Electronics > Repair
Wavetek 2520a 0.2-2,200MHz RF sig gen repair
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dazz1:
Hi
I replaced my nvram with a modern equivalent, but I didn't pre-load it with the contents of the old nvram.  As a result, I lost a feature.

I suggest to remove the nvram and copy its contents, so when it fails, you can program its replacement.

The firmware on the ROMs is the one thing that definitely can't be 2nd sourced.  Closely behind that is specific contents of the nvram not regenerated by the diagnostics routines. 
dazz1:
Hi
Progress on the repair of the Wavetek 2520 has stalled for months while I waited for hardware from China. 
The hardware and software were copied directly from https://sites.google.com/site/gogleoops/mega-nvram-prog.
The only tweak I made was to use a 28pin socket rather than a 40pin socket. 

I could have purchased a general purpose programmer, but I would never use it again.  I elected to make a single purpose nvRAM programmer based on an Arduino Mega board.  I doubt the nvRAM programmer will never be used again, but I expect the Arduino will find a useful place to be.

I also fitted an IC socket to the 6502 micro-processor board so I can remove/reprogram/replace the nvRAM if needed. 

I have successfully programmed the nvRAM but not yet installed and tried it.    I used an image from the original factory fitted nvRAM to copy across to the new DS1225 nvRAM.  The old factory fitted nvRAM battery was well past its expected life, so it is entirely possible that the original factory image is corrupted.    Fortunately the nvRAM records configuration and calibration data.  The firmware is now located in OTP EPROM to replace the factory fitted EPROM (UV erasable).

dazz1:
Hi
First power-on test indicates that the repairs, including the programming of the nvram, are 100% successful.  I need to run full diagnostics and calibration to confirm, but it is looking good so far.
dazz1:
Hi
OK so I still have at least one more fault.
I was able to get through all of the self-tests OK.  No errors.
I was able to calibrate the oscillators and output level control.

Further testing showed AM and FM modulation is not working.  It was, but not anymore.
The FM modulation sort of works, but there is a step change at a certain modulation frequency.
Enabling AM modulation does not produce side bands, but it does change the output level. 

The LCD display is showing what I would expect to see.   That indicates that the firmware is OK.  I haven't created a fault with the nvRAM.
The manual has a diagnostics section for modulation, so I will work through that.   


So, do I expect to need or use FM/AM modulation?  NO. 
Do I need to fix it for any other reason?  NO.
Am I going to fix it?? YES of course. 
It would just annoy me to know that it is broken when I know I can fix it. 

The block diagram, with annotated modulation signal flow, is attached.

dazz1:
Hi
OK did some some fault finding today.  With the assistance of my trusted hairy assistant, I identified the problem as zero volts on the -15VDC supply to the circuits for AM/FM modulation.  The good news is that finding this fault shows the programmed nvRAM is working as it should.   

Previous experience shows this is most likely due to failed Tant caps.   When I went looking, my trusty assistant pointed out the toasty brown one in the picture.  Well at least I think he did.  Unfortunately, inspection of the circuit diagram showed that the toasted Tan cap was on the +15VDC supply. 

The time taken to lift and test Tant caps is about the same time as replacing it.  I have just gone out and purchased about 20 Tan Caps to replace the toasted +15V and all the -15V caps. 

Some of you might be thinking that I should go through and replace every Tant Cap.  I have considered that but there are well over 100 of them on the boards.  I have decided to replace them one supply at a time.  If one fails, I just replace all the Tant Caps on that supply.   As I learn more about this piece of kit, I can isolate faults relatively quickly.  I have long lost count of how many faults I have fixed, but it is a lot.   It is likely that as I replace more failed Tant caps, I will get to the point where there aren't many more to replace. 

As an aside, I now think it most likely that the options are coded into ROM, and not enabled by some secret value loaded into nvRAM.  This could be confirmed by looking at the 6502 code that handles front panel button presses.  If there is a secret combination of button presses, they will be found there.  This means investigating or modifying the Wavetek would require reverse engineering the 6502 code.  Something I will not be doing.
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