Greetings to the Group:

?? I may have botched the search function, as I can't find any actual discussions of repairs to this instrument.?? If so, I apologize for taking up the bandwidth.

?? However, I do seem to need a bit of help with this instrument.?? The unit will not zero on the 1 volt / 0 dB scale; it reads anywhere from about 1/3 scale to 2/3 scale, depending on phases of the moon or whatever.?? The reading decreases proportionally on the higher voltage scales.?? On the .3 volt / -10 dB scale, the zero is pretty close, and remains so on the more sensitive scales.

?? On the .3 volt / -10 dB scale, the meter cannot be calibrated; with R29 at the maximum position and .3 volts applied, the reading is .18 volts.?? R29 seems to have little effect on the meter level.?? It can be used to bring the meter to read .3 volts on the 1 volt scale as per the manual. but the meter reads only slightly less with no signal applied and the input terminals shorted.

?? The manual suggests that the trouble is in the power supply, and indeed, I see a 60 Hz signal in the last two stages of gain (V4 and V5).?? Unfortunately, the power supply looks very clean with my oscilloscope.?? I note that there is only 3 mV of ripple permitted, so I had to use a series .1 uF capacitor to protect my scope from the B+ when using a X1 probe.?? At 5 mV per division, I see only about 1 mV (barely discernible).?? I have two of these instruments and have them opened up side by side on the bench.?? One works well and the other is failed as described.?? The two instruments have very similar power supply ripple, with the good instrument possibly being a bit worse than the failed one.

?? Filament supply DC ripple is more pronounced, but is no worse than (and may be slightly better if I recall correctly) than the working unit.

?? Grounding the grid of V2 gets rid of the noise and allows the instrument to zero.?? V1 and V2 have both been replaced with no improvement.?? All of the tubes have been pulled from the instrument and checked with a TV-7 B/U tube tester.?? All check good for shorts, mutual conductance and gas.

?? I am looking for any suggestions at this point.... I seem to be completely stumped.

? Thanks for reading and for nany suggestions you may have.

73,
--
Jim Thorusen
KB6GM

The HP 8900B manual with circuits diagrams is available from BAMA;

David

The CPU board slides out from the back. The ram slot is available through a cutout on the CPU board. The CPU is a 68000 and HP made only 256k and 1M cards for these while other vendors made 2M and 4M versions. I think other card where not available due to ram cost.? Not sure that the system can handle more then 4M but the CPU is able to address up to 16M of address space.?

Razvan:

The 3GHz board is a divide by 128, with a max output of 23.4375MHz at 3GHz.
That has to get to the main board through the ribbon cable, not the best way to connect.
At 12GHz the output would be almost 100MHz so I think that the higher frequency option boards likely have an additional divider to keep the output at a reasonable frequency.

There are two pins (CH3CODE0 and CH3CODE1) on the connector that are used to tell the main board that one of the options is installed.

I would think that a combination of those pins would select one of the three options or no option.

I will try to see what effect they have later this weekend.

ed

Yves:

Thanks for the update.

Do the boards count randomly when there is no input?

Were these the OEM options or clones?

ed

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In metric-land I got a bunch of 1mm ID 2mm OD watch stem seals, eBay or Amazon I seem to recall. That was about 10 years back on an 8566A - still working fine - done several other attenuators since.
However while they seem OK at the moment, nowadays having had cheap plumbing 'rubber' seals go gooey after about 5 years, I have little faith in unbranded stuff and I'll bet it's just a matter of time until they start to return to their original liquid hydrocarbon form.
Probably worth finding some of that size from a known (and trusted) manufacturer?

hello friends,

I got a 8593A with a unknown problem,
to me it looks to be a failes display board. Any tips to short the search ?
thank you.

see the short video I have made from that :-)


greetings from germany
Martin

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I literally gave away several 8620A’s at hamfests, hoping Roy didn’t trip over one of mine! I have one lone 8620C with 2-18GHz plug in and maybe anther plug in or two, and several scales. Every now and then I fire it up for sentimental reasons!

I did a bulk buy a few years ago which made a few folks happy. They came from: ?

 Precision Associates Inc. ()

Part number:?

23-41 5747
70 Duro Black Buna N

.023 ± .002 x ID .041 ± .003

Quite cheap if you got 500 then, no idea now...
Dan

Is the attenuator the same form factor as the 8568/8566 attenuator - HP 33321A? If so the attenuator o-rings are spelled out in the 8566 attenuator notes on this site. They are n-butyl rubber and are available from Apple Rubber Co. In New York. They may also still be available from Keysight.

As far as I can tell (from the pics)... it's an 8620A... and getting a scale to match the plug in is not a "... minor issue." (at least it wasn't for me.)
Am I missing something... or is there something special about this particular 8620. (Owned by Bill Hewlett?)
Despite the well staged, and sumptuous color photos... it looks like any other clapped out 8620X, one used to trip over at a ham fests.?

On 5/3/24 19:11, Maciej Kawalkowski, Liberty Electronics Design and Service LLC wrote:

Dave McGuire
Do you still have these ?? Could? you test one in the 3852A?

The ones that I had left are installed in HP-9000 systems at LSSM, but I could borrow one and try it. My 3852A is in a rack, though, how do you reach that board? Can I do it without taking it out of the rack?

Oh yes, also...these boards are 7.5MB; didn't you mention earlier that the 3852A wouldn't address that much RAM?

-Dave

--
Dave McGuire, AK4HZ
New Kensington, PA

On 5/3/24 20:05, Maciej Kawalkowski, Liberty Electronics Design and Service LLC wrote:

Moved the fuse to the top

I made the same change to my version. My intention was to build a bunch of these via pick-and-place; components on the bottom with all that open space up top was...not welcome. ;)

-Dave

--
Dave McGuire, AK4HZ
New Kensington, PA

Moved the fuse to the top

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Dave McGuire
Do you still have these ?? Could? you test one in the 3852A?

Maciej

Does anyone know what size and where I could get "O" rings for my HP 8714 VNA.Attenuator??
One is not there and attenuator is not working properly.

wally KC9INK

Howdy folks,

? ?I am selling the subject line parts, and I understand this group is not primarily for selling stuff, but it *IS* for?
repairing equipment.? ?Since I am willing to part out the sweeper, I figured I would quickly post a link to it here
in case anyone is looking for parts.? ?There are more details and pictures in my original post here:



? To keep down the clutter, I'll take any responses THERE, not in this group.? ?Thanks!

?Dan