Electronics 29


Next I removed the KHz turret assy and was left with the amplifier chassis, several shots above. I checked all the caps and resistors on the amplifier assy ckt board, caps were all good but every one of the carbon comp resistors was excessively high. All the resistors were replaced. You can see the board with carbon resistors in pic 2 and the board with the new resistors in pic 3. Also, in pictures 1 and 2 I labeled the contacts for reference so others don't have to painstakingly figure it out. I cleaned all the contacts and set it aside for reassembly.


Finally I gave some attention to the KHz Turret assy. Checked all the caps, all checked good and cleaned up the contacts. In the pic above I labeled the different decks so others didn't need to painstakingly figure these out either.	Above just a couple shots of the entire thing disassembled. Next the whole thing got reassembled and tested.

I did an alignment next and plotted a lot of data to reference how well alignment and fiddling was proceeding. I wanted to make sure I could actually see if what I was doing was for the better or the worse. In retrospect I probably should have fixed the synthesizer problem first so I could see the overall radio sensitivity as I progressed through this as well. Will remember that next time. As a result I was really only able to see performance from an RF block perspective rather than the entire radio perspective. Things worked out in the end though as I am getting 1uv sensitivities from the radio after all problems were fixed. Additionally, I didn't plot any performance data before I took it apart and refurbished/rebuilt it. So again I don't have that comparison. I did plot performance after it was rebuilt and before alignment, so that's my starting point. You can download the entire excel spreadsheet of data I plotted here, it may help as a reference if you tearing into your RF Amp.

Test Equipment

For a Sig Gen I primarily used an HP-8640B and occasionally an HP-8443A Tracking Generator on an HP-141T Spectrum Analyzer. I used the 141T Spec An as my measurement piece. Both the Tracking Gen and the HP-8640B are connected to an external standard, URQ-10, so I'm reasonably sure I'm working with accurate signal frequencies. Levels? Well that's another story. I have no idea when the last time was my test equipment was calibrated and it costs a fortune to get this stuff calibrated! It's all reference anyway right? My input reference level for all the data plots and tests was -60dBm. I made my measurements using the141T at TP3 and TP4 on the top of the module. TP3 being the output of the 1st RF Amp and TP4 being the output of the 2nd RF Amp. Be sure and use the Ground test points next to TP3 & 4 when making measurements, it does make a difference as I found out the hard way.

Plotting the Data

I plotted the data in Excel and all the data is plotted the same way. My intent was to then take the data and make a graph so I could graphically view the gain/loss of; 1) the 1st RF, 2) the second RF, and 3) the overall gain/loss of the "box". Basically it's the long way of getting a swept view of the RF Amp. I set the spreadsheet up so all I had to do was enter the input signal level and the signal levels read at TP3 & 4 and formulas in the gain/loss cells would automatically calculate the appropriate gain/loss. To produce the graph I used the Freq column as the X axis and gain/loss as the Y axis on the graph. I also added a trend line to each plot to give an overall view for the plot. Each plotting of data produces three graphs, 1st RF, 2nd RF, and Overall. Here's the entire spreadsheet if you want to download it. R1051 RF Amp Perf Data I'm only going to show the graphs, not the raw data, in the following.

This first set of plots is the RF Amp Performance after I did the refurb/rebuild but before doing any aligning and fiddling with it. For this test I had the RF Amp in the radio, signal was injected at the antenna connector at -60dBm and outputs taken at TP3 & 4. I read the dB levels directly off the Spectrum Analyzer' screen. This was my reference for things getting either better or worse with playing. Keep in mind, none of these plots were to see what the smallest signal I can put in and still get a discernable output. The intent is to see how gain/loss changes.

1st RF Output

2nd RF Output

Overall

Couple of observations on this first pass after refurb/rebuild. First RF Amp has a dip at 12 and 19 MHz. Overall gain of the 1st RF declines as frequency increases but overall the gain stays positive through out the range. 2nd RF gain stays level


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