Hi everyone,

I recently repaired a non starting up HP 8568A.

Now the whole machine boots up but i have YTO UNLOCK after start.

The system sweep quite fast but that's not the normal rate, it's like 10 sweep a second.

On the A6 YTO PLL board i have a steady YTO UNLOCKED & HIGH FILTER led and a flashing YTO LOOP CLOSED
On the A8 249Mhz PLO board i have a flashing 249 MHZ UNLOCKED led.
And on the A22 Frequency Control board i have a flashing STOP and HIGH led.

YTO problems could be a million things, and for now i have trouble narrrowing it down, i have to deep dive into the instrument, if there's 8568A expert, any advices is welcome.

开云体育

After a while, i'm back at the repair of the analyzer.

I have done some more testing by injecting signal into the instrument, here's what i observed:

For span greather than 20 MHz, i have YTO UNLOCK, 10 sweeps per second and the frequency measurement lacks of precision.
For span between 1MHz and 20 Mhz, the analyzer, just doesn't do anything, it sweeps every once in a while, keyboard, rpg aren't responding, just like if the analyzer was spending all of it's time trying to lock the system, without succeding so far.
For span less than 1 Mhz, same symptoms as for the > 20 MHz span.

It seems to be FM Coil related to me, and doesn't smells good as well.

From that point, i moved on the "SHIFT + CF STEP SIZE" command to manually input the YTO frequency and checked it with the "1st LO OUT" output at the back of the instrument, here's what i found:

For YTO Tune DAC value between 0-240 255-750 770-1023, everything is responding normally, the frequency move from 2.0Ghz to 3.875Ghz with 9 dBm, as fragged.
But for 240-255 and 750-770, it's another matter, the thing goes crazy and reach frequencies like 15GHz to 26 GHz and stable.

That might be my problem. I checked the voltage levels at the output of the DAC, perfectly fine, 0 to +10.23V

Check the 8568A manual troubleshooting for YTO unlock. It is very good and will isolate it to a component if you follow the tree correctly.

That is what i've done so far, but i'm still unsure of my conclusion.

That's why asking if someone ever had the same symptoms. Because i want to buy a new YTO and i want to be sure that it is the YTO given the fairly expensive price of that part.

On Fri, Apr 19, 2024 at 11:17 AM, <ThirtykayDEV@...> wrote:

That is what i've done so far, but i'm still unsure of my conclusion.

That's why asking if someone ever had the same symptoms. Because i want to buy a new YTO and i want to be sure that it is the YTO given the fairly expensive price of that part.

==
I debugged a similar issue on an 8566B (fortunately it was LODA and not YIG) and I believe 8568A shares most of the architecture with 8566B. On 8566B:
- There is a jumper to disable YTO unlock interrupt to the CPU. This solves the unresponsive keyboard and rotary knob issue.?
- There is another jumper to force PLL control voltage to mid point. In your YTO sweep you may need to disable PLL to prevent interfering with the YTO sweep.

Is there a service manual available online for 8568A? I can provide the jumper information of 8566B for reference or if there is a 8568A manual I can check if similar jumpers exist.

Ozan
?

Interesting,

So it was your LODA that was faulty in the end or something Elise ??

I know about the "SHIFT +FREE RUN" command to ignore any YTO UNLOCK, but i dont know about the other one, i'll give a look at that, thanks !

Yes there's a service manual available online for the 8568A , at around 10 dollars

On Fri, Apr 19, 2024 at 01:26 PM, <ThirtykayDEV@...> wrote:

Interesting,

So it was your LODA that was faulty in the end or something Elise ??

I know about the "SHIFT +FREE RUN" command to ignore any YTO UNLOCK, but i dont know about the other one, i'll give a look at that, thanks !

Yes there's a service manual available online for the 8568A , at around 10 dollars

=====
Shift-Free Run is assigned to another function on 8566B but I see 8568A can disable unlock from keyboard.?

On my 8566B LDO coupled output of LODA was not functional, it can be checked with a power meter. Replacing LODA fixed YTO unlock problem.?

Ozan

Thirty Kay,
The RF sections of the 8568 and 8566 are 2 different beasts. There is very little the same in the RF paths. The control, sweep, and tune circuitry is similar in some respects, but different in other respects. Do check the RF block diagrams. This will greatly aid you in troubleshooting your problem.

Don Bitters

Did some testing this morning, my conclusion are that the YTO FM Coil is very likely to be failed.

After looking at all the test point on the A22 board, i almost sure everything is working fine on that board.

Next to test, is the A9 and A10 with a power meter, if they work perfectly fine, let's go for a YTO.

A9 and A10 working just fine, definitely YTO related now.

Quick log entry.

Received my new YTO, the failure is still there and there's very high chance that my problem is in the A23 RF converter, which is very expensive.