I'm trying to revive this old mini-scope unit that uses rechargeable batteries. They are lead acid and shot. I'd like to remove them from the circuit and operate it solely from the charger. Here is the schematic, the ps area is in the lower right hand corner of the diagram:
Will simply eliminating the batteries and MJE210 do the trick?
If you eliminate the batteries, the unit will probably work ok if there are no line voltage transients. The presence of the batteries acts like a giant capacitor if there is a sudden spike in the line voltage that the MJE 210 can't handle.
It looks like the batteries were at nominally 2 volts each with three cells, they would be at about 2.2 - 2.3 volts per cell while charging, and that is what the input to the 5V regulator Q18, Q19, Q20 would be expecting.
If I was doing what you are proposing, I would put as large a capacitor as I could fit into the battery space, something rated at 10
- 20 Volts and as many mfd as I could get into the space, to help cut down the chances of any power surges getting thru to the 5V regulator circuit.
Thanks Bob. What you propose is to simply install 3 large value radial capacitors in the slots assigned for the batteries? The batteries are in fact 3- 2V lead acid cells by Gates (now Cyclon). I believe they are 0800-0004 type, but I don't currently have the scope to confirm, might be next larger size. These are listed in their catalog as having 3.5 mOHM (is that 3500 ohms?) resistance, should I account for that also in some fashion?
I do have a 6V lead acid motorcycle battery I can connect to take place of the batteries. What I'm trying to do is confirm the unit works before spending too much time on parts for it (new batteries are about $10 each). The batteries are installed, but have leaked and have also damaged some other components.