smps trouble

hello to everybody ,
around the transformer there is one mosfet on the heatsink, the lm339 (probably for the LX 50 hall sensor on the 400v dc bus )which i think it does not do much, biger caps are changed , diods ok . The source has 2 paralell smd rezistors giving ~ 7 ohm. They are smd type labeled  18R7 .
No ic , only 2 smd tranzistor like components .
I can't imagine how this power supply circuit looks like
. Are these power supplies something old school?
The inverter is 15kW , made in usa in 1996.
I would like to repair it because it is a good work horse , nor the 3 ph rectifier nor the 3 ph inverter is damaged . The caps on the dc bus look also real good
the control logic looks like this 

It is quite well made compared to nowdays inverter
I have 2 working and this would be the spare . 
I am also stunned that it does not seem do simple v/hz control. I believe that is doing some kind of  vector control on the cip. Something  like field oriented vector control or space vector modulated pwm .
I have great torque control on both accel/decel transient states over every speed set. And the year is 1996  

Just looked at the photo. It looks like there is no smoothing choke on the output. If it runs straight off the rectified mains its probably a self oscillating flyback type if there is no dedicated controller IC. Does it just have one power switch? is it a mosfet? if so look for a low value resistor in the source lead and check it hasn't gone high resistance. This monitors the switch current and would cause hiccupping if it has gone high in value. Other thing to look for is the auxiliary supply that provides startup power before the auxiliary winding has come up to its normal voltage. Look for a high value resistor (220k, 470k or so) between the high voltage DC input and the control circuit, this may have increased in value. Also check the diode and capacitor that provide this auxiliary supply, if these go bad this would also cause hiccupping.

Linear regulators can be made to operate as SMPS regulators if they are allowed to squegg/oscillate. There was a design in the 1990 ARRL handbook for a switching regulator using an LM317. Is there a filter choke on an iron powder ring? is there a diode after this choke?

The LM338 is a voltage regulator

https://www.ti.com/product/LM338?utm_source=google&utm_medium=cpc&utm_campaign=app-null-null-GPN_EN-cpc-pf-google-eu&utm_content=LM338&ds_k=LM338&DCM=yes&gclsrc=aw.ds&gclid=Cj0KCQjwpreJBhDvARIsAF1_BU3zMPVOXQatZMuRARQuGAB1gp4e7ty6rMrCFsMAtqBz2MMaKkgx3RMaAqXxEALw_wcB

So I think that your PSU may be a linear one

hello to all
i have an unusual smps trouble. Actually it is the power supply for the electronics of an inverter that keeps cycling like it would like to start but it changes mind and blocks over and over. 
As usual i jumped on the filter cap's and changed them . Most cases this solved the problem but now when i powered on same thing happened  Next i checked the power mosfet and is ok . When i look at the suspected smps driver IC surprise   it ' s a lm338 op amp . I look desperately on the board after a smps driver ic but nothing    . If there is not one hidden between the pcb layers then there is no such IC.
What kind of power supply is this ?