Prev: Revox B225 Service/User Info
Next: Fuse or fusible resistor?
From: Arfa Daily on 29 Jun 2010 12:36


"William R. Walsh" <wm_walsh(a)hotmail.com> wrote in message
news:4f83d666-93a0-4a06-8944-b4239971c92a(a)t10g2000yqg.googlegroups.com...
> Hi!
>
>> So what does the cap do in the circuit ? Is it a big secret, or
>> something ?
>
> No, should it be? :-)
>

Well, not really. I was just trying to find a way to better help you with
your diagnosis. Twice I had asked what the function of the cap was, and
twice you had not replied.


> I don't know for certain what it does. I haven't explored it that
> closely. I only noticed it being bloated out of the corner of my eye,
> so I said "that should be replaced".

Fair enough. A reasonable assumption, given the amount that's said on here
about bulging caps being at the bottom of many faults.

I assumed that as you had the service manual, and were engaged in fault
finding to component level, that you would have sufficient understanding of
circuit design to be able to just look at the cap on the schematic, and
determine its function without a second thought. It seems to me, pretty
fundamental to this process, that you know the function of the cap you have
replaced, considering that the fault symptoms changed dramatically after
replacing it. You seem to have put this down to coincidence, and indeed, it
may be, but from many long years of experience, it usually isn't. Either
something went wrong with the process of changing it - print damage,
component damage, via damage, un-noticed solder splat, etc, we've all been
there - or the fact that it was faulty before was masking some other
problem, that is now the dominant one. That was the reason that I felt it
important to know what the function of that cap is. I don't know how much
experience you have of component level faultfinding, but if it's not that
much, don't be afraid to admit that. There are plenty on here with a
lifetime's experience who will try their best to help, given the necessary
information, which ultimately helps others to learn.

>
> I've put it aside for now. Maybe another day

OK, no probs.


>
> TEAC has been no help. Although they did respond to my request asking
> for more information, they recommended "shotgunning" the unit. (Yeah,
> really.)
>
> William


It depends what they meant by "shotgunning". Once it has been determined
which stage a fault lies in, sometimes the best approach is to shotgun all
the electrolytics, or sometimes all the transistors. Semiconductors will
sometimes 'hide' their faultiness in that they've been weakened by obviously
failed nearby devices. This can - and does - result in bounced repairs,
sometimes with a cascade failure that you thought you'd successfully
repaired, repeated all over again ... :-(

So usually, if a manufacturer recommends "shotgunning", it's for a good
reason. Of course, whoever was replying to you might just have been being
facetious, and was actually suggesting that you stood it on a post at the
end of the yard, and took a 12 gauge to it .... :-))

Arfa

From: Adrian C on 29 Jun 2010 13:37
On 27/06/2010 17:47, Arfa Daily wrote:

> Knowing that may be helpful to assess whether there may be an immediated
> connection between the previous and current symptoms, or whether we're
> in 'red herring' territory here ...

Well, I'm a bit bored at the moment so having a peek. :-)

The service manual is up on eserviceinfo. Page 13 is relevant.

http://www.eserviceinfo.com/downloadsm/25360/TEAC_AG-790.html

This (with the exception of the resistors) is all in output protection
past the final stage.

7C26 is in reverse series with 7C27 (i.e. non-polarised configuration)
and I guess may be short, open or leaky? Change it? What's across them
looks to be a summed fraction of the output of both L&R amplifiers, and
also a POW signal to the system micro.

The burning of the resistors first mentioned is indicative of an
overload prior to the final output transistors (7V19 to 7V22). Don't
know what that is all about, unless the amp power rails themselves have
been up to some silly voltage they shouldn't have been.

Seems enough discretes there to enjoy (!) a good troubleshoot, with
similar and isolate-able sections.

--
Adrian C
From: William R. Walsh on 29 Jun 2010 16:01
Hi!

> Twice I had asked what the function of the cap was, and
> twice you had not replied.

I am sorry about that. I must have missed seeing it.

> I assumed that as you had the service manual, and were engaged
> in fault finding to component level, that you would have sufficient
> understanding of circuit design to be able to just look at the cap on
> the schematic, and determine its function without a second
> thought.

You could say that. But I'm still *very* much in the learning phase. I
started with studying basic home appliances and household electricity
long ago. I started to study the workings of transistors,
semiconductors and other components after that. (This particular
process continues today.) Then, after much trial, I decided that I was
going to learn how to solder properly. And I finally did. My plan is
to know how to repair things even if nobody cares to. I don't plan to
do this sort of thing professionally. I'm getting there...I've built
simple electrical devices to solve problems or improve things in some
way, modified circuits, mended broken up circuit boards a few times
now and done "other stuff".

I haven't really looked into what that cap is doing...yet. I've been
very busy with many other things, including the repair of cars,
computer consulting work and "other jobs". I just haven't had the time
to sit down with the schematic and look at what's going on.

> You seem to have put this down to coincidence, and indeed, it
> may be, but from many long years of experience, it usually
> isn't.

No, I don't think it's a coincidence. I'm familiar with the concept of
more than one part being bad, and more faults surfacing as damaged
circuitry is repaired.

> Either something went wrong with the process of changing it
> - print damage, component damage, via damage, un-noticed
> solder splat, etc

Well, I'm pretty sure I didn't do any of those things. When I first
noticed the behavior change, I looked over my work very carefully. I
wanted to be sure that I had not induced this problem by my own doing.

> I don't know how much experience you have of component level
> faultfinding, but if it's not that much, don't be afraid to admit
> that.

It's probably on the lower end of intermediate, or just out of the
"total beginner" range. Of the things I've tried, though, I've had a
100% success rate...including the diagnosis of my linear tracking
turntable's front panel control failure.

> OK, no probs.

As it is, I've got enough stereo receivers, some of which I have
repaired. Maybe I have too many...at last count I'd repaired at least
four units in some way or another. One was a case of the previous
owner experiencing "operator error" so maybe it does not really count.

> It depends what they meant by "shotgunning".

This is a direct quote. Note the style as well, which has been left
intact.

"HI William.We only have what's in the manual too.Since you already
have the service manual,it shouldn't be too difficult to
troubleshot.Power supply rails o.k.?,D.C. offset?
Other than "shotgunning"parts,I don't know what else to recommend."

The power supply rails seem to be OK...they're all doing something
(don't recall the exact measurements). I wasn't sure where exactly
they wanted me looking for DC offset...I have some idea what it is,
and that's about it.

Maybe I'm reading more into it than I should be, but I take what was
written to mean "oh, we really don't repair those when one comes in
under the terms of the warranty". And maybe they don't?

I asked if there was any documentation for the error codes, or if
anyone there might know what they meant. I felt, perhaps incorrectly,
that knowing what an error code meant would help. (I do realize that
error codes are not always definitive and that they could send me
barking up the wrong tree.) I also asked if my manual was
complete...if maybe there was a piece I didn't have.

I never heard back.

I don't mind "shotgunning" parts of a circuit, but I have reservations
about doing the whole darn unit. I'd have to think whether it is worth
that to me...and I'm not convinced that it is.

> Of course, whoever was replying to you might just have
> been being facetious, and was actually suggesting that you
> stood it on a post at the end of the yard, and took a 12 gauge
> to it ....    :-))

Being as it was through e-mail, I can't reliably know the intention of
what was written. I like to see the best in people, so I assume it
probably was serious.

Compared to the other receivers I have worked on, I don't think this
one is as good of a design. I may not be qualified to say that, but
then again, I did fix the others.

William
From: Baron on 29 Jun 2010 16:36
Arfa Daily Inscribed thus:

> So usually, if a manufacturer recommends "shotgunning", it's for a
> good reason. Of course, whoever was replying to you might just have
> been being facetious, and was actually suggesting that you stood it on
> a post at the end of the yard, and took a 12 gauge to it .... :-))
>
> Arfa

Thats probably what they did mean... ;-)

--
Best Regards:
Baron.
From: William R. Walsh on 29 Jun 2010 17:03
Hi!

> 7C26 is in reverse series with 7C27 (i.e. non-polarised'
> configuration) and I guess may be short, open or leaky?
> Change it?

I had this feeling that I should change that cap, if for no other
reason than its immediate neighbor was bad. I'd even go so far as to
say that said feeling just had something about it, telling me that I
shouldn't ignore it.

No, I'm not superstitious or anything like that. But I know when I
have a gut feeling, and I *do*.

So I'm going to do that and see what happens.

> The burning of the resistors first mentioned is indicative of an
> overload prior to the final output transistors (7V19 to 7V22).

I don't know either. The only inkling I have is that someone played it
very, very hard. (The previous owner didn't want to talk about what
happened, but I don't think they were inside it. So it's another one
of these feelings.) One channel still played without issue when I got
it.

As best I can tell, the final transistors are good.

The only thing that's really driving me batty about this is the fact
that it's a pain to do any testing, since I have to put the board back
in. I really don't want to rearrange all the internal parts for
testing purposes, because I am afraid I will lose some of them.

William
First  |  Prev  |  Next  |  Last
Pages: 1 2 3 4 5
Prev: Revox B225 Service/User Info
Next: Fuse or fusible resistor?