Mastech Hy6003D power supply

I had my Mastech HY6003D Constant-Current Constant-Voltage power supply smoke and quit working one day after a year of use. I use the HY6003D to constant-current charge and equalize my aircraft battery. Aircraft batteries are very expensive like $400+ so getting extended life out of them is important. I have gotten over 9 1/2 years on the last battery so I am confident that the equalizing with constant-current is helping with longevity although some do not buy into this theory.

I acquired a schematic for a HY5003 power supply as the manufacture's rep said it was functionally equivalent, but I am not so sure. See attached schematic. The smoke came from the two .82 ohm 5 watt ceramic resistors, R12A and R14A, on the collectors of the output transistors, V21 and V20. After many component checks both in and out of the circuit board, I finally found the 741 op amps, N1 and N2, bad and all is well with new resistors and op amps. I did notice the center tap of coil N5 of transformer T1 does not go to ground as the schematic shows, but I don't know if that is specific on the HY5003 or is a misprint, but it did cause some problems for me until I used another known ground for testing purposes. This is a tricky power supply in that it has 4 relays, K1a, K2A, K3a, K4a, that actually switch as the constant voltage is increased with the switching going to different diode rectifiers, D1-D2 and D3-D4. I haven't figured out how that exactly works.

Also V29A shows as a PNP transistor, but C708 is a NPN and in a TO-92 can form whereas the transistor on the board was not a can. I am assuming C708 translates to a 2sc708. The other C708, V28A, shows as a NPN so I imagine the Chinese person drawing the schematic was slightly confused. When testing V29A and V28A, they both tested as NPN transistors.

This was an interesting exercise to say the least
Dick Welsh