HP 8753C sampler replacement - Page 1
Forum section: electronics / testgear

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 08:19:07 pm
content: I just picked up an 8753C, and unfortunately, the sampler card for Port "A" is DOA. I verified this by swapping the card to the "B" port and the problem followed the card.I've looked through the service manual and it's a bit unclear if the 8753C needs calibration after replacement of this card, or only the A and B models. Does anyone have experience replacing one of the A4/A5/A6 cards in an 8753C that could shed some light on this?If not, it's going back to the seller. Which I'd hate to do as it's a super clean unit that already has an LCD upgrade, but there's no way I can justify paying for a full calibration nor do I have the equipment to do it myself.Thanks!

author: KE5FX
author_rank: Super Contributor
timestamp: June 22, 2024, 11:21:17 pm
content: Strongly doubt there is any reason to recalibrate anything after swapping the sampler card. Any additional errors will be taken out when you calibrate as usual.That's a nice display, I'd keep it.The LCD is far better than the crappy color CRTs they used.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 22, 2024, 11:32:00 pm
content: Well hello in this thread! I'm keeping you busy this evening, aren't I?I much appreciate the reply. My thoughts are, even if it's a bit out of CAL I'm sure it will still be far good enough for hobby use. I'm not using it to land on Mars.Thank you for backing that thought up.

author: KE5FX
author_rank: Super Contributor
timestamp: June 22, 2024, 11:52:54 pm
content: I'm subconsciously seeking distractions from whatI'mtrying to troubleshoot, I guess...

author: Bud
author_rank: Super Contributor
timestamp: June 23, 2024, 12:03:52 am
content: You can send a PM to Dr. Joel Dansmore (user Joel_Dunsmore), the designer of the 8753C. Joel hangs out in RF and Microwave subforum.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 23, 2024, 12:15:16 am
content: Glad I could be of service!

author: W4PJB
author_rank: Regular Contributor
timestamp: June 23, 2024, 12:15:55 am
content: Oh wow, good to know! Thank you!

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: June 23, 2024, 01:05:39 am
content: I got the email but figured I would reply here so all can see; from the email, and it reminded me I haven't looked in on the EEVblog for some time:another member here suggested you may have just a "bit" of insight on the 8753C. I recently picked up a unit and unfortunately the A5 "Sampler/Mixer" card on Port A is dead. I confirmed this by swapping it with Port B, and the problem followed the card. My question is, if I replace the card (mine are all labeled 5086-7413 on the sampler) with a random unit, what kind of trouble can I expect if I don't do have it lab calibrated? This is just for home-lab use, nothing critical and nothing that requires traceability.Will the standard user Open/Short/Load/Thru CAL's get the unit close enough for most use? Or is there enough variance from part to part that it can cause issues?Short answer: It should be fine.Long Answer (digging back to 1983 when I designed that sampler...) The diode bias doesn't change much but does effect the absolute conversion gain. The different samplers will have slightly different frequency responses so that they might not pass the absolute conversion loss test or the frequency response test. But these limits were set based on the HP8754 which did not have a computer or user error correction and at the time the thought was "some people will want to use the raw performance just like the 8754 so we need to make it just as good". So this was the first time anyone had put in a "factory calibration" with numeric correction in an instrument (well VNA anyhow).However, since you can correct for all that (and in fact if you use it with a test set the test set will dominate the response anyhow), you should be fine to just use it as is. You can hook up a power splitter and measure A/R and B/R to compare the two receivers (you always have to have a signal in the R channel; no such thing as freq offset mode back in the day).Likely the A receiver has a blown 50 ohm resistor (well, I think it more like 47 ohms) in the front stage. People were always blowing it up by overpowering the receiver. In fact we did a study on the kind of damage we can find vs. power applied, and we could tell if it was 1 W, or 10W or 100W that was put into the receiver by the kind of damage on the that thin-film resistor (e.g., if there was no resistor but charcoal on the lid => more than 100 W).The lid is epoxied on so it will come off if heated to about 125 C and you can look inside. See if the input resistor is intact. The next thing would be the common base transistor that follows (it was an HP Tech center part TC16). The final thing to look for is a bond wire from one of the DC feeds that might have popped loose. If it is the transistor, any chip transistor with Ft over about 20 GHz should work. If it is the resistor you can try bodging in an 0201 or other small 40-50 ohm resistor (47 was the design value to make up for the 3 ohm input looking into the emitter of the CB stage) and see if it comes back to life.

author: W4PJB
author_rank: Regular Contributor
timestamp: June 23, 2024, 02:02:50 am
content: Joel,Thank you VERY much for your reply! And also, thank you for posting this in the public forum. That was very thoughtful, and I'm sure others will benefit from this post in the future. My suspicion was a full calibration would not be necessary for most use, only to get the last ounce of performance, or to verify performance and have traceability for a commercial application. You seem to have confirmed this suspicion was correct.It's fascinating that you actually learned to identify how much power the input saw simply by the condition of the resistor. That tells me you must have seen a lot of blown samplers in order aggregate all of this data. It would be interesting to know how many resistors got damaged from RF, and how many were smoked by users characterizing capacitors that they "forgot" to discharge first. I'm sure more than a few samplers lost their lives to the latter.As for repair I plan to simply replace it with a used one. But I will be sure to do an autopsy on the current one when it's all finished and post the results here. That will be interesting now that I know what to look for.And welcome back to the forum!

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: June 23, 2024, 07:40:30 pm
content: Quote from: W4PJB on June 23, 2024, 02:02:50 amWell, it was driven by one larger customer who insisted the failures were not his fault. We actually blue up a bunch ourselves to check the damage we saw compared to his units. It was on the order of 10W case, and surprise, his amplifiers had a +43 dBm output power spec. And so that is how I was assigned to design a limiter (11930A) to try to prevent accidental damage. In fact the limiter damage level was about +36 dBm, but the sampler damage is on the order of about +25, so gave an 11 dB margin. Further the limiter failed "safe" with a short to ground if it was damaged, so you could replace the limiter rather than repair the VNA.

author: Bud
author_rank: Super Contributor
timestamp: June 24, 2024, 07:48:30 pm
content: What a fascinating bit of history! Thank Dr. Joel !

author: W4PJB
author_rank: Regular Contributor
timestamp: June 28, 2024, 08:17:52 pm
content: The replacement card showed up this afternoon, and I'm happy to report the 8753C is back up and running! Thank you to everyone on the forum for your replies.Joel_Dunsmore, I opened up the sampler and did not see anything obviously smoked. I will post some pictures below; two microscope photos, and an iPhone image of the complete unit. Some of the components are downright microscopic. Incredible work Dr. Joel, amazing to think this is early 1980's technology.

author: Joel_Dunsmore
author_rank: Regular Contributor
timestamp: July 04, 2024, 03:50:32 am
content: Yes. it all looks pretty good. It might be the bias board that gave up the ghost. The microcircuit looks like everything is in place, no lifted bond wires and the input resistor looks OK.