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2SC1971 Amplifier problem

geccoo

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Hello! This is my first post, so sorry if it's a bit substandard.

I have just started getting into RF electronics, and I really wanted to try and build an amplifier for the FM band, since I have a small 250mW PLL FM transmitter.
I found a PCB layout on the internet, that looked to me, pretty simple and easy to put together. I gathered the parts and threw it all together, and I think it turned out OK.

IMG_20191106_231152.jpg

I connected a 50 ohm dummy load, the PLL 250mW transmitter via a smaller gauge coaxial cable to the input & 12V regulated supply, current limited to 1.5A (if anything goes wrong) since the 2SC1971 is only rated for 2A, and it shouldn't draw more than 600mA when operating, with an input of 500mW from what I've read. And I'm only giving it 250mW (which in the datasheet says should give me around 4W of output which is enough for me).

The problem is that as soon as I turn the thing on, the power supply maxes out at 1.5A and the transistor get's pretty hot. I have checked for shorts multiple times and it's driving me crazy since there are none.
If I turn the voltage down from 12V to around 8V the current drops from 1.5A to around 600mA/500mA but it's still not outputting any RF, when the voltage is dropped even lower to around 5V it's not drawing any current at all. Is the drive to small or is there something wrong with the biasing? :???:

The two 220pF capacitors in the beginning are substituted for one 470pF and the 1.2nF for two 1nF, could this be the cause of the problem? And also, there's 50mW less of drive power. I asked one of my buddies who's a lot more into the RF section of things, and he told me it shouldn't cause any problems. Otherwise is the 2SC1971 fake? I ordered them from China a while ago, and to me they look like the Eleflow copies, which I've heard to work OK.

Here's the schematic:
6-watt-fm-transmitter-amplifier-schematic (1).jpg
 
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BigBoss

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This is C-Class Power Amplifier and it shouldn't draw any current unless the Input Power reaches to sufficient level.
So, since there is no RF signal at its Input, the quiescent current will be zero.It might probably be a fake transistor ( as very usual at Chinese internet shops ) or due to unknown reasons it oscillates or there are errors on PCB or components..
https://www.youtube.com/watch?v=_OVJ-87jcw8
 

srizbf

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Cannot access the attachment you have specified.

[Moderator action: Image was re-attached in post #1]
 

geccoo

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Okay, thank you for the answer.

I will try to change out the substituted parts for the correct ones and see if that makes a difference.
The video you linked, the transistor on the right looks exactly as the one that I've used. I will try ordering a new transistor from a more reputable source when I get home.
 

betwixt

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I haven't checked the transistor data sheet to be sure but isn't the tab normally the collector connection on TO-220 RF power transistors. If it is, I can't see any insulating washer beneath it!


Brian.
 

geccoo

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I checked a couple of datasheets from 3 different manufacturers, and they all said that the tab is connected to the emitter, which is the center pin.
 

srizbf

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you can try the following till you get a new 2sc1971.

Have a small rectangular piece of plate(copper or tin whichever is in your junkbox.).

The sheet length is that of the distance betwn your transistor mount and back side of the case.
height is you box depth.

So divide the compartment into two at the transistor emitter point with the plate and ground it .

The plate is a temporary solution to isolate your wiring already done. (which is the root cause for instability and current shootup.)

Test whether you have lesser no signal current .
 

BigBoss

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There are many mistakes on your PCB layout.
-You didn't use double sided PCB so Microstrip Lines have no sens at all..
-GND Connections are very poor so decoupling caps and other stuffs have not been well connected..There must be many VIAs around them..
-RF Output/Input are very bad, you should use appropriate connectors that are tightly screwed to chassis
-Screws are very close to RF Lines, they shouldn't be..
 
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geccoo

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Okay! Thank you for pointing it out.

I do have proper connectors which I just haven't mounted to the chassis yet.
What do you mean that the ground connections are poor for the decoupling capacitors?

For the PCB, I will make a new one, using double sided copper board and remove the screw that is close to the RF line, though the one in the corners are they also a problem? Since the chassis is also grounded, it shouldn't matter if the screws touch the ground on the board, right?
 

BigBoss

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Okay! Thank you for pointing it out.

I do have proper connectors which I just haven't mounted to the chassis yet.
What do you mean that the ground connections are poor for the decoupling capacitors?

For the PCB, I will make a new one, using double sided copper board and remove the screw that is close to the RF line, though the one in the corners are they also a problem? Since the chassis is also grounded, it shouldn't matter if the screws touch the ground on the board, right?
-Screws have to touch to the GND if they are mounted on Top GND layer..( preferable )
-If you will re-design the PCB, place numerous Top-to-Bottom VIAs and place them especially close to De-Coupling capacitors.
-Use Connectors.. this is important..
-If you're able to use SMD components, use them..
 

schmitt trigger

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I checked a couple of datasheets from 3 different manufacturers, and they all said that the tab is connected to the emitter, which is the center pin.
Can you attach those datasheet files?
I'm not saying it is not possible, but I've never, ever seen a TO220 transistor whose tab wasn't the collector.
The reason is very simple: heat transfer is optimized that way.

EDIT: I found the datasheet. It says that the tab is indeed the emitter!View attachment datasheet.pdf
WOW! one learns something new every day.
 

geccoo

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-Screws have to touch to the GND if they are mounted on Top GND layer..( preferable )
-If you will re-design the PCB, place numerous Top-to-Bottom VIAs and place them especially close to De-Coupling capacitors.
-Use Connectors.. this is important..
-If you're able to use SMD components, use them..
Okay, I will place an order tomorrow for some new SMD parts and create a new PCB on a double sided board. I really want this thing to work.
Thanks!!
 

Easy peasy

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try running off 6V initially - this xtor does not have a high Vce, class E will ring the collector up to 2 x Vin...
 

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