Help reclaiming the magic smoke that came out of my 50W PA #help #magicsmoke #troubleshooting

Charles Johnson

Greetings all,

In going to adjust the bias for the 40m 50W PA that I've built, I began with R5 turned all the way down as per the instructions. Putting the QXC+, which was outputting 4.24W into a QRP Labs dummy load (supply voltage 13.84V, measured voltage on the load 20V), into Practice Mode, I connected the PA (supply voltage also 13.84V, from the same PSU) in series between the QXC+ and the dummy load. WIth R5 all the way down, I measured 93mA. Turning it up, once I hit 95mA, I nudged it back and the meter read 93mA. Taking the QXC+ out of Practice mode, I then depressed my straight key and measured 4.16V on the dummy load. Unfortunately, I'm pretty sure that the VOM was set to read AC voltage (it defaults to AC), so I have no idea what the actual output was. Since I thought it was DC voltage, though, I decided to have a second go at calibrating the PA.

For my second attempt, it was identical, save I mistakenly did NOT put the QXC+ in Practice mode. I did, though, have a dummy load connected to the amp. With R5 turned all the way down, the multimeter was reading 990mA on key down. I turned R5 up a bit - no change. I then began to turn R5 up some more...and then whatever series of events happened (me looking at the meter and/or applying too much torque on the screwdriver or...), the result was that R5 was turned up quite a bit and some smoke and a burning smell came out from just forward of R5, between it and IC1 (it looked like it was coming from beneath R5 and IC1 looks just normal). Interestingly enough, the position was right near the setting from the previous calibration. Turning R5 all the way down, when I keyed the QXC+, things appeared to work - the PA was drawing 900mA while idle and when I keyed the QXC+, the current draw went up to ~990mA. Beginning to turn R5 up again, things seemed to work...until I hit the (potentiometer) position roughly where the puff of smoke had come out. At that point, current consumption dived down to ~15mA (effectively 0).

As I'm not sure where to start troubleshooting this - nowhere else on the amp seemed to be affected - I would appreciate any and all help. I have not looked at the underside of the PCB, so I do not know if there was any damage there. I can supply pictures of the PCB, if desired, but nothing looks wrong at least on the top side.

Charles Johnson

PS As the multimeter that I was using to read the current consumption is new to me, I suspect that I had the range set differently during the first measurement. I know that the numbers written down for the second measurement are accurate (ie if I say that the meter read 990mA, I mean that it was showing 0.990 when set to the 10A setting).

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