Hot (but working) FETs

[gatorjj]

Hello all,

Long story (TL;DR below) but I replaced the RF board of my low-band (tr)usdx, and the FETs get extremely hot on receive with 12v or 9v input power. They actually cool down some while transmitting. They are working as I've been able to hit Germany on the RBN and have made an SSB contact as well to check, but I really don't want to blow it up again. Any suggestions of what I can check inside?

TL;DR
I've had the assembled (tr)usdx from the approved Ebay seller for a couple weeks, made a bunch of good QSO's and was having a great time with it until I accidentally put it on the wrong antenna, and the SWR >2:1 appears to have blown the FETs (I went from ~5W to 0.0W). I got some replacements on Ebay but they were probably counterfeit as they literally caused flames when I powered it up. I reversed the next FET replacements and just got smoke, but obviously not good.

I happened across someone selling an unassembled RF board (also from the China seller) and bought it, put it together and I'm getting around 3W at 12V (65-ish efficiency) which I'd be okay with at this point, but I noticed the case got extremely hot on receive so I started to look into it. The FETs stay cool at 5-6V on receive but heat up almost instantly at 9V or 12V (Q3 is hottest, Q2 is hot, Q1 stays cool). I'm now switching the battery pack on to transmit and shutting it off to receive with USB power only to keep it from blowing up again, but that's not a great long-term solution obviously

Any thoughts would be appreciated, I'd like to get this back to a good state and get back to having fun with it!

Thanks very much,
J.J.
KF4VE

[gatorjj]

I thought I would document my further experiences with this radio in case it helps someone else. The good news is though they get extremely hot on receive (too hot to touch), the BS170s have not blown yet and I've made many contacts both on CW and SSB. I have been made aware that the SSB audio has become quite horrible with "whistling" artifacts ("sounds like RF"), and through trial and error listening to SDR's hundreds of miles away I believe it is related to the heat. When I power up with 12V from a cold state the audio is fine for a few minutes (no artifacts present) but once it heats up they get worse and worse. Same happens at 9V, the audio stays good for a bit longer but then goes bad. For CW it works fine until it gets too hot and then starts crashing a lot. Unplugging the power cord when receiving (USB only) seems to help them cool down enough to extend the operating time.

So for you folks having bad audio reports and crashes, it may be heat that is giving you a problem. FWIW I did wire up a cheap Retevis mic, but the audio with the onboard mic is actually better. The same condition happens whether it is the external or internal mic.

I'm not the most technically savvy at troubleshooting the electronics, so I'm still hoping someone has a thought of what can be checked or done to reduce the BS170 heat on receive.

[LILO]

I would check DRIVE signal (fet gates) in RX mode first.
BTW what is current consumption on receive?

[gatorjj]

Thanks for the help!

Current @12.5V is 180ma on receive, 380ma on transmit with 2.5w. At 5.8v current is 120ma on receive.

[LILO]

Way too much. Should be ~80mA on RX at 12V.
Probably faulty fets. Check DRIVE signal also.

[gatorjj]

Thank you so much for responding! After reading your reply I did some deep digging in this forum looking for "drive" and found somewhere in the threads a similar problem to mine, where the advice was the hot FET was bad and to remove it. So I decided to pull out the back FET (leaving the other 2) and now I have a cool and working radio! It also seems to have solved my whistling audio problem, and the efficiency went up as well.

It's currently putting out +/- 3 watts (which it also was with the hot FET) which I am okay with, after the literal flames from replacing twice with a bad batch of new FETs I'd rather just keep it where it is than blow more things up. I'm assuming it's okay with just the 2 FETs? If not I'm sure I will find out

J.J.
KF4VE

[LILO]

All capacitors and inductors values on the board are calculated for 3xBS170. So, now your circuts are out of tune. Not sure could it be somehow compensated by spreading/compressing windings on toroids.

[dl6sez]

Hello Guys,

if it is okay now for you with 3W and two FETs you can leave it if your current consumption at TX is not higher than 400mA @12V.
The circuit is not really out of tune, this is not true!
The LPF works still when it was in tune before with 3 FETs before, maybe the efficiency is not the best possible. Remember the "Pi" CLC combination farer away from the FETs is the notch and is independent of the Fets output characteristics. The Cx0 (parallel Cx1), Cx2 (parallel Cx3) and Lx1 are the two components playing the mayor role for the output impedance match of the FETs (series resonance at TX frequency and impedance transformation).
The problem we all have is that there are many different FETs sold declared as BS170....
We only now that Mouser was a serious seller where the BS170 have been real Onsemi(ex. Fairchild) FETs.
So it is a bit of a lottery to get the same performance when you have others.
If possible for you to get 3 new FETs from Mouser would be the safest solution,
but you still have to tweak the windings normally a little bit with original replacement FETs, because output characteristics of the FETs have small changes from one batch to the other.

73 de Chris

[LILO]

So, it's ok now with 2 fets instead of intended 3. But you still have to tweak the windings if you put 3 original ones, because they all have slightly different characteristics?
Nice )

[gatorjj]

Thank you again, I've learned a lot from this. I originally was going to replace the FETs with ones from Mouser but they show a 6 month backlog. So I've been trying to work around that somehow after the 'eBay' batch I got blew up.

I will certainly replace them all when the supply chain straightens out, but 3w is a fine compromise in the meantime. I still need to measure the current draw (TX/RX) but I greatly appreciate the guidance here in what to look for.

73,
J.J.
KF4VE