DIYbanter - View Single Post - How does the gain of a transistor go down ... ?

N_Cook

On 16/04/2014 01:46, wrote:
On Saturday, April 12, 2014 10:44:41 AM UTC-4, wrote:
On Wednesday, April 2, 2014 3:41:42 AM UTC-7, Arfa Daily wrote:

Hartke AH500 on the bench this morning. It struggled to come out of protect

from cold, but if it did make it, then it worked ok, and mostly stayed on

producing clean power of the right order, although the owner had complained

that it had cut out during a gig on more than one occasion.

As the thing powered up there was a small(ish) DC offset of a little under

negative 2 volts at the output stage midpoint, and it was this that the

protect circuit was rightly objecting to. With + / - 90 volts across the

output stage, it doesn't take too much imbalance to produce a 2 volt offset

...

As it warmed up, the amount of offset varied a bit, so I went in with a can

of freezer to see if I could see anything that was particularly sensitive.

That led me back right to the front end where one transistor in a

long-tailed pair had a significant effect when sprayed, the offset rising to

around negative 4 volts. So I pulled both out and checked them on my cheapo

component tester. Both correctly registered as NPN transistors, and the BCE

pinning agreed. Both had 645 mV B-E voltage, but one had a beta of 215, and

the other, just 35.

A new pair of matched transistors had the offset down to a few mV, and the

protect circuit was happy with that. But it got me to wondering what could

be the failure mechanism that resulted in a transistor still being a

recognisable transistor in that the tester still saw it as one, and it still

basically worked in the amp, but had a very low gain ?

Arfa

You need to look at those transistors on a curve tracer. I bet one of them is leaky - acts similar to having a resistor paralleled from E-C.

G²

It's too bad that you don't still have them. It might have been a interesting "forensic" experiment. I still have two bulged electrolytic caps that checked good on ESR but when replaced fixed the set. I'm holding to them until I can get to a friend's place who has a Sencore analyer. I'm interested to see exactly what the analyzer says the failure was. Lenny

Its always been an operational policy of mine to put pulled parts in the
box of screws/subparts of a repair item. When reassembled those
failed/suspect/good parts placed in another small box, unless so bad it
goes in my black museum. Then when that box is full then safe enough to
dispose of the lower accumulation. Also taking notes on a pad , along
the way, then sometime when I get time, typing them up to wwwland.