Is measuring HFE a valid test to verify transistors?

[rob_gould]

Hi,

I have a circuit which emits a high pitched screaming noise when engaged.  I've verified component placement and solder joints etc so now I'm looking for a faulty part.

I've pulled out 6 transistors and measured them, and the HFE reading on my cheap digital multimeter indicates that they are 'good' because they all give an HFE reading in broadly the same range.  Is this enough to confirm that the transistor is working 100% as it should?

Thanks.

 

[JohnRoberts]

Probably good unless the fault is something very subtle. I would look elsewhere for problem.

JR

 

[PRR]

If the transistor was dead, it would not "scream".

Scream is more likely layout, or that wiring error you have not found yet. In very marginal cases it *might* be a transistor that is "too good"; however I would bark every other tree first.

 

[ricardo]

Was the device/circuit working OK before?

What is it?

 

[emrr]

In very marginal cases it *might* be a transistor that is "too good"; however I would bark every other tree first.

There are definitely plenty of old circuits designed around transistors of lower hfe, and if you are cloning one it is possible you can't get weak enough transistors for the job today.  We need to know more, as Ricardo said. 

 

[Arkestra]

I had a Neve module doing this recently. Was a grounding/feedback issue. I usually check Q's for leakage but usually not needed unless its a germanium.

 

[rob_gould]

Hi All,

Thanks for the replies.  This is the project :

groupdiy.com/index.php?topic=54964.0

It is a new build, so it has never worked properly. 

If the transistor was dead, it would not "scream".

Scream is more likely layout, or that wiring error you have not found yet. In very marginal cases it *might* be a transistor that is "too good"; however I would bark every other tree first.

Yes, the general consensus is that the problems are related to a solder bridge or bad joint.  Because of how the boards have been manufactured, creating a bridge is very easy.

Good point that if a component was faulty, it probably wouldn't do anything at all.

My next step is to probe around the circuit trying to find out where the noise is introduced.

 

[ricardo]

This is the project :

groupdiy.com/index.php?topic=54964.0

How about posting the EXACT circuit you think you have built?  Did you use PCBs supplied by irfrench or is it your own layout?

That thread has mods & other stuff.  Also a bypass switch.

Is the problem still there when bypassed?

 

[rob_gould]

This is the project :

groupdiy.com/index.php?topic=54964.0

How about posting the EXACT circuit you think you have built?  Did you use PCBs supplied by irfrench or is it your own layout?

That thread has mods & other stuff.  Also a bypass switch.

Is the problem still there when bypassed?

Hi,

Thanks for your input.  I typed that in a rush yesterday and the post was a little lacking in info admittedly.

Here's the schematic : https://drive.google.com/file/d/0Bx2bMNFdYu3YMGZNQkhCQXozVVk/edit?usp=sharing

It works fine in bypass so I know that the INA and DRV chips are OK.

I built the board from the PCBs organised by Ian (project creator).  They've been built by taking a blank piece of PCB and milling it to make the tracks.  The PCB takes solder very easily and it seems that a few people have had problems with solder bridges etc. 

The fault is that when the circuit is engaged, a high pitched squeal is heard when turning the mix knob clockwise.  So at 100% wet, I hear my signal going through the unit wit squeal over the top.  What's interesting is that compression seems to be working - I can hear the effect of the compression on the signal and also the compression seems to duck the squeal too. 

So far I have :

- verified all component placements three or more times

- checked over and over again for solder bridges / dry joints with a jeweller's loupe

- resoldered lots of the components anyway

- replaced the TL074 twice.

- replaced the J112 FET with a 2N5457

- removed  all 8 transistors from the circuit, checked their HFE to make sure they're not dead, then put them back in.

Now I will probe round the circuit trying to find the place where the noise is introduced. 

I have

 

[ricardo]

Here's the schematic : https://drive.google.com/file/d/0Bx2bMNFdYu3YMGZNQkhCQXozVVk/edit?usp=sharing

This is a very obfuscating schematic.

To start with, the power arrangements are suspect/unclear.  The thread implies the original was powered with a single 9V rail.  This one appears to have +/- rails.

It's not clear what's connected to what.
___________________________________________

Contact someone who has built a working example and get DC voltages at VR, ??(T), VA, VB, the power pins & outputs on all the ICs, Collector & Emitters of all the transistors.  AND MARK THEM ON THE SCHEMATIC.

This will certainly help anyone else working on the circuit even if you don't benefit.
_________________________________

Double check the orientation of your transistors.  Check the datasheet for these again.

Do you know how to use a multimeter to find Base, Emitter & Collector on a BJT?
_________________________________

What you  are hearing is something oscillating.

- It could be caused by the poor decoupling of the zillion power rails and/or poor earthing arrangements.

- C4 & C7 show the designer had serious problems with instability with the original and its layout.  You may have to increase these for your layout.

 

[rob_gould]

hi Ricardo,

Resurrecting an old thread here cos I've only just had a chance to dig back into this after having a new baby.

I built a second unit which works fine.  I took voltages at all the points you specified, and they match to within hundredths of a volt between the two units.

The  oscillation seems to change pitch when I wiggle C8.  I've replaced C8 and resoldered all the components around C8 but there's no change.

VA  is 8.98V
VB is 7.45V

The voltages going in and out of the board at + / - 16V pins are all correct.

I need to check the TL074 output voltage, but I've replaced that already so I don't think it is faulty.

What kind of component is most likely to generate oscillation?

I'll find the time to print off a copy of the schematic and mark on the voltages later tonight hopefully...

Thanks!