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Repair HP3455A Multimeter

 

Hi,
my 3325A is driving me crazy and no one could help yet, so I went over to a faulty HP3455A multimeter. It worked many years, but now it comes up with a OL display in DC.
Cal test stops at 11,10,9,8,5,4,3,2,1,0 and a digital value 0.062.
?
All voltages are stable with nearly zero ripple. Vref is 10V, all perfect.
The good service manual (no extrem testing procedure with a signature analyzer as in the 3325A manual)? has many tables with listings of voltages for the individual cal number. The cal feature switches many fets to ground, vref etc.
?
I found suspect voltages at the outputs of U6 a LM339 and replaced it, this fixed some cal errors. The meter now has only cal error #10. Checking the flowchart for #10 and the voltage matrix a found a suspect output on Pin 14 of U5, so I replaced it with no success. The voltage is 8.8V, that seems to be the Gate Bias voltage after 100KOhms, i lifted the pin and supplied shortly -24v, but this was not suceccessful either.
To pin 14 there is Q21 connected, i suppose it does not switch off, Pin 14 should be -24V.?
?
Next I will? replace Q19, the part number is 5070-7074, but I can't find any information and it is not listed in the HP crossreference list. I has 4 pins, may be one pin is can and gate is tied to taht pin as i can measure the gate bias voltage at the can.?
?
Has anyone a spare part or a hint which fet i can use to replace the Q19?
?
Ralf
?
 

Ralf,

Unless I am very mucht mistaken (...) the 3325 uses the chassis as GND, and **all** screws fastening the PCB should better be nice and tight. The PCB has no overall GND itself. This can drive you nuts, for obvious reasons.

Wilko
(who hopes he has remembered correctly)
 

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I don't find a 5070-7074 in the 3455A manual I downloaded, Manual P/N 03455-90003. Looking for Q19, I find A10Q3, 4, 15, 16, 18, 19, 21? which is a 5081-7074 and A15Q19, 20, which is a 2N4391. ?

?You might try swapping Q19 with a different 5070-7074. You might pass diagnostics, or at least find a different failure to troubleshoot.

John??? KK6IL

On 3/28/2025 4:15 AM, Ralf Rosche via groups.io wrote:

Hi,
my 3325A is driving me crazy and no one could help yet, so I went over to a faulty HP3455A multimeter. It worked many years, but now it comes up with a OL display in DC.
Cal test stops at 11,10,9,8,5,4,3,2,1,0 and a digital value 0.062.
?
All voltages are stable with nearly zero ripple. Vref is 10V, all perfect.
The good service manual (no extrem testing procedure with a signature analyzer as in the 3325A manual)? has many tables with listings of voltages for the individual cal number. The cal feature switches many fets to ground, vref etc.
?
I found suspect voltages at the outputs of U6 a LM339 and replaced it, this fixed some cal errors. The meter now has only cal error #10. Checking the flowchart for #10 and the voltage matrix a found a suspect output on Pin 14 of U5, so I replaced it with no success. The voltage is 8.8V, that seems to be the Gate Bias voltage after 100KOhms, i lifted the pin and supplied shortly -24v, but this was not suceccessful either.
To pin 14 there is Q21 connected, i suppose it does not switch off, Pin 14 should be -24V.?
?
Next I will? replace Q19, the part number is 5070-7074, but I can't find any information and it is not listed in the HP crossreference list. I has 4 pins, may be one pin is can and gate is tied to taht pin as i can measure the gate bias voltage at the can.?
?
Has anyone a spare part or a hint which fet i can use to replace the Q19?
?
Ralf
?
 

I'd try 2N4119A or PN4119A.? I might have some...

Mouser?are mighty proud of them.

On Fri, Mar 28, 2025 at 4:15?AM Ralf Rosche via <ralf.rosche=[email protected]> wrote:
Hi,
my 3325A is driving me crazy and no one could help yet, so I went over to a faulty HP3455A multimeter. It worked many years, but now it comes up with a OL display in DC.
Cal test stops at 11,10,9,8,5,4,3,2,1,0 and a digital value 0.062.
?
All voltages are stable with nearly zero ripple. Vref is 10V, all perfect.
The good service manual (no extrem testing procedure with a signature analyzer as in the 3325A manual)? has many tables with listings of voltages for the individual cal number. The cal feature switches many fets to ground, vref etc.
?
I found suspect voltages at the outputs of U6 a LM339 and replaced it, this fixed some cal errors. The meter now has only cal error #10. Checking the flowchart for #10 and the voltage matrix a found a suspect output on Pin 14 of U5, so I replaced it with no success. The voltage is 8.8V, that seems to be the Gate Bias voltage after 100KOhms, i lifted the pin and supplied shortly -24v, but this was not suceccessful either.
To pin 14 there is Q21 connected, i suppose it does not switch off, Pin 14 should be -24V.?
?
Next I will? replace Q19, the part number is 5070-7074, but I can't find any information and it is not listed in the HP crossreference list. I has 4 pins, may be one pin is can and gate is tied to taht pin as i can measure the gate bias voltage at the can.?
?
Has anyone a spare part or a hint which fet i can use to replace the Q19?
?
Ralf
?
 

Hi Orin,?
thank you for the hint,? i ordered 2N4149A from Dugikey, hope this will solve my cal #10 error.
Ralf
 

2N4119.
A bit more investigation this morning.
I have on error 10 ?A10U6 and A10U5 Pin 13 the correct 10 Volts, the drivers U16... are ok.
The inputs are correct as listed in the manual.?
On A10U5 PIN14 I see 8 Volts, should be -24 Volts,?
May be its an error in the manual?
If I open Q21, which is connected to PIN 14, then error 8 and 5 comes up, as expected.
Connecting shortly PIn 14 of A10U5 to -24V clears ?the error 10.?
Hmm very strange.?
I tested Q4 and Q19, they are ok with multimeter, as JFets can be tested with a simple multimeter.
However, I am waiting now for the 2N4119 and will replace them. This are the only involved cal switches mentioned in the flowchart.
Ralf
 

Hi have a pdf from the flowcharts, group B schematics and the groupB service excerpt.

HP3455A_groupB_Schematics.pdf
HP3455A_groupB_Schematics.pdf
HP3455A_groupB.pdf
HP3455A_groupB.pdf
HP3455A_flowcharts.pdf
HP3455A_flowcharts.pdf

 

Hi,
yesterday i got the Jfets from digikey, i replaced Q19,Q21,Q4 ,Q22 but the result is the same, still error 10. According to the flowchart the advice is to check Q4,Q19 and components around.
?
There are many components around, so could anyone look at this part of the schematics and give an advice?
?
May be U3??
?
And I have a curious 8.75 V at pin 14 of U5, should be -24V, but not? in focus of cal test 10.? (I replaced already U5,U4,U6)
?
Ralf

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