Regulated Power Supply Using LM 723

This week I’m helping friends repairing their Regulated Power Supply. Both units using IC LM 723 as it main controller for regulating process. And coincidentally they have identical schema but different PCB layout. The first Regulated Power Supply unit is owned by my local orari chief, YB3CC. It’s a Bell BL-1030A. The second unit is Sumura AR 1030A. I’m not sure it’s an original Bell manufactured since the identification badge just made from a small sticker. Everyone can make it easily.

Both regulated power supply are having 4 pcs 2N3055 transistor as the final stage. Drivered by 2SD313 which directly controlled by LM 723. The Sumura’s transistors are RCA. Bad luck for the Bell, since previous transistors are not from same factory, I replaced all 4 transistors by 2N3055 produced by ST.

The weird thing of their schema  is how the Regulated Power Supply Designer want to control over current feature. It has too tight amperes allowed to run. I think it’s around 5-6 amperes only. Above that, LM 723 will reduce it voltage output and will cause deep hum to my Yaesu FT-7800. So I decide to disable this over current protection with make pin #2 open. Then the unit will give max amperes without voltage drop. And now it rely to fuse to protect from short problem or over current.

Transformer of both Regulated Power Supply also too high output on secondary side. It has 24 v AC. So after being converted to DC and hum filtered, it become around 32V DC. It’s too high voltage to 2N3055 and 2SD313. Be aware that those transistor then will generate heat as compensation of difference between input voltage and output voltage then multiply it by active current. I think bot Regulated Power Supplies are intended for general purposes rather than specifically designed for amateur radio need. Perfect transformer output is 18-19V only, that is the minimum requirement for LM 723 to work (+4V above of output voltage).

The other lesson I got is how essential to check current balance on every 2N3055 transistor. Equal load sharing will protect those transistor being over loaded. If you use common emitter schema, you will be haunted by transistor short even and will feed 32V to your transceiver! Your transceiver will stop working and commonly you will see smoke come up and smell something burned out. So don’t hesitate to check your transistor current balance. Modify those 5w resistors value to achieve it. Try to achieve as equal load as possible.

I add an LM 7818 to downgrade input voltage of 2SD313. Before, this transistor generate excessive heat even it has aluminum heat sink. After voltage downgraded, 2SD313 generate little heat even on full load capacitance.

If you interest to add a 12V air cooler, my suggestion is don’t direct connect it to 12V. Because it will generate noise sound to your microphone. You can use LM7810 to give 10V to that air cooler. So the noise sound not as high as 12V feed.

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