Could transistor still be damaged if it passes diode test?

We have an amplifier here and we've had no luck in getting an amplified output.

The manual says to place a dummy 100 ohm resistor in place of the transistors to adjust the bias voltage and we did that and we set it to 0.7V. We removed that dummy resistor and soldered the MRF454 NPN transistors in place. The biasing is correct but the transistors dont seem to come on because they're very cold and there are some parts of the circuit which are very hot like some of the resistors. We thought it may be the voltage regulator but it's providing the necessary 0.7V bias to the base when we tested it with the dummy resistor.

We removed the transistors and did a diode test and it was showing 0.7V drop across BE and BC so it passed the diode tests. Could the transistors still be damaged even if they pass the diode test? We don't know what's wrong because the biasing is working correctly and we get no output. The amplifier is the AN762 by Motorola. The transistors are the MRF454 NPN.

Did you check the resistance between CE?

Post your schematic.

wmodavis said:

Did you check the resistance between CE?

Click to expand...

No I did not. But I did do a diode test and it showed nothing.

As for schematic, please see the attached file:

A lot of meters have sockets for a Hfe check.
I have found transistors with a Hfe of 1 which is clearly wrong.

Would an hfe check be the most accurate check to see if a transistor is good or bad? What is a good range of hfe value for an RF transistor (MRF454) ?

Generally small transistors will read a hfe of 100-300 or so on a multimeter hfe test.

It's unlikely your transistors are dead and still measuring ok on a diode test B-E. That can happen in high voltage apps after enough time but is very unusual for short term faults in low voltage apps. Your transistors are probably ok.

The most likely problem is you have miswired the circuit or have the transistors in the wrong orientation, or if this is an RF design it might be failing due to poor layout and not following good RF layout techniques. A PHOTO would be of great help!

zero_coke said:

No I did not. But I did do a diode test and it showed nothing.

As for schematic, please see the attached file:

Click to expand...

This application note describes how to setup the bias for the transistors. It will depend upon whether you want to run the amp in Class A, Class AB, etc. Because there is no emitter degeneration, the adjustment of the bias voltage is critical, so it must be done carefully.

Hi guys:

The photo of the amp we built is attached.

@w2aew: We did follow the application note and there were two things we had to do. One was to set the bias voltage by placing a dummy resistor from base to collector and adjusting the trimpot till we get 0.7V and this passed successfully. Then we had to place the transistors on the board and connect the +Vcc to a multimeter as an ammeter and measure the bias current to be 100mA or more. We couldnt do this...the meter was showing 0mA. I don't know why it was doing that.

I know my partner always plays with the trimpot when the amplifier is connected to the Vcc and has an RF input as well. You shouldn't be playing with the trimpot like that or it will change the bias right?

I don't see L3 and L4 populated on your board.

w2aew: We didn't install those because we thought they were optional probes for current measurement...do we need them in there? Jeez if this is the problem then I'll shoot myself...not really but 1 week of troubleshooting would've been gone to waste!

Yes, they are absolutely needed. That's where the collectors get their bias. Be sure to use the correct beads as shown in the parts list. The only optional part is constructing them with a longer wire to be able to use a current probe.

Wow ok. Thanks w2aew! This is most probably why it hasn't been working! Jeez....thanks again!

zero_coke said:

Wow ok. Thanks w2aew! This is most probably why it hasn't been working! Jeez....thanks again!

Click to expand...

It definitely won't work without them. Be sure to carefully set the bias after you install them. Setting the bias too high could damage the transistors.

The manual says to set the collector current bias to >=100mA or >=200mA total for both transistors. We will set to 150mA each I guess. Thanks once again w2aew! And also a warm thanks to The_RB for the photo request!

@w2aew: Uh oh, we've run into a completely different problem now. And this problem, thanks to a mistake we made otherwise we wouldn't have ever figured it out, is with the transistors being mounted on the heatsink.

According to the datasheet, the base metallic part of the transistor which is mounted onto the heatsink is not associated with any of the base/emitter/collector pins. However, when we screw it down, it shorts out the base pin to 0V. We put some isolation between the heatsink and the transistor by using 3M thermally conductive rubber pads instead of the heatsink paste and this prevents the base package from touching the heatsink but now the screws are the problems. The screws touch the heatsink and this is unavoidable unless we use nylon/teflon screws which we don't have.

First of all, why in the world are we running into this problem? I've never heard of such thing...the metallic base of the transistor is not connected to any of the pins (we also tested using multimeter continuity check). I don't understand why it short circuits the base pin to 0V when it touches our heatsink (which is grounded, of course).

Anyone have any suggestions? Could it be a manufacture defect of this transistor because seriously I have not found anything else wrong with the circuit. I replaced the old transistors with new ones ($40 a pop!) and still the same problem

We wrapped the screws with electric tape and screwed them in and then we also placed that isolative thermal conductive rubber instead of the heatpaste but now our transistors starts smoking when we reach an output of 20V RMS into a 50 Ohm load (~8-10 Watts). I'm speculating that the rubber pads are not providing enough thermal conductivity which is why we see smoke at low input powers even.

zero_coke said:

We put some isolation between the heatsink and the transistor by using 3M thermally conductive rubber pads instead of the heatsink paste and this prevents the base package from touching the heatsink but now the screws are the problems. The screws touch the heatsink and this is unavoidable unless we use nylon/teflon screws which we don't have.

Click to expand...

They make bushings to prevent a screw from touching the transistor or the inside of the ring it passes through. This is the wrong transistor type but just for example:
http://www.ebay.com/itm/50-Units-He...hing-And-Film-TO-220-/150471475680#vi-content

I'll be curious if someone has an answer on this because I've had similar issues in the past where I was not able to tighten the screws very much or the transistor would make contact with the heat sink.