keithley 2303 OVERFLOW in current line

[maaii]

My problem is that the current line of the digital display is reading OVERFLOW. When the instrument wasn't wired to anything I was getting a current reading that jumped between 1-2 A so I wired a 10 ohm, 5 W resistor and set the voltage to 1V. I assumed this would give me a current of .1A (.1A *10ohm = 1V) with a power drain of 0.1W (0.1A^2 * 10ohm = 0.01 *10 = 0.1W).

The current line has been reading OVERFLOW no matter what I do. The current limit is set to 5A and, though I am certain this is a display issue rather than a screwup in the circuit (since the DMM is reading correctly).

Please advise.

Thank you,

[picburner]

Can you take a pic of how you connected the load to the rear terminal board?

[maaii]

Hi picburner

Thank you for reply, i took this PSU to my home and use DC FAN to test.

i can read Voltage and  Current on Meter. i don't know why 2303 always show OVERFLOW of current on display.

[picburner]

The load connection you made is correct.
From the 2303 user manual: Overflow indicates that voltage or current reading exceeds the measurement range of the instrument.
But the settings of the instrument you made would not seem to lead to this.
Perhaps the instrument could have a problem.
The service manual is easily found on the web but, unfortunately, does not contain the schematic which makes the search for a possible fault complicated.
I sold my 2303 a long time ago and therefore I can't make measurements for comparison.

[maaii]

Hi picburner

OK, Thank you. I hope someone could help me to solve this problem.

Many thanks!!

[TiN]

Sorry, I did not do any repairs on my K2304, so I'm not familiar with this failure.

I'd perhaps start with testing power supply rails and then pass transistors/current sensing circuits in the PSU.
Check resistance on the output, maybe some output capacitor get damaged and shorted the output?.

[Chris56000]

Hi!

The way to trace this type of fault is to locate the current–shunt in the main PSU output circuit, and trace the leads across to the front–panel metering circuitry.

Once you have done this, try shorting the leads out to see if you the current indications on the LCD reads zero – if you still get "OVERFLOW" with the leads from the current shunt shorted the fault is in the front–panel metering itself – is there any suggestions of a previous overload that might have damaged components on the front panel itself?

Chris Williams

PS!

I couldn't find any of the diagrams that the Service Manual claims are provided – can anyone who's printed a copy out confirm they're actually in it please?

I've no printing facilities at home at the moment!

[maaii]

Hi TiN

I saw some capacitor on the PCB with a little different colors, is this the bad capacitor ??

[maaii]

Hi Chris56000

After i shorted the leads out, i still got "OVERFLOW" on LCD panel.

so, what should i do ?

Thanks for reply.

[Chris56000]

Hi!

You'll need to dismantle and investigate the front–panel metering circuitry!

I'm not sure if the 2303 is a one channel psu or two, if it is one channel you'll not have a comparison side, so what I suggest is to pay particular attention to:–

1) A VERY CAREFUL examination of the front panel components for damage – Keithley have at least provided a full component–level parts list so you don't have to guess what any charred remains were!

2) All voltage–supply rails going up to the front–panel – loss of a negative supply feeding an op–amp may unbalance it's output and cause the a/d converter in the front–panel uC to overflow – op–amp supplies are +ve on 7 (single) or 8 (dual) and –ve on pin 4;

3) Any voltage–reference diodes or ICs on the front panel PCB – the parts list will help here!

4) Make sure there's no open–circuit wiring or yakky connector–plugs between the main and front panels!

I don't think it's likely to be the a/d converter or the front–panel uC itself causing this, but if you're really unlucky and can't find an obvious defect in the items I mentioned above, you might need to ask on here and see if another Member with a comparison unit can offer any voltage readings on all the front–panel devices with the unit providing a known load, e.g., 5V at 0.1A.

Wether you can get individual bits and pieces out of Tek now since they took over Keithley is a moot point and one I have distinct misgivings about!

Unfortunately I can't afford one of these units to use for drawing out as even "parts mule" units are going for ridiculous bloody postage charges – I saw £650 postage PLUS import charges from one US seller not long back!

Chris Williams

PS!

I've just realised I think it's all on one large main PCB with only possibly the LCD display connections going to the front panel – the LCD module wiring would NOT cause a fault like this as a problem in the LCD wiring would cause garbage, black blocks in all the character positions or nothing at all!

DO replace that discolored tantalum capacitors and the MAX5. . . device next to it to begin with tho!

[maaii]

Hi Chris56000

Where is front panel ? 2303 only have one output channel and two PCB. One is analog board the other is digital board. Could you help me to figure out which components need to be measured?

[Chris56000]

Hi!

On the Digital Board, I would replace:-

1) C129 – that discolored tantalum capacitor;

2) The MAX532BCPE U110 (Keithley have provided pads to fit an SMD package one if you can't get a DIP one and you're confident soldering SMD);

3) The four HPCL2601 and HPCL2631 optos.

Can you set up the unit to give 5V at 0.1A and tell us what ALL the test–point voltage readings are after you've done that?

Chris Williams

[maaii]

Hi Chris56000

I will try to replace the tantalum capacitor first.

Thanks~~

[maaii]

Hi Chris

I repaced the tantalum capacitor and MAX532BCPE U110, but no effect  , so i measured all test point of digital board and analog board as below.  I setup up 5v 1a on front panel and connect with DC 12v fan.

After checked the analog board, the voltage of TP307 and TP311 seem not correct.  do you have any idea about this ??

Thanks for your help.

2303 digital board            
Test Point   Name        V      Test point with GND Pin
TP100    RAMPB      1.06            TP109
TP101    ISNSB      0.055    TP109
TP102    REG_IN      6.555    TP106
TP103    GDBHI      6.408    TP109
TP104   "+5VD"      5.006    TP106
TP105   RAMPA      0.994    TP109
TP106   DGND         
TP107   ISNSA      0.008    TP109
TP108   GDAHI      6.6            TP109
TP109   BGND         
TP111   "+24VA"      24.27    TP112
TP112   AGND         

2303 Analog board            
Test Point   Name         V       Test point with GND Pin
TP301    VOUT      4.998    TP317
TP302   "+12VB"      11.99    TP306
TP303   "-15VA"      -14.99    TP317
TP304   "+5VA"      4.985    TP317
TP305   OVER-I      0.424    TP306
TP306   BGND         
TP307   I-SNSB      0.057    TP306
TP308   "-10VA"      -9.86    TP317
TP309   15VA      15.41    TP317
TP310   AGND         
TP311   I-SNSA      -0.01    TP306
TP314   GBL                4.1            TP318
TP316   GAL                4.5      TP318
TP317   AGND         
TP318   BGND         

[Chris56000]

Hi!

If the current display changes to "0" or similar TP307 & TP311 are short-circuited, the fault lies in the current-sense amplifier between the output-current shunt and the test-points TP307/TP311, whereas if short-circuiting these two TPs together makes no difference to the "OVERFLOW" indications, the fault is between the TPs and the ADC/microprocessor circuitry.

It seems like there's an open-circuit somewhere in the current monitoring loop after TP307/TP311 causing the ADC/uC to overflow!

Unfortunately I can't offer much more assistance beyond this, as it really needs someone with one of these units to start reverse-engineering the main PCB between TP307/TP311 and the ADC/microprocessor & LCD circuitry - we need to identify what circuitry is between TP307/TP311 and the ADC, and also the current-amplifier circuits between the current-shunt and TP307/TP311!

Don't forget tho', there is a full component-level parts-list given by Keithley in the service manual, my recommendation being to print out those pages of the parts-list covering the "300" block of components, and then start testing all the resistors and capacitors from R301/C301 upwards for faults against the stated values in the parts list - these will cover a lot of the current-measuring circuitry.

If you can't find anything amiss with the Rs and Cs, repeat with the diodes and discrete-transistors in the "300" block of components.

You can use a Chinese Tester or a DMM with a Capacitance Test range for these checks, then anything wildly out can be removed for an out-of-circuit test!

I don't have, and can't afford to buy, one of these units at the moment, so can I pass this thread over to someone who has a 2303 and is willing to consider a rev. project please?

My apologies!

Chris Williams