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Hi Peter:

When I get a few moments to take a look with a scope I will. This problem seems very much temperature related. My plan is to let it run for a good couple of hours, and then probe the A15W1 Feedthrough points on the SMPS.?

My only concern is, I may not see a lot of ripple with the SMPS unloaded. I have had two cases in various instruments where the switcher fooled me like this. It was not until I loaded it down did I begin to see a lot of ripple.?

v/r,

Rich?

On Aug 27, 2022, at 7:22 PM, peter bunge <bunge.pjp@...> wrote:

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Rich; I would be interested?in what you find. I plan on putting my 8753 on a variac to run with lower voltage. If it was a failing reservoir?capacitor in the pre-regulator this should show it up. The other thing to check is to 'scope the ripple on the pre-regulator.

Keep me informed.

Peter.

On Sat, Aug 27, 2022 at 7:16 PM Rich Miller via <av8torrich=[email protected]> wrote:

Peter:

I have it back and running. It was the A8 board, despite the service manual pointing to the post regulator board. I replaced the SMPS, and I am back in business. I opened the SMPS, and did not find anything out of the ordinary. I may plug this in, leave it sit for a few hours, and then begin probing the output.?

v/r,

Rich

On Aug 27, 2022, at 3:30 PM, Richard Miller <av8torrich@...> wrote:

?Hi Peter:

Have an update for you. The fault started again. If you follow the troubleshooting guide on Section 5-8 - Page 271 it would leave you to believe I have a post regulator problem (A8).?

Swapped A8 out, and the problem precise. When I unplug A15W1 from the post regulator, the problem disappears. Being that I have a known good post regulator board to swap out, I think the problem is the A15 Pre Regulator (SMPS). Getting ready to swap that out and see what happens. ?

On Aug 27, 2022, at 2:44 PM, peter bunge <bunge.pjp@...> wrote:

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Rich;

I think this is a case of "nothing works Thursday". It happens sometimes and maybe this only applied to 8753s. The electronics Gods work in mysterious ways.

I read the manual and it says surging is the post regulator circuit sensing an overcurrent and shutting down the power supply repeatedly to protect it. Maybe a cap reforming after months of non use? I had disconnected the S-Param Test set and thought it may have been the culprit but reconnecting it made no difference. It still works.

Peter

On Sat, Aug 27, 2022 at 2:02 PM Rich Miller via <av8torrich=[email protected]> wrote:

LOL - I just got to my office with a replacement PSU, and Post Regulator Board. This was an hour drive.?

?Turned on the VNA, and its fine now. This maybe temp related, so I am going to run some more measurements and wait for the fault to manifest once again. A little annoyed, but such is life I suppose.

I was in the middle of an important measurement when this happened yesterday, so I am going to take it while I can. I will keep you updated, as I learn more.

Rich

On Aug 27, 2022, at 12:53 PM, peter bunge <bunge.pjp@...> wrote:

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I took the cover off and everything works. I have no idea what the problem was.

This unit had a manufacturer's fault on it for 30+ years. The problem showed up once and fixed itself after re-seating?boards but I could not confirm what fixed it. Two years later the fault re-appeared and it was these two bad solder joints. It is not the problem this time because they were re-soldered and the symptoms are different anyway.

So the fan was surging and it would not start up. I will 'scope the filter caps before putting the cover back on.

How are you making out with yours?

On Sat, Aug 27, 2022 at 7:48 AM peter bunge via <bunge.pjp=[email protected]> wrote:

My HP8753B suddenly started doing this surging with all the front lights on (I will have to re-check that statement but it is what I remember).

This was several weeks ago and I have not had time to check it. If you start on yours take the top lid off and look at the row of lights on the power supply board. They should be green if OK and red to indicate a bad power supply (I'm going from memory so anyone correct me if I am wrong). Check the?power supplies with a good digital meter BUT DO NOT ADJUST ANY, especially if they are marginally off,?you do not want an additional fault of your own doing. Check each one with a 'scope and I would not be surprised if you find high ripple on one or more in which case replace the bad capacitor.? The manual will tell you what they should look like. Also there is?a start up sequence for the?CPU row of red lights. Check the manual. The fans are special on these?and are?controlled by a circuit. The VNA will not turn on if the?fan is bad and it?must be replaced by the correct one.

I will be starting on mine next week, possibly Monday. I have family visiting right now.

Peter.

On Fri, Aug 26, 2022 at 7:35 PM Rich Miller via <av8torrich=[email protected]> wrote:

All:

My HP-8753E started demonstrating what I believe is a PSU related fault. When I switch the VNA on, it attempts to boot up, but before it can, I hear the fan stop, and then turn back on, and the VNA attempts a start again. This cycle goes on forever at this point.

I have taken a close look at the spare PSU, and there are no fuses externally. I also note there is a Voltage Regulator Board. I am wondering if anyone has had this issue, and was it the PSU itself or the Regulator Board?

My thinking is if this was a hard fault in the PSU something would have self destructed, or blown a component/fuse internal to the PSU. That being said, I have never seen the inside of the PSU to know if there is some sort of soft restart, or a fuse which should blow.

I would also be curious if there is some other protection feature in the 8753E which would cause it to power cycle like I describe above?

Thanks in advance!

Rich