| Electronics > Repair |
| Wavetek 2520a 0.2-2,200MHz RF sig gen repair |
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| wn1fju:
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. |
| dazz1:
--- Quote from: wn1fju 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. --- End quote --- 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 |
| dazz1:
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 |
| dazz1:
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 |
| wn1fju:
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! |
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