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

TheDefpom:
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.
dazz1:
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

dazz1:
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.
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