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+++ b/content/hp8662a.rst Sat Mar 24 01:43:02 2018 +0100
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+===============================================
+ HP8662A Synthetized Signal Generator - Part 1
+===============================================
+
+:Author: David Douard
+:Category: Electronics
+:Tags: HP8662A, repair, test equipment, HP, Generator, RF
+:series: HP 8662A
+:series_index: 1
+
+I recently pulled an HP 8662A I bought a while ago from my cellar. I don't
+remember exactly in what condition it was advertized when I purchased it, but
+it was most probably advertized as non working (otherwise it would have been
+cheap enough, more, what the fun in buying properly working test equipment?)
+
+I wanted to make a break in my current other repair project, the `EIP 545B
+Microwave Fequency Meter <{filename}eip545b.rst>`, also having a properly
+working unit such as this incredible signal synthetizer would be quite useful
+to perform several adjustement tasks on the EIP 545B.
+
+Carrying the unit from my cellar to my appartement was some kind of a job: it's
+a robust 30kg device I had to climb the 7 levels: no lift in the old
+parisian building I live in...
+
+Overall state and disassembly
+=============================
+
+The unit was in reasonably good condition, but is missing a few parts: the
+feets, 2 of the plastic corners on the back, and the nice quick sliding guide
+normally located on the bottom. Buttons however were very hard for most of
+them but 2, which were very soft and sloppy.
+
+I think I tried to power it up when I received it a couple of years ago, and
+that it did start, but the keybord did not respond smoothly.
+
+So when I started to look at it two weeks ago, I opened the upper cover and the
+protection plate above the PSU, and tried to plug it and power it up.
+Obviously, a small magic smoke escaped.
+
+So I gave a more detailed look inside the unit, not only the PSU. I found the
+unit so dirty inside I decided to take it apart as far as I can to clean it
+before even attempting to fix the PSU.
+
+.. image:: {filename}images/hp8662a/dirty_hp.jpg
+
+
+And I must say that if most of the unit is very easy to service due to the
+modular design of all RF modules, making is very easy to remove all the boards
+and modules, there are also many parts that are connected together with wires
+soldered point to point, which makes disassembling the case, motherboards and
+so very tedious.
+
+.. image:: {filename}images/hp8662a/top_before_2.jpg
+
+.. image:: {filename}images/hp8662a/bottom_before.jpg
+
+.. image:: {filename}images/hp8662a/front_after.jpg
+
+Also, there are hundreds of screws in this unit! it's insane.
+
+.. image:: {filename}images/hp8662a/top_after.jpg
+
+Whatever, I finally got to a point I could clean most of the parts of the unit,
+so I started to reassemble it. Unfortunately, I did not took enough pictures
+while the disassembling, so it took me some effort to have something that looks
+like an HP 8662A...
+
+.. image:: {filename}images/hp8662a/reassembling_2.jpg
+
+Once I had the structure of the generator back togother, with the back plane in
+place but no other board nor module plugged in, I started to take care of the
+PSU.
+
+.. image:: {filename}images/hp8662a/reassembling_1.jpg
+
+The PSU
+=======
+
+As I said, the first attempt to plug (and not even power on) the unit ended
+with a small magic smoke. The PSU consists in 2 main parts:
+
+- A small linear power supply that is always powered to keep the OCXO
+ tempearure regulation on all the time (so it's not necesary to wait an hour
+ before being able to actually use the device when turned on), This also power
+ a small part of the control circuit of the main PSU.
+- The main switching power supply. The high voltage section is allways on, and
+ the mani switch on the front panel only activate the switching.
+
+.. image:: {filename}images/hp8662a/PSU_simplified_schematic.jpeg
+
+The 4 boards the PSU is made of are:
+
+- a main board (A7A4) on which are the first stages of the AC rectification and
+ filtering,
+
+- a daugther board (A7A3) on which is the inverter, generating several
+ unregulated voltages: +5.2V (used as control rail for the feedback loop),
+ +23V, -13V and -43V; note that this board doaes not have the switching
+ control circuitry,
+
+- a daughter board (A7A1) with the linear regulators to produce very clean and
+ quiet power rails for the RF modules; its output voltages are +20V
+ (+/-100mV), -10V (/- 40mV) and -40V (+/-200mV),
+
+- a daughter board (A7A2) with the switching control circuit. The switching
+ regulation being made on the 5.2V rail, which is not regulated by A7A1.
+
+.. image:: {filename}images/hp8662a/PSU_block_diagram.jpeg
+
+So I gave a close look at thes boards, beginning by the main board, the I found
+the culprit for the smoke: the input protection thermistor. Strangely, it still
+measures a decent resistance value, but hey.
+
+The thermisor is descrived the old fashion way: it's a 5ohm@25°C with a
+temperature coefficient of -3.8%/°C. I did not find an exact replacement in my
+junk parts; only a slighty highter value one (a XXX, which is more like
+20ohms@25°C). Not ideal, but for now, it seems to work fine.
+
+.. image:: {filename}images/hp8662a/smoking_thermistor.jpg
+
+After this first step forward, I was not confident enought to plug the boards
+back in place and try again (I am always worried when I repair switching
+PSUs... they can fail in flame and glory sometimes). So I checked all the
+diodes and transistors of the PSU, but none looked short.
+
+I first applied power to the main board: nothing exploded, I was now having
+a nice + and -160VDC and the oven power was on spec.
+
+Next step, I checked transformers of the inverter board before plugging it
+back, still no catastrophic failure. The inverter alone cannot do anything
+without the control board.
+
+So I quickly checked the 2 other board before plugging them back. So far so good.
+
+But when I tried to turn power on, the voltege OK green LEDs flickered once but
+did not stay on, and the error LED did turn on. All the voltages produced by
+the inverter board were very low. Somethin was wrong.
+
+Looking at the schematics and the boards, I did find several failure points:
+
+- one of the 2 150µH inductors on the 160VDC rails (L6 anf L7 on A7A3) ) looked
+ a bit odd to me: it was not sitting flat on the PCB, which it should for this
+ kind of inductor; and when I gently tried to wiggle it, it appeared a leg was
+ broken, unfortunately too short to be able to solder a piece of wire, so I
+ replaced it with a brand new inductor I recently bought when I repaired an
+ old analog scope (Metrix OX734) for a friend of mine (I allways buy several
+ more parts than I really need, when these are cheap, just in case; seems to
+ be a good idea),
+
+- there are 2 small 400mA fuses on the inverter board to protect the base of
+ the 2 switching (bipolar!) transistors (first time I see the base of an NPN
+ transistor protected by a fuse, but it's also the first time I repair a
+ switching PSU which the main switching transistors are bipolar ones and not
+ MOSFET or so). These are small XXX fuses, they look nice and so but are very
+ expensive (and a bit hard to find): something like 15€ each. So for now, I've
+ replaced it with a simple 5x20 glass fuse: not as pretty but does the job for
+ a few cents.
+
+.. image:: {filename}images/hp8662a/broken_choke.jpg
+
+After these fixes, I reassembled the PSU and tried again: the result was quite
+promising, but not a complete success: the PSU seemed to start, but very
+quickly stops itself with an error LED on: the overvoltage protection was
+activated.
+
+I tried to check several things on the error detection circuits with no succes,
+and followed the service manual instruction, until I found this in the service
+manual, about this overvoltage error:
+
+ If red LED in the upper left hand corner of the board is lit indicating the
+ input voltagr from the A7A3 Inverter assembly was too high and the supply was
+ shut down, it is most likely a problem with the regulator not drawing enough
+ current.
+
+Just had to plug back enough boards and modules for the PSU to be happy again!