4.7K linear trimmer potentiometer R5. Smoked it. Smoked it real good, like a Marlborough cigarette. Is there a part number anyone knows of off hand or where I can get a replacement without sending to

#50wpa #pa #power #qcx40 #amplifier

The output stage could be oscillating, but not at the required frequency. Put a 'scope on the output, or input. If you have a cheap SDR dongle, use it to look at the transmitter output. Feed the TX ou

#pa #u3s

+10 !

#pa #u3s

If you can look at the drive signal at the gates with a scope and compare it to the other two that could be helpful. Be sure to use a 10x probe and a good ground for it. Steve KY4GX. Glory to God in t

#pa #u3s

Hans, I'd suggest you go back to a single FET to see if there's anything external to the FETs themselves. Care must be taken; sacrifice the component, not the PC board. Another suggestion is to instal

#pa #u3s

For a project I've just built three Ultimate3S kits. Two work fine with 3xBS170 in the PA, with powers ranging from 1000 to 100 mW depending on the band. The three FETs get warm, but not hot with the

#pa #u3s

Roy, Congratulations on the successful repair! I would agree that replacing the 4 BS170s at this time does not make sense. 73 Evan AC9TU

#qdx #pa #30m

For those still following this thread, I can report that my QDX is providing normal service once again. The current PA configuration is two renewed BS170's for the failed Q8 and Q10 and the original Q

#qdx #pa #30m

Yes, that's mostly wrong. I worked in the semiconductor business (engineer and operations manager) for more than 30 years ... bipolar and FET discrete devices. It is true that some devices have multip

#qdx #pa #30m

So true!

#qdx #pa #30m

BS170 is an N-channel MOSFET so the electrons actually go up the drain :-) Alan G4ZFQ wrote:

#qdx #pa #30m

Tony, Replacing the Si5351 is possible, several here have done it on various rigs. I've done a QCX and synth board https://sites.google.com/site/g4zfqradio/qrplabs_replace_si5351 shows how badly it ca

#qdx #pa #30m

@ Alan G4ZFQ. Oh yes - I agree Alan. My old battle scarred QCX 40 which suffered the IC3 (driver chip) failure, is still not back on the air. I cast it into a box in disgust after replacing the IC3 dr

#qdx #pa #30m

Tony, This also has happened a few times with the QCX, on at least two occasions the 12 volts must have gone back to the 5 volt line and/or the Si5351 through the driver chip. For most of us that woul

#qdx #pa #30m

Curt, I wonder what parameters you measured to match them? I did also wonder if matching really makes any difference. They are (should be) switched on and off almost instantaneously by square waves. I

#qdx #pa #30m

I also like the resistive SWR bridge very much. I always use it when tuning my antenna for QCX or QDX regardless of tuner type, manual or autotuner. I use a circuit similar to this: http://www.zerobea

#qdx #pa #30m

Auto-Tuners are great for higher power rigs that have SWR protection circuits, but BS170 killers. While the tuner is going through its “let’s see if this setting makes things worse. Oh it sure does, l

#qdx #pa #30m

I swapped out one BS170 then another and then I pulled all four and tested about 20 new BS170’s and came up with a matched group of four. Prior to this I had been using an auto tuner. Both BS170’s fai

#qdx #pa #30m

I don't have a QDX, but one problem of gate to source failure in the PA mosfets in the QCX family can be that the failure puts the full PA voltage onto the driver gate and this can feed back through t

#qdx #pa #30m

Dave, Is that so? I was under the impression that many semiconductors were actually multiple devices where excessive stress damages some of them leaving the component still working but vulnerable to f

#qdx #pa #30m