Author Topic: Wavetek 2520a 0.2-2,200MHz RF sig gen repair  (Read 3860 times)

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Offline dazz1

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Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« on: April 30, 2021, 10:07:20 am »
Hi
I am trying to repair an old Wavetek 2520a sig gen.
I have narrowed the fault to the main board.  This board drags the +/-18V supplies to near zero.  There is an almost dead short on the board.
The +18V regulator  on the power supply is hotter than the -18V regulator.  The regulators are based on LM395 so I can run the power supply into a dead short. 

The +8V supply feeds the +5V regulators on the main board.  That is OK.

The +/-18V supply feeds the +/- 15V regulators on the main board.  The input to the 15V regulators is near zero, but the regulators are not even warm.   The outputs are less than 0,5V.   None of the ICs I can touch are warm (there are some under a shield).  There are no burn marks or signs of the release of the magic smoke. 


I have a scan of the service manual but it appears that two schematic drawings are missing.  There are parts on the board that have no purpose.

I don't have a thermal camera to trace the current flow.
I have a Fluke 77 so not ideal to track the voltage drop along tracks on the double sided PCB. 

Does anyone have a scan of the missing schematics?
Does anyone have suggestions on fault finding this board?

Dazz

« Last Edit: July 10, 2021, 10:06:03 am by dazz1 »
 

Offline wn1fju

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #1 on: April 30, 2021, 11:14:04 am »
Your description confuses me.  You say that the input to the regulators is near zero.  So why would you expect the output of the regulators to be anything other than near zero?  That doesn't necessarily imply a short on the main board.  If you had said the input to the 15V regulators was the normal 18V and the output was zero, then I would agree with you.

You should first unload the power supply by disconnecting the various boards and make sure all the power supply voltages are correct.  On my similar Wavetek 2510 unit, the Molex connector from the output of the main transformer to the rest of the power supply had corroded and the power supply didn't work correctly.  (I ended up removing the connector and soldering the wires since no amount of connector cleaning seemed to cure the problem).

There very well may be a short on the main board (or other boards).  Invariably it will be a capacitor hanging off the power rails.  You can start lifting one lead of the capacitors while watching an ohmmeter on the rails.  When the ohmmeter jumps away from zero, you've found your short.
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #2 on: April 30, 2021, 09:10:17 pm »
Hi
I started by isolating the power supply.  With an open output, the voltages are all good.  The LM395 transistors on the power supply limit current to about 2.2A.  I can safely connect the power supply to a dead short.  Good for testing purposes.   The power supply is fully operational. 

I then isolated the fault to the main board.  When the main board is connected, the +/-18V supplies are near zero.  As a result the outputs of the +/-15V regulators on the main board are also near zero.    Something on the main board is taking about 2A.  None of the chips have a high enough supply voltage to operate, so there is no signal to trace and no function to check.

The 5V supply on the main board is OK so all of the digital logic devices are also OK.   None are exposed to the +/-18V supply.

Temperature measurements on the power supply shows that the +18V regulator is running hotter than the -18V regulator.    This could simply be because of the design loading on the +18V circuits is more than the -18V circuits.

There are several hundred parts on the main board so randomly lifting parts would be the option of last resort. 
I have started by marking the components supplied from the +/-18V including the +/-15V regulators and attached parts.  Without the 2 missing schematics, this is difficult to finish.

Ideally I would have a thermal imaging camera to follow the current along the heated tracks to the faulty component, but I don't have one. I am considering converting an old webcam to infrared.

Today I am going to try and trace voltage gradients along the PCB tracks, but I don't have the PCB drawing or a really sensitive volt meter.  I will pay particular attention to capacitors.  There are no electrolytics on the main board.

Dazz
« Last Edit: June 09, 2021, 10:28:59 am by dazz1 »
 

Offline wn1fju

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #3 on: April 30, 2021, 11:21:08 pm »
You're right, I apologize.  I just quickly looked at the mainboard schematic (at least the pages that aren't missing!) and saw the 15V regulators.  It didn't occur to me that the +/-18V was also propagated throughout the board. 

There may be no electrolytics, but it looks like there are quite a few (1 uF, 10 uF) tantalums which are notorious for going (near) short.  That would be my first guess.
 
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #4 on: May 01, 2021, 01:45:19 am »
Hi
The regulated +/-18V from the power supply is applied to 7815/7915 regulators on the main PCB.   There are two separate +18V regulators on the power supply, presumable because someone really wanted to use the LM395 transistors, and one couldn't supply enough current.   The power supplies are double regulated.  Definitely over engineered.

I am having a go at hacking a old webcam to make an infra-red camera.  It that works, I should be able to "see" where the current is going.

Dazz
« Last Edit: July 10, 2021, 10:06:40 am by dazz1 »
 

Online tautech

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #5 on: May 01, 2021, 02:26:55 am »
As wn1fju says Tants !
DO NOT overlook them !
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #6 on: May 01, 2021, 08:05:27 am »
Hi
Visual inspection revealed one Tant split in half that was missed on the initial look over.  Clearly not the source of the short, but if one has gone, the others are probably near or past failure.

I tried a webcam hacked to block visible and allow infrared.  The infrared filter was replaced with crossed layers of polarizing film.  Not surprisingly it doesn't work at light frequencies low enough for fault finding.  It does show sub-visible infrared which equates to a temperature to just below red hot.  It does not detect a soldering iron at 350 deg C.  I would need a real infrared camera for that.

There are about 30 tant caps shown on the parts list, but naturally these are all on the missing circuit diagrams.

Dazz
« Last Edit: July 10, 2021, 10:07:00 am by dazz1 »
 

Online tautech

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #7 on: May 01, 2021, 09:06:11 am »
Never 2nd guess Tants.
In Pt 1 Defpom replaced a split and burnt one but still had a short on a rail. Guess what he found:

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Offline TheDefpom

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #8 on: May 01, 2021, 11:16:30 am »
Just chuck your meter into diode test mode and put it across each tantalum, that will tell you if one is shorted out completely.

If you get any odd readings, reverse the probes and retest, if it is a short you will get the same reading again, if it is just some other component effecting the reading, then you will likely get a different reading, or even see it changing as a capacitor charges up.
« Last Edit: May 01, 2021, 11:20:55 am by TheDefpom »
Cheers Scott

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Offline wn1fju

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #9 on: May 01, 2021, 11:24:13 am »
Although I actually have the thermal cameras and micro-ohmmeters, I've often found it simpler to hook up an ohmmeter across the shorted power rail(s) and start lifting one end of the tantalums until the short disappears.  You can usually do this from the component side of the board without major damage or the need to pull the board out.  It can be a little difficult, however, if they folded the leads on the foil side. 

The last time I did this was on an HP 1345A digital display module (used in many vintage HP pieces) that had shorts on both 15 and -15 volt rails.  There were about a dozen tantalums (those little yellow cylindrical jobs) and I think it took me all of about 5 minutes to find the two shorted ones.  What is funny is that after I replaced the two bad ones, I powered the display up successfully and observed that the graphics/text was quite fuzzy.  Turns out I forgot to solder the lifted leads of the other capacitors back to the board!!!
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #10 on: May 03, 2021, 03:33:46 am »
Hi
I tried lifting the 4x  10uF 25V Tants on the 18V supply.  One was blown apart. The other 3 are OK.  With these tants connected to air, the short is still there.
There are still 26x  1uF 25V tants on the 15V supply. 
The search continues.

Dazz
« Last Edit: July 10, 2021, 10:07:28 am by dazz1 »
 

Offline Chris56000

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #11 on: May 03, 2021, 06:12:41 pm »
Hi!

Could you please possibly email me a copy of, or provide a link to, the incomplete manual please?

If one of these generators comes up in the future that I can buy than I can draw out the missing sheets again!

Chris Williams
« Last Edit: May 03, 2021, 06:19:32 pm by Chris56000 »
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #12 on: May 03, 2021, 08:53:55 pm »
Hi!

Could you please possibly email me a copy of, or provide a link to, the incomplete manual please?


The manual is too big to email or post.
Based on the figure numbers (different to the drawing numbers), it appears the missing drawings were not included in the original manual.
Reconstructing the missing drawings would be made more complex because some parts of the circuit are fitted but not used.

Dazz
« Last Edit: July 10, 2021, 10:07:50 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #13 on: May 23, 2021, 06:26:37 am »
Hi
I discovered that the main board has connectors on the bottom side that connect through to PCBs inside the enclosures on the other side of the equipment.

When I pulled the main board the short disappeared. 
I isolated the short to a Tant in the Oscillator module. 
When I refitted the mainboard and applied power, a Tant on the main board released the magic smoke. 
All of the failed 25V Tants are on the +18V rail.   I have replaced them with 35V Tants.

So now all the power rails are at the correct voltage. 

The self test and auto-calibration has failed. 
There are multiple control loops so now I need to work my way around these to find the next fault.

Dazz


« Last Edit: July 10, 2021, 10:08:11 am by dazz1 »
 
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #14 on: May 23, 2021, 10:47:36 am »
Hi

A local RF company http://www.rftest.co.nz/  with a ton of test equipment and expertise let me use a Thermal Imaging Camera on the main board. 

This was before I knew that there were still connected boards below the main board.  The thermal imaging camera was very sensitive and easily showed temperature differences of 1 degree C.  It did sense the slight temperature variations of current carrying tracks and components.  It correctly showed no hot components on the main board.

It was the camera that convinced me I needed to look harder at the equipment to trace what turned out to be a shorted Tant cap on another board.

Dazz

« Last Edit: July 10, 2021, 10:08:32 am by dazz1 »
 

Offline wn1fju

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #15 on: May 23, 2021, 12:11:59 pm »
Glad you finally found the shorted tantalum.  Had to be that.

I will tell you what was wrong with my unit when it quickly failed the auto-calibration and threw an error.  The main loop uses a VCO mixed with a quadrature DDS pair, then divided down to the phase detector and loop filter.  The DDS circuitry was outputting zero - not good!  I substituted my own quadrature signals from an external source and then everything worked.  So clearly the DDS was malfunctioning.  Wavetek uses a ROM-based look-up table that contains a (digital) sinewave.  Turns out that the ROM was not being addressed correctly by the bank of 74HC583 BCD adder chips.  And indeed, a logic analyzer showed a stuck bit on one of the chips.  A new 74HC583 fixed the unit.

Also, I found that the main board is really wedged tightly around the metal bulkheads surrounding it.  I had to pry the board out with a screwdriver (ouch).  And I found that Wavetek was fond of placing the 3-terminal regulators at the perimeter of the board, right at the bulkheads, potentially shorting them.  I found a couple that were almost touching so I moved them back a little.
 
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #16 on: May 23, 2021, 08:30:23 pm »
Glad you finally found the shorted tantalum.  Had to be that.

I will tell you what was wrong with my unit when it quickly failed the auto-calibration and threw an error.  The main loop uses a VCO mixed with a quadrature DDS pair, then divided down to the phase detector and loop filter.  The DDS circuitry was outputting zero - not good!  I substituted my own quadrature signals from an external source and then everything worked.  So clearly the DDS was malfunctioning.  Wavetek uses a ROM-based look-up table that contains a (digital) sinewave.  Turns out that the ROM was not being addressed correctly by the bank of 74HC583 BCD adder chips.  And indeed, a logic analyzer showed a stuck bit on one of the chips.  A new 74HC583 fixed the unit.

Also, I found that the main board is really wedged tightly around the metal bulkheads surrounding it.  I had to pry the board out with a screwdriver (ouch).  And I found that Wavetek was fond of placing the 3-terminal regulators at the perimeter of the board, right at the bulkheads, potentially shorting them.  I found a couple that were almost touching so I moved them back a little.

Hi
I read about your fault/repair on your website:  https://emperoroftestequipment.weebly.com/ 
You are lucky.  There is no way I would get approval from SWMBO to have a fraction of your collection.

I suspect I have a similar problem to your Wavetek.  The autocal for the main (coarse) loop should be able to get the oscillators somewhere close to their respective ranges.  The autocal faults on the first osc so either the first osc has failed or (more likely) one of the control loops has a fault. 

I get Error 1 on Osc 1, but I cannot find an explanation of the error codes.  No helpful.

The Wavetek also has an original Dallas 1225Y NVR with internal battery.  This is now 32 years old and may have passed its use by date  :)
If I can get the Wavetek working again, the plan is to replace the DS1225Y with a new DS1225AD, the modern equivalent.   


Dazz
« Last Edit: July 10, 2021, 10:08:50 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2025a 0.2-2,200MHz RF sig gen repair
« Reply #17 on: May 24, 2021, 11:11:18 pm »
Hi
Looks like I could also have a fault in the DDS section.
I am not seeing a sine wave output on the output of IC215, an op-amp found on Schematic 4 of 4 : Digital Synthesis.  I am seeing -1V DC.

Before I dive down that rabbit hole, I need to check the inputs to that whole section.   I don't have a logic analyser so in-circuit checking of chip functions will be harder than it should be.
I need to start by checking that the Data, Clock and Strobe inputs are OK.  If the input is rubbish, the output will be to.

I am seeing clock signals on the Buffer Registers (74273).  I just haven't looked deeper yet.

Dazz
« Last Edit: July 10, 2021, 10:09:05 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #18 on: May 28, 2021, 08:00:26 am »
Hi
I think I am making some progress.
I have not found a fault in the DDS.  Re-checks showed all the right wave forms where expected.

The Operator Manual includes info on the Auto Calibration feature.
This goes through a process of finding the upper/lower voltage ranges to control each of the 4x VCOs. Within each voltage range, the WaveTek finds the control voltage necessary to set a course frequency.   

The AutoCal fails on the first VCO.  When set to operating mode, the display indicates that stored settings are not being used.
I did think this could be due to a failed battery in the Dallas 1225 NVR (32 years old) but the user input settings are remembered between power cycles.  The NVR still seems to be working, but probably not for too long.

There is no serial port available for loading the NVR but there is an option to set factory defaults plus AutoCal.  I think both options were used in the factory to calibrate the Sig Gen (along with some other specified test equipment).  I think the user AutoCal is the same as the initial factory calibration.  If so, it means that I can replace the NVR with a DS1225AD, the modern version of the DS1225Y.  The NVR would then be loaded with the factory defaults and AutoCal.


The VCO control voltage is set by a DAC.  The DAC output is static during AutoCal.  I would expect the DAC output to be hunting up and down as a search is made for each coarse tuning point (35 points for the first oscillator). 

The data side of the DAC is also static during AutoCal.

Both the DAC input and output are also static when the frequency is changed by the operator. 

The input to the DAC is a 74HC595 serial to parallel register on the main board. 
The serial data comes from the 6502 microprocessor. 

So the next line of investigation is down the data side of the VCO control DAC.  This is where the  Logic Analyzer I don't have would be ideal.  I do have some Raspis and Arduinos so I may end up writing some simple code so they can simulate serial to parallel converters.   They could read the data and I could then compare the simulated output to the hardware output.  Any difference would reveal a fault.

I look with envy at the range of high quality logic analyzers available on e-bay, but the cost and risk of shipping them here would be more than SWMBO would ever approve. 
So the hunt continues for the next fault without the right tools to do the job.  I will just have to make do with what I have.

Dazz


« Last Edit: July 10, 2021, 10:09:18 am by dazz1 »
 

Offline wn1fju

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #19 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.

Yes, there are benefits to living here in the USA where there is an abundant supply of test equipment on eBay with reasonable shipping costs.  I won't go into the disadvantages of living in the USA as I don't want to start an argument!
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #20 on: May 29, 2021, 12:42:27 am »
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.
...

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
« Last Edit: July 10, 2021, 10:09:41 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2025a 0.2-2,200MHz RF sig gen repair
« Reply #21 on: May 30, 2021, 06:57:51 am »
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
« Last Edit: July 10, 2021, 10:09:53 am by dazz1 »
 

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #22 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.......
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #23 on: May 30, 2021, 08:49:34 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.......

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
« Last Edit: July 10, 2021, 10:10:11 am by dazz1 »
 

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Re: Wavetek 2025a 0.2-2,200MGHz RF sig gen repair
« Reply #24 on: May 30, 2021, 07:12:22 pm »
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.......

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.
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.
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.
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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