Positive voltages at V- and AV-

[Bill]

I am getting V- = 0.63V and AV- = 0.83V instead of the negative voltages I should get. I have inspected all my solder joints and after some fixing, they all look OK to me. The attached photo show my analog board. I've rechecked the resistor and capacitor values and they all check out correctly. I have measured the voltage at pin 8 of U5 since a previous post suggested this might be a problem and it is 8.18V. Is there anything else I can do to troubleshoot the problem?

My analog board

[Petroid]

Check C10 and traces from C10 to U5.
Check if Pin 3 of U5 connected to GND.

[Bill]

I have resoldered C10 and still have the same voltages. I have continuity between Pin 3 of U5 and the GND pad.

I don't know what you mean by check traces from C10 to U5.

[jan]

U5 pin 8 and V+ should be the same , verify first.

have you checked U5 pin 5 ( Vout ) ?

If pin 5 is not at -7.8 there will be no AV- as the 7905 is responsible for that.

are you sure about the orientation of C10 and C11 ?? they are both needed to make the 7660 to work and generate a negative voltage so they need to be in the correct orientation .

[Bill]

C10 and C11 orientation

U5 pin 8 measured 8.17 v and V+ measured 8.19v so I think they are the same to the limits of my meter.

pin 5 reads 0.63 which is the same as I measure at V-

I've attached a closeup of C10 and C11 with the neg mark of all 3 caps facing the same direction which is towards the JYE TECH symbol. Is this correct?

[Ric82]

Hello
Yes the good side for the caps
regards Eric.

[Petroid]

I don't know what you mean by check traces from C10 to U5.

I mean check connectivity between C10 "+" terminal and U5 pin 2; between C10 "-" terminal and U5 pin 4.
While you're at it, also check connectivity between C11 "+" terminal and ground; between C11 "-" terminal and U5 pin 5.

[Bill]

I have checked the 4 connectivities that Petroid ask about and they all are connected. I also resoldered the connections for C8 but that didn't change the voltages at V- and AV- that I am measuring.

[Petroid]

Well, I'm out of suggestions. The U5 seems to be dead.

[Ric82]

Power supply is < 10 volts, ok?
Take a look for a bad solder on jack power J7 too (one of mine was bad and there were troubles on some voltages on main board) perhaps it could send bad voltage too on analogic...

SWITCH on GND position to check reference voltages


So finaly as indicated in the FAQ for troubleshootings at the end of the DSO page


"....
No negative power supply. V- and AV- have a positive value less than 1V.

1 ) First check the voltage at U5 pin8 to make sure power supply is connected to the chip. This voltage should be the same as V+.

2 ) Visually check all pins of U5 to see whether all of them are soldered good. Check the soldering of C12 and C13 as well.

3 ) Check C12 and C13. First use an ohm-meter to check whether they are shorted inside. If they are shorted replace them. If they are not shorted then connect in parallel with a similar value capacitor and see if you can get V-. If you do replace the old capacitor.

4 ) Disconnect one end of R27 and see if the you can get negative voltage at V-. With R27 being open the loading is cut. This is to eliminate the possible impact from loading.

5 ) If V- is still missing with R27 being disconnected it should be considered to replace U5.
...."

so do the check for each point, then as said in the last post, it could be capacitor dead or U5 dead....

rgds Eric