Hello,

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I'm fixing up a magnetic step attenuator salvaged from an 8555A SA plug-in, to use in my TG project. I have some of these nice old attenuators, and the special rotary wafer switches that encode the binary sequences for the 10/20/40 dB step coils, and trigger the drive pulse. After some grief trying to figure out how the coil driver is supposed to work, and modifying the switch, I've got it going nicely. I built a pulse driver to run the coils according to the operational info in the 8555A manual - the coils take 12V, with the polarity selected by the switch, and are pulsed for about 150 mSec.

In the 8555A, the attenuator was only used over a 0-50 dB range, but I want the full 0-70 capability for the TG. This required some simple mods to the switch. First, the CCW stop on the shaft was eliminated, giving eight positions instead of six. 0 dB is at full CW, which is what I wanted. When I took it apart the first time, it looked like the switch was fully set for all the coil states, but when I got the whole thing operational for experimenting, it still only had the 0-50 dB range - the last two added spots were dead. On closer inspection, I found that one tab on one of the 10 dB wafer rotors was not included, but only on one coil side - the other side's rotor has all four tabs, and properly connects, for some reason. After some consideration, I decided the easiest fix was to do some simple switch surgery, to add a couple of contacts on the wafer, to pick up another tab when it went by, so the last two steps would fully encode. These contacts are redundant with some of the others, except in the last two spots, where they do their thing. It worked beautifully.

I'm working on optimizing the timing now, so am wondering about how well the original 8555A implementation worked - user interface-wise. Since the coil drive relies on open-loop triggering and pulsing, it's possible to overrun the state control by turning the switch knob too quickly, which doesn't hurt anything, but may result in the attenuator's actual state not matching the switch position. For example, if it's rotated quickly through a few steps, it can misfire. Changing it one more step either way will trip the cycle and restore it to the right condition.

Does anyone who has or has used the 8555A (or anything else with a similar setup) remember if this was an issue or minor nuisance? I had some many years ago, but don't recall being able to spin the knob fast enough on the cramped front panel, to misfire. Also, it only had five steps that could be rapidly switched anyway, since the last one had a manual release button to enable the 0 dB setting. On the TG, the knob has lots of room, so it's easy to whip through the steps fast, and I have to remember to take it easy, and be more deliberate in changing the setting. It seems to work fine up to about three or four steps per second. Of course, beyond that, there's less and less time to put out valid coil pulse duration and reset everything.

For the coil driver, I made a small board that attaches right to the coil pins on the attenuator. The board space is kind of tight, and it needs to run from the raw +33V supply, and include various protection circuitry. I decided to go old-school, and made the pulse driver with a relay and capacitor, where the cap discharges through the coil resistance for timing. The trigger from the switch fires the relay coil initially through a small cap, then the relay hooks up the main cap, and applies the juice to the attenuator coils through the encoding switch. At the end of the cycle, the relay flips, disconnecting the load, and connecting the timing cap back to the recharging source. The way the 8555A did it, and the way the switch is set up, is that all three coils are fired simultaneously, forcing it to agree with the setting. The coils are about 70 ohms each, so firing them all takes about half an amp at 12V. I didn't want to add extra power supplies or circuits for a lower voltage, so I took advantage of the non-continuous and pulsed nature of the load, and used the resistive drop of a string of PTC thermistors from the raw supply, with a big cap in the middle to give some isolation against the edges of the pulsed load. The PTCs provide intrinsic overload protection, in case something gets stuck, or in case someone like me gets carried away and spins the knob too often and too fast - the PTCs are clumped together, and will gradually all heat up and trip out. It ain't efficient, but it is simple - there's not a transistor or IC in there, just a few diodes and Zeners. The only active component is the relay.

Anyway, if anyone is contemplating re-using any of these fine old attenuators, this info may help. I have had to learn a lot about them lately, and have renewed appreciation for how they work.

Ed

Beginning at around 47:50, in the following video,

he discusses HP tc and diode power sensors, and shows a strip down of a diode one.
There is some focused discussion about the theory of operation (for tc and diode types)… and you can see the tools he uses to examine the blown diode sensor, for possible repair.
Roy VE3RFT

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Here is a video on repairing one of these attenuators:



The O-Rings are 2mm od x 1 mm id x 0.5mm thick.
Theses are used in watch making.



Hope this helps

Glenn

--
-----------------------------------------------------------------------
Glenn Little ARRL Technical Specialist QCWA LM 28417
Amateur Callsign: WB4UIV wb4uiv@... AMSAT LM 2178
QTH: Goose Creek, SC USA (EM92xx) USSVI LM NRA LM SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"

Hey Jeff. Have you measured one to check the size? I have several
sources that might be able to supply them -
73,
Bo L.

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Hi All!

Dont ask why, but I am resurrecting several HP 8660D generators with the HP86603A plugin. The attenuators are "sticky", get stuck in a certain position. If you have experience with this exact type of stick attenuator (long silver block) with the 3 - 40 dB, 20 dB, & 10 dB sections and can advise how to make a reliable repair, I would love to hear about it. Several that I cleaned the solenoid shafts on seemed to be working only to stick again. I do not know how to search the archives as well, can someone advise?

Sounds like the universal problem of decaying O-rings Jeff.? They are easier to replace than source.
Dan

Hi Jeff,
I would be very grateful to receive it, whether it is for a 606A or 606B. Beggers can't be choosers! I'll await your advice. (Fingers crossed!).

Hi Kim,
It may also be possible for me to make up a serviceable (if uncalibrated) 606B replacement unit from a combination of your 606B unit and Jeff's unit, if Jeff's is for a 606A.

Thanks to you both for your kind offers which are gladly accepted. What a great day!!

Graeme

Hi Ken

If you google "HP 1703a" the first hit is an operating and service manual. It's actually a 35MHz scope.

The HP memory project also has a nice article on the 1700 series.


Paul

--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group, Inc. | Security, Systems & Software
paul@... | Unix & Windows

Hello,

Final (?) findings.

Interactive IO in the Agilent Connection Expert seems not sufficient...
Running the code, accessing the VISA library immediately and _not_ using viScanf, but viRead seems to have done the trick. I can now talk to and _read_ (not scan?) from the 436A - Yippie!

Many thanks to all you people involved for suggestions, comments and taking the time to comment!

Regards,
? Staffan

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some good reading here.



Easier to search for "boonton principles of power measurement"

Boonton has always made some very nice equipment, I own a Boonton peak power meter that also reads CW power, down to about -70dBm and up to +10dBm, with provision for external 30dB pad via switch.

No CRT or solid state display, it has a numerical display. Don't know how old it is, but the last time I used it (quite a while ago) it worked perfectly.? That URL (the big long one) has a LOT of good info.

--Doug, WA2SAY, retired RF engineer

Hello,

Absolutely - just how to do this on the 82357B? Hopefully very simple, but seems not to be available on the interactive IO. Any hints would be very much appreciated!

Regards,
? Staffan

Just get the controller to "un-talk/un-listen" the 436, or use the GPIB
low level "Go To Local" function.? It'll then un-freeze it's display.

Regards.

Dave B (G0WBX)

--
Created on and sent from a Unix like PC running and using free and open source software.
::

Hello,

I can't argue against, but my only experience is that I _can_ set the instrument in the state I want (REN active) and can read the output data when I deactivate REN, but not otherwise. I have tried to follow section 3.
What I would really like is a description of how to communicate with this instrument using the 82357.

Regards,
? Staffan

At 09:49 AM 11/20/2018, David C. Partridge wrote:

You mean apart from the fact that they are the only truly accurate RMS RF sensors, and that all calibration is referred back to this type of sensor?

Note also that some thermocouple power meters, such as my HP 434A Calorimetric Power Meter, can be completely calibrated with some basic equipment, such as a distortion analyzer, a 'scope, and (most important) a calibrated true RMS DMM of sufficient range and accuracy.

Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA

Thermocouple power sensors can be referenced to the DC volt, and
as such give you a connection between AC power and the standardized
DC volt.

You can't do that reliably with the diode type sensors.

-Chuck Harris

n2msqrp wrote: