rprosperi wrote: ↑
Wed Jun 30, 2021 5:28 am
I would not use a 3A supply. As I posted above, the printer manual provides these power specifications:
9 - 12 V AC or DC, 500-1500mA.
While I can understand the "it probably won't hurt" thinking, I'd assume HP specified the 1500 mA limit for a reason.
I hope not, but perhaps you'll find out why if you do use that... Note that several users have reported failures of 82240B printers while in use during the last year, though none that I recall specified using external power supplies. Remember that these devices are 30-40 years old and probably sat unused for most of that time; I'd treat them gently.
Again, I Am Not An Electrical Engineer, but from what I understand (and please correct me if I'm wrong)...
The original power specifications mean 9-12V, can be either AC or DC, minimum 0.5A but ideally 1.5A. Since typical constant-voltage regulated PSU would output constant voltage and up to
rated current, using 3A PSU doesn't mean it will actually feed all those 3A into the device, it only means that if the device pulls all 3A PSU can oblige up to that limit, and after that limit, depending on the voltage regulator, either the voltage will sag or some other undesirable effect will happen - e.g., PSU will start overheating.
Pulling some datasheets for the power circuit elements on the schematics, I see that MAX4193 is a switching power regulator and DC-DC converter. So that doesn't tell me much, but also probably isn't all that important for the conversation. On the other hand, 7806T sitting between PSU and battery pack is a linear voltage regulator. Pulling its specs looks like its input voltage needs to be at least 2.5V higher than output, meaning at least 8.5V - that's the lower bound for the PSU voltage. Output current is almost the same as input current, and can be as high as 1.5A. That doesn't necessarily mean the whole of printer will be limited to 1.5A, as there is still a battery pack connected after 7806T, but it does imply no more than 1.5A will be pulled from the PSU, and that's the upper bound on the PSU current - if your PSU can provide more than that you're probably just overpaying for a PSU that's more powerful than you can theoretically make use of. Finally, since 7806T is a linear voltage regulator, the power difference between output with constant voltage and consumed current, and input with same current as output but higher voltage has to be dissipated as heat on the regulator itself, which is where the upper limit of 12V comes in - more than that and you're just heating the device needlessly, even though 7806T can take input voltage up to 35V.
So ideal PSU would probably be 9V DC capable of supplying at least 1.5A current. Voltage up to 35V could be tolerated, but is not desired due to excess heat generated. Higher current is not going to be consumed, so if buying a dedicated brand new PSU there is no point in paying extra for something rated more than 1.5A.
As to the devices dying - most common cause that I've heard about so far (and I'm still in the beginning of my retro-computing journey of restoring old machines) is electrolytic capacitors failing. I've seen several of those when I was operating on the circuit board, but I did not try checking if their characteristics are within their nominal specs. That said, there were no visible signs of them failing - no deposits of any sort, no physical deformations. I did not specifically check the one capacitor that on schematics precedes the 7806T, the 1000uF one used to smooth out rectified AC current after the diode bridge, but if it looked to me like a capacitor failing due to PSU that would probably be my first suspect. If the fault is not due to PSU, but on batteries... The second most common cause I've heard about so far (and encountered when trying to repair ThinkPad A20p) is DC-DC converter failing, which could imply MAX4193 as one of possible suspects.
If someone here is
an electrical engineer and sees anything wrong with the above - please, please correct me.