latzka wrote:...Are there any other transistors driving the ones on the H-bridge?
Indeed there are. Four can be seen on the partial H-bridge sketch provided (as a link) to you yesterday. Here is a copy of it taken from post: viewtopic.php?p=92759#p92759
By this time it should be obvious to you that the sketch is showing solder pads on the top side of the main_PCA, and all the dashed-lines indicate component cases beneath the board -- actually they all are leaded through-hole mounting devices.
Transistors, (xstrs), Q72, Q73, Q87 & Q88 are necessary to control the four power xstrs that form the bridge. Perhaps two of them are more critical that the other pair. I do not have that detail for Roomba R3, but I do for Roomba R2. To view the R2 bridge circuit simply search this rr-board for schematic2
In R2's schematic2 one can see a pair of smaller xstrs, just outside the H-bridge central area, that serve a signal splitting function and also a signal buffering function, since the MCU's driving capability is too wimpy to directly drive bases of B772 and D882 power xstrs. By studying either splitter xstr, say Q31 (upper right) one may see that when signal voltage on its base goes HI it makes Q31 turn on (i.e., emitter and collector currents flow). Emitter current through resistor R118 creates a voltage signal on node #120, thus causing current to flow through resistor R130 and into the base of NPN xstr Q28 which turns it ON.
Back over at the collector of Q31, the xstrs HI base voltage causes collector current through R119 to develop a base voltage on PNP power xstr Q27 that is low enough to make base current flow out of the xstr and turn it ON.
With both Q27 and Q28 conducting collector current, that current courses through the motor, "M", and the motor is expected to drive its wheel in reverse
(opposite to the green "Forward" arrows in the drawing).
Notice that Q30, at right in schematic2, must be held OFF for the above to happen -- so the MCU pulls Q30's base down to near zero volts.
But if R2's MCU commands FORWARD wheel rotation, it is expected to first command Q30 fully ON to clamp the base of Q31 to just a few tenths of a volt greater than GND, an action which prevents Q31 from erroneously turning ON.
Back to the R3 circuit now. It has four outlier xstrs shown 'wrapped around' the H-bridge components in the above sketch. I can only think that two of those provide split-voltage drives to bases of the power xstrs, and the remaining two (TO-92 cased) serve in the same manner as R2's Q30, to clamp low one or the other splitter bases (to safely hold OFF the side that must remain OFF).
Now, with that introductory material behind us, I suggest that you shift from resistance measurements on those bridge components to voltage measurements
while running the several wheel-drive BiTs. The goals would be to:
1) Verify that a pair of PNP/NPN power xstrs have their base voltages lowered & raised, respectively, relative to PCB-GND for one commanded drive direction.
2) Verify that the other pair of PNP/NPN power xstrs have their base voltages lowered & raised, respectively, relative to PCB-GND for the inverse commanded drive direction.
3) If improper base signal is found on any power xstr shift the voltage test point to the relative splitter xstr's base to see that the MCU is passing an inverted signal to it.
If you learn that the MCU is driving a splitter correctly, but the splitter does not drive the PNP/NPN pair correctly, you have a fair chance of fixing the main_PCA.
If you learn that a properly driven PNP/NPN pair fails to pass current through the wheel motor, you have a fair chance of fixing the main_PCA.
If you learn that the MCU fails to output any of the four necessary control signals to H-bridge components your main_PCA is toast. IMHO, that is.