MRF186 amp problem

[OgreVorbis]

Hi,

So I am having an issue with one of those chinese amp boards using an mrf186 mosfet. The board used to produce the proper 120w, but suddenly one time after powering down a problem arised. I remember it being perfectly fine on powering down, but the next time I powered it up, it only produced 30w. I measured the bias voltage and it was substantially lower than before (about half a volt) and I had it set to 4.4v which is how it came stock from china. I tried to raise the bias by turning the pot up, but now it maxed out at about 2.4v and I could only push about 60w out. By bypassing a resistor in the bias supply circuit, I got it a little above 3v and pushed 80w for several hours with no problems.

So my question is: what the hell is going on? I tried to replace some of the components in the bias supply, but the problem wasnt fixed. Now I am thinking it must be that the transistor is dodgy, but I've never experienced this type of problem where the transistor seems to work, but it pulls the bias voltage too low. Has anyone experienced this type of problem before? I dont want to go on and replace the mosfet if thats not the problem.

[teckniqs]

It's probably just the dodgy little BIAS pot needs replacing, they can easily fail after a slight knock, replace that first...

[Maximus]

Probably a dodgy transistor. Had the same problem with the used ones which are supplied with the kit. Switched from Motorola to Freescale and never had a problem since.

This is one of the reasons I'm reluctant to buy the kits, plus half of the parts are missing.

Much prefer the Dutch gear.


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[Analyser]

One of the issues with these Chinese designs is the bias circuitry, it isn't temperature compensated and the bias on these transistors is extremely sensitive. As the transistor warms up the bias will naturally increase and may lead to blowing an input (gate) of the fet which can fail semi-short circuit. This would give the symptoms you're describing.

You should try carefully lifting the gates of the FET from the PCB and see if the bias voltage at the PCB pads returns to normal, if not it's probably a dodgy bias pot as Teck says.

[OgreVorbis]

Yeah, I already changed the pot with no results. I guess the next step is to disconnect the gate pads and see if the voltage goes back up as someone said. I also ordered some mrfs that look new from the pic on ebay. I also got some 9180s from enigma which are the freescale and look new. I can't seem to find any of the 186s by freescale. It looks like those were only produced by motorola?

[radio-berlin]

As you slowly turn up bias measure current draw to your amp, if it starts to flat line whilst still turning the bias up then your power supply might be current limiting..

Also check filter capacitors havnt frazzled, a lot of them kits have atc equivalent caps which are not rated high enough

[Maximus]

Yes it was 9180s that I was using. Had 2 of the Motorolas from the Chinese supplier which lasted about 5 minutes.

Bought a Freescale from Enigma and never had a problem. Even forgot to connect the dummy load on one occasion and it still didn't pop.





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[thewoodstarr]

Hi,

So I am having an issue with one of those chinese amp boards using an mrf186 mosfet. The board used to produce the proper 120w, but suddenly one time after powering down a problem arised. I remember it being perfectly fine on powering down, but the next time I powered it up, it only produced 30w. I measured the bias voltage and it was substantially lower than before (about half a volt) and I had it set to 4.4v which is how it came stock from china. I tried to raise the bias by turning the pot up, but now it maxed out at about 2.4v and I could only push about 60w out. By bypassing a resistor in the bias supply circuit, I got it a little above 3v and pushed 80w for several hours with no problems.

So my question is: what the hell is going on? I tried to replace some of the components in the bias supply, but the problem wasnt fixed. Now I am thinking it must be that the transistor is dodgy, but I've never experienced this type of problem where the transistor seems to work, but it pulls the bias voltage too low. Has anyone experienced this type of problem before? I dont want to go on and replace the mosfet if thats not the problem.

Watch the SMD Resistor network on the bios, ie the 470 Ohms SMD`s, they break under expansion and contraction with the heat of the device. Hope you sort it.

[s2000]

One of the issues with these Chinese designs is the bias circuitry, it isn't temperature compensated and the bias on these transistors is extremely sensitive. As the transistor warms up the bias will naturally increase and may lead to blowing an input (gate) of the fet which can fail semi-short circuit. This would give the symptoms you're describing.

You should try carefully lifting the gates of the FET from the PCB and see if the bias voltage at the PCB pads returns to normal, if not it's probably a dodgy bias pot as Teck says.

Interesting reading that, I have had a similar problem when I messed around with a ready made pallet amp a while back. I'd set the amplifier up for the correct quiescence for the device but as the amp got hotter the bias current draw would increase until it was drawing way too much!

I have heard of temperature compensation before but whats the best way to implement it into the bias circuit?

Ive noticed a few amps that use a cheap voltage regulator in the bias cct, always thought it was to drop the amplifier supply voltage to a more suitable level for the bias, could it be configured to current limit to prevent the above?

Or maybe a thermistor to limit the current when the device gets hot? I remember seeing one schematic with one in somewhere.

[Shedbuilt]

One of the issues with these Chinese designs is the bias circuitry, it isn't temperature compensated and the bias on these transistors is extremely sensitive. As the transistor warms up the bias will naturally increase and may lead to blowing an input (gate) of the fet which can fail semi-short circuit. This would give the symptoms you're describing.

You should try carefully lifting the gates of the FET from the PCB and see if the bias voltage at the PCB pads returns to normal, if not it's probably a dodgy bias pot as Teck says.

Interesting reading that, I have had a similar problem when I messed around with a ready made pallet amp a while back. I'd set the amplifier up for the correct quiescence for the device but as the amp got hotter the bias current draw would increase until it was drawing way too much!

I have heard of temperature compensation before but whats the best way to implement it into the bias circuit?

Ive noticed a few amps that use a cheap voltage regulator in the bias cct, always thought it was to drop the amplifier supply voltage to a more suitable level for the bias, could it be configured to current limit to prevent the above?

Or maybe a thermistor to limit the current when the device gets hot? I remember seeing one schematic with one in somewhere.

From a bit of "googling", it looks like those Chinese amps use a 5v1 Zener diode; followed by a dropper ladder. From memory (hopefully someone will correct me if I'm wrong), Zener diodes have a positive thermal coefficient (Zener voltage goes up with increasing temperature); meaning the bias voltage will also (slightly) increase, with increasing temperature; thereby worsening the situation (assuming a degree of thermal coupling), and heating of the Zener itself possibly contributing slightly more.
I "think" the three pin regulators (eg 7805 or 78L05), have negative thermal characteristics, which should help to some extent.
Using a 7805 (rather than L805), would allow it to be bolted to the same heatsink as the FET.
You could also / instead, change all or part of the shunt resistor in the dropper ladder, for a NTC thermistor. Again, to make it reasonably accurate, the ratio (of resistor vs NTC thermistor), would need to be worked out (and/or tweaked) carefully, and could only be made truly accurate if the thermal characteristics of the FET are linear (haven't looked into this in depth), and the NTC device is tightly thermally coupled.

[Maximus]

Here's a picture of one of the supplied 9180s. The marking on the white outer casing was there from the start, which suggests that it probably came from a 'shit bin' of about 1000+ pieces which were ready to pop.

Btw apologies for the previous amp pic, was using a well worn out soldering tip.




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[shorty]

Is there any technical difference between the blue and green variants of the 170w amplifier for sale on ebay, some sellers show both in the same add, they seem to be a good buy at that price even with a new 9180 from enigma, it's 50 quid all in.

[Analyser]

The bias drift of these FETs should normally be in the region of 2mV/ deg. C which means you need to compensate with a negative bias voltage of the same amount to keep the bias stable with temperature.
There are various ways of doing this, some simple and others more complex but you can use things like a P-N junction of a transistor/ silicon diode, or a thermistor with carefully worked out resistor values. Some people use a temperature sensing element followed by an op-amp which means they can adjust the "slope" of the compensation to get it spot on.

What Shedders said is correct, a zener has a positive voltage slope with temperature so could worsen the problem. I think voltage regulators are temperature compensated (not 100% sure but think they are, for obvious reasons).
Normally for FM use none of this would be an issue because the amplifier doesn't need to be linear and a drifting bias won't change the performance. There are plenty of amplifiers out there with a simple zener voltage reference or resistor ladder which work fine.

The issue comes when you need a lot of bias to get the huge gain (remember 200W out from 1W in??) coupled with the fact that for some reason these UHF FETs seem to be very bias sensitive and a small change in gate voltage has a massive increase in drain current. This means that without careful adjustment it's quite easy to set a huge bias current which tips the device over the edge and pops it when it gets hot!

[Albert H]

From a bit of "googling", it looks like those Chinese amps use a 5v1 Zener diode; followed by a dropper ladder. From memory (hopefully someone will correct me if I'm wrong), Zener diodes have a positive thermal coefficient

Weirdly, 5V1 and 5V6 zeners have virtually zero temperature coefficient. I've used this property for temperature stable bias circuits in commercial products.

Voltage regulator ICs have temperature compensation. However, the 78XX series do drift a bit with temperature (usually upwards with rising temperature). When I've needed real precision (such as tuning a musical instrument), I've designed discrete (or op-amp based) Voltage regulators for the utmost precision.

[rigmo]

Hi Guy do any body try this baord withs short coax..
i have confirmation, i n that case perheps mrf9180 go 200W without diodes
mrf186 150W 22V input 3W

[rigmo]

Hi I never seen this board before? ware you get it?
by the way input balun to much turn swr is above 2.1
use only 2 tur, on this site from photo only one turn to see.

[Electronically]

Hi I never seen this board before? ware you get it?
by the way input balun to much turn swr is above 2.1
use only 2 tur, on this site from photo only one turn to see.

I remember I had mrf186 on that board. Trouble was I had to input about 8 Watts of drive, to get about 100 Watts. Power supply was about 28volts.

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[rigmo]

Hi, Today I play A lot with mrf186 and get 150W 26V idq 200mA... I do not wont try at 28V
This guy get power with self oscillating... opposite of mene work - expirance.. see link
Need to be end of coax Add 1nF on both channel 25R, China diy kit give only 4X120Pf ATC.. its to low value...

[Electronically]

Hi, Today I play A lot with mrf186 and get 150W 26V idq 200mA... I do not wont try at 28V
This guy get power with self oscillating... opposite of mene work - expirance.. see link
Need to be end of coax Add 1nF on both channel 25R, China diy kit give only 4X120Pf ATC.. its to low value...

Try the chips nrg mentioned you'll be surprised at how well those chips work.youll find you'll have less issues if inserted into one of those boards. So forget about mrf186 and 9180.

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[rigmo]

Try the chips nrg mentioned you'll be surprised at how well those chips work.youll find you'll have less issues if inserted into one of those boards. So forget about mrf186 and 9180.

Tru but can you named specify?
In one of hand I cant because if all good with match and tune working perfectly.. cost 35usd... in diy kit...