Wondering about transistor age effects

[Mark Hammer]

I picked up a Yamaha G100 head last weekend at a garage sale.  This is the early single-channel issue of the amp, which would make it 30 years old or so.  All the controls work just fine, but the output power is equivalent to 2W or so, through an efficient 12" speaker, and the built-in clipping circuit only provides a bit of volume increase and no clipping.

I'm wondering if the input stage, that uses a K30A FET and a C1000 bipolar is simply not driving the rest of the circuit hard enough.  I know I improved the performance of my Heathkit TA-16 by replacing the input transistors, but that thing was a good 15 years older than this amp.

Can transistors go off spec over time/use?

[GibsonGM]

I've never heard of any real "degredation" other than in a device that's dying, Mark....anyone?
Good question! 
What if the 'dying transistor' took a LONG time to crap out?  *shrug*

[armdnrdy]

Transistor aging anyone?

https://www.google.com/search?sourceid=navclient&ie=UTF-8&rlz=1T4GGHP_enUS450US450&q=transistor+ageing

I breezed through a few things...but what caught my eye was accompanying resistors aging and losing resistance. 

[CodeMonk]

Well, we all know that Electos can go bad with age.
A GF of mine several years ago (she was an EE), told me something like this:
"A component. primarily a microprocessor, that gets damaged by heat or over-voltage, but still functions just fine.
The damage may be small, but over time it can get worse and kind of prematurely "age" the chip".

I dunno if I'm explaining it right though, so here are a few analogies...

Kind of like if you get a cut and it gets infected and you don't do anything about it, the infection spreads, was sort of the way it was described to me.
Or if a piece of iron gets a little water on it and gets a small rusty spot, that rust can spread.

[PRR]

There are LOTS of reasons a 30 year old amp may be down on power.

1970s+ transistors would be way-way-way down the list.

You know the process. Check for way-wrong voltages. (This finds a LOT of trouble.) Find schematic and data, look for slightly-wrong voltages. Clean connectors. IIRC, this series had the power amp on a 13-pin plug-in, and those contacts go bad.

[R.G.]

Transistors do have an aging mechanism. Eventually the dopants diffuse out and the junctions become not-junctions. This happens on a time scale that's more geological than human, though.

However, you can slowly damage them over time. One way is with penetrating radiation. This randomizes the junctions too. Another way (yeah, yeah, I'm getting there) is with poor circuit design. There is a damage mechanism where especially low noise, high gain bipolars are damaged slightly by every time you reverse break the base-emitter junction. They get noisier, and eventually lose gain. Static sparks into the tip of your guitar cord won't make your NPN-input amplifier stop working. But over time it gets noisier and less well behaved in the circuit sense.

Worse is when the circuit itself has capacitors in unfortunate places that will blip a little dose of charge through the emitter to base every time the circuit is turned off. It happens, and it's not uncommon.

This is one reason old effects sometimes get revived by new input transistors.

But as Paul says, it's more likely to be other stuff like drifted resistors and bum electros in their senescence.

[bool]

A simple protection diode (or the simplest form of a baker clamp) can prevent some of the damage to Ln bjt's that Mr. R.G. described..

[Jdansti]

Maybe the old circuit has one or more solder joints that have deteriorated or maybe the PCB has been damaged.  I also wonder about the pot that controls the clipping.  Also as mentioned, electo caps.  It just seems to me that these types of problems would be more likely than semiconductor aging problems.

[GibsonGM]

A cap job would certainly be a good investment for ANY item from the 70s, 80s...can't tell ya how many times I've gone thru things and done it, and been surprised by just how MUCH goodness comes back....

[R.G.]

A simple protection diode (or the simplest form of a baker clamp) can prevent some of the damage to Ln bjt's that Mr. R.G. described..

Yep, that was the recommedation in the text I read. Well, "Baker clamp" to me means a diode from base to collector. The diode needed here is reverse biased across the base-emitter. It clamps the reverse voltage to less than a diode drop and prevents any possible reverse break.

Of course, it also adds the diode's leakage and reverse capacitance, which may or may not matter at all in the circuit at hand.

[amptramp]

If you restore old radios, it is a standard procedure to recap (replace the capacitors) and check resistors for being excessively out of tolerance.  There are judgement call to be made here - if a plate resistor for a 12AX7 or its triode - double diode equivalent, the 6AV6 is supposed to be 330K and it is actually 200K or 500K, the circuit will still work, but resistor drift could continue, so you may replace resistors even if there is no problem that is apparent.  You can see an example of how I did it here:

http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=130714

Transistors have ageing mechanisms that are different for bipolar, JFET and MOSFET.  MOSFET's can absorb charged particles and the usual transparency of the packing is greater for electrons than protons, so the gate appears to go negative, turning off n-channel devices and turning on p-channel devices.  MOSFET's are not particularly affected by neutron bombardment.  JFET and bipolar devices are not affected by charged particles since they get swept away in operation, but they are affected by neutrons because that affects the junctions and can cause doping anomalies.

MOSFET's also have failure mechanisms related to excessive gate voltage and since the power required to punch through gate oxide is low, people use conductive foam or shorted leads to protect the devices before they are used.  The failure may not be immediately obvious, but minor oxide damage can accumulate into major oxide damage if precautions (like diodes to the rails and series gate resistors)are not taken.

Junction devices have a problem with heat - excessive heat may cause solid diffusion to accelerate.  This affects abrupt junction devices like varicap diodes (used to tune frequencies in radios that don't have variable capacitors or inductors) first, but these are normally used in feedback loops in frequency synthesizers, so the effect may be a change in bias voltage for a given frequency but not a difference in circuit operation.  Germanium transistors have a problem with leakage that increases with temperature.  Abrupt junctions like varicaps and zeners appear to survive radiation damage better because of the reduced probability of a radiation-induced defect landing in the junction.

[bool]

@Mr. R.G.: this exactly, yes.

I'd add my own concern regarding reverse bias diode junction noise, but that may be irrelevant. Also of concern may be reverse junction "varicap" modulation (and miller-esque consequences) in "baker" diode (a small schottky usually).

These observations are targeted towards common-emitter circuit.