sci.electronics.repair@googlegroups.com

Today's topics:

* Mixing 4 audio channels to 3? - 8 messages, 4 authors
http://groups.google.com/group/sci.electronics.repair/t/a9c921002b2ed750?hl=en
* Searching for a AM broadcaster electronic kit manual C6752 - 1 messages, 1
author
http://groups.google.com/group/sci.electronics.repair/t/b92439b741998d27?hl=en
* Tektronix 2215A worth repair? - 2 messages, 2 authors
http://groups.google.com/group/sci.electronics.repair/t/fc4cb167c05d84fd?hl=en
* Hot electrolytics - 1 messages, 1 author
http://groups.google.com/group/sci.electronics.repair/t/abec0b759dbe9370?hl=en

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

== 1 of 8 ==
Date: Wed, Nov 9 2011 2:45 pm
From: DaveC

> I was going to suggest that, since the mixer is going to be external
> to the amp, he use a couple of wall-warts to get the dual supplies,
> but I like your solution a lot better. :-)

Enough to suggest a nice circuit? ;-)

Thanks,
Dave

== 2 of 8 ==
Date: Wed, Nov 9 2011 4:43 pm
From: "krw@att.bizzzzzzzzzzzz"

On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. <bespoke@invalid.tv> wrote:

>> What does U4 do?
>
>Provides a Vcc/2 local "ground" so I can use these op amps with a single
>supply voltage.

Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's confusing
and someone along the line might get hurt. Grounds should be.

== 3 of 8 ==
Date: Wed, Nov 9 2011 7:10 pm
From: DaveC

> Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's
confusing
> and someone along the line might get hurt. Grounds should be.

Yeah, I wasn't sure about that. How should I have indicated 2 separate
grounds?

Thanks.

== 4 of 8 ==
Date: Wed, Nov 9 2011 9:02 pm
From: isw

In article
<0001HW.CADF4AA000BC0A04B02069DF@news.eternal-september.org>,
DaveC <invalid@invalid.net> wrote:

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

Distortion performance might be better if you run the amps in inverting
mode. If you run the audio into the non-inverting inputs, the inverting
input follows and the amp has to operate throughout it's common-mode
range. If you use the amps in inverting mode, both the + and - inputs
will stay very close to ground.

Isaac

== 5 of 8 ==
Date: Wed, Nov 9 2011 9:07 pm
From: isw

In article <4ebae303.100219082@news.eternal-september.org>,
spam@spam.com (Don Pearce) wrote:

> On Wed, 9 Nov 2011 12:10:19 -0800, Bob E. <bespoke@invalid.tv> wrote:
>
> >> What does U4 do?
> >
> >Provides a Vcc/2 local "ground" so I can use these op amps with a single
> >supply voltage.
>
> More useful to let it oscillate as a square wave generator at 100kHz
> or so, and rectify the output into a negative 15V rail. That way you
> can run the op amps the way they are meant to be run.

??

What is wrong with using an op-amp to "amplify" DC? Surely they are
"meant" to do that.

Isaac

== 6 of 8 ==
Date: Wed, Nov 9 2011 9:12 pm
From: DaveC

> What is wrong with using an op-amp to "amplify" DC? Surely they are
> "meant" to do that.
>
> Isaac

I think he means that better audio results can be achieved by providing true
dual-voltage supplies and eliminating all coupling caps.

The "DC amplifier" is a fine design, but will result in inferior audio
performance due to the caps.

Now, if someone would volunteer such a negative voltage generator circuit...
;-)

Thanks.

== 7 of 8 ==
Date: Wed, Nov 9 2011 9:21 pm
From: "krw@att.bizzzzzzzzzzzz"

On Wed, 9 Nov 2011 19:10:58 -0800, DaveC <invalid@invalid.net> wrote:

>> Hint: Don't use the chassis ground symbol as a "Vref" symbol. It's
>confusing
>> and someone along the line might get hurt. Grounds should be.
>
>Yeah, I wasn't sure about that. How should I have indicated 2 separate
>grounds?

It's not ground. It's Vcc/2. I generally call it Vref, or some such thing.

== 8 of 8 ==
Date: Wed, Nov 9 2011 10:43 pm
From: spam@spam.com (Don Pearce)

On Wed, 09 Nov 2011 20:49:28 +0000 (GMT), Stuart
<Spambin@argonet.co.uk> wrote:

>In article <4ebae303.100219082@news.eternal-september.org>,
> Don Pearce <spam@spam.com> wrote:
>> >Provides a Vcc/2 local "ground" so I can use these op amps with a single
>> >supply voltage.
>
>> More useful to let it oscillate as a square wave generator at 100kHz
>> or so, and rectify the output into a negative 15V rail. That way you
>> can run the op amps the way they are meant to be run.
>
>KISS

Exactly. Much easier than all these halved supplies, coupling caps and
multiple grounds.

d

==============================================================================
TOPIC: Searching for a AM broadcaster electronic kit manual C6752
http://groups.google.com/group/sci.electronics.repair/t/b92439b741998d27?hl=en
==============================================================================

== 1 of 1 ==
Date: Wed, Nov 9 2011 3:42 pm
From: gccradioscience

I am searching for a AM broadcaster electronic kit manual from a
Chaney Electronics kit. I have bought the kit back in April of 2010,
and I got the kit working, BUT I am having a problem finding my copy
of the lost AM electronic kit manual. I am right now searching high
and low in my room finding the kit manual, still no luck yet. The
kit part number is C6752 and I tried to give the company a call, but
they just told me that the kit was discontinued and there is no
further support for this electronic hobby radio kit. Please
contact me if you have a copy of the manual you want to send me. I
need this for back up purposes so just in case the electronic kit
malfunctions or ceases to work. I hope Chaney Electronics brings this
worthy kit back in their kit line. Thank you, I would appreciate
it.

Adam E
Va.
gccradioscience

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

== 1 of 2 ==
Date: Wed, Nov 9 2011 4:42 pm
From: Geoff

"Phil Allison" <phil_a@tpg.com.au> wrote in
news:9huignFj3nU1@mid.individual.net:

>
> "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
>
>

I took the case off and no difference, your implied suggestion
that a change in magnetic field caused this matches what I
observe, ie no electronic settings change the waveform, it looks
like when someone shoves a magnet near a crt. I have not removed
the tube screen but I have shaken it and nothing like a magnet
bouncing around can be heard, I may just give up now, it is not
a paying job just an old CRO we use occasionally for R&D work,
we have newer better ones around including tek analog CRO's.

== 2 of 2 ==
Date: Wed, Nov 9 2011 11:31 pm
From: "Phil Allison"

"Geoff"

> I took the case off and no difference, your implied suggestion
> that a change in magnetic field caused this matches what I
> observe, ie no electronic settings change the waveform, it looks
> like when someone shoves a magnet near a crt. I have not removed
> the tube screen but I have shaken it and nothing like a magnet
> bouncing around can be heard,

** The CRT is a *mesh PDA* type - right ?

There must be a chance the steel mesh has become magnetised.

Suggest you play around with a small magnet near the tube face and see what
happens.

FYI:

There is a fine steel mesh just behind the face of the tube that has final
acceleration voltage applied to it - if you de-focus the beam you can
usually see the pattern of the mesh projected onto the phosphor.

... Phil

==============================================================================
TOPIC: Hot electrolytics
http://groups.google.com/group/sci.electronics.repair/t/abec0b759dbe9370?hl=en
==============================================================================

== 1 of 1 ==
Date: Thurs, Nov 10 2011 12:32 am
From: "N_Cook"

John Robertson <spam@flippers.com> wrote in message
news:O7GdnV9LGpJNQSfTnZ2dnUVZ5v-dnZ2d@giganews.com...
> N_Cook wrote:
> > How much is down to conflict between power supply designer and EMC
> > compliance bod?
> > Designer: I want convection vent holes here, here and here, through the
> > shielding
> > EMC bod: No way, I can let you have a few holes here and thats your lot
> >
> > Just repaired an LCD monitor, probably started failure with ESR'd caps
then
> > ohmic zener and eventually blown backlight inverter pair of transistors
and
> > blown fuse. Plenty of holes top, side and bottom of the shield on the
> > digital processing side but only holes at the top on the ps side. So air
> > comes in on the colder side at bottom rises to the top by about mid
board
> > and little or no air traversing the hottest section, that whole section
in a
> > dead-zone in more ways than one, brilliant
> >
> >
>
> Any cap with shrinking plastic film is ready to be replaced. Figure out
> what circuit it is in and find either a low ESR or low Inductance (or
> both) cap to replace the poorer original one.
>
> We put fans on any LCD screens that we install in any sort of cabinet.
>
> It would be nice if tiny fans were installed by the manufacturers...
>
> John :-#(#
>
> --
> (Please post followups or tech enquiries to the newsgroup)
> John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
> Call (604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
> www.flippers.com
> "Old pinballers never die, they just flip out."

They would not have to be high revs, just enough to circulate air in or out
of dead zones like between heatsink plates. In this case the hottest as
evidenced by the shrunk sleeves , were from between a U of heatsinkery. I
will try controlled heating of one of the other removed caps of the same
make to see what temp it must reach to axially shrink the wrappers.
Do they make miniature
longitudinal fans say .5 inch diameter x 1 or more inches long ? Failing
that it would be some ducting associated with standard 1 inch fans.

If no fans allowed I would have placed the ps board over the "scaler" board,
assuming the whole monitor is not used horizontally

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