LM317/LM337 Bipolar Power Supply Not Regulating (Mackie HUI)

[Maniac303]

Greetings folks. I have _two_ Mackie HUIs with PSUs that are not regulating on the +/- 15 VDC rails. They are based on LM317 and LM337. I have found and changed a few bad caps (changed all electrolytics regardless), refreshed diodes in bridge, checked all diodes, all resistors etc., replaced the LM317 and LM337 and tested with supplied transformer (providing ~37VAC, once rectified and split this gives ~24VDC per side). The PSU does not regulate, even when loaded with 220 Ohm 5W resistors, what goes in comes out the other side. Regulators were tested in simple breadboard circuit prior to install!

I am seeking hints. Also trying to understand the purpose of R15 and R16, 30 Ohm power resistors bridging each input and output. These were not in the breadboard test circuit.

Attaching a simplified schematic derived from the one in the service manual.

Thanks in advance for any advice!

[edavid]

The shunt resistors reduce the power dissipation in the regulators.

The downside is that the regulators won't work until there's a 9V drop across those resistors (load resistance must be < 50 ohms).

[Maniac303]

I may actually understand this....

24V-15V = 9V drop before the regulator kicks in?

9V across 30 Ohms is 0.3 A current to get the 9 V drop.

15 V @ 0.3A occurs when load is 15V/0.3A = 50 Ohms?

So I have been banging my head against the wall when the circuit was working properly once I recapped and refreshed the diodes (diode drops were wonky). I was simply testing with a load that was too low to get the regulators to kick in .

Thank you so much for your reply. FWIIW, the circuit regulates fine with one leg of the shunt pulled on my test jig.

[David_AVD]

Wow, that circuit seems like it could cause all sorts of mayhem !