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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
On 4/2/2014 7:36 PM, Arfa Daily wrote:
"David Platt" <dplatt@coop.radagast.org> wrote in message
news:joeu0b-i81.ln1@coop.radagast.org...
In article <MPG.2da66547ae8296b9989856@news.eternal-september.org>,
Maynard A. Philbrook Jr. <jamie_ka1lpa@charter.net> wrote:
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
zenering the emitter will over time, kill Beta.
It might be worth considering the schematic, and seeing what the amp's
power-up and power-down behavior is like. Depending on how the rails
bounce around, and on what's plugged into the input, the input side of
the differential pair might end up being zenered in this way.
Adding some B-E clamp diodes might not be a bad idea.
OK. All good stuff. There appears to be neither clamp diodes nor current
limiter Rs in the bases. That said, it's quite an old design, and this
amp is quite an old example that's been working ok to date, and is
working ok again now, so I'm reluctant to start modifying it by adding R
into the circuit, but reverse diodes across the B-E junctions might be
worth doing.
Cheers all for the insights and suggestions.
Arfa
I don't know the makeup of your transistor (or any), but I ran across
this picture showing electromigration, a following post says " So the
symptom was gain loss due to loss of emitter area? That's subtle; I was
looking for a short. I don't know what I was thinking. Great pictures, by
the way."
Page down about 4/5 to see the gold and orange picture with arrows
pointing to the migration.
http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=196236
btw: this guy is doing some amazing reconstruction of unobtainium HP
parts.
Here's one thread, http://www.antiqueradios.com/forums/vie ... p?t=190955
Mikek
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.
G˛
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.
G˛
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
On Saturday, April 12, 2014 10:44:41 AM UTC-4, stra...@yahoo.com 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.
G˛
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
G˛
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.