content/hp8904a_2.rst

Tue, 15 Mar 2016 16:23:22 +0100

author
David Douard <david.douard@logilab.fr>
date
Tue, 15 Mar 2016 16:23:22 +0100
changeset 38
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parent 24
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[10MHz] typos

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1 ========================================================
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2 HP8904A Multifunction Synthetizer Repair: 10MHz output
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3 ========================================================
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5 :Author: David Douard
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6 :Date: 2016-03-06
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7 :Category: Electronics
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8 :Tags: HP8904A, repair, test equipment, signal generator
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10 After having `fixed the PSU of one of my HP8904A`_ signal generator, I
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11 discovered that the other unit had its 10MHz reference output
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12 dead flat.
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14 I've disassembled the main logic board, but unfortunately, the
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15 schematic of this later A2 board (as it is referenced in the servce
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16 manual) is notavailable in any available HP's manual...
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18 The board I have is the second revision:
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20 .. image:: {filename}images/hp8904a/A2_top.jpg
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21 :alt: The logic board (A2) of the HP8904A signal generator.
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22
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23 And the bottom side of the PCB:
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24
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25 .. image:: {filename}images/hp8904a/A2_bottom.jpg
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26 :alt: Bottom side of the A2 board.
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28 According to the presence of soldering flux, it looks like the U46
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29 chip (a 74ALS73CN) on this board has been replaced:
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31 .. image:: {filename}images/hp8904a/A2_U46.jpg
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32 :alt: The U46 chip seems to have been replaced.
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34 After having moved my scope probe around the 10MHz output (J102), I've
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35 found that the most probable culprit is the Q106 transistor (which
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36 seems to be a bipolar one). The 10MHz arrives to the base signal, the
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37 emitter is grounded, but nothing on the collector...
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38
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39 Unfortunately, HP had the awful habit of taging most of the parts in
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40 their test equipments with custom references, even if most of them are
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41 in fact regular parts.
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43 The problem was then to identify a reasonable replacement part for
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44 this puppy. It presents itself as a Motorola with weird reference
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45 ``3 0659``:
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47 .. image:: {filename}images/hp8904a/HP8904A_Q106.jpg
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48 :alt: The failing transistor.
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50 Having no luck searching the net, I've asked the `wonderful EEVBlog
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51 community`_ for some help identifying this transistor. My question
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52 obviously was answered very quickly by several fellows. First I've had
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53 the correct reference for the transistor. Its HP part number is in
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54 fact 1853-0659 (notice the 3-0659 as stamped on the transistor case),
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55 and it is i fact a simple 2N3906.
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57 But another forum member also noted that a wire was broken, as it is
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58 clearly visible on the picture above.
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60 So I did a very quit fix for this problem by simply resoldering the
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61 broken leg of the transistor, and bingo, the 10MHz output is working
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62 again!
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64 This EEVBlog community really is awesome!
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65
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66 .. _`fixed the PSU of one of my HP8904A`: {filename}/hp8904a.rst
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67 .. _`wonderful EEVBlog community`: http://www.eevblog.com/forum/repair/repair-of-an-hp8904a-signal-generator
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