sci.electronics.repair@googlegroups.com

Today's topics:

* Thoughts on this little oddity, anyone ...? - 6 messages, 4 authors
http://groups.google.com/group/sci.electronics.repair/t/9f831e8e8fd1b96e?hl=en
* Tektronix 2215A worth repair? - 3 messages, 3 authors
http://groups.google.com/group/sci.electronics.repair/t/fc4cb167c05d84fd?hl=en
* Mixing 4 audio channels to 3? - 1 messages, 1 author
http://groups.google.com/group/sci.electronics.repair/t/a9c921002b2ed750?hl=en

==============================================================================
TOPIC: Thoughts on this little oddity, anyone ...?
http://groups.google.com/group/sci.electronics.repair/t/9f831e8e8fd1b96e?hl=en
==============================================================================

== 1 of 6 ==
Date: Tues, Nov 8 2011 6:18 pm
From: "Arfa Daily"

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:9htrqeFo4sU1@mid.individual.net...
>
> "Arfa Daily"
>
>>
>> I fired it up again, and put a sine wave in, then hooked up two channels
>> of my 'scope, one to the "A" output and one to the "B", both with the
>> same polarity, and was surprised to find that the two signals were
>> completely anti-phase,
>
>
> ** When servicing audio, knowing the polarity of the test signal and if
> there is significant phase shift is important. A scope triggering off the
> incoming wave will it not reveal this in single channel mode - so AF used
> both channels.
>
> A much better way is to use the External Synch input on your scope and
> link it to a fixed output on your bench audio oscillator ( create on if
> you have to). This way, you see instantly if the signal's phase has
> reversed or has a large phase shift.
>
> Also, triggering will be rock steady with nearly any kind of distorted,
> noisy or contaminated signal on the scope screen.
>
> Try it out for a day or two - you will never go back.
>
>
> ... Phil
>
>

Interesting. Are you squaring up the generator's sine output, to give a good
sharp transition for syncing the scope ? I was looking at the phase
reversals on this particular unit, by using 2 of the channels on the scope,
and using internal auto trigger derived from CH 1. The sync circuitry on
this Hitachi scope is very good, and will produce a perfectly stable trigger
point from virtually any waveform, no matter how complex or scruffy, but I
can see where it would be useful to be able to have a fixed known trigger
point, and from there, be able to look at relative phasing between amps, and
phase shift within an amp, using a single scope channel.

Arfa

== 2 of 6 ==
Date: Tues, Nov 8 2011 6:48 pm
From: "Mark Zacharias"

"N_Cook" <diverse@tcp.co.uk> wrote in message
news:j9b9kd$su9$1@dont-email.me...
> Mark Zacharias <mark_zacharias@labolgcbs.net> wrote in message
> news:4eb918cc$0$28491$a8266bb1@newsreader.readnews.com...
>> "Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
>> news:Xc0uq.25576$gy7.110@newsfe28.ams2...
>> > Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has
> two
>> > completely separate and identical power amps, one of which was running
>> > very hot very quickly. No schematics, but problem was quickly traced to
> an
>> > open circuit resistor, by comparison ohms checks between the two amps.
> In
>> > order to replace the resistor, it was necessary to remove the offending
>> > ("B") power amp to get to the board underside. The power rails and
> ribbon
>> > from the mixer desk section were on removable connectors, but the 4
>> > pole
>> > speakon connector was bolted to the rear panel and the wires were
> soldered
>> > directly to the board. When I came to unsolder these wires from the
>> > socket, I noticed that the "B" channel one was wired the opposite way
>> > round to the "A" channel one. I noted this down to check on later.
>> >
>> > When the amp was tested ok, I bolted it back in, and reconnected the
>> > speakon socket the same as it was when I unsoldered it. The wires lay
>> > exactly where they originally were, and looked absolutely
>> > factory-original, even down to the one which went to the uppermost
>> > connector tag, having a rubber sleeve on it, exactly the same as on the
>> > "A" channel. Except the two channels were definitely wired
> arse-about-face
>> > to one another. On both channels' speakons, the "1-" and "2-" pins were
>> > linked, as were "1+" and "2+". But on channel "A" the ground wire was
>> > connected to the "+" pair, and on the "B" channel, the ground was on
>> > the
>> > "-" pair.
>> >
>> > I fired it up again, and put a sine wave in, then hooked up two
>> > channels
>> > of my 'scope, one to the "A" output and one to the "B", both with the
> same
>> > polarity, and was surprised to find that the two signals were
>> > completely
>> > anti-phase, which meant that with the reversed connections to the
> speakon
>> > sockets, they would be back in phase again to the outside world.
>> >
>> > I then went back to the power amps' front ends near where the ribbon
>> > connector was, and found the same thing. At the same point on each amp,
>> > the signals were in opposite phase to one another. I then spotted three
>> > little 3 pin header blocks, with one pin pair on each designated "B",
> and
>> > the other pair "A" and a little 1 or maybe an I next to the "A" in a
> sort
>> > of 'to the power of' position. On the B amp, these three jumpers were
> set
>> > to "B", and on the A amp they were set to "A", so clearly, these
>> > jumpers
>> > reverse the phase of the signal coming in from the desk section. The
>> > two
>> > amps then process the signals in antiphase to one another right to the
>> > final output, where the 'correct' phase relationship is again restored,
> by
>> > wiring the speakon connector 'wrongly'.
>> >
>> > I have puzzled over this, and can think of no good reason for doing it
>> > that way. I did wonder at one point if it was anything to do with being
>> > able to make the amps bridgeable, but I downloaded a copy of the user
>> > manual, and there is a dire warning paragraph towards the end,
>> > specifically saying that under no circumstances should any attempt be
> made
>> > to operate the amps in bridge. It also warns against making any
>> > connections between the two amps or any speakers connected to them,
>> > although in actual fact, the 'ground' side of each output is truly
> ground,
>> > and is common to both amps, the whole (dual) power supply, and all
>> > metalwork.
>> >
>> > What am I missing here ? Not of any real consequence, as the unit is
>> > mended and back in its original condition, but for academic interest,
>> > it
>> > would be good to understand the designer's thinking.
>> >
>> > Arfa
>>
>> Carver has also done this on some models. Not so good when someone tries
> to
>> demonstrate through a common grounded speaker switcher...
>>
>> Mark Z.
>>
>
>
> Someone last week asked my advice on bridging high power amps. My advise
> was
> don't - bridge inputs if you must, thinking that any fault tends to lead
> to
> spectacular failure all round, I'd forgotton about this ,antiphase,
> reason
> for not bridging
>
>

Such amps are naturally adaptable for bridging.

Mark Z.

== 3 of 6 ==
Date: Tues, Nov 8 2011 7:14 pm
From: "Phil Allison"

"Arfa Daily"
> "Phil Allison"
>> "Arfa Daily"
>>
>>>
>>> I fired it up again, and put a sine wave in, then hooked up two channels
>>> of my 'scope, one to the "A" output and one to the "B", both with the
>>> same polarity, and was surprised to find that the two signals were
>>> completely anti-phase,
>>
>>
>> ** When servicing audio, knowing the polarity of the test signal and if
>> there is significant phase shift is important. A scope triggering off the
>> incoming wave will it not reveal this in single channel mode - so AF used
>> both channels.
>>
>> A much better way is to use the External Synch input on your scope and
>> link it to a fixed output on your bench audio oscillator ( create on if
>> you have to). This way, you see instantly if the signal's phase has
>> reversed or has a large phase shift.
>>
>> Also, triggering will be rock steady with nearly any kind of distorted,
>> noisy or contaminated signal on the scope screen.
>>
>> Try it out for a day or two - you will never go back.
>>

>
> Interesting. Are you squaring up the generator's sine output, to give a
> good sharp transition for syncing the scope ?

** FFS - that processing is inside any decent scope.

Supply it with 0.5volt rms sine wave and you are away.

> The sync circuitry on this Hitachi scope is very good, and will produce a
> perfectly stable trigger point from virtually any waveform, no matter how
> complex or scruffy,

** Stable maybe, phase locked to the audio generator NO.

> can see where it would be useful to be able to have a fixed known trigger
> point, and from there, be able to look at relative phasing between amps,
> and phase shift within an amp, using a single scope channel.

** Most scope have an internal /external switch you can flick anytime.

If you ever deal with tape recorders or tape echos, there is a big
difference with a play back signal.

... Phil

== 4 of 6 ==
Date: Tues, Nov 8 2011 7:19 pm
From: "Phil Allison"

"Arfa Daily"
"Dave Platt"
>>
>> As a result, an amp which was designed with a "not-so-stiff" power
>> supply system, may be able to deliver a slightly higher amount of peak
>> power before it begins to clip and distort.
>> Dave Platt
>
> Thanks Dave. This seems to be what everyone is thinking, and makes a lot
> of sense when you stand back and think about it

** Dave is wrong.

The only time the OOPS idea increases power is when the test frequency is
low, less than 120Hz.

This is why Bob Carver did it - to reduce by half LF ripple on the PS
voltage at frequencies between 20Hz and 120 Hz to meet his published power
specs.

... Phil

== 5 of 6 ==
Date: Tues, Nov 8 2011 8:26 pm
From: Cydrome Leader

Arfa Daily <arfa.daily@ntlworld.com> wrote:
>
>
> "Cydrome Leader" <presence@MUNGEpanix.com> wrote in message
> news:j9aahv$k3m$1@reader1.panix.com...
>> Arfa Daily <arfa.daily@ntlworld.com> wrote:
>>> Had an Allen & Heath PA20-CP 2 x 500 watt powered mixer in today. Has two
>>> completely separate and identical power amps, one of which was running
>>> very
>>> hot very quickly. No schematics, but problem was quickly traced to an
>>> open
>>> circuit resistor, by comparison ohms checks between the two amps. In
>>> order
>>> to replace the resistor, it was necessary to remove the offending ("B")
>>> power amp to get to the board underside. The power rails and ribbon from
>>> the
>>> mixer desk section were on removable connectors, but the 4 pole speakon
>>> connector was bolted to the rear panel and the wires were soldered
>>> directly
>>> to the board. When I came to unsolder these wires from the socket, I
>>> noticed
>>> that the "B" channel one was wired the opposite way round to the "A"
>>> channel
>>> one. I noted this down to check on later.
>>>
>>> When the amp was tested ok, I bolted it back in, and reconnected the
>>> speakon
>>> socket the same as it was when I unsoldered it. The wires lay exactly
>>> where
>>> they originally were, and looked absolutely factory-original, even down
>>> to
>>> the one which went to the uppermost connector tag, having a rubber sleeve
>>> on
>>> it, exactly the same as on the "A" channel. Except the two channels were
>>> definitely wired arse-about-face to one another. On both channels'
>>> speakons,
>>> the "1-" and "2-" pins were linked, as were "1+" and "2+". But on channel
>>> "A" the ground wire was connected to the "+" pair, and on the "B"
>>> channel,
>>> the ground was on the "-" pair.
>>>
>>> I fired it up again, and put a sine wave in, then hooked up two channels
>>> of
>>> my 'scope, one to the "A" output and one to the "B", both with the same
>>> polarity, and was surprised to find that the two signals were completely
>>> anti-phase, which meant that with the reversed connections to the speakon
>>> sockets, they would be back in phase again to the outside world.
>>>
>>> I then went back to the power amps' front ends near where the ribbon
>>> connector was, and found the same thing. At the same point on each amp,
>>> the
>>> signals were in opposite phase to one another. I then spotted three
>>> little 3
>>> pin header blocks, with one pin pair on each designated "B", and the
>>> other
>>> pair "A" and a little 1 or maybe an I next to the "A" in a sort of 'to
>>> the
>>> power of' position. On the B amp, these three jumpers were set to "B",
>>> and
>>> on the A amp they were set to "A", so clearly, these jumpers reverse the
>>> phase of the signal coming in from the desk section. The two amps then
>>> process the signals in antiphase to one another right to the final
>>> output,
>>> where the 'correct' phase relationship is again restored, by wiring the
>>> speakon connector 'wrongly'.
>>>
>>> I have puzzled over this, and can think of no good reason for doing it
>>> that
>>> way. I did wonder at one point if it was anything to do with being able
>>> to
>>> make the amps bridgeable, but I downloaded a copy of the user manual, and
>>> there is a dire warning paragraph towards the end, specifically saying
>>> that
>>> under no circumstances should any attempt be made to operate the amps in
>>> bridge. It also warns against making any connections between the two amps
>>> or
>>> any speakers connected to them, although in actual fact, the 'ground'
>>> side
>>> of each output is truly ground, and is common to both amps, the whole
>>> (dual)
>>> power supply, and all metalwork.
>>>
>>> What am I missing here ? Not of any real consequence, as the unit is
>>> mended
>>> and back in its original condition, but for academic interest, it would
>>> be
>>> good to understand the designer's thinking.
>>>
>>> Arfa
>>>
>>
>> are these amps run from one power supply?
>
> Sort of. It's one bloody great torroidal tranny, for sure, but it looked as
> though there was enough filter caps on the board for it to be otherwise two
> independant supplies. Certainly, two sets of + / - / gnd cables came out of
> the psu for the two amps, but I must admit that I did not look at its design
> closely enough to see if there was two sets of secondary windings, and two
> rectifiers. The ground was certainly common to both amps, and one leg of the
> output on each amp was that same ground.
>
> Arfa

It looks like others answered what I sort of suspected where they wanted
to try to balance out the + and - rails when the audio peaks. I've never
actually seen this before though.

That reminds me- does anybody unnderstood what Carver's "power steering"
was supposed to be or mean in their amps?

== 6 of 6 ==
Date: Tues, Nov 8 2011 9:20 pm
From: "Phil Allison"

"Cydrome Leader"

> That reminds me- does anybody unnderstood what Carver's "power steering"
> was supposed to be or mean in their amps?

** Bob Carver was an expert at coming up with high falutin' names for
various clever dodges he used to save weight and cost.

His " Magnetic Field " amps were nothing of the kind. They simply employed
a triac based voltage regulator in the AC supply to improve the regulation
factor of a ridiculously small iron transformer.

In his M400 and PM-2 designs, these puny transformers would emit cracking
noises and *smoke* if you ever bench tested them at rated power for more
than 60 seconds. No thermal shut down was fitted to save the trannies
either.

From the blurbs on "Power Steering" it applies to 5.1 channel amps and
allows one audio channel to dominate the load on PSU at any given time.
Likely a simple boost inhibit circuit that allows the use of a puny AC
transformer again.

... Phil

==============================================================================
TOPIC: Tektronix 2215A worth repair?
http://groups.google.com/group/sci.electronics.repair/t/fc4cb167c05d84fd?hl=en
==============================================================================

== 1 of 3 ==
Date: Tues, Nov 8 2011 6:49 pm
From: "Mark Zacharias"

"Geoff" <public@email.com> wrote in message
news:Xns9F97CA1381610Geoff@88.198.244.100...
>I have a tek 2215A CRO with following fault..
> Left side of trace has a ripple, about 1/2 division vert
> amplitude and about 25% of horizontal is affected.
> Both channels the same
> When vert offset is manually swept, the pattern of ripple
> changes, still both channels the same
> Timebase does not change shape or size of ripple
> ripple is 1/2 or 1 cycle but is like someone has put a magnet on
> the side of the tube.
> Changing attenuator makes no difference.
> Was OK a few months ago when last used
>
> If this is a tube fault, I will junk it, what do others think?

Always start with the power supply.

Mark Z.

== 2 of 3 ==
Date: Tues, Nov 8 2011 9:47 pm
From: "Phil Allison"

"Geoff"
>
> Left side of trace has a ripple, about 1/2 division vert
> amplitude and about 25% of horizontal is affected.
> Both channels the same
> When vert offset is manually swept, the pattern of ripple
> changes, still both channels the same
> Timebase does not change shape or size of ripple
> ripple is 1/2 or 1 cycle but is like someone has put a magnet on
> the side of the tube.

** Could the case be magnetised in one area ?

... Phil

== 3 of 3 ==
Date: Wed, Nov 9 2011 12:27 am
From: "N_Cook"

Geoff <public@email.com> wrote in message
news:Xns9F97CA1381610Geoff@88.198.244.100...
> I have a tek 2215A CRO with following fault..
> Left side of trace has a ripple, about 1/2 division vert
> amplitude and about 25% of horizontal is affected.
> Both channels the same
> When vert offset is manually swept, the pattern of ripple
> changes, still both channels the same
> Timebase does not change shape or size of ripple
> ripple is 1/2 or 1 cycle but is like someone has put a magnet on
> the side of the tube.
> Changing attenuator makes no difference.
> Was OK a few months ago when last used
>
> If this is a tube fault, I will junk it, what do others think?

I would suspect something in the flyback suppression/blanking area

==============================================================================
TOPIC: Mixing 4 audio channels to 3?
http://groups.google.com/group/sci.electronics.repair/t/a9c921002b2ed750?hl=en
==============================================================================

== 1 of 1 ==
Date: Tues, Nov 8 2011 9:07 pm
From: DaveC

Latest rev:

<http://i40.tinypic.com/35m026h.jpg>

>> What needs to be reference to the new "ground"? Everything between the
>> input
>> caps and output caps?
>
> Pretty much... each of the op amps' noninverting inputs, and the
> "bottom ends" of the potentiometers, as you have drawn them. *NOT*
> the V- input to the op amp(s), of course.

You do mean each of the op amps' *inverting* inputs, yes?

> You might want to add "pop preventer" resistors at the inputs and
> outputs... say, 100k to DC ground, from the "outside" end of each of
> the DC-blocking capacitors.

Is this what you mean (see link)?

Thanks.

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