For the field I use an old UPS with the battery removed, I connect the battery cables in the UPS to the car's battery. I never had a problem with equipment powered by a UPS,
just take a Salicru or APC from trash (discarted because the lead battery was dead) , remove its internal batt and just connect the internal wires going to the internal batt to car's batt (see polarity!). You'll have a high quality mains for what you want, for free.
To connect the UPS to the cars batt use cables at least as thick to not melt cable insulation, use double thick if cables are long.
The UPS may take a lot of power from car's batt, so of course you may discharge batt to a point you may not be abble to start the car, you may use a "Low Voltage Battery Disconnect" device that cuts supply when battry is low enough, or at least you should have an emergency jump starter. ...or you may power the UPS batt from the jump starter, not messing in the car, ...or you may bring an additional car batt, which is what I do.
In your case the problem seems based on radio-interference., either emitted out of the casing, or via the mains cable (that is why some switching supplies got chokes in their mains cable).
In general equipment is designed to work with a nominal Effective Voltage (220, 110 AC) and with a nominal frequency (50Hz or 60Hz) and with sinus wave.
When you don't have a sinus wave, say a 60Hz square wave, you can think you have a perfect 60Hz sinus wave added with additional waves of higher frequencies, those sinus waves of higher frequencies are named Harmonics:
https://en.wikipedia.org/wiki/Harmonics_(electrical_power)
A problem is that different components (resistors, inductors, capacitors) behave different at different frequencies. With same effective Voltage a resistor will deliver the same power at 50Hz than at 60Hz: a filament bulb will work the same in 50 Hz than in 60HZ, but not a motor that is an inductive load, a lower frequency allows to pass more current in the coils of the motor, so an AC motor designed for the US may burn in the EU, and a motor for the EU will underpower at 60HZ. Capacitors is the counter, a capacitor takes more current as frequency increases.
Now we have the two ingredients:
1) components behave different depending on frequence
2) a non sinus wave introduces harmonics that are sinus-waves of higher frequencies, so those harmonic components of higher frequency overpower capacitors and underpower inductors.
If harmonics presence is important and we have a device that has capacitors and inductors (motors) directly connected to mains then those components will work a bit out of specs, the real impact depends on how conservative desing is.
Quality of AC power supply is measured with THD %, this is Total Harmonic Distortion (
https://en.wikipedia.org/wiki/Total_harmonic_distortion), so the power not related to the fundamental sinus wave.
A UPS for informatics may deliver 1% to 5% depending on load.
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Electronic gear:
Control electronics work DC, so first a device does is sourcing DC from mains. Today it's very easy/cheap to implement a high quality DC power supply, an ATX power supply is a marvel, for $30 (new) you have a 800W war machine delivering +3.3, +5, +12 and -12.
What happens with harmonics? OK if the Power Supply is modern (probably good) it has a high switching frequency and a good stabilization. An antique power supply designed to operate from regular AC mains may have side effects with harmonics, it may deliver less power an it may not filter well ripples, even it can be damaged as a capacitor can be overloaded.
See here difference between $5 vs $15 PS :
(Even the $5 is pretty good, the $15 is better)
