HP6201B power supply


The 6201B is a power supply which was sold by HP, and is by far the oldest piece of test equipment I own. I guess this model was built around the time when men first reached the moon, but it is still in working condition. I got it from Ebay for 29 USD with shipping.

Does it come with a LC Display and a powerful feature-rich user interface? No. It comes with the following controls:



Why have I choosen to show this very basic and very old meter here? Because it is a very good power supply, even for todays use. Why? Because the controls for the voltage and current are high quality potentiometers and the allow to set whatever value I want very precisely. Wait: Precision with an analog meter? No, not really. But if I combine the 6201B with the 3478A multimeter this power supply will really start to shine. The supply can be tuned very precisely and the meter will give me the precision.

And I think this is the biggest drawback of todays cheap new power supplies: the have fancy digital controls, but either the resolution of the controls is lacking, or the potentiometers the use are cheap low-quality single-turn ones for a huge output voltage range of for example 30V: that means if you want to tweak the output voltage to a precise value like 2.58V you can basically forget it. If you just touch the potentiometer you will jump all over the place. Better power supplies have at least a coarse and fine tuning knob.

Do I think cheap power supplies from today should not be used or bought? No. I have one myself. It just depends on the use case. In fact I will use the cheap supply from today more for supplying my circuits. But if I want to set a specific bias-voltage for an opamp-circuit or some other input voltage which should be precise I will use the 6201B.

The only point I want to make with this page is that most of the people forget about older test equipment, or the think because the old stuff does not have a fancy menue that the old stuff can not have good performance in electronic terms, but I think this is sad and also not allows true. In fact I think this old meter will wipe the floor with cheap supplies from today in terms of regulation and electronic performance.



The build quality is very high and even test points are nicely marked on the PCB. It is also interesting and almost unbelievable for today to see capacitors marked with "Made in the USA". Even thought it is amazing that this supply still works after 46 years (how long will the cheap one from today work?) be very careful when handling old equipment like this:

Of course some of this points made are also true for very cheap supplies from today. You also do not know always if the are safe to operate or when the will start to fail. It is just something to be careful about, and what everybody has to decide for himself.

Line voltage modification

I saw a lot of different solutions in HP equipment how the user can change the line voltage selection setting when moving to another country. For example the 3325A has 2 internal switches and the 3478A has a jumper wire which is connected to a different pin header. The 6201B voltage selection is done a little less comfortable, or how the service manual describes it: cut this two PCB traces, and make a new connection between this 2 points. This is also what I did afterwards. The power transformer has two equal primary windings and by doing this change the windings are changed from being in parallel (US) to a series connection (Europe). Secondly I removed the old US style power plug and replaced it with a European one, because the power cable is fixed to the power supply. Last but not least the line fuse was changed with the new current rating for 240V (very important).