# Ambika double voltage readings

I’ve just finished assembling an Ambika motherboard and and having problems getting past the testing phase.

My first problems is that I seem to have a short between the -8v (blue point on the PSU testing diagram) and the GND of 145ohms

I also get a reading of 145 ohm across capacitor c7 which I replaced and get the same readings

My second problem is I get strange voltage readings
GND // +5V = 10.5V
GND // +3V = 6.6V
GND // +8V = 16.8V
GND // -8V = 0V

can anyone point me in the right direction. All the capacitors and diodes are facing the right way and the IC 9, 10 and 11 are in correctly.

Do you have a second multimeter to confirm the readings of the first?

no but I’ll get hold of one and double check. The readings are repeatable tho

I managed to get hold of another meter and run the test again. The new meter (it’s the same model) gives the same results.

GND // +5V = 10.6V
GND // +3V = 6.6V
GND // +8V = 16.9V
GND // -8V = 0V

and a resistance of 145 ohms between the GND and -8.

I also tried with another PSU. (9v AC, centre +)
Is the -8 used much on the board? I can only really find it at C7? any ideas welcome. I can post some pictures if that helps

1

here’s the link to the photos

anyone …

You obviously have a short between -8V and GND.

-8V is sent to IC1 pin 4, or to the headers. This might be where the short is located.

“I also tried with another PSU. (9v AC, centre +)”

There is no such thing as a center positive AC supply, AC supplies are not polarized, only DC ones are. Are you sure you actually have an AC supply and not a DC supply?

^This!

Hey thanks for everyone for getting back. been trying to figure this out but really struggling.
-altitude, it’s definitely a 9V AC power supply from Farnell. http://www.idealpower.co.uk/external-power-supply/fixed-plug-top-ac-ac-linear/77DB-06-09. the output current looks a bit low actually as only 670mA. The ambika draws 1A ?
-pichenettes. I’ve checked IC1 and get 16V on one pin only (I’m guessing this is +8 not -8).
I’ve checked the headers and they pretty clean.
I also tested the PSU fairly extensively as the -8 is near the ground there too. All the diodes test fine (in circuit) but get a funny reading on IC5. All the other transistors read about 670kOhms on the collector and emitter but IC5 reads 240kOhms between one of the pins and the base. the other pins reads 670kOhms as the others. Does this looks like a faulty reading. I might try swapping this out

to clarify when I say all the transistors I mean in the power supply - IC5, IC6 and the one in the large heatsink