changeset
[hp8662a] new series on the HP 8662A generator
Sat, 24 Mar 2018 01:43:02 +0100
- author
- David Douard <david.douard@logilab.fr>
- date
- Sat, 24 Mar 2018 01:43:02 +0100
- changeset 92
- ce8a1de27226
- parent 91
- d2632122f838
- child 93
- 6233ca28a05c
[hp8662a] new series on the HP 8662A generator
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/content/hp8662a.rst Sat Mar 24 01:43:02 2018 +0100 @@ -0,0 +1,175 @@ +=============================================== + 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!
