RF amplifier board repair - Page 1
Forum section: electronics / repair

author: otpisani
author_rank: Newbie
timestamp: August 31, 2023, 09:14:44 am
content: I need to repair this RF amplifiers, although it never work, it's cheap one, and you don't have any schematic at all, and take care for heat dissipation, you need to buy MRF300An and MRF300BN mosfets, which I did but when everything connected, and board need two power supply one for drain and one for gate side, so I put voltage regulator on power supply for drain side, which need 53V, and downgrade voltage around 10.5V for gate side, but when is everything connected I don't have any current flowing!!!???,I Can't regulated voltage with trimmer more than 2.2V on both side, and on amplifier board says that bias voltage need to be 2.7V or 0.3A. When I turn voltage around 2.3V on both side, than starts sudden drops in voltage on power supply, and current jumps around, but I regulated (blocked) with power supply max of 0.5A to let on amplifier board, just because that mosfets don't burn, so breaking point for both mosfets are around 2.3V, but again both side AN and BN are connected, you can't regulated with trimmer slowly current, although this trimmer are for fine step by step regulation.Strange is also that I can continuity (diode mode) from ground to gate side, but I don't have continuity from ground to drain side.Mosfets are new, and like to determine where is issue if someone can helpamplifier boardhttps://www.ebay.com/itm/265724892694?hash=item3dde701616:g:sV0AAOSwSr5indG-&amdata=enc%3AAQAIAAAAwMlqL9LUPMxJ7vX5cVq4AD9uX8CvQYPnzLC0lb8GGvtLQFW%2FZ%2FFOFwWcfSDaVYn2WJHKgl0y9ol%2B7a1p0IRnYJM3dP8k7sEEGCHQv7WVB8j9ojqE0vJj3foHZQtAMqs%2Fj4iBo%2FyRRjtOIE4QG3ZKPEKtExYG36w7z1QpCosvrgMsSpuzOClcChlshcC1FH6jAEQznxMQDgE%2FeiWnCd%2FzFOsBykqckfs4GfZC%2B%2BATgKME7hzjkfF4OTU%2Fs0ei%2FkaCLg%3D%3D|tkp%3ABk9SR-TrgKzJYgMRF300AN datasheethttps://www.nxp.com/docs/en/data-sheet/MRF300AN.pdf

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: August 31, 2023, 05:36:05 pm
content: Post a picture of the RF amplifier and your voltage regulator

author: otpisani
author_rank: Newbie
timestamp: August 31, 2023, 06:49:57 pm
content: Voltage regulator is this one , but the same is also when I put power supply 12V, everything is new, and main power supply running drain side (53V) I blocked current up to 0.8A, that I'm sure that mosfet will not be burned when configuring them, although I suspect that something already burned ;-)https://vi.aliexpress.com/item/1005004983920053.html?spm=a2g0o.order_list.order_list_main.39.6f471802ewFBd8&gatewayAdapt=glo2vnm

author: Hamelec
author_rank: Regular Contributor
timestamp: August 31, 2023, 08:10:27 pm
content: MRF300AN / BN from Ali? Fake part?AN and BN mixed up on the board?Source of MRF300xN not properly grounded?measure Gate voltages..

author: otpisani
author_rank: Newbie
timestamp: August 31, 2023, 08:34:21 pm
content: Quote from: Hamelec on August 31, 2023, 08:10:27 pm

author: langwadt
author_rank: Super Contributor
timestamp: August 31, 2023, 08:58:03 pm
content: Quote from: otpisani on August 31, 2023, 08:34:21 pmwhere did you get the 2.7V from? the datasheet just has a 1.7-2.7V threshold and a 100mA@2.5V typical

author: otpisani
author_rank: Newbie
timestamp: August 31, 2023, 09:07:25 pm
content: Presume that this board is some version of this one in this articie, and actually on the board is label, between mosfets, that bias gate voltage need to be 2.7V or 300mAhttps://qrpblog.com/2019/10/a-600w-broadband-hf-amplifier-using-affordable-ldmos-devices/

author: langwadt
author_rank: Super Contributor
timestamp: August 31, 2023, 09:25:54 pm
content: Quote from: otpisani on August 31, 2023, 09:07:25 pmignore the voltage and adjust it to 300mA

author: RFDx
author_rank: Regular Contributor
timestamp: September 01, 2023, 04:08:43 am
content: The board implies a 12V supply for the gate voltage circuit. Are the 10.5V you are using enough for that? The amp has it's own voltage regulator with temperature compensation for the gate voltage. The exact gate voltage is secondary, you have to adjust the two trimmers for a specific drain bias current (e.g. 2 x 300...500mA) with no drive signal.You have to bare in mind that, because of the two seperate bias drain chokes, the amplifier can not put 600W out. This modules will do about half the power in linear mode. The transistors are made for ~400W in linear (class-AB) mode. The isolation pads under the MOSFETs are not needed and are counterproductive. Typically you would find a copper heatspreader under the transistors.

author: otpisani
author_rank: Newbie
timestamp: September 01, 2023, 03:54:19 pm
content: QuoteYes, board has voltage regulator WS78M05A2218, presume that is based on L7805 voltage regulator, but can't find datasheet for my exact regulator, and L7805 has internal division, but according LM7805 min input voltage can be from 7V, although can be different because of the several species, max can be, again different because of lot sub species from 13V-20-25V, but no matter what offer as Vi you will always get Vo around 5V, not sure about current, but I measure voltage, which is 5V.QuoteI will remove thermal pads, but I'm not sure about copper underneath, service is grey color, but maybe they using paint

author: otpisani
author_rank: Newbie
timestamp: September 01, 2023, 05:34:39 pm
content: I can confirm that removing thermal pads achieve something, now I have continuity between left side ground (53V) and drain, and right side (12V) and drain, before removing pads I only have continuity between both sides and gate, but it's the same in the value on the both side, as it should be around 1900, and for gate side continuity values depends from that blue trimmer, which is also correct, know that because have amplifier with worse features.Removing pads didn't change anything about suddenly drops of voltage from 53V to 12-8-6, and that happened always when I set trimmer on both side around 2.2-2.3VBefore de soldering mosfets plan to move step voltage regulator and put 12V power supply for gate side, if any one have some idea before de soldering mosfets....

author: dietert1
author_rank: Super Contributor
timestamp: September 01, 2023, 07:03:18 pm
content: I remember reading that these RF power stages need termination on input and output. If you leave the output without load or with a bad SWR, that can destroy the transistors. Also if you connect a multimeter to measure gate voltage, you should use a test point that has some 1 KOhm resistor to avoid direct contact of probe and its cable to the mosfet gate.As you noted, once you get close to the gate threshold, strange things can happen (oscillation at very high frequencies).Regards, Dieter

author: otpisani
author_rank: Newbie
timestamp: January 09, 2024, 08:36:06 pm
content: Hello,I would like to re post old thread, now amp is build as it should,Issue is that signal don't boost at all, I have one power supply, for drain and gate. On gate side I have voltage down converter, and can confirm that voltage converter can supply actually around 12-14V 1.5A under working condition, because I try with some cheap amp operating around12-14V, and voltage converter starts acting funny around 1.5-1-6A. From manual gate side needs around 1A and 12V.I actually don't have proper power supply on drain side, it's some cheap Chinese Voltage/current adjustable power supply, but can provide only 4A at 53V, and manual says 15A and 53V, don't know if fet needs that much current when actually starts, although MRF300AN and MRF300BN are not toy gadgets.Manufacturer didn't provide any manual/debug manual, I don't know how to connect output coaxial cable, now is only central wire connected, usually ground and plus are connected on second magnetic coil (before output SMA, like on picture).

author: RFDx
author_rank: Regular Contributor
timestamp: January 10, 2024, 01:29:02 am
content: Quote from: otpisani on January 09, 2024, 08:36:06 pm...that is because you have short circuited your coax cable. Turn the board 180° around and make a picture so the output balun can be seen. You connect your coax cable where the transmission line (blue coax) of the output 1:1 current balun ends.Quote from: otpisani on January 09, 2024, 08:36:06 pm12V/1A is just a specification, the gate side needs in reality very little current.