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:
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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
?
