loop is a condition where the ground connections on your equipment inadvertently form a big loop antenna. A famous example.  

Your problems sound to me like a ground loop.  It may be an unwanted ground connection on your modem or serial cable.  The only solution is to break the loop, either by ungrounding something, putting an isolation transformer somewhere, or re-arranging all the ground connections so that the wires form a star topology, all connections radiating from a single point.  Your modem probably has an isolation transformer in it; if there is a break in its insulation it can introduce a spurious ground connection through the phone line.

[Zac67]

mrad wrote:

Conducted emissions can come through the power lines.  It seems unlikely to me that conducted emissions would affect both your modem & your Amiga though.

Hate to disagree, but a flaky PSU can very well cause fluctuations in the serial control or data lines, ranging from mis-handshaking to data loss, causing transmission interruptions or PPP errors...

Get a multimeter, set it to AC(!) in 0.2 to 2 V range and check the 5V and 12V power lines, at best on the mainboard, while switching the lights. Obviously the power should be DC only and AC component should be zero. If you do get (intermittent) readings above some few mV, the power is flaky.

[rkauer]

 I bet in an almost dead Amiga PSU.

 Move electrical appliance from one to another mains plug does nothing if the circuit is the same (means, in the same breaker/fuse).

 Telephone lines are unshielded (never notice one shielded in my whole life, at least). But you can get a filter (buy one on the nearest Radio Shack or equivalent) to minor those problems.