| Electronics > Repair |
| Wavetek 2520a 0.2-2,200MHz RF sig gen repair |
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| dazz1:
Hi As I feared, the root cause of the problem with the non-functional external FM/AM modulation traced back to the firmware. It turns out that the order of going from one modulation to another is important. The attached drawing shows the permitted sequential steps in green, and the ones that don't work in red. I can go from Internal AM to Internal FM, but I can't go diagnonally from Internal AM to External FM. So all the hardware symptoms I saw were due to a firmware "feature". The Wavetek also has an option button for external phase modulation, but this button does nothing I can see and this oiption does not show up in the self test. There is a test that shows the options installed. Also the manual indicates phase modulation requires an additional PCB module, which is not present. The front panel has a covered hole for an option for pulse modulation of the RF output. It also needs added circuitry. I definitely don't have this option fitted. So the plan is to soak test the Wavetek 2520 tomorrow to try and flush out any latent defects before I put the covers back on. Then over the coming weeks, I will try and get access to enough equipment to do a basic calibration, focusing on crystal frequency reference and output level. I think I started this repair about a year ago. I never expected to find so many faults in one piece of equipment. It has been a challenge but I learned electronics at around the period this was designed and built, so it has a familiarity to it. It has definitely been hand built. The circuit boards are dated and signed by real people. No stick-on labels here. Unlike old HP gear, it looks like Wavetek didn't quite finish design and development before it went into production. The optional modules are solder-in not plug-in, and probably needed to be factory fitted, including new EPROMs with updated/modified firmware. |
| tautech:
--- Quote from: dazz1 on June 18, 2022, 08:28:44 am ---I think I started this repair about a year ago. --- End quote --- You did, maybe a little over a year and July last was when I came to Wellywood and stopped by with Defpoms programmer. Never did I imagine it would take you this long to get to where you are however life has this habit in getting in the way of projects don't it ! Keep at it Daz ! |
| dazz1:
Hi Last night, I ran the built-in calibration. This sets the range of each of the voltage controlled oscillators. This calibration means the Wavetek can select an oscillator, then apply a frequency control voltage that is close to the required control voltage. Any remaining error is eliminated by the closed loop frequency control system. There is another test where the Wavetek RF output level is checked at 60 points across the frequency range. I used my Siglent SA to check the levels. At each point, the RF level is adjusted to a 0dBm output to a resolution of 0.1dBm. Between points the Wavetek interpolates the output level. As a result, the output level should be stable across the entire frequency range in a way that would be difficult to achieve with traditional analogue control methods. Today I left the Wavetek running as a short soak test to try to flush out any latent defects. The test was carefully supervised by His Majesty, the Right Royal and Honourable Sir Fluffy who showed true dedication and stayed the entire day. I am now ready to put the covers back on. The final step is to see if I can get access to equipment that I can use to verify the output level across the frequency range, and to adjust the reference oscillator. |
| dazz1:
Hi So the Wavetek 2520 project to complete all the repairs and get it back to a fully operational state ended today. RF Test Solutions at https://www.rftest.co.nz/ kindly helped me do a limited calibration with a range of test equipment I could only dream about. I was able to tune the 10MHz oven controlled crystal oscillator to within 0.28Hz, which corresponds to a 60Hz error at 2.2GHz. I ran the built-in calibration procedure to adjust the output level from 200kH to 2.2GHz to within +/-0.1dB at 0dBm output. So the Wavetek is probably better than factory, because way back in the 1990's they probably didn't have the quality of test equipment that I had access to today. So thanks to the two Dave's at RF Test Solutions. I couldn't really finish off this project without them. It has been well over a year since I started and now I can move onto something different. I have already. Now that I have restored the Wavetek, I just need to find a reason to use it. At present it is a solution looking for a problem. |
| Jeff eelcr:
Looking at your PCB I see a burnt ? near reg IC left side Middle Jeff This was in responce to post 7 picture 3. |
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