John

I am still back at U700 which seems to be working but I can't get enough drive into it.

The oscillator circuit is not tuning properly and I suspect it is the crystal. It has been changed sometime in the past and I not sure characteristics of the crystal are the same. I will see if I can trace? details of what has been put in it.

Thanks

Dave

On 12/04/2019 1:55 pm, johncharlesgord via Groups.Io wrote:

Dave,
I think I had an HP856x with a similar 600 MHz problem.? It turned out to be a failed RF amp on A15. (U501?? U701?? U702?)
--John Gord

On Thu, Apr 11, 2019 at 06:16 PM, Dave Ireland wrote:

Amir

?

A lesson in the need for methodical fault finding techniques methinks.

?

Checking out the RF section block diagram, trying to follow your advice, I noticed the 600Mhz drive into the Second Converter. The 600Mhz is derived from the 100MHz VCXO that the regulator transistor had failed on. The 600MHz out of A15 is about 15dB low and when I injected 600Mhz at the right level into the second converter the level has come right up. Still about 20dB down but more promising.

?

I will go back now and go through the A15 board and get the outputs correct and then after that maybe check out A14 for the mixer and converter bias volts.

?

Thanks

?

Dave

?

?

On 11/04/2019 10:29 pm, amirb wrote:

I forgot to ask, what is your noise floor level at full span?



On Thu, Apr 11, 2019 at 10:27 AM, amirb wrote:

I assume there are no error messages appearing anymore, right? Any error message about Log amp or step gain amplifiers?
if there is no error and you are sure the attenuator and the input AC/DC coupling are ok (you can even check with oscilloscope by injecting a signal)
then the best and perhaps only way to isolate the problem is by injecting signal at various points and measuring power levels according to the service manual

I dont think this is due to low LO amplitude in any of the converters. otherwise you would get an error message and besides
90dB down is way too much for that. I suspect this is due to a fault in one of the 3 converters or in the Log amp (but not in any of the LO drives)

since you dont have a second spectrum analyzer, I suggest you inject a signal at say -20 or -21dB to the first mixer (make sure span is set to zero)
and then measure the 310.7MHz second IF with an oscilloscope (I hope you have a 400-500MHz scope?) or even a power meter.
if the amplitude is way off then the problem is either the first mixer or the second converter. By the way, you can check the mixer bias voltages coming from A14
In some versions even the first mixer has bias voltage.

Next you can inject a clean 310.7MHz at around -35 or -36dBm to the third IF input (you might have it on the front panel) (you must choose it first in the menus and also choose zero span)
and then you can measure the 10.7 Mhz final IF with an oscilloscope it must be around -15 to -17dBm I think.



On Thu, Apr 11, 2019 at 09:44 AM, Dave Ireland wrote:

I am repairing a HP8560E and until now have been doing well.

The unit on delivery had the classic distorted display and no apparent response to any input though it did sweep it was very slow. 12 Alarms were reported.

Choose the "600 MHz Reference Oscillator Unlocked" first and replacing failed 2N2222A regulator on the 100MHz oscillator which bought the display back and all the alarms disappeared. Best 40c I've ever spent.

Was it completely fixed? No unfortunately not, the input reads about 90dB low. Every thing else looks fine, the resolution band width, span and frequency accuracy all look good. Checked the attenuator and the filter and even went straight into the RF port on the first low band mixer but still 90dB down.

I am at an impass now until I can borrow a decent SA as I do not have anything to measure 3-6GHz.

I did the LO feed through test in the service manual ie centre freq 0Hz, Span 1MHz, input attenuator 0dB and it says the feed through should be between -6 and -30dBm. It says if it is inside this range the RF path following the Low band mixer is operating properly.

Mine is -26dBm, so in the range.

My question is does that test mean that the the Yig, the LO distribution amp, the first mixer LO and IF ports and all the following mixers are operating correctly albeit not 90dB out? If so, the only thing I can think is a faulty RF port on the mixer (sounds unlikely). Also, I suppose if the LO drive was low, maybe 10 dB or so, there would still be feed through but may be not enough level to bias the mixer to mix.

Any though while I wait for a better spec an?

Dave

?

?