Hello All, this is my first post here. I recently acquired an HP 8566B with a some gremlins. I've replaced a few bad caps in the display section (one literally went up in smoke within 10 minutes of getting it home, another within 10 seconds after replacing the first one) and that's all working fine now.?

The low band works perfectly, as do all attenuator steps. But there is an issue with the high-band. The TLDR is this: Voltages in the power supply of the RF section seem good to within a few mV, except the floating 18V on the A18, which is 16.2 (the -10V side of that test pin is perfect to GND). The C4 and C19 on the A21 board are sketchy but measure good with an LCR meter out of circuit. I will need to order some as I have nothing close enough on hand, and "proper" wet tantalum caps are $40 each (I'll test with AL caps). I've tested the RF switch with another SA with TG (it's good), and the 1ST LO out is spot on 321.4 MHz + center frequency, amplitude is +5 dBm as expected.?

Finally, after days of thinking there was no signal on the high band, I input a 0 dBm, 3 GHz signal, set center frequency to 3 GHz, span to 10 kHz, slowed down the sweep time slow enough to watch, and set a peak marker to grab a measurement. Finally, I was able to detect an itty bitty peak about 4 dB above the noise floor, which is at about -97 dB at this bandwidth.?

And this is where I am now. I have no errors on the screen, just a very weak signal. If anyone has thoughts on where to go next, I'm all ears!?

Thanks!

-Pete W4PJB

Pete, How versed are you with the operation of the 8566B?
Try this first, there is a key on the RF section (it might be a shifted key) call PSP. This is preselector peaking. Do this at 2.6 GHz or 3 GHz, then at 6GHz (this is close to the band edges of the 1st preselected band), if you can see a signal after that, that is much higher in amplitude then you may have an alignment problem. Look up how you do manual preselector peaking and for each of the upper frequency bands (2-6.2GHz, 5.8-12.5GHz, 12.5-18.6GHz, 18.6-22GHz). You should check the low, middle, and high end of each preselected band. Ideally the DAC numbers should all be 32 +/- 10 (range is 0-63), anything outside of this range indicates a frequency realignment may be needed. If you still cannot see a signal that is close to the correct amplitude then you may have a component failure in the preselected RF/uWave path.
Pleas post your findings so that we can point you in the right direction .
Don Bitters

On Sun, Jun 23, 2024 at 02:34 PM, Don Bitters wrote:

Don Bitters

Hi Don, thank you for your reply. I'm not very versed at all in the 8566B; I recently picked this unit up locally for a very good price, but was not able to test the 2-22 GHz section until I got it home. So I'm coming into this was no experience on the 8566. I appreciate your suggestions and will give them a try. 6 GHz is the highest source I have to test with at the moment.?

I'll be sure to post my findings, though it might be a day or two until I can get to it. I also have a thread on the EEV blog that includes everything I've done up until this point.

Did you ever run the Preselector Peak routine?

Vladan

Thanks everyone, I tried running the Preselector Peak Routine, no success. Also, just for kicks, I swapped out the YIG with a supposed "tested good" unit. Also, no change, although I didn't expect this to be the issue.?

The SA works perfectly on the low band. There are no errors such as YTO unlock, PL1/PL2, REF Unlock (except when the Oven light is on), M/N unlock, HET unlock, etc. It simply gets a very, very weak signal on the high band. Like -94 dB weak. So it's something in this path for sure. My signal generator only goes to 6 GHz, but now that I think of it, I have an Anritsu SA with TG that goes to 7.1 GHz. Perhaps I can try injecting a 7 GHz signal with the Anritsu to see if it works on the next band up.?

I tested the RF switch with another SA with TG to 7 GHz, there is no attenuation in the switch on either band. I should also add, I did put another SA on the "1ST LO OUTPUT", set a really narrow sweep span on the 8566B (might have been 0), set attenuation to "0", and was able to measure my input signal (3 GHz) + the 321.4 MHz signal, at full strength from the LO OUT. So we've got? great signal at least to there.?

Follow this sequence:

Right next to the 1st LO OUTPUT connector (front panel) is the IF jumper. Remove the jumper and check it for continuity. The one in the picture on EEVBLOG does not look like the factory original, so maybe someone made a replacement and it's open.
Connect your troubleshooting SA to the "IF OUTPUT" SMA connector. Is the 321.4 MHz signal present?
If you still have a good 321.4 MHz signal there, reconnect the jumper. Does the IF signal make it through the 1.5 GHz low pass?
If it does, and you are sure the cable between A16 LPF and A4 SECOND CONVERTER is good, then the problem is in or around A4 Second Converter. That microcircuit contains a switch. Check the d.c. drive signal to the switch by changing between low band (0-2.5) and high band (2-22). Every time you change bands, the control voltage should do something.
If the control voltage is working, and you have a 321.4 signal going into the second converter microcircuit, then I would suspect the switch inside.

Vladan

Hi All,?

Thanks again for the suggestions. I'm calling it a night, but I wanted to update this thread before turning in.?

I have already run the test on the LO OUTPUT, and I do get a mixed 321.4 MHz s + my center tuning frequency at the proper amplitude. For example, a 3 GHz input signal, with center tuning at 3 GHz, yields a 3.3214 GHz output on an SA. I did just quickly try putting a short 3 inch SMA jumper to the IF In/Out and there was no change in performance.?I will run the other checks tomorrow.?

One thing I did discover this evening is the red LED on my A6A7 YTX Current Driver is lit. Not very brightly, which is probably how I didn't see it until this evening, but it is lit. I haven't had any time to dive into that yet but I suspect it isn't helping. But I'm also getting a proper, full amplitude signal to the LO OUT, perhaps this isn't affecting anything regardless. To be determined another evening.?

I really appreciate all of the ideas, and I am testing every theory as time allows. Thank you!?

On Tue, Jun 25, 2024 at 06:45 PM, W4PJB wrote:

......

One thing I did discover this evening is the red LED on my A6A7 YTX Current Driver is lit. Not very brightly, which is probably how I didn't see it until this evening, but it is lit. I haven't had any time to dive into that yet but I suspect it isn't helping. But I'm also getting a proper, full amplitude signal to the LO OUT, perhaps this isn't affecting anything regardless. To be determined another evening.?

=====
Red LED says heater is working hard to bring up temperature of YTX. It should turn on bright at power up, and turn off when YTX temperature is within target. I just checked on my 8566B, red LED was on at power up and was out in less than 1 minute. On your unit does it start bright and get dim later or is it always dim?

As Jeff wrote LO output signal doesn't need YTX. However, if YTX is not working high band will not down convert input RF to IF.?

In case it helps your debug: I measured resistance of YTX pins to ground on a spare YTX of unknown state. ?

pins 2 and 5 are two coils in series ~ 60-ohms across
pin 3 heater drive: 238-ohm to ground
pin 4 heater sensor: 455-ohm ground

Ozan

Hello, All,?

I am happy to report a successful repair! By a stroke of pure luck, I found a parts 8566B locally for a very attractive price. It was missing the entire A11 section, and the 2nd converter, but was otherwise in-tact.?

I wound up replacing the A6A12 YTX driver card with one from the donor unit. This was a partial success, as it would finally display a meaningful signal, albeit 30 dB low and not 90 dB low. By performing a factory reset of the YTX current (SHIFT, PRESEL PEAK) the error improved to -20 dB, and by adjusting the DAC control (SHIFT, GHz) I was able to get a full amplitude signal, although it was non-linear as I changed the input frequency.?

Ok, so on the right path, but something is still off.?

During this time I found R106 was burned on my original A6A12. Concerned this could have been caused by a damaged coil in the YTX, I decided the next logical step was replacing the YTX with one from the donor unit. Success! I was able to achieve a full amplitude signal across all tested frequencies.?

TLDR for those reading this from the future, looking for help with their 8566B. The conclusion is a failed coil in the YTX that overloaded the A6A12 driver card. Replacing the YTX and A6A12 solved the issue. R106 could likely be replaced, but as I had a spare card, and one that was tuned to the YTX, I elected not to repair the original.?

Thank you everyone for all of your help and advice!?

开云体育

Re...

On 09/07/2024 21:01, [email protected] wrote:

During this time I found R106 was burned on my original A6A12. Concerned this could have been caused by a damaged coil in the YTX, I decided the next logical step was replacing the YTX with one from the donor unit. Success! I was able to achieve a full amplitude signal across all tested frequencies.

I wonder if a shorted turn in the YTX device could cause that?

Was there any noticeably different behaviour at faster sweep speeds than slower?

Regards to All.

Dave B.

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

Hi Dave,?

That's a great question. There is no way of really knowing if a shorted coil in the YTX caused an overcurrent damaging the A6A12, or if an overcurrent in the A6A12 caused a shorted coil in the YTX. It could have been a failure upstream of R106 that caused the damage. In fact, the more I think about it, I'm inclined to think that was the case. Failure of that coil seems unlikely given how well protected and low power it is. It seems more likely that something in the current regulating circuity shorted, sending high power levels through R106, burning it and overheating the coil. Chicken and egg question, I suppose. As such, I have no intention of attempting to replace the resistor and re-use the driver card. It's going straight into the parts bin.

Also, I did not notice any difference in sweep speed.?

It was a thread I had read a few weeks ago on the EEV blog that made me think of the coil being damaged:

I should pull the old YTX apart some time, it's not like it will ever be going back into service.?

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Might it be worth repairing as to the cost of Yig units? Thousands of dollars?