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@@ -0,0 +1,69 @@
+========================================================
+ HP8904A Multifunction Synthetizer Repair: 10MHz output
+========================================================
+
+:Author: David Douard
+:Date: 2016-03-06
+:Category: Electronics
+:Tags: HP8904A, repair, test equipment, signal generator
+
+After having `fixed the PSU of one of my HP8904A`_ signal generator, I
+discovered that the other unit had its 10MHz reference output
+dead flat.
+
+I've disassembled the main logic board, but unfortunately, the
+schematic of this later A2 board (as it is referenced in the servce
+manual) is notavailable in any available HP's manual...
+
+The board I have is the second revision:
+
+.. image:: {filename}images/hp8904a/A2_top.jpg
+   :alt: The logic board (A2) of the HP8904A signal generator.
+
+And the bottom side of the PCB:
+
+.. image:: {filename}images/hp8904a/A2_bottom.jpg
+   :alt: Bottom side of the A2 board.
+
+According to the presence of soldering flux, it looks like the U46
+chip (a 74ALS73CN) on this board has been replaced:
+	 
+.. image:: {filename}images/hp8904a/A2_U46.jpg
+   :alt: The U46 chip seems to have been replaced.
+
+After having moved my scope probe around the 10MHz output (J102), I've
+found that the most probable culprit is the Q106 transistor (which
+seems to be a bipolar one). The 10MHz arrives to the base signal, the
+emitter is grounded, but nothing on the collector...
+
+Unfortunately, HP had the awful habit of taging most of the parts in
+their test equipments with custom references, even if most of them are
+in fact regular parts.
+
+The problem was then to identify a reasonable replacement part for
+this puppy. It presents itself as a Motorola with weird reference
+``3 0659``:
+
+.. image:: {filename}images/hp8904a/HP8904A_Q106.jpg
+   :alt: The failing transistor.
+      
+Having no luck searching the net, I've asked the `wonderful EEVBlog
+community`_ for some help identifying this transistor. My question
+obviously was answered very quickly by several fellows. First I've had
+the correct reference for the transistor. Its HP part number is in
+fact 1853-0659 (notice the 3-0659 as stamped on the transistor case),
+and it is i fact a simple 2N3906.
+
+But another forum member also noted that a wire was broken, as it is
+clearly visible on the picture above.
+
+So I did a very quit fix for this problem by simply resoldering the
+broken leg of the transistor, and bingo, the 10MHz output is working
+again!
+
+This EEVBlog community really is awesome!
+
+.. _`fixed the PSU of one of my HP8904A`: {filename}/hp8904a.rst
+.. _`wonderful EEVBlog community`: http://www.eevblog.com/forum/repair/repair-of-an-hp8904a-signal-generator
+
+