Guy that I know brought me a Polytone Mini Brute to look at earlier this
week. He reckoned that it hummed, but otherwise worked ok. I slung it up on
the bench today, and he was right. It had a noticeable raspy sort of
background noise that was controlled by the master volume. Otherwise, it
worked ok. I also discovered that the level of the noise could be altered a
little by flexing the preamp board. When I got it out to look at, it had
previously had some pretty poor work done, so I first tidied that up. You
could tap the board pretty much anywhere, and the level of the noise
altered, but never went away. I did a blanket solder up of the most
sensitive area, and the noise was then at a fixed level. Not sensitive to
tapping any more.

The preamp has its own simple dual rail power supply with a bridge, a couple
of filter caps, and a pair of 78 / 79 15 volt regs. Scoping the positive
rail, there was a fair bit of ripple on it - certainly more than on the
negative rail. This was still there checked at an op amp, which I thought
was odd, as those 3 terminal regs are usually pretty good at ripple
rejection. I then checked the actual voltages at the IC.
-15 volts at pin 4 ...

.... and +28 volts at pin 8 !!!!

So the poor little TL072 ICs on there had 43 volts across them ! A quick
check back at the 7815 showed 28 volts in and 28 volts out. A new regulator
restored the voltage to +15 volts, and the whole amp became almost totally
quiet.

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins - and it was a measurable dead short. I was also amazed that the poor
old '072s had survived having all those volts across them, absolute max
rails being quoted at + / - 18 volts ...

Arfa

Arfa Daily wrote:
Guy that I know brought me a Polytone Mini Brute to look at earlier this
week. He reckoned that it hummed, but otherwise worked ok. I slung it up on
the bench today, and he was right. It had a noticeable raspy sort of
background noise that was controlled by the master volume. Otherwise, it
worked ok. I also discovered that the level of the noise could be altered a
little by flexing the preamp board. When I got it out to look at, it had
previously had some pretty poor work done, so I first tidied that up. You
could tap the board pretty much anywhere, and the level of the noise
altered, but never went away. I did a blanket solder up of the most
sensitive area, and the noise was then at a fixed level. Not sensitive to
tapping any more.

The preamp has its own simple dual rail power supply with a bridge, a couple
of filter caps, and a pair of 78 / 79 15 volt regs. Scoping the positive
rail, there was a fair bit of ripple on it - certainly more than on the
negative rail. This was still there checked at an op amp, which I thought
was odd, as those 3 terminal regs are usually pretty good at ripple
rejection. I then checked the actual voltages at the IC.
-15 volts at pin 4 ...

... and +28 volts at pin 8 !!!!

So the poor little TL072 ICs on there had 43 volts across them ! A quick
check back at the 7815 showed 28 volts in and 28 volts out. A new regulator
restored the voltage to +15 volts, and the whole amp became almost totally
quiet.

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins - and it was a measurable dead short. I was also amazed that the poor
old '072s had survived having all those volts across them, absolute max
rails being quoted at + / - 18 volts ...

Arfa

I've not seen a 78/79xx go dead short either, but it happens often enough
on other series pass power supplies.

It all circles back to "check the power supply". The bad -5 volt rail in
my bench supply where I assumed the completely missing zener was just an
option left out at the factory was pretty quickly solved when I did
finally check all voltages in the service guide.

Not checking that first was almost inexcusable, the power supply had two
indentical, isolated outputs 1 and 2 and the the voltages at the connector
going to the control board behind the bezel were clearly marked with what
voltage should be present.

Hats off to the designers at TTI in the UK for doing their part right.

"Arfa Daily" wrote in message ...

Guy that I know brought me a Polytone Mini Brute to look at earlier this
week. He reckoned that it hummed, but otherwise worked ok. I slung it up on
the bench today, and he was right. It had a noticeable raspy sort of
background noise that was controlled by the master volume. Otherwise, it
worked ok. I also discovered that the level of the noise could be altered a
little by flexing the preamp board. When I got it out to look at, it had
previously had some pretty poor work done, so I first tidied that up. You
could tap the board pretty much anywhere, and the level of the noise
altered, but never went away. I did a blanket solder up of the most
sensitive area, and the noise was then at a fixed level. Not sensitive to
tapping any more.

The preamp has its own simple dual rail power supply with a bridge, a couple
of filter caps, and a pair of 78 / 79 15 volt regs. Scoping the positive
rail, there was a fair bit of ripple on it - certainly more than on the
negative rail. This was still there checked at an op amp, which I thought
was odd, as those 3 terminal regs are usually pretty good at ripple
rejection. I then checked the actual voltages at the IC.
-15 volts at pin 4 ...

.... and +28 volts at pin 8 !!!!

So the poor little TL072 ICs on there had 43 volts across them ! A quick
check back at the 7815 showed 28 volts in and 28 volts out. A new regulator
restored the voltage to +15 volts, and the whole amp became almost totally
quiet.

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins - and it was a measurable dead short. I was also amazed that the poor
old '072s had survived having all those volts across them, absolute max
rails being quoted at + / - 18 volts ...

Arfa




Very rare, but a while ago I had a Linndrum where the very very ancient 5v
regulator was merrily supplying its input 12v or so to all the 5v CPU, ROM
and logic chips.
http://www.vintagesynth.com/linn/linn2.php


Poor Linndrum.
Still, it was in lovely condition.


Gareth.

Arfa Daily wrote:

Guy that I know brought me a Polytone Mini Brute to look at earlier this
week. He reckoned that it hummed, but otherwise worked ok. I slung it up
on the bench today, and he was right. It had a noticeable raspy sort of
background noise that was controlled by the master volume. Otherwise, it
worked ok. I also discovered that the level of the noise could be
altered a little by flexing the preamp board. When I got it out to look
at, it had previously had some pretty poor work done, so I first tidied
that up. You could tap the board pretty much anywhere, and the level of
the noise altered, but never went away. I did a blanket solder up of the
most sensitive area, and the noise was then at a fixed level. Not
sensitive to tapping any more.

The preamp has its own simple dual rail power supply with a bridge, a
couple of filter caps, and a pair of 78 / 79 15 volt regs. Scoping the
positive rail, there was a fair bit of ripple on it - certainly more
than on the negative rail. This was still there checked at an op amp,
which I thought was odd, as those 3 terminal regs are usually pretty
good at ripple rejection. I then checked the actual voltages at the IC.
-15 volts at pin 4 ...

... and +28 volts at pin 8 !!!!

So the poor little TL072 ICs on there had 43 volts across them ! A quick
check back at the 7815 showed 28 volts in and 28 volts out. A new
regulator restored the voltage to +15 volts, and the whole amp became
almost totally quiet.

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins - and it was a measurable dead short. I was also amazed that the
poor old '072s had survived having all those volts across them, absolute
max rails being quoted at + / - 18 volts ...

Arfa
Sure they do that, if for some reason the input voltage drops to fast
and still has output voltage on the reg, it can back feed and short at
times. It depends on what is happening on the input side, like maybe a
short circuit yanked it down real quick.

Normally a back feed diode is installed for those regs to stop that.

Of course, you can only verify this if pin 1 and pin 3 are ohmed
shorted. It could also be the common leg becoming detached inside. I've
seen that happen do to poor mechanical mounting practices and near by
vibration.

Jamie

"Cydrome Leader" wrote in message
...
Arfa Daily wrote:
Guy that I know brought me a Polytone Mini Brute to look at earlier this
week. He reckoned that it hummed, but otherwise worked ok. I slung it up
on
the bench today, and he was right. It had a noticeable raspy sort of
background noise that was controlled by the master volume. Otherwise, it
worked ok. I also discovered that the level of the noise could be altered
a
little by flexing the preamp board. When I got it out to look at, it had
previously had some pretty poor work done, so I first tidied that up. You
could tap the board pretty much anywhere, and the level of the noise
altered, but never went away. I did a blanket solder up of the most
sensitive area, and the noise was then at a fixed level. Not sensitive to
tapping any more.

The preamp has its own simple dual rail power supply with a bridge, a
couple
of filter caps, and a pair of 78 / 79 15 volt regs. Scoping the positive
rail, there was a fair bit of ripple on it - certainly more than on the
negative rail. This was still there checked at an op amp, which I thought
was odd, as those 3 terminal regs are usually pretty good at ripple
rejection. I then checked the actual voltages at the IC.
-15 volts at pin 4 ...

... and +28 volts at pin 8 !!!!

So the poor little TL072 ICs on there had 43 volts across them ! A quick
check back at the 7815 showed 28 volts in and 28 volts out. A new
regulator
restored the voltage to +15 volts, and the whole amp became almost
totally
quiet.

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins - and it was a measurable dead short. I was also amazed that the
poor
old '072s had survived having all those volts across them, absolute max
rails being quoted at + / - 18 volts ...

Arfa

I've not seen a 78/79xx go dead short either, but it happens often enough
on other series pass power supplies.

It all circles back to "check the power supply". The bad -5 volt rail in
my bench supply where I assumed the completely missing zener was just an
option left out at the factory was pretty quickly solved when I did
finally check all voltages in the service guide.

Not checking that first was almost inexcusable, the power supply had two
indentical, isolated outputs 1 and 2 and the the voltages at the connector
going to the control board behind the bezel were clearly marked with what
voltage should be present.

Hats off to the designers at TTI in the UK for doing their part right.


Agreed, checking the supply rails is fundamental, and I guess that I should
have, except there was no particular reason to on this one, as it appeared
to all be working except for this slightly 'raspy' buzz, that wasn't
actually loud enough to be causing a problem, but was annoying to the owner,
as it shouldn't have been there.

Arfa

"Jamie" t wrote in message
...
Arfa Daily wrote:

Guy that I know brought me a Polytone Mini Brute to look at earlier this
week. He reckoned that it hummed, but otherwise worked ok. I slung it up
on the bench today, and he was right. It had a noticeable raspy sort of
background noise that was controlled by the master volume. Otherwise, it
worked ok. I also discovered that the level of the noise could be altered
a little by flexing the preamp board. When I got it out to look at, it
had previously had some pretty poor work done, so I first tidied that up.
You could tap the board pretty much anywhere, and the level of the noise
altered, but never went away. I did a blanket solder up of the most
sensitive area, and the noise was then at a fixed level. Not sensitive to
tapping any more.

The preamp has its own simple dual rail power supply with a bridge, a
couple of filter caps, and a pair of 78 / 79 15 volt regs. Scoping the
positive rail, there was a fair bit of ripple on it - certainly more than
on the negative rail. This was still there checked at an op amp, which I
thought was odd, as those 3 terminal regs are usually pretty good at
ripple rejection. I then checked the actual voltages at the IC.
-15 volts at pin 4 ...

... and +28 volts at pin 8 !!!!

So the poor little TL072 ICs on there had 43 volts across them ! A quick
check back at the 7815 showed 28 volts in and 28 volts out. A new
regulator restored the voltage to +15 volts, and the whole amp became
almost totally quiet.

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins - and it was a measurable dead short. I was also amazed that the
poor old '072s had survived having all those volts across them, absolute
max rails being quoted at + / - 18 volts ...

Arfa
Sure they do that, if for some reason the input voltage drops to fast and
still has output voltage on the reg, it can back feed and short at times.
It depends on what is happening on the input side, like maybe a
short circuit yanked it down real quick.

Normally a back feed diode is installed for those regs to stop that.

Yes indeed - I have seen such diodes fitted, but not very often. I have to
say though, that I can't ever remember having one short like this, although
I have changed many many of them over the years for no output or low output
or intermittent output and so on.

Of course, you can only verify this if pin 1 and pin 3 are ohmed shorted.
It could also be the common leg becoming detached inside. I've
seen that happen do to poor mechanical mounting practices and near by
vibration.


Looking at the internal circuit, it has to be the series pass transistor
that has failed as you can measure a short straight between pins 1 and 3,
and the only thing in the way other than the transistor, is the 0.33 ohm
sense resistor.

Arfa
Jamie

"Arfa Daily = ****WIT "

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins


** Bragging about what you have not seen proves only that you are blind,
stupid and not a real tech.

Shorted 78xx series regs are not uncommon faults.




.... Phil

"Phil Allison" wrote in message
...

"Arfa Daily = ****WIT "

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins


** Bragging about what you have not seen proves only that you are blind,
stupid and not a real tech.

Shorted 78xx series regs are not uncommon faults.




... Phil


Maybe down in that steamy ******** where you live, Philip, but not here.
Bragging about the everything that you know all the time, just reinforces
what we all know. That you are a gobby total **** who should never be let
loose with a screwdriver, let alone a soldering iron. Now **** off back to
your kangaroo shagging.

Arfa

"Arfa Daily"
"Phil Allison"
"Arfa Daily = ****WIT "

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins


** Bragging about what you have not seen proves only that you are blind,
stupid and not a real tech.

Shorted 78xx series regs are not uncommon faults.


Maybe down in that steamy ******** where you live, Philip, but not here.
Bragging about the everything that you know all the time, just reinforces
what we all know. That you are a gobby total **** who should never be let
loose with a screwdriver, let alone a soldering iron. Now **** off back to
your kangaroo shagging.


** Even a lunatic asylum would turf this stinking pile of pommy garbage out.

AD would last about 10 minutes in Australia.

Before some one decked him.



.... Phil

Ummmm, don't these things almost always have a Zener on the output ?

That would have had to gone open. Usually I see them dead shorted if they get overloaded. That's because the pass transistor is shorted which made the Zener short, which means it is shorted all the way around.

Of course on any given Thursday run there could be some units that have an open Zener. And you'll never know it unless the rest of it fails.

Just love this business.

wrote:
Ummmm, don't these things almost always have a Zener on the output ?

That would have had to gone open. Usually I see them dead shorted if they get overloaded. That's because the pass transistor is shorted which made the Zener short, which means it is shorted all the way around.

Of course on any given Thursday run there could be some units that have an open Zener. And you'll never know it unless the rest of it fails.

Just love this business.

I've never seen integral over voltage protection on the 78/7900 devices,
but they all behave differently since they're so generic any that you can
get known good ones as well as sketchy ones.

Fairchild even claims this on their datasheet (from
http://www.fairchildsemi.com/ds/LM/LM7805.pdf)


The LM78XX series of three terminal positive regulators
are available in the TO-220 package and with several
fixed output voltages, making them useful in a wide
range of applications. Each type employs internal current
limiting, thermal shut down and safe operating area pro-
tection, making it essentially indestructible.

I doubt "essentially indestructible" applies to the no-name ones with
poorly molded cases and crappy printing. If you can barely package a
semiconductor, there's no way you care about the dies going inside them.

In really plain open style frame linear power supplies, I've seen the 723
regulators blow up far more often than the aux wimpy outputs with the
single to220 regulators. If only switching power supplies all had marked
parts and were fixable.

Agreed, checking the supply rails is fundamental, and I guess
I should have, except there was no particular reason to on this
one, as it appeared to all be working except for this slightly
'raspy' buzz, that wasn't actually loud enough to be causing
a problem, but was annoying to the owner, as it shouldn't have
been there.

Did you say whether you found the source/cause of the buzz?

"William Sommerwerck" wrote in message
...
Agreed, checking the supply rails is fundamental, and I guess
I should have, except there was no particular reason to on this
one, as it appeared to all be working except for this slightly
'raspy' buzz, that wasn't actually loud enough to be causing
a problem, but was annoying to the owner, as it shouldn't have
been there.

Did you say whether you found the source/cause of the buzz?


Yes, it was the 7815 that was short input to output, and shoving 28 volts up
that rail to the preamp opamps ...

Arfa

"Phil Allison" wrote in message
...

"Arfa Daily"
"Phil Allison"
"Arfa Daily = ****WIT "

I don't think that in over 40 years of being in this game, I can ever
remember a 78xx series device going short between its input and output
pins


** Bragging about what you have not seen proves only that you are blind,
stupid and not a real tech.

Shorted 78xx series regs are not uncommon faults.


Maybe down in that steamy ******** where you live, Philip, but not here.
Bragging about the everything that you know all the time, just reinforces
what we all know. That you are a gobby total **** who should never be let
loose with a screwdriver, let alone a soldering iron. Now **** off back
to your kangaroo shagging.


** Even a lunatic asylum would turf this stinking pile of pommy garbage
out.

AD would last about 10 minutes in Australia.

Before some one decked him.



... Phil



Strange that you seem to have lasted so long then ... :-)

Arfa

Did you say whether you found the source/cause of the buzz?

Yes, it was the 7815 that was shorted input to output, shoving
28 volts up that rail to the preamp opamps...

That was clear. I should have been more specific -- what was the mechanism?

"William Sommerwerck" wrote in message
...
Did you say whether you found the source/cause of the buzz?

Yes, it was the 7815 that was shorted input to output, shoving
28 volts up that rail to the preamp opamps...

That was clear. I should have been more specific -- what was the
mechanism?


Oh, I see. Sorry. You could measure a proper short between the input and
output pins of the reg, so I guess the mechanism there was a short series
pass transistor. That obviously led to the full input voltage appearing at
the output, and hence the opamps, so these had -15 volts at one leg, and +28
volts at the other. This, in turn, caused the opamps to draw excess current
from their positive rail. As the rails were only expected to be light
current suppliers, the designer only put small caps - 470uF - on them for
smoothing. When the opamps were supplied correctly with + / - 15 volts, this
was adequate, but with the big imbalance that the faulty regulator was
causing, the excess positive current was enough to make the rail about twice
as 'scruffy' as the negative rail. The fact that there was now effectively
also no regulator in the line to add its inherent ripple rejection, meant
that the opamps now had one correct and clean rail, and one excessive and
dirty rail. This was reflected in them introducing enough of that dirty rail
onto their outputs to be audible. Remarkably, it was actually barely visible
on the 'scope.

Arfa