Help! I am a Mechanial Engineer trying to fix an Electronic control.
When I power up the spa hear the hi limit relay pull in then 10 or 15 secs I hear the circ pump relay but no pump activity at all. touching the pump button activates the hi (2nd) speed relay again no pump activity.
blower does not work light does work.

Spa Data:
ThermoSpa, 1900D Balboa control, about yr.2000?,
230V AC Input,
1 Pump 2 speed
Heater Type : 2.5" Stainless Flow Thru, heater 240V 5.5 KW

Problem:
Started when I my kids ran the spa on 104 for near two hours straight they said it just "died". Think system may have overheated because slow blow fuse blew. I tried to replaced fuse and then HI limit relay would not stay on. there was also an SN error so I replaced the hi limit sensor. I think it is the control board

Fixes I have tried:
replaced hi limit sensor
replaced pump relays (Zettler American)
Replaced opto-isolator chip.


Fixes not tried:
Flow sensor
replace triac
replace darlington arrays ICs
replace driver ICs
(I work for an electronics MFG company so we have these parts on hand)

I am working my way back to the the programmed micro replacing every part

Observations:
pump works w/o control.
heater measures 10 Ohms
very low or no voltages a relay outputs or heater line.
transformer has one black and two yellow wires on output side.
No voltage from output but relays still fire.
cannot seem to find a schematic at ThermoSpa site.
I do not want to spen $400 on a replacement control.
There is a programmed micro on the board.

I am running out of Ideas and I gues I do not want to spend the 20 bucks I some one wiser tells me I am wasting my time and that the the micro is fried.

I am sure I will have Good luck getting the hex code out of Balboa to flash a new micro.
Correct.