Total Newbie here with using Oscilloscopes to work on Amateur radios. - Page 1
Forum section: electronics / rf-microwave

author: KG5RPZ
author_rank: Newbie
timestamp: July 29, 2018, 07:39:11 pm
content: Hello everyone,I'm a General class license holder, I have always had an interest in Radio since I was 14, I am a 20 year Industrial Electrician who has always been a hobbiest electronics technician, but have no formal training in the field other than my 5 year Apprenticeship becoming a journeyman Wireman aka Industrial Electrician.I'm working on diagnosing the final section of a Hybrid radio, specifically the Kenwood 520. As you may or maynot know these radios have very high voltage in the final PA section of the Transmit. 900 Volts down depending on what part of the radio your working on. So being careful is the most important thing, which I always am.The radio itself has bias current of 60ma as it should, The tubes all light up the driver and the finals, I can place the radio in "Tune" mode and see a faint blue glow from one of the two finals, a very faint glow from the other one and the driver tube as well... So its basically not putting out the right Transmit current or wattage at all, its unable to be tuned, because other than the High Voltage meter showing 850 Volts and the Bias showing 60ma, when you go into the Tune procedure, it does nothing, on any band at any time, even in CW mode nothing...It does receive, and if you dial the tune knob the receive comes up and gets louder, so something is working on the recieve in the tube section, but not on the transmit at all. I have replaced all 3 tubes with known working ones, same problem so its not the tubes...I have a very elementary issue stopping the entire system from working on Transmit, and need to isolate the issue. My intention was to take the scope or a multimeter and check for RF voltage via the schematics before the driver, at the driver, and finally from the driver section to the final section.I have looked over the boards, I see no evidence of the "Smoke" getting out of any components, and I have did a total recap of the entire radio, in the process, I have not lost anything that was working, however I have not gained anything but receive on all bands, at first it only received on one band, I cleaned each of the "wafer" switches by hand and that brought all band recieve back. I have also taken out both Relays RL1,RL2 and hand cleaned them with CorrosionX and a dollar bill, also with Deoxit, and a dollar bill, and checked the relays out of the circuit and they open and close as they should both in and out of the circuit . I have watched them with a flashlight and had my wife hit the "Send" switch... So all the relays and wafer switches are good to go. I have replaced the final section capacitors, resistors, and double checked all the resistors value. I bought the Final section repair kit, and capacitor kit from Hybridestore online shop, and a few other parts from a fella named KEN KE4EEA i think it is.I am to the point, I think its "Gremlins" lol.... anyone have a tip? Also how could I use my new Fluke/Philips Combiscope PM3394 200mhz 4 channel scope in aiding me with finding my issue?

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2018, 01:58:41 am
content: You need to be really careful probing with an oscilloscope into high power amplifier like that even without the high voltages from a tube transmitter. High power RF into a standard x10 or x100 probe can short the probe and then destroy the oscilloscope. 900 volts is beyond the rating of a standard x10 probe and most x100 probes must not be used with AC coupling.I would probably take a close look at the exciter and solid state output stage. I assume you have a proper power meter and dummy load.

author: CopperCone
author_rank: Super Contributor
timestamp: July 30, 2018, 02:12:30 am
content: also if you probe RF you need to check your probes rating with AC. The voltage rating goes down at high frequencies.