Electronics > Beginners
Boonton 102F on Ocilloscope
DannyKlenz:
Forum,
I recently acquired a Boonton 102F. I hooked up to my oscilloscope and I am getting an unusual signal back from it. Im not sure if its my cabling causing reflection or what. My goal is to fix this and learn as much as I can while doing so. Electronics have always been an interest of mine but I never found the time to really dig in to a project like this.
Things I have tried:
1. I have tried every combination of probe/terminator combo I have on hand and still get the same result.
2. I recapped the power supply using Nichicon caps. Original values were 2x 2400uf 40v DC for the +15 and -15 rails and 1x 5300uf 20v-30v Surge DC for the +5 These were replaced with 2x 2700uf 50v DC and 1x 5600 35v DC
3. Adjusted the power supply so all voltages are within spec
4. Bought a copy of the service manual.....
Below is a link to a youtube video I just posted showing the results I am getting on my scope.
https://youtu.be/hgCUPs4UhLA
Mr Evil:
It looks like its clipping. Does the problem go away if you reduce the output amplitude? How much DC offset is there?
DannyKlenz:
The link below is a picture of the lowest output I can measure with the scope. Sign wave is still distorted. I used the top measurement which I think is the DC offset? There are a dozen or so tantalum caps throughout the device. Even on the power supply board. I've read that they can be an issue but only in certain situations. There are also a couple electrolitic caps as well. Maybe these are part of the problem?
http://imgur.com/a/J023PMg
Mr Evil:
I don't know what "top" is, but it looks like you've set it to DC-coupled, and the signal is only a few mV from perfectly centred, so the DC offset is ok, on the output at least. It does still look distorted, but it isn't toally flattened on one side like in the video. If you run it like in the video so the peaks are flattened, then reduce the amplitude a little at a time, does it become less distorted, like the peak amplitude on one side is limited?
I'm thinking that somewhere along the signal chain there's a faulty coupling capacitor, so the signal to the following stage is no longer centred. Presumably the service manual will have detailed enough schematics to let you locate the coupling capacitors. Tantalum capacitors can fail short, so checking those might reveal something.
rf+tech:
Hi Danny,
I've serviced a number of the Boonton 102 A through D. The wideband output amplifier and ALC detector are both ahead of the step attenuator, so the distortion seen will be present at all steps regardless of range. ALC loop alters drive level via a PIN diode attenuator, located ahead of the wideband amp input.
With the output step attenuator set to the highest level, vary the vernier level control, while observing the distortion. Let's see if reduced drive to the wideband amp resolves the distortion. From experience, it would not be unusual to find one or more carbon composition resistors in the wideband amp with increased resistance, altering bias on the final as the culprit.
RF+ Tech
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