OK... after the first repair - replacing ERA5 and BLF404 on the Driver board it worked perfect on a Bench test

After on test for a while at 300>400W it got hot - now the max I can get out from it is about 40W

After doing some measurements it looks like the BLF404 has blown again.
When I replaced it I did the best I could regarding the mounting and cooling recommendations.
I have one more but I recon that one will blow just the same as well.
I suspect that the problem is the cooling of the BLF404.
Question could I use another power MOS transistor mounted on a separate heatsink instead.
Any suggestions of what I could try ?
I am willing to experiment I can't make it worse


.

The BLF404 is a rather low power device, so its cooling requirements are not too special.
Could it be too much bias? Maybe the setting potentiometer is scratchy, and with temperature changes it may go open briefly and overbias the transistor.

do you sure your P1 is ok? i do not like this type of voltage regulation for bias drive..

I would make yourself a balun , seriously it stopped rf coming back down my rf coax into the PA and blowing my two MRF300’s, i’m betting you have the same issue and a simple diy balun will do the trick, I made mine using 9 ferrite cores (the ones with an internal diameter that matches the diameter of RG8 coax , you can have it right at the back of the pa (or at the antenna if you prefer)

Krakatoa wrote: ↑Fri Aug 25, 2023 7:00 pm The BLF404 is a rather low power device, so its cooling requirements are not too special.

According to the pfd

Philips Semiconductors Product specification
UHF power MOS transistor BLF404
MOUNTING RECOMMENDATIONS
Both the metallized groundplate and leads contribute to the heatflow. It is recommended that the transistor is mounted
on a grounded metallized area of a maximum thickness of 0.8 mm on the printed-circuit board, equipped with at least 12
(0.5 mm diameter) through metallized holes filled with solder.
A thermal resistance Rth(mb-h) of 5 K/W can be achieved if heatsink compound is applied when the transistor is mounted
on the printed-circuit board.

This is the board before the BLF404 was replaced.

Krakatoa wrote: ↑Fri Aug 25, 2023 7:00 pm Could it be too much bias? Maybe the setting potentiometer is scratchy, and with temperature changes it may go open briefly and overbias the transistor.

As far as I know the bias pot seems to be OK, when it was working (before the fault) I measured a steady +0.44V on the Gate.

.

I see a multiturn pot, they are usually quite reliable.
But under the BLF404 there are not enough ground vias to help evacuate the heat. Maybe soldering some copper tabs next to the transistor's grounded terminals could help as an improvised heatsink.

Krakatoa wrote: ↑Sat Aug 26, 2023 4:23 pm I see a multiturn pot, they are usually quite reliable.

Krakatoa wrote: ↑Sat Aug 26, 2023 4:23 pm But under the BLF404 there are not enough ground vias to help evacuate the heat. Maybe soldering some copper tabs next to the transistor's grounded terminals could help as an improvised heatsink.

I have some 0.5mm flat copper plate I could cut some shapes and use that

.

Do you try heat elements and see if have anomaly... to much bias? ..

rigmo wrote: ↑Sun Aug 27, 2023 3:50 pm Do you try heat elements and see if have anomaly... to much bias? ..

I have fixed the new BLF404 in much better job this time
It was well seated and soldered onto the print 100% this time.
New R12 12K
New P1 10K - set at the lowest bias setting ready for a test outside the main TX.
Gonna see how much current it draws.


.

OK....

R11 12K = New
P1 10K = New

With P1 set at to lowest bias setting (basically short circuit) the TX works fine tried it all the way up to 500W
On a soak test at 150W for two hours = no drift in RF output, constant mains consumption and steady Temp.

I have searched everywhere on the Net but I can't find any info on what the BLF404 Bias should be set at.

What would you guys suggest to set it at ?
.

BriansBrain wrote: ↑Sat Sep 16, 2023 1:14 pm OK....
What would you guys suggest to set it at ?

from mr. GEORGE
"Two possibilities. One is dead the first driver ERA5 is dead and the second BLF404 is dead. I see all this problems in last 20 years."
MRF 245 final driver need 1W max.. BLF202 - 404

rigmo wrote: ↑Mon Sep 18, 2023 4:28 pm from mr. GEORGE
"Two possibilities. One is dead the first driver ERA5 is dead and the second BLF404 is dead. I see all this problems in last 20 years."

Well they both are new and they both give gain

rigmo wrote: ↑Mon Sep 18, 2023 4:28 pm from mr. GEORGE
"Two possibilities. One is dead the first driver ERA5 is dead and the second BLF404 is dead. I see all this problems in last 20 years."
MRF 245 final driver need 1W max.. BLF202 - 404

OK......... I did a lash-up with a RD15HVF1 and Bingo everything came to life

So..... decided to do it properly all seems good, stable RF out up to 500W no problem
Managed to now get 20V P-P or 1W now for the BLF245 final driver.
Best results with the Bias set to 2.8V, temperature and power consumption steady also.
Going for a full long term test tomorrow

Awaiting Your Criticism


.

So....... BLF404 changed for a RD15HVF1.

First low power test today - 150W RF out for 560W Mains consumption.
110W less than before with the BLF404.

Biggest difference of all running at only a Temperature of 41.2°C.
Compared to before +/- 55°C @ 150W out with the BLF404.

After 1hr all measurements stable......... unlike with the BLF404.

.

You might find that the heatsink you've put on to the RD15HVF1 is a bit small - you'll find that the power out of the device will reduce as it gets hotter. FETs don't (usually) suffer from thermal runaway because of this negative temperature coefficient, which causes supply current consumption (and therefore power output) to fall as temperature increases.

I remember the first VHF power amps I built using FETs. The first one used a VMP4, and wasn't too great, but had quite a lot of useful gain. The second one used an MRF150, and I was delighted to discover that I could get well over 100 Watts out for just under 3 Watts in. When compared to the bipolar VHF transistors I'd been using before then (mostly with gain - at best - of ~10dB), they were amazing - more like a valve than a solid state device, often with gains of 16 - 19 dB!

Albert H wrote: ↑Thu Sep 28, 2023 12:40 am You might find that the heatsink you've put on to the RD15HVF1 is a bit small - you'll find that the power out of the device will reduce as it gets hotter.

I have beamed it after 1hr of it running with my Infrared Thermometer Gun result MAX 38°C

Also don't forget this device is only 6cm away from the incoming air from the front mounted
PAPST 120mm x 120mm 24V 11W fan.
The RD15HVF1 gets the full force of the air flow.


.

You'll be fine with the fan that close! It looks like you've found a viable solution.

Albert H wrote: ↑Sat Sep 30, 2023 4:24 am You'll be fine with the fan that close! It looks like you've found a viable solution.

Thanks

shorten the legs and lay it horizontally. make some room for the RD15HVF1

rigmo wrote: ↑Sat Oct 07, 2023 7:10 am shorten the legs and lay it horizontally.

Not possible without a board re-design