****************** SOLVED THE PROBLEM *********************

I am fixing a Fluke 8840A's PS output +7.5V (TP604).

I changed the 7.5V Zener diode (CR613) 12 times from 3 different suppliers and from two other MBs. Always I got around +4V (3.8~4.5) never over 5v.

I put new resistor (R601) and capacitor (C609).

I always checked the part before I put in and after I pulled out to see if I put too much soldering iron heat and damaged.

All the voltages out of TP601 through TP607 has no ripple.

At last I pulled out the the resistor (R601) and the Zener from the MB. Put the two parts on a BB and connected my Lab PS to 15V. My DMM shows 7.69V across the Zener!

No clue what I have to now.
Any suggestion will be big help, sure!

 

Assuming the voltages at TP 603, 605, 606 are correct, then bad or leaky components(s) in the circuit down stream are drawing down the voltage.

 

You have to snip the traces to the load branches then re attach in order and isolate the one that is drawing the excess current and overloading it.

 

Thanks for the reply.
All the other out puts are OK.
Only 3 Fluke custom ICs use +7.5V of which two, u302 and u303 I changed from other MB. Do you think these SW IC can make such problem?

See enclosed file; 3 ICs are left side down. I marked 7.5v pins.

 

I pulled IC u303 out of socket and now I am getting +7.3V!
Faulty IC?
Thank you very much.

 

I pulled IC u303 out of socket and now I am getting +7.3V!
Faulty IC?
Thank you very much.

Sounds like it could be bad. Is that IC getting hotter than normal when running? That could indicate it's fried.

 

The temperature is OK; same with other two SW ICs.

I did the continuity test with my DMM.

1) No power on, IC in the socket and out of the socket, IC pins and socket holes. No shorted pins with no 20 pin which is +7.5

2) With power on; no pin has same voltage with pin 20 which is +4.48V now.

I put same kind but different IC but same result. Maybe I have to try with more IC?

I will have to pull out more ICs from other MB and try them later today.

Thank you very much.

 

IC 303 controls Q314. Try disconnecting there.

Check that area, 2 zeners, etc...

 

Whenever I switch on my meter, CR309 & CR311 values change a lot.
ex) CR309 -1.15~-0.43v, Cr311 -1.15~0,
5 min later I power off and turn on; CR309 -1.15~-0.43v, Cr311 -1.15~0,
And after another booting; CR309 -2.94`-3.7 CR311 -2.94~0

By the way Q314 is not hot.

Which pin trace from Q314 I should cut and test CR309 and CR311 voltage? No 7?

 

Hi,
I pulled out Z303 and CR311 still same 4.xV.
-3.9V for U307 out pin 6 and U303 pin 19 which is same spot.

Any idea?

 

Don't need to cut traces yet. U303 acts like 4 low value resistors switched in/out. The back to back zeners limit/clamp the voltage at 3.3v + the diode forward drop in both directions.
Take various voltage readings at the U303 switch input/output (circle-x), and the pins on Lf351-U307 both with U303 in and out of circuit.
U303 interacts with some control(s) on the meter - which I can't say, maybe someone else knows.
You could test the 3.3 v zeners out-of-circuit as you did the other zener.

Oh, I couldn't quite follow post #7

 

Whenever I switch on my meter, CR309 & CR311 values change a lot.
ex) CR309 -1.15~-0.43v, Cr311 -1.15~0,
5 min later I power off and turn on; CR309 -1.15~-0.43v, Cr311 -1.15~0,
And after another booting; CR309 -2.94`-3.7 CR311 -2.94~0

By the way Q314 is not hot.

Which pin trace from Q314 I should cut and test CR309 and CR311 voltage? No 7?

Hi,
I pulled out Z303 and CR311 still same 4.xV.
-3.9V for U307 out pin 6 and U303 pin 19 which is same spot.

Any idea?

Can you clarify?
Check voltages at inputs to U307 and Q314.

And we still have 7.5 vdc ok with no U303, and low ~4.5 with multiple U303's inserted?

 

What is the part number of U303?
I would suggest looking downstream of the U303 for a problem.
What are the voltage readings on pins 12 to 20 on U303?

 

Last time it was moving around 6.9V~7.3V and 3.8~4V chip inserted.

While I was checking the voltages around U303, I smelled burning. Q305 was very hot. I checked Q305's 3 pins. Looks OK but in the afternoon, I will change that from the other MB. And I will check all voltages on U307 and Q314.

Thank you very much.

 

Are the ICs in sockets? If so remove U307 and see if the 7.5V supply is ok.

 

I put back CR311 and network resistor Z303. Q305 and Q306 were replaced from another MB.
Then I installed IC sockets in U307 and U303 with these interchanges:

With Without Voltage
U303 U307 6
U307 U303 7.3

Without Without Voltage
U307 U303 7.1

With With Voltage
U307 U303 4.1

 

Here are the voltages for each pin on U303 and U307:

U303
1 4.89
2 0.18
3 4.3
4 0.18
5 4.89
6 4.88
7 2.78
8 0.18
9 3.19
10 0.03
11 -7.9
12 -3.7
13 0.3
14 0.2
15 1.95
16 0.22
17 0
18 0.2~1.8
19 -1.7~0.2
20 4.5~5.1

U307
1 -15.4
2 12.7
3 12.7
4 -15.4
5 -15.4
6 -1.76~0.05
7 15
8 0

 

Anyone? Any idea?

 

Don't know. Tough one. What is the part number of U303?
I had suspected that the output voltage of U307 pin-6 was exceeding the limits of U303.
But now you show that even U303 by itself is pulling the +7.5V supply down to 6V.

We need to calculate the max supply current and figure out where the current is going.

Max current in 13.4mA.

Next, we need to know current drain of U301, 302, 303.

 

Thank you very much for your time, effort, and concern.

U301~303 is same type Fluke custom "analog quad chip."

I checked voltage U302 removed. Same 4.x Volt
I am getting about +7.5V only if I remove U303.