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| 1 =============================================== | |
| 2 HP8662A Synthetized Signal Generator - Part 1 | |
| 3 =============================================== | |
| 4 | |
| 5 :Author: David Douard | |
| 6 :Category: Electronics | |
| 7 :Tags: HP8662A, repair, test equipment, HP, Generator, RF | |
| 8 :series: HP 8662A | |
| 9 :series_index: 1 | |
| 10 | |
| 11 I recently pulled an HP 8662A I bought a while ago from my cellar. I don't | |
| 12 remember exactly in what condition it was advertized when I purchased it, but | |
| 13 it was most probably advertized as non working (otherwise it would have been | |
| 14 cheap enough, more, what the fun in buying properly working test equipment?) | |
| 15 | |
| 16 I wanted to make a break in my current other repair project, the `EIP 545B | |
| 17 Microwave Fequency Meter <{filename}eip545b.rst>`, also having a properly | |
| 18 working unit such as this incredible signal synthetizer would be quite useful | |
| 19 to perform several adjustement tasks on the EIP 545B. | |
| 20 | |
| 21 Carrying the unit from my cellar to my appartement was some kind of a job: it's | |
| 22 a robust 30kg device I had to climb the 7 levels: no lift in the old | |
| 23 parisian building I live in... | |
| 24 | |
| 25 Overall state and disassembly | |
| 26 ============================= | |
| 27 | |
| 28 The unit was in reasonably good condition, but is missing a few parts: the | |
| 29 feets, 2 of the plastic corners on the back, and the nice quick sliding guide | |
| 30 normally located on the bottom. Buttons however were very hard for most of | |
| 31 them but 2, which were very soft and sloppy. | |
| 32 | |
| 33 I think I tried to power it up when I received it a couple of years ago, and | |
| 34 that it did start, but the keybord did not respond smoothly. | |
| 35 | |
| 36 So when I started to look at it two weeks ago, I opened the upper cover and the | |
| 37 protection plate above the PSU, and tried to plug it and power it up. | |
| 38 Obviously, a small magic smoke escaped. | |
| 39 | |
| 40 So I gave a more detailed look inside the unit, not only the PSU. I found the | |
| 41 unit so dirty inside I decided to take it apart as far as I can to clean it | |
| 42 before even attempting to fix the PSU. | |
| 43 | |
| 44 .. image:: {filename}images/hp8662a/dirty_hp.jpg | |
| 45 | |
| 46 | |
| 47 And I must say that if most of the unit is very easy to service due to the | |
| 48 modular design of all RF modules, making is very easy to remove all the boards | |
| 49 and modules, there are also many parts that are connected together with wires | |
| 50 soldered point to point, which makes disassembling the case, motherboards and | |
| 51 so very tedious. | |
| 52 | |
| 53 .. image:: {filename}images/hp8662a/top_before_2.jpg | |
| 54 | |
| 55 .. image:: {filename}images/hp8662a/bottom_before.jpg | |
| 56 | |
| 57 .. image:: {filename}images/hp8662a/front_after.jpg | |
| 58 | |
| 59 Also, there are hundreds of screws in this unit! it's insane. | |
| 60 | |
| 61 .. image:: {filename}images/hp8662a/top_after.jpg | |
| 62 | |
| 63 Whatever, I finally got to a point I could clean most of the parts of the unit, | |
| 64 so I started to reassemble it. Unfortunately, I did not took enough pictures | |
| 65 while the disassembling, so it took me some effort to have something that looks | |
| 66 like an HP 8662A... | |
| 67 | |
| 68 .. image:: {filename}images/hp8662a/reassembling_2.jpg | |
| 69 | |
| 70 Once I had the structure of the generator back togother, with the back plane in | |
| 71 place but no other board nor module plugged in, I started to take care of the | |
| 72 PSU. | |
| 73 | |
| 74 .. image:: {filename}images/hp8662a/reassembling_1.jpg | |
| 75 | |
| 76 The PSU | |
| 77 ======= | |
| 78 | |
| 79 As I said, the first attempt to plug (and not even power on) the unit ended | |
| 80 with a small magic smoke. The PSU consists in 2 main parts: | |
| 81 | |
| 82 - A small linear power supply that is always powered to keep the OCXO | |
| 83 tempearure regulation on all the time (so it's not necesary to wait an hour | |
| 84 before being able to actually use the device when turned on), This also power | |
| 85 a small part of the control circuit of the main PSU. | |
| 86 - The main switching power supply. The high voltage section is allways on, and | |
| 87 the mani switch on the front panel only activate the switching. | |
| 88 | |
| 89 .. image:: {filename}images/hp8662a/PSU_simplified_schematic.jpeg | |
| 90 | |
| 91 The 4 boards the PSU is made of are: | |
| 92 | |
| 93 - a main board (A7A4) on which are the first stages of the AC rectification and | |
| 94 filtering, | |
| 95 | |
| 96 - a daugther board (A7A3) on which is the inverter, generating several | |
| 97 unregulated voltages: +5.2V (used as control rail for the feedback loop), | |
| 98 +23V, -13V and -43V; note that this board doaes not have the switching | |
| 99 control circuitry, | |
| 100 | |
| 101 - a daughter board (A7A1) with the linear regulators to produce very clean and | |
| 102 quiet power rails for the RF modules; its output voltages are +20V | |
| 103 (+/-100mV), -10V (/- 40mV) and -40V (+/-200mV), | |
| 104 | |
| 105 - a daughter board (A7A2) with the switching control circuit. The switching | |
| 106 regulation being made on the 5.2V rail, which is not regulated by A7A1. | |
| 107 | |
| 108 .. image:: {filename}images/hp8662a/PSU_block_diagram.jpeg | |
| 109 | |
| 110 So I gave a close look at thes boards, beginning by the main board, the I found | |
| 111 the culprit for the smoke: the input protection thermistor. Strangely, it still | |
| 112 measures a decent resistance value, but hey. | |
| 113 | |
| 114 The thermisor is descrived the old fashion way: it's a 5ohm@25°C with a | |
| 115 temperature coefficient of -3.8%/°C. I did not find an exact replacement in my | |
| 116 junk parts; only a slighty highter value one (a XXX, which is more like | |
| 117 20ohms@25°C). Not ideal, but for now, it seems to work fine. | |
| 118 | |
| 119 .. image:: {filename}images/hp8662a/smoking_thermistor.jpg | |
| 120 | |
| 121 After this first step forward, I was not confident enought to plug the boards | |
| 122 back in place and try again (I am always worried when I repair switching | |
| 123 PSUs... they can fail in flame and glory sometimes). So I checked all the | |
| 124 diodes and transistors of the PSU, but none looked short. | |
| 125 | |
| 126 I first applied power to the main board: nothing exploded, I was now having | |
| 127 a nice + and -160VDC and the oven power was on spec. | |
| 128 | |
| 129 Next step, I checked transformers of the inverter board before plugging it | |
| 130 back, still no catastrophic failure. The inverter alone cannot do anything | |
| 131 without the control board. | |
| 132 | |
| 133 So I quickly checked the 2 other board before plugging them back. So far so good. | |
| 134 | |
| 135 But when I tried to turn power on, the voltege OK green LEDs flickered once but | |
| 136 did not stay on, and the error LED did turn on. All the voltages produced by | |
| 137 the inverter board were very low. Somethin was wrong. | |
| 138 | |
| 139 Looking at the schematics and the boards, I did find several failure points: | |
| 140 | |
| 141 - one of the 2 150µH inductors on the 160VDC rails (L6 anf L7 on A7A3) ) looked | |
| 142 a bit odd to me: it was not sitting flat on the PCB, which it should for this | |
| 143 kind of inductor; and when I gently tried to wiggle it, it appeared a leg was | |
| 144 broken, unfortunately too short to be able to solder a piece of wire, so I | |
| 145 replaced it with a brand new inductor I recently bought when I repaired an | |
| 146 old analog scope (Metrix OX734) for a friend of mine (I allways buy several | |
| 147 more parts than I really need, when these are cheap, just in case; seems to | |
| 148 be a good idea), | |
| 149 | |
| 150 - there are 2 small 400mA fuses on the inverter board to protect the base of | |
| 151 the 2 switching (bipolar!) transistors (first time I see the base of an NPN | |
| 152 transistor protected by a fuse, but it's also the first time I repair a | |
| 153 switching PSU which the main switching transistors are bipolar ones and not | |
| 154 MOSFET or so). These are small XXX fuses, they look nice and so but are very | |
| 155 expensive (and a bit hard to find): something like 15€ each. So for now, I've | |
| 156 replaced it with a simple 5x20 glass fuse: not as pretty but does the job for | |
| 157 a few cents. | |
| 158 | |
| 159 .. image:: {filename}images/hp8662a/broken_choke.jpg | |
| 160 | |
| 161 After these fixes, I reassembled the PSU and tried again: the result was quite | |
| 162 promising, but not a complete success: the PSU seemed to start, but very | |
| 163 quickly stops itself with an error LED on: the overvoltage protection was | |
| 164 activated. | |
| 165 | |
| 166 I tried to check several things on the error detection circuits with no succes, | |
| 167 and followed the service manual instruction, until I found this in the service | |
| 168 manual, about this overvoltage error: | |
| 169 | |
| 170 If red LED in the upper left hand corner of the board is lit indicating the | |
| 171 input voltagr from the A7A3 Inverter assembly was too high and the supply was | |
| 172 shut down, it is most likely a problem with the regulator not drawing enough | |
| 173 current. | |
| 174 | |
| 175 Just had to plug back enough boards and modules for the PSU to be happy again! |
