Author Topic: Wavetek 2520a 0.2-2,200MHz RF sig gen repair  (Read 4170 times)

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Online tautech

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #50 on: June 22, 2021, 05:43:53 am »
See you Thursday arvo with the rescue package.
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #51 on: June 23, 2021, 06:53:38 am »
Hi
I identified 5x 1uF Tant Caps connected to the +15V regulator output.   At least one of them had a short.
I went through and destructively checked each cap.  I cut a lead and lifted each cap for testing.
Naturally it was the last cap that I tested that had the short.  This positively confirmed I had identified the faulty component.

It takes me not very much time to remove the main pcb having done it a few times. 

I am now replacing all 5 tant caps.  If there are no further faults, the 2520A should be fully operational again. 

Dazz
« Last Edit: July 10, 2021, 10:14:54 am by dazz1 »
 

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #52 on: June 23, 2021, 11:40:14 am »
I've seen Defpom replace caps only soldering the top side pad but providing sufficient dwell time to let the solder wick through to the pad on the other side so to not need to remove PCB's that might be difficult to remove.

Guess you got my mail explaining I how won't be done until Welly sorts out their Covid situation.  :scared:
2 weeks I guess should have it sorted.
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #53 on: June 24, 2021, 01:13:17 am »
I've seen Defpom replace caps only soldering the top side pad but providing sufficient dwell time to let the solder wick through to the pad on the other side so to not need to remove PCB's that might be difficult to remove.

Guess you got my mail explaining I how won't be done until Welly sorts out their Covid situation.  :scared:
2 weeks I guess should have it sorted.

Hi
The leads are bent over on the underside.  I only have a rubbish single shot solder sucker, so clearing holes is a 2 sided operation.   Taking out the main pcb and cutting the leads was going to be easier that trying to do it all from the top. 
I have now got to the point if I find one failed Tant Cap, I replace all on the same power supply.  Another reason why I did destructive testing.    I am trying to avoid replacing every Tant Cap.  They are everywhere.

Testing converted a hypothesis (that a Tant Cap had failed) into a fact. Similar symptoms could have been produced by a failed device.

I'm looking forward to meeting with you but Covid disruptions are now a normal part of life.

Dazz
« Last Edit: July 10, 2021, 10:15:09 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 #54 on: June 24, 2021, 07:20:22 am »
Hi
I think I have almost cleared the last fault.  I still have to replace the Dallas nvRAM.   Attached are images from the Siglent spectrum analyser with FM and AM modulation.  I used the internal signal (400Hz or 1000Hz) source, and the external 1000Hz modulation feature.  The external sig-gen is an Elector XR2206 based design.   OK but not a pure sine wave generator. 

The attached screen shots are functional tests only.  I was just looking to see if I have more faults to deal with.

Internally, the 400Hz/1kHz oscillators produce a 10V signal at full scale.
The external modulation signal input only requires 2.5V to match the internal signal.  There are a couple of images showing a 5V input signal, over modulating and over driving the Wavetek.

The modulated spectrum is quite messy as the modulation depth rises but I probably wouldn't use the modulation feature.  If I did, it would probably be good enough.
I haven't looked to see if there is another fault. 

The envelope shape for FM might be distorted because the internal/external modulation oscillators aren't great.  I wouldn't want to dive too deeply into finding a fault unless I can get access to a low distortion audio sig-gen.

Dazz



« Last Edit: July 10, 2021, 10:15:19 am by dazz1 »
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #55 on: July 01, 2021, 07:04:17 am »
Hi
Ran the Wavetek 2520a and connected Siglent SSA all day to do a burn in test to flush out any more defects on the 2520a, and to stabilize the crystal osc on both instruments.
The 2520A is fitted with a 10MHz OCXO. 

The tolerance for the frequency is +/-2.5ppm.    This translates to +/-25Hz for the 10MHz reference.

The measured relative difference between the 2520A and the Siglent is 12.6Hz, but I don't know what the Siglent reference error is.    What I can say is that the OCXO is likely to be within tolerance, and if it isn't, there is a calibration and frequency adjustment procedure I can follow based on a more precise time reference.

Of course all of this points to falling down Alice's rabbit hole trying to find true time.

So all tests done to date indicate the 2520A of fully functional with all faults cleared except for the failing nvram.  A replacement is on order.

Dazz
« Last Edit: July 10, 2021, 10:15:36 am by dazz1 »
 

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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #56 on: July 01, 2021, 07:59:56 am »
Of course all of this points to falling down Alice's rabbit hole trying to find true time.

Come back Dazz !  :-DD
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Re: Wavetek 2520a 0.2-2,200MGHz RF sig gen repair
« Reply #57 on: July 01, 2021, 01:54:11 pm »
Wow I didn't know there was such as device capable of going from 0.2MGHz which is IR, through to 2200MGHz, in the hard X-ray region. :P
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #58 on: July 10, 2021, 09:59:47 am »
Hi
For the benefit of others that may need to repair/restore a WaveTek 2520A, attached is a copy of all the eproms of my one.
These are version 5.1 of the firmware. 
The file names include the chip number on the PCB and the component id.

There is one eprom that stores the values of sine waves, for the frequency synthesis.
There is one eprom that is installed on the GPIB board, if this option is installed.  It probably is.
There are 3x eproms that hold the firmware on the board with the 6805 processor. 

The files are in intel hex format.

I have copied the eproms onto Atmel TMS27C256-R-70PU one time programmable devices.  They are almost pin for pin compatible.
I replicated the firmware from the TMS27C128 devices because pin 27 is address A14 or PGM depending on the device.

For the TS27C64 replacement by the TMS27C256, I soldered a link between pins 26 and 27 directly on the memory device.  This meant I could fit either the original or the replacement device without modifying the PCB.  It should be possible to link these pins on the pcb and still swap because on the 64kb eprom, pin 26 is NC.

I took a great deal of care to avoid risk of static discharge wrecking the chips.  Now I have a copy of the original eproms, the plan is to replace them with the otp eproms.  If I have a problem with those, I can revert to the original eproms. 

Thanks to Tautech for organizing the eprom programmer loan from The Def Pom.  They used to be so common but are now surprisingly rare.

I have programmed the new otp eproms, but I have not yet installed them.  The 2520A has developed a new fault.  Prior to removing the original eproms to copy them, I switched on the 2520A to check that it was OK.  This showed the LCD display flashing randomly.  The LCD display segments are driven by a number of driver chips, so whatever is causing the problem affects them all at the same time.  This points to a bad joint or an unstable power supply.  I need to investigate.

Doing a power-on check proved to be a really good idea, because I know that the fault was not caused by removing, copying and replacing the eproms.  The symptoms did not change.  If I had corrupted the firmware while copying, the 2520A would be bricked.  The amount of effort required to reverse engineer the firmware to find and fix the corrupted code would have been too much effort to justify.

I suspect this 2520A has been sitting around for years and things have deteriorated.
Applying power has revealed a cluster of defects.


Dazz
« Last Edit: July 11, 2021, 09:38:26 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 #59 on: July 10, 2021, 10:18:02 am »
Wow I didn't know there was such as device capable of going from 0.2MGHz which is IR, through to 2200MGHz, in the hard X-ray region. :P

Do you know how long it took me to edit my posts ???    :(

Dazz
 

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #60 on: July 11, 2021, 09:36:21 am »
Hi
I did some fault finding on the flickering LCD display.    As usual, I started in the middle of the circuits that could cause the problem.  This just happened to be a single row idc interconnect flat ribbon cable between the main cpu board and the lcd display board.
I noticed that the first and last pins weren't there.    They looked like they had been cut off at the factory.  I remembered that one pin had always been missing but this is the first time I had noticed 2 pins missing.  Some of the connectors on the 2520A have pins cut and sockets blocked as a crude way of keying connectors.

After a further study of the circuit diagrams, I concluded that one pin was NC, and the other was an address line for a serial to parallel device that controlled the LCD display drivers.  A missing pin would leave the address line floating, consistent with the flickering LCD display symptoms I had seen.  I think I must have broken at least one of the pins while isolating boards to find other faults. 

Attached is part of the circuit diagram for the CPU board, alongside the diagram of the LCD display board.    The blue lines show the flat cable connections.  The red lines show signals disconnected by the broken pins.    Clearly these diagrams have been drawn by different designers.   The single row connector wasn't a great choice because it is relatively fragile, without a restraint, and non-polarized.    The photos show the problem.

By starting in the middle of the signal path between the CPU and the LCD, I happened to land directly on the fault.  I hadn't even checked the power supplies (normally step 1).    To fix this fault properly, I need to find a replacement flat cable, or replace the single row PCB sockets and cable with something different. 

Once I find and fit a replacement interconnect cable to get the 2520A fully functional again, then I will fit the newly programmed otp eproms.  Programmed thanks to the help of tautech and the def pom.

Assuming the otp eproms are correctly programmed, I will then replace the nvram with a modern version.

There is no connector that would allow external access to the nvram.  This makes it unlikely that the nvram has any factory data that cannot be generated by the calibration process in the manual.  I anticipate that I will not loose anything by replacing the nvram.
After the nvram is replaced, I will aim to fully calibrate the 2520A.  Initial testing indicated it should be within the original specs. 

I have lost count of the number of faults I have found and fixed on the 2520A but I hope they have all revealed themselves.


Dazz
 

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #61 on: July 13, 2021, 07:11:35 am »
Hi
I have been looking for a replacement cable assembly to replace the one I broke,  This has turned out to be much harder that it should be.  The original cable has male connectors, but I could not find an available replacement.

I could have just made up a custom cable assembly with some wire and Du Pont style connectors and that would work.    Du Pont type connectors are used elsewhere in the 2520A so they would not look out of place if I used them.

I like to keep things looking factory and original.  I found this Santek cable assembly : https://nz.element14.com/samtec/idss-16-d-05-00-g/cable-assy-16p-idc-rcpt-rcpt-127mm/dp/3551322?ost=3551322
which has female connectors to mate with a pinned header.   So the plan is to replace the sockets on the PCBs with male headers to mate with the Santek cable assembly.

That will take a while because I need to order at least $50 of parts to avoid shipping costs.  Once I get the 2520A fully functioning again, I will continue with repairing the known outstanding defects and replace the eproms with the programmed otp eproms..

Dazz

 

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #62 on: July 13, 2021, 08:48:02 pm »
It looks like it is only using every 2nd conductor of that ribbon cable, so my suggestion would be to fit an IDC connector and use a long pin header to plug the two together, and cover/plug the unused holes in the ribbon plug.

OR

Swap the PCB's IDC socket to be a pin header, and fit a IDC plug on the ribbon as I mention above, or try to find one similar to the one you found, the one you found is a 0.1" ribbon spacing, so it may not clamp on the ribbon due to the extra conductors in between the pins.
« Last Edit: July 13, 2021, 08:53:22 pm by TheDefpom »
Cheers Scott

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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #63 on: July 13, 2021, 09:50:50 pm »
Hi Scot

Thanks for your suggestions.  I am basically going for the 2nd option.  It may not be clear from the e14 catalog but the Santek part is a complete cable assembly.  I just need to swap the female connectors on the pcb for headers. 
That is a good thing because the factory ribbon cables and connectors are rubbish.  They should have used standard 2 row, 16 pin keyed and locked connectors, like they have used elsewhere in the 2520A.

That I think is the cheapest and easiest solution.

Dazz

 

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #64 on: July 24, 2021, 10:57:16 am »
Hi
I received a complete IDC cable assembly sample from Samtec. It was exactly what I needed but the cable ends were fitted with female sockets, for headers. 
The factory fitted female sockets were on the PCB.

I removed the sockets on the PCBs and replaced them with headers.  A much better arrangement than the factory version.  The headers and the new cable assembly looks factory original, but better.
I had to take the display/key panel apart to get to that socket on the PCB.  I fitted the new header on the other side of the PCB to make future access better.  No need to disassemble the front display/key panel.

Once I confirmed that the display was now working again, I started with testing the otp eproms.  This took quite a while because I swapped one eprom at a time, then ran a full set of diagnostics tests.  They all worked perfectly, so I have left them fitted in place.  Now that I have file copies of the eproms, I don't need the originals.   

The only fault left now is the nvram.   The battery isn't a battery anymore.  I need a proper solder sucker work station to remove the original nvram.  Once that is replaced, I will then be able to do a calibration, if I can get access to suitable test equipment.

So I seem to be approaching the end of fault finding/fixing on this old piece of test equipment.   I suspect it was sitting in storage for a long time, and accumulated faults while there.  Despite its age, it is still a useful  item to have.  Maybe it will still be going in another 30 or more years. 

The first photo shows where the PCB mounted socket was.  To unplug the connector required taking the front panel apart.   Now a new header is soldered on the other side of the board.

The second photo shows the front panel removed from the chassis and opened up to reveal the LCDs.  Under the LCDs is an electroluminescent panel that provides a feeble back light.  In 1989 that was high tech stuff. The LCD and inside glass panel were quite grubby so this was a good opportunity to clean them.
« Last Edit: July 24, 2021, 09:54:37 pm by dazz1 »
 
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Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #65 on: July 24, 2021, 11:00:09 am »
Some more photos

The 1st photo is the new cable assembly in-place.  It is and looks better than the factory original.
The 2nd photo shows 4 of the 5 otp eproms installed.

All of the otp eproms are 256k compared to the original 128k.  The only thing needed to make these work was to fill up the 256k devices with duplicate copies of what was on the 128k device. 

The sine wave table for the synthesizer was stored on a 64k eprom.  For this, 4 copies of the eprom fitted in the 256k device. I also soldered a bridge wire directly across pins 26 and 27 of the 256k otp eproms. No changes required to the pcb.
« Last Edit: July 24, 2021, 10:00:48 pm by dazz1 »
 

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #66 on: July 27, 2021, 01:14:08 am »
Hi
Just a couple of photos.

The wire link soldered directly to the 26-27 pins of the 256k otp eprom (follow arrow) to make it plug-in compatible with the 64k eprom it replaced.

Confirmation that all the otp eproms are working.  There are 3 ROMs with code in them.  They switched in/out depending on the mode.  It is possible that the 2520A could have passed all its self-tests in calibration mode, while the operating ROM was defective.
Note that the 2520A passed all the eprom (parity ??) tests so that possibility should be impossible. 
 

Offline dazz1

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Re: Wavetek 2520a 0.2-2,200MHz RF sig gen repair
« Reply #67 on: Yesterday at 09:34:20 am »
Hi
I replaced the Dallas battery backed nvram today with a modern equivalent.  When the 2520A was powered up, it displayed error messages. Not surprising.
I went through the diagnostics routines which includes loading default settings.
Everything went swimmingly except for one small detail.  The "Options" self test failed.


I suspect that the nvram still retains some memories, which includes the options are installed on this particular 2520A.  When I swapped the nvram, the memory of those options remained on the old nvram. 
Given that there is no connector that would allow an external device to read/write to the nvram, and given the vintage of the 2520A, I suspect that the front panel keys are the only way to write to the nvram. 
If that is true, then it is likely that the ROMs include the routines necessary to make the options usable.   

My theory is that there is a secret command entered on the key pad that enables specific options.  It is probably a variation of the key commands to run the diagnostics and calibration routines.   Testing that theory probably requires disassembly of the firmware code.  I think that would be challenging to say the least.  Additionally, I don't know what options are now missing.  There are factory stickers on a couple of option keys.  One is for external phase modulation.  The other is to specify units of rad.  It is unlikely I would use either of those options, which I think are related and part of the same option.   

It would be nice to restore the 2520A to its full operation.


EDIT

I found a description on the Phase  Modulation feature in the Operator Manual, now attached.


« Last Edit: Today at 10:03:21 am by dazz1 »
 


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