I was most surprised to see my name in the Road Testers list.
Totally unexpected!
Many thanks to the sponsors and element14!
I have encountered a lot of vintage electronic devices that make loud noises or produce smoke and glowing fireballs when powered up due to aging.
I'd like to test the claims that polymer capacitors can be used as superior capacitor replacements to extend these devices' longevity and improve their operational stability and safety, whilst using fewer capacitors.
In the early 1980's I was one of the proud parents of Australia's Microbee computer.
It is a Z80 based computer for schools and hobbyists that has a unique feature of having non-volatile memory.
One could work on a BASIC program at school and disconnect it and take it home to continue working on it.
It was a precursor to the Laptop, Notebook and Tablet computers of today.
Many of these computers still exist but like many other devices of this era time has not been kind with components - particularly Tag Tantalum capacitors failing spectacularly with flying fireballs accompanied by the mandatory bang and magic smoke.
When electrolytic capacitors of this era blow up then tended to fill the air with a feltly like substance. This is the remnants of the paper insulation.
Also back in this time EMI, EMC and signal integrity were not seemingly of importance due to the incorrect technical assumptions of this time.
Many engineers and PCB designers incorrectly based their calculations upon the clock frequencies present and not slew rates.
Another issue was the layout of the voltage rails; they were often laided out like parallel branched open ended irrigation channels (that can create extreme ground bounce).
Other factors such as limitations and the costs of PCB production contributed towards this view.
The microbee was powered by various sources during its evolution; initially with a 10VAC with a transformer, then 9VDC with a transformer and finally in some instances 8VDC with a switch mode power supply.
The initial design used AC voltage to be able to derive a negative voltage for RS232 signalling.
These input voltages fed two or three 7805 voltage linear regulators depending upon the design revision.
At their inputs they had varying sizes of input filter capacitors to match the provided power supply.
Multiple voltage regulators were introduced used because of the change of the microbee casing.
The original case had a large aluminium base that doubled as a heatsink. It's updated casing was made of ABS plastic so the metal keyboard frame was used.
With two linear voltage regulators present, the outputs of them were separated from each other to ensure that the voltage regulators worked correctly without the need for any even power distribution circuitry.
For the transformer based power sources a large 4700uF @25V was used. For the switch mode power a smaller 220uF @16V was used.
The reason for this is that many switch mode power supplies of the time misbehaved when presented with a large capacitance.
The theory was that the large capacitance presented as a transient short circuit that overly stressed the switch mode power supply.
And another reason for the capacitor replacement was the significant reduction in cost.
The large input filter capacitor was accompanied by smaller value but less inductive Tag tantalum capacitors at the voltage regulator inputs.
The theory is that these capacitors responded faster to higher frequency voltage changes.
At the output of the 7805 linear voltage regulators are a few Tag Tantalums with a sprinkling of ceramic capacitors called bypass or decoupling capacitors.
This term is actually a misnomer and should be called" Columb buckets". They are reservoirs of electric charge to cater for short fleeting local power draw requirements.
An example of this is when a logic IC changes from sourcing current to sinking current where its demand on the power supply changes within nanoseconds.
If the power supply can not provide enough charge for the demand the voltage of the power supply will unwantedly sag (i.e. be lowered for a positive power supply or be raised for a negative power supply).
The repeated action of the power sags and power recoveries is called ripple and can cause unwanted side effects.
These changes to the the voltage level create EM wave fronts that propagate through the path of least impedance. This may or may not include the the radiation of EM waves into the air.
I just powered up a microbee for this challenge to find that there was unwanted ripple on its video output.
The ripple doesn't immediately appear. It appears about a minute of power on leading me to suspect it a heat or signal routing (or both!) related issue.
Figure 1. The noisy video output
The slight additive brightness indicates that amongst other things it causes additive interference to the luminance level.
So to summarise, in this case the electrical noise comes from three main sources:
- Low frequency AC Hum from the AC to DC power supply
- High frequency Digital gate switching
- External sources
