There was a very nice 189/89 IV repair manual once published on the forum (see the links to the posts):
Check the manual carefully before repairing.
auto-translated documents in english
Which was very helpful, but did not help much. All the voltages seem to be OK. Input path is OK, ceramic resistor network is fine
.
I actually have another 189 (this time a a real 189) that I could compare.
(It was also sick, but soaking the PCB in IPA for a day as suggested by the manuals above seem to have fixed it.)
So after much poking around, I found 6 traces going from the measurement IC to the uC (see the image below).
(Luckily these signals have accessible vias (and wire-wraping wire nicely fits in), so these is no need for fancy grabbers to grab the data.)
My 189 has pulses on five of them (the one remaining I marked "SPI GND(?)").
The "READY (?)" pin has double pulses synchronized with the SPI.
So you can see that 189 has "fancy" bus with a lot of data, while 89 IV has a very short repeating pattern.
And 89 IV has no signal on the mysterious "READY" pin!
I also dumped the SPI Flash initialization sequence, and it seems that both meters read mostly identical addresses and values.
I can use settings, switch modes and ranges, turn on the backlight, etc. So it seems that everything works except that the value is always 0.
Unless I am missing something, I guess that the measurement IC is blown. Could you suggest other ideas?
Upd: Added another photo to show that uC is on the other side of the board and it's not the TIR1000.