Bruel & Kjaer Microphone Amplifier 2604
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and the
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Of the pair, I looked
at the later Model 2604 first because the meter dial glass had
become detached and needed attention. The design of the equipment
follows almost identical lines to the Model
1014 which I've recently overhauled. I picked both a Model
2603 and a 2604 which are almost identical apart from having
different frequency coverages.
The 2603 covers 2Hz to 40KHz
and the 2604 covers 20Hz to 200KHz
Both use valves and in my search
for information I tried using one of the AI tools which swore
blind the 2603 was valve-based and the 2604 used transistors
(in fact even suggesting from the production date they were germanium
transistors.. maybe later productions were redesigned to use
transistors as in fact did the 1014 ?).
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The meter was in a dreadful
condition because mice had been messing it up and with the
loose glass the interior was in bad shape. The first step was
to remove it from the equipment in the same manner as I'd detached
the 2014 meter then remove the front escutcheon and clean off
all the old glue etc. Once clean I used a thin line of superglue
to fix the glass in place.
Fortunately the movement seems
to be fine and the case polished up well.
Below, the front panel cleaned
and the meter refitted.
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In terms of its usefullness
either the 2903 and 2604 can be used as an RMS voltmeter although
nowadays a handheld multimeter is much more convenient.
The two equipments are bacally
similar although they have diffrent frquency responses. The
2603 has a lower range from 2Hz to 40KHz and this 2604 from
10Hz to 200KHz.
Below is the only circuit diagram
I can find and this is for the 2603
Click
to see a larger drawing
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