Problem with the Manson EP-920 power supply feeding a Yaesu FT 857
My power supply showed a very irritating behaviour in combination with my FT 857 transceiver. I have heard that the same behaviour was noticed with the units bigger brothers FT847 and EP-925.
When switching the power supply on, or switching the rig on using the power button on the FT 857, the PSU's overload function cuts in, causing a fast oscillation (on-off-on-off) of the output.
I am not sure what causes this, apparently the set is consuming some high peak current when switched on. The power supply reacts on this and regulates the output voltage to zero ===>the current is gone ===> output voltage comes up again ====> high peak current ====>overload cuts in ===> etc.
After I got a copy of the schematic
– tnx Ruud – of the EP-925 (which is the 25A version,
electronically mostly the same, with some more power transistors and
bigger transformer), I found a way to solve this rather anoying
The output current is measured by a comparator (¼ of IC2, a LM339). When the output current is reaching a certain treshold value (controllable by VR2), the output is forcing pin 13 (called COMPENSATION, actually the base of the output transistor driving stage) of the LM723 voltage regulator IC to ground, bringing the output voltage of the power supply to zero.
The comparator has a small 500pF capacitive feedback, making it act as an integrator, with a very short time constant. I didn't bother to calculate how short, but short enough to react on the current peak caused by my FT 857. The idea is to make this integration time longer, to give the set time to startup, without being interrupted by the overload protection of the power supply.
After some experimenting with various values (22nF was too small, 2200nF was too big) I found that 150nF worked fine. The set switches on neatly, without all that on-off stuff. The short circuit protection is still fast enough. This capacitor can easily be soldered on the back side of the pcb, so it is not necessary to disassemble the front part.
Some pictures (better then 1000 words):
Part of the schematic, with the modification.
Close-up of the modification.
Wider overview picture.
FT857 sitting on top of the EP-920.