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Advice on how to use an SA for test point troubleshooting

 

Hi Goran,

The tuning voltage to the HF oscillator seems to be constant at -12V at 500MHz and does not vary until the set frequency drops down to 750MHz where it flips to +13V and stays there. Nothing in between. I guess this is my problem?

BW

tony
 

Hi Tony, well, it is a symptom. This loop lacks conditions required for locking. You need to trace the 50 MHz signal from the reference oscillator to the A8 power splitter. Next trace the 60 to 110 MHz oscillator signal to the A8 mixer, see block diagrams BD1 - 3. Are they present in the first place? And then move on, what do you find?

G?ran
 

Thanks Goran - very helpful to have some pointers for where to look. I¡¯m away for the next couple of days but will post an update when I have some
progress.

Tony
 

It is the SYMPTOM - probably a problem in the phase detector or anything between the pD and the oscillator drive.

You need to verify the PD inputs and output.

Cheers!

Bruce

Quoting Tony <tonycox01@...>:

Hi Goran,

The tuning voltage to the HF oscillator seems to be constant at -12V at 500MHz and does not vary until the set frequency drops down to 750MHz where it flips to +13V and stays there. Nothing in between. I guess this is my problem?

BW

tony
 

Thanks Bruce. These are complex beasts - it¡¯s a bit hard to know where to start the trouble shooting. I¡¯m going to check the 50MHz through the freq multipliers and if there is nothing amiss there I¡¯ll move to the PD section.

thanks

tony
 

And stepping through the full output frequency range might give some interesting results.
G?ran
 

Seems like a reasonable plan. The observation you gave is an indicator that the PD is not controlling the VCO. Disd the VCO frequency change when the drive voltage changed?

Cheers!

Bruce

Quoting Tony <tonycox01@...>:

Thanks Bruce. These are complex beasts - it¡¯s a bit hard to know where to start the trouble shooting. I¡¯m going to check the 50MHz through the freq multipliers and if there is nothing amiss there I¡¯ll move to the PD section.

thanks

tony
 

I looked at this the other way around and stepped through the frequencies observing?the tune voltage (is that what you mean by drive voltage?). It was constant through the tuning range starting at 500MHz at about -12V and suddenly swung to?+13 at about 750MHz and stayed there for the rest of the range up to 990MHz.

Tony


On Mon, 20 Feb 2023 at 21:58, Bruce <bruce@...> wrote:
Seems like a reasonable plan.? The observation you gave is an?
indicator that the PD is not controlling the VCO.? Disd the VCO?
frequency change when the drive voltage changed?

Cheers!

Bruce

Quoting Tony <tonycox01@...>:

> Thanks Bruce. These are complex beasts - it¡¯s a bit hard to know?
> where to start the trouble shooting. I¡¯m going to check the 50MHz?
> through the freq multipliers and if there is nothing amiss there?
> I¡¯ll move to the PD section.
>
> thanks
>
> tony
>
>
>








 

QuotiTony -
Yes - drive = tune.

My GUESS (that and $4 will get you a Starbucks) is that the problem is the PD. Check that the drive is adequate for both signals and try stepping your way through the PD circuitry. Hopefully it is discrete - if not it is a bit more of a problem to analyze. What I would expect to find is that one of the two outputs is working and the other has a problem.

Cheers!

Bruce


ng Tony <tonycox01@...>:

I looked at this the other way around and stepped through the frequencies
observing the tune voltage (is that what you mean by drive voltage?). It
was constant through the tuning range starting at 500MHz at about -12V and
suddenly swung to +13 at about 750MHz and stayed there for the rest of the
range up to 990MHz.

Tony


On Mon, 20 Feb 2023 at 21:58, Bruce <bruce@...> wrote:

Seems like a reasonable plan. The observation you gave is an
indicator that the PD is not controlling the VCO. Disd the VCO
frequency change when the drive voltage changed?

Cheers!

Bruce

Quoting Tony <tonycox01@...>:

Thanks Bruce. These are complex beasts - it¡¯s a bit hard to know
where to start the trouble shooting. I¡¯m going to check the 50MHz
through the freq multipliers and if there is nothing amiss there
I¡¯ll move to the PD section.

thanks

tony
 

Can you send a picture of the PD schematic ??

Cheers!

Bruce

Quoting Tony <tonycox01@...>:

I looked at this the other way around and stepped through the frequencies
observing the tune voltage (is that what you mean by drive voltage?). It
was constant through the tuning range starting at 500MHz at about -12V and
suddenly swung to +13 at about 750MHz and stayed there for the rest of the
range up to 990MHz.

Tony


On Mon, 20 Feb 2023 at 21:58, Bruce <bruce@...> wrote:

Seems like a reasonable plan. The observation you gave is an
indicator that the PD is not controlling the VCO. Disd the VCO
frequency change when the drive voltage changed?

Cheers!

Bruce

Quoting Tony <tonycox01@...>:

Thanks Bruce. These are complex beasts - it¡¯s a bit hard to know
where to start the trouble shooting. I¡¯m going to check the 50MHz
through the freq multipliers and if there is nothing amiss there
I¡¯ll move to the PD section.

thanks

tony
 

Hi Bruce,

I attach both the block diagram and service sheet schematic for the PD. Apologies but the only service manual PDF document I have splits the schematics across 3 pages.

regards

Tony

SS10.pdf
SS10.pdf
PD-block.pdf
PD-block.pdf

 

Hi Tony,
I think the problem is related to the high frequency loop. However, you can first check the reference and low frequency loops.

On schematic 13 you find the phase detector U19 (ECL EXOR circuit). On pins 4 and 5 you shall have 10 MHz signals which are locked and 90 degrees out of phase. It is assumed that you have a 10 MHz reference signal connected to the instrument.

On schematic 10 you find the phase detector U17A and U17B. On pins 3 and 11 you shall have 100 (or 250) Hz signals which are locked and in phase.

G?ran
 

OK, the manual gives the answer.
The required probe ia a HP 1250-1598. This is just a pir of pins to coax lad an connector (SMC according? to the description).
So basically a direct connection to the spectrum analyser.
Here is a description of a homeberw one:


Robert G8RPI.

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