Electronics > Repair
Wavetek 2520a 0.2-2,200MHz RF sig gen repair
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dazz1:

--- Quote from: wn1fju on May 28, 2021, 03:13:19 pm ---Sounds like you are getting close!

Although I do have a logic analyzer, and actually used it to fix my Wavetek unit, I've often found it simpler just to use a couple of scope probes and look for I/O lines that are not changing or are at an intermediate undefined logic level.  Sometimes one gets lucky.  It's usually a big hassle to set up the logic analyzer and get all the clips attached to the chip.  I use it only as a last resort.
...

--- End quote ---

Hi
The 74HC595 is a serial to parallel register.
The plan is to use the data write line connected to the external trigger of my ancient analog scope.  That will give me the Ch A and Ch B scope inputs to probe data and other pins to look for digital activity. 

The register outputs are all Lo so if I see any serial Hi data written into the register, I will know it is faulty.

Logic analyzers are really cool tech but with chip level integration and serial data streams the norm, they are not so useful any more.  I still want one.

Dazz
dazz1:
Hi
My testing of the circuits and devices that convert serial commands from the 6502 to parallel inputs to the DAC that controls the course frequency did not reveal any faults in the digital circuits.
I did find that some of the pins on a PCB interconnect are not the same on the circuit diagram and the actual PCB.     These differences probably affected my test results. 

The photos show the test setup to test the serial to parallel devices.  I connected the ext trigger to the data strobe input to the serial/parallel 595 device.  The A ch is the serial data.  The B ch is one of the parallel outputs.  It almost made a 2 channel logic analyzer.    I was able to confirm that the serial input matches parallel output.    This confirms that the 6502 has control of the inputs to the coarse frequency DAC.


The inbuilt diagnostics includes DAC test routines but the documentation only discusses the test showing the right result on the LCD.  I found that the DAC tests also write to the respective DACs.  So I can check the DACs for faults with the DAC test routine. 

The problem with fault finding on the 2025a with the limited test gear I have is that the 2025a requires a series of circuits to work in multiple loops.  Finding a fault requires proving that each circuit works using the previous one to provide a test signal.

The AutoCal software routine uses a binary input (output from a comparator) to confirm the course frequency range.  That binary input is at the end of a chain of circuits in the loops.

The search for the fault(s) continues.

Dazz
tautech:
A DSO would be much easier to fault find this.  :P
I'm down your ways in a couple weeks if you want to have a look at something.......
dazz1:

--- Quote from: tautech on May 30, 2021, 07:38:49 am ---A DSO would be much easier to fault find this.  :P
I'm down your ways in a couple weeks if you want to have a look at something.......

--- End quote ---

Hi
A DSO and all the other test equipment I don't have would make this easier. 
Rationally I should throw the 2520a in the bin.  The value of the time to fix it is far greater than the value of a 32 year old piece of test equipment.  It is a solid instrument built by engineers, not the marketing and accounting dept.  It was built to be fixable.

So far, I am still making progress, very slowly. 

If you want to drop by, that would be fine, but be prepared to be underwhelmed by my ancient and inadequate range of test gear. 

Dazz
tautech:

--- Quote from: dazz1 on May 30, 2021, 08:49:34 am ---
--- Quote from: tautech on May 30, 2021, 07:38:49 am ---A DSO would be much easier to fault find this.  :P
I'm down your ways in a couple weeks if you want to have a look at something.......

--- End quote ---

Hi
A DSO and all the other test equipment I don't have would make this easier. 
Rationally I should throw the 2025a in the bin.  The value of the time to fix it is far greater than the value of a 32 year old piece of test equipment.  It is a solid instrument built by engineers, not the marketing and accounting dept.  It was built to be fixable.
--- End quote ---
You can certainly afford to spend some amount of time on it as 2.2GHz sig gens are not cheap.  :o

Have you tried to use the TG on your SSA as a RF source ?


--- Quote ---So far, I am still making progress, very slowly.
--- End quote ---
Repairs can take a disproportionate amount of time however the personal development is often worth the effort and stands us in good stead for decades to come. 


--- Quote ---If you want to drop by, that would be fine, but be prepared to be underwhelmed by my ancient and inadequate range of test gear.
--- End quote ---
Everyone starts with old gear so never be ashamed of what you have if it does the job.
I'll drop you an email when I have some definite dates to check our contact details for you are still current and make some plan to drop by.
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