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| 1 ======================================================== | |
| 2 HP8904A Multifunction Synthetizer Repair: 10MHz output | |
| 3 ======================================================== | |
| 4 | |
| 5 :Author: David Douard | |
| 6 :Date: 2016-03-06 | |
| 7 :Category: Electronics | |
| 8 :Tags: HP8904A, repair, test equipment, signal generator | |
| 9 | |
| 10 After having `fixed the PSU of one of my HP8904A`_ signal generator, I | |
| 11 discovered that the other unit had its 10MHz reference output | |
| 12 dead flat. | |
| 13 | |
| 14 I've disassembled the main logic board, but unfortunately, the | |
| 15 schematic of this later A2 board (as it is referenced in the servce | |
| 16 manual) is notavailable in any available HP's manual... | |
| 17 | |
| 18 The board I have is the second revision: | |
| 19 | |
| 20 .. image:: {filename}images/hp8904a/A2_top.jpg | |
| 21 :alt: The logic board (A2) of the HP8904A signal generator. | |
| 22 | |
| 23 And the bottom side of the PCB: | |
| 24 | |
| 25 .. image:: {filename}images/hp8904a/A2_bottom.jpg | |
| 26 :alt: Bottom side of the A2 board. | |
| 27 | |
| 28 According to the presence of soldering flux, it looks like the U46 | |
| 29 chip (a 74ALS73CN) on this board has been replaced: | |
| 30 | |
| 31 .. image:: {filename}images/hp8904a/A2_U46.jpg | |
| 32 :alt: The U46 chip seems to have been replaced. | |
| 33 | |
| 34 After having moved my scope probe around the 10MHz output (J102), I've | |
| 35 found that the most probable culprit is the Q106 transistor (which | |
| 36 seems to be a bipolar one). The 10MHz arrives to the base signal, the | |
| 37 emitter is grounded, but nothing on the collector... | |
| 38 | |
| 39 Unfortunately, HP had the awful habit of taging most of the parts in | |
| 40 their test equipments with custom references, even if most of them are | |
| 41 in fact regular parts. | |
| 42 | |
| 43 The problem was then to identify a reasonable replacement part for | |
| 44 this puppy. It presents itself as a Motorola with weird reference | |
| 45 ``3 0659``: | |
| 46 | |
| 47 .. image:: {filename}images/hp8904a/HP8904A_Q106.jpg | |
| 48 :alt: The failing transistor. | |
| 49 | |
| 50 Having no luck searching the net, I've asked the `wonderful EEVBlog | |
| 51 community`_ for some help identifying this transistor. My question | |
| 52 obviously was answered very quickly by several fellows. First I've had | |
| 53 the correct reference for the transistor. Its HP part number is in | |
| 54 fact 1853-0659 (notice the 3-0659 as stamped on the transistor case), | |
| 55 and it is i fact a simple 2N3906. | |
| 56 | |
| 57 But another forum member also noted that a wire was broken, as it is | |
| 58 clearly visible on the picture above. | |
| 59 | |
| 60 So I did a very quit fix for this problem by simply resoldering the | |
| 61 broken leg of the transistor, and bingo, the 10MHz output is working | |
| 62 again! | |
| 63 | |
| 64 This EEVBlog community really is awesome! | |
| 65 | |
| 66 .. _`fixed the PSU of one of my HP8904A`: {filename}/hp8904a.rst | |
| 67 .. _`wonderful EEVBlog community`: http://www.eevblog.com/forum/repair/repair-of-an-hp8904a-signal-generator | |
| 68 | |
| 69 |
