Accu-Trainer AT1 repair
The Voltcraft Accu-Trainer AT1 is a charging device for AA and AAA NiMH and NiCd cells. It comes with an integrated switching power supply for direct mains connection instead of using a wall wart. The AT1 and it’s relatives AT2, AT3 and AT8 are popular chargers, at least in Germany, with the reputation of treating accus nicely and not frying them.
A few month ago I got this piece at our local Conrad Electronic store in Hannover. They have these plastic boxes located near the cashpoints where you can find faulty or B-stock pieces at reduced prices. This stuff is definitely worth a look as very often you will get properly working units with only a few cosmetic drawbacks. BTW: You know that you got an „out-of-the-plastic-box“ unit if there is a big cross scribed somewhere on the housing with a screwdriver or knife. Keep an eye on it, maybe this is your next eBay snip.
Well, this time it was not cosmetic but a real fault. So I disassembled the AT1 and put it on the desk with a breadboard as isolator and powered up the isolating transformer to go for error search. There was neither smoke nor smell nor any other sign of life, nothing obvious. The switching power supply did not look as if this job would be fun, anyway.
Switched off and checked the semiconductors with ohmmeter and diode tester, checked resistors and capacitors without result. To check the capacitors in-circuit I used a new toy, the Elektor In-Circuit Kondensator-Tester and found one capacitor with ESR value 1Ω to be possibly critical, two others had ESR value 0Ω which is optimal.
Powered it up again and started measuring voltages. This is always a bit fiddly with mains on the board, even behind an isolating transformer. Beneath the controller PIC16C72A there was a three legged something hidden under a shrink hose that looks like a voltage regulator. On supposed input there were 14 volts, on output there was nothing and the output is connected to Vcc of the controller, hmm. Cut the shrink hose to have a look at the labeling.
Found an LM340T5 which is better known as 7805 voltage regulator. This is an easy one; you may be out of toilet paper but you will never miss a 7805 in your spare parts inventory. After replacing I did a quick check using a dead AA accu sticked with tape between the contacts and… yup!
Pulled over a new shrink hose, reassembled the AT1 and that’s it. If you own a dead AT1 maybe this post could be helpful. Although integrated in a very small housing and with internal power supply the device gets hand-hot normally which is a good perspective for long life and reliability.
