Teac AG-790 has me stumped...
William R. Walsh
I received a Teac AG-790 stereo receiver with burn damage to the PCB
underneath resistors 7R41, 7R47, 7R?? (unmarked, next to 7R47) and 7R48. I
repaired all of this damage, cleaned up the carbon that had been left behind
and soldered to the nearest remaining good portion of the board.
Up to this point, the receiver did what it has done since I got it. It
played through the right channel only.
I kept on looking and found a capacitor at location 7C27 (220uF, 16V) that
had blown out the rubber plug at the bottom. So I replaced that. Now nothing
works. I know the new cap to be good. The unit will power up just as it
always has, there are no blown fuses and my repair job seems to be
bulletproof (and correct, per the schematic in the service manual). The
audio is gone, replaced only by a faint humming on speakers and headphones
alike, in the right channel. Turning the volume dial does not change this.
There is still nothing from the left channel. The unit is not in protection
and nothing is getting hot, burning up or worse.
In addition, the display on the front of the unit will now indicate "E0" (or
maybe it's an O...no way to tell). This is not mentioned in the service
manual, nor did I find anything in a web search. Operations from the front
panel remain possible.
I'm beginning to think that it may be too far gone to fix. Anyone out there
with an eleventh hour idea?
William
Jamie
did you put the new cap in backwards?
Also, is it possible that maybe the protection circuit is now
working since you put the cap in, which is why you are now
getting the E0 error. This could be caused by yet, a problem that
remains in the bad channel..
Assuming this is like most protection output circuits, you may
want to test the output of the bad channel before the relay circuit
that joins the speakers. .YOu could have a channel that is outputting
DC above the safe point for the initial on cycle. Which would lead
to say that you still have a problem, ofcourse!
stra...@yahoo.com
'blown plug' usually means the acid is out and quite possibly eating
away the traces near by. Replacing the cap may have finished off a
nearly gone trace. Do you have the prints so you can 'ohm it out' and
verify the connections are actually there?
G²
Baron
Just asking, you did put the cap in the right way round !
--
Best Regards:
Baron.
Arfa Daily
"Baron" <baron....@linuxmaniac.nospam.net> wrote in message
news:i07qvk$dja$1...@news.eternal-september.org...
Agreed on that, but seems to me that the fundamental question is not being
asked. In what part of the circuit is the replaced cap ? Can you determine
its exact function from the schematics ?
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 ...
Arfa
William R. Walsh
Thanks to everyone for the replies...
The new cap is installed properly. I made careful notes of how the old
one had been installed and the schematic is in agreement with my
notes.
I do believe the "E0" indication is some kind of error. But what it
means is a mystery, and the service manual does not help. I'm going to
ask TEAC, although so far they have not been helpful at all. The
output of the amplifier does not appear to be blocked--it is
amplifying this humming sound. It's not outputting DC to the speaker
connections.
Outside of the burned area, no traces on the board were ruined.
If I knew what E0 meant, I suppose it might be very helpful.
I may try pulling the cap (since the original was completely open
circuit, I doubt it can hurt) and seeing how the behavior changes.
William
Geo
<newsg...@idontwantjunqueemail.walshcomptech.com> wrote:
>I kept on looking and found a capacitor at location 7C27 (220uF, 16V) that
>had blown out the rubber plug at the bottom. So I replaced that. Now nothing
>works. I know the new cap to be good.
Is it a double-sided (or worse - multilayer) pcb?
If the pcb designer used one or both of the capacitor pins as vias then you
could possibly have removed sufficient of the hole plating to lose e.g a power
or ground through connection.
--
Geo
Arfa Daily
"William R. Walsh" <wm_w...@hotmail.com> wrote in message
news:016c0e5e-f07c-4ecb...@r27g2000yqb.googlegroups.com...
So what does the cap do in the circuit ? Is it a big secret, or something ?
Arfa
William R. Walsh
> So what does the cap do in the circuit ? Is it a big secret, or
> something ?
No, should it be? :-)
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".
I've put it aside for now. Maybe another day.
TEAC has been no help. Although they did respond to my request asking
for more information, they recommended "shotgunning" the unit. (Yeah,
really.)
William
William R. Walsh
> Is it a double-sided (or worse - multilayer) pcb?
No, it appears to be a simple PCB with one layer on one side.
I would not call this receiver the greatest design I have seen.
Compared to the Sherwood and Sony receivers that I have been working
on, it strikes me as being unnecessarily complex. The main PCB is
littered with all sorts of components where the others are much
cleaner and simpler.
William
Arfa Daily
"William R. Walsh" <wm_w...@hotmail.com> wrote in message
news:4f83d666-93a0-4a06...@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
Adrian C
> 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
William R. Walsh
> 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
Baron
> 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.
William R. Walsh
> 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
b
I'd retrace my steps a bti by desoldering that last cap and checking
it out.use junk speakers just in case.
-B
Arfa Daily
"William R. Walsh" <wm_w...@hotmail.com> wrote in message
news:1de39763-9576-4ab5...@d37g2000yqm.googlegroups.com...
> 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.
OK, that's fair enough. At least we now have an idea of your capabilities.
When Teac asked you to check for DC offset, they were referring to a
standing DC component at the power amplifier midpoint. This is at the
junction of 7R43 / 45 for one channel, and 7R44 / 46 for the other channel.
The voltage with respect to chassis measured at this midpoint, should be
zero. Any positive or negative standing voltage at this point, is termed a
"DC offset". If large enough, it could be damaging to a loudspeaker, so for
this reason, there is circuitry to sense any offset, and if there is one,
block a relay from operating. This relay is normally in series with the
speaker feeds. As you are hearing some hum through one channel, this would
suggest that the relay is closing (can you hear it close after a short delay
from switching on ?) and if it is, that might suggest that there was no DC
offset . But, if the protection circuit was not working correctly, it might
just allow the relay to close with an offset present. As 7C27 is in inverse
series with 7C26, I would be inclined to replace them as a pair. Your first
move though, would be to check if an offset is present on either channel.
Arfa
William R. Walsh
> As you are hearing some hum through one channel, this would
> suggest that the relay is closing (can you hear it close after a
> short delay from switching on ?)
Yes, the relay closes. I have replaced 7C26 just now. The old cap
tested OK per the cap checker built into my multimeter, but the new
caps were cheap and I don't want it causing trouble. I've just found a
47uF 80V cap that is also bad. I don't have that one in my stash of
parts, so I may have to order one. It appears that the heat which
burned the circuit board also dehydrated this cap. Removed from the
circuit, it tests open.
There is soft hum from the working right channel. I have yet to hear
anything from the left channel. When it was playing (through the right
channel only), it was possible to just barely hear distorted music
through the left.
I did not notice any DC voltage at the speaker connections, if that's
an acceptable place to take a measurement. I tested that before I ever
hooked any speakers to the unit.
William
Arfa Daily
"William R. Walsh" <wm_w...@hotmail.com> wrote in message
news:3896589f-88b1-428b...@t10g2000yqg.googlegroups.com...
Yes, as long as the relay is closing, you can measure offset at the speaker
terminals, with one slight reservation. In the case that you can hear
nothing at all from the speakers, but you hear the relay close, you need to
make sure that having closed, the relay is actually staying closed.
Sometimes, you can be fooled, as on some designs, the protection relay will
close momentarily, and immediately reopen if there is a fault.
What's the circuit designation of the new cap that you've found to be bad ?
I note you say that you are using the cap checking facility on your
multimeter. What parameter does this measure ? Capacitance maybe ? If so,
and you are aiming to do a moderate amount of repair work, you would do well
to consider investing in an ESR meter, such as the "Blue" version of Bob
Parker's original design, now marketed as an easy to build kit, by Anatek
Corporation in the U.S. Although capacitance readings can be useful, to get
such a reading necessitates removing the cap from the board. Apart from any
other reason, this can be a tedious procedure if there are a lot of caps to
check. However, a much more valid reason for owning one, is that the ESR of
an electrolytic capacitor is, these days, one of its most important
parameters. A lot of modern circuitry - particularly in switch mode power
supplies - relies absolutely on the ESRs of the caps being within tolerance,
and a basic capacitance meter cannot tell you this. I have seen
electrolytics which show no signs at all of heat distress or bung popping,
and which read almost spot on for value, but whose ESR is out of the window.
If I was recommending test equipment on a usefulness basis, then an ESR
meter would be top of my 'must have' list.
Arfa
William R. Walsh
> Yes, as long as the relay is closing, you can measure offset at the
> speaker terminals, with one slight reservation. In the case that you can
> hear nothing at all from the speakers, but you hear the relay close, you
> need to make sure that having closed, the relay is actually staying
> closed.
I am pretty sensitive to the noises of relays and such, and if one
does something out of the ordinary, I usually catch it in a sort of
"and what do you think you're doing?" moment. (My hearing is also good
enough to catch a lot of high frequency noises that some circuits
make, and that has clued me in to a few problems on other repairs.)
This one really does seem to stay closed. It produces pretty
noticeable clicks and clacks.
> What's the circuit designation of the new cap that you've found to be
> bad ?
I am not near the receiver now and will have to look.
> I note you say that you are using the cap checking facility on your
> multimeter. What parameter does this measure ? Capacitance
> maybe ?
While the meter itself seems to be of decent quality and pretty
accurate, the directions that came with it are poor and do not discuss
how to do anything except voltage and resistance checks. Even those
are very poorly expressed by way of poorly written or translated
instructions.
The display reading in the "cap test" mode is expressed in nano or
microfarads, although the test seems to be limited to low voltage and
capacitance levels. So it would appear to be useless outside of small,
low voltage capacitors.
For those that seem to exceed the ceiling on the meter, I've done a
simple resistance test on the caps. A neighbor suggested this method,
and stated that a good cap would slowly charge up and that the
resistance reading would gradually increase while it did so. This
seems to be true. Every "bad" cap that I've found using this method
has either been open circuit or shown a high resistance reading that
did not change much/at all from the initial reading.
At some point I do want to invest in a proper ESR meter, and the Bob
Parker meter is the one I planned to get. If and when finances
allow...
Every suspicious cap has been removed from the circuit for testing.
This most recent one had a heat-shrunken plastic jacket.
William
Arfa Daily
"William R. Walsh" <wm_w...@hotmail.com> wrote in message
news:a5cd455f-6fdc-4b58...@q12g2000yqj.googlegroups.com...
Heat distress like that is always a good reason to reach for the ESR meter
... Seriously though, as soon as you can afford it, invest in one. The
"Blue" is very reasonably priced, and is a fine performer. You'll never
regret a day of owning it. The thing that makes an ESR meter different from
the static DC tests that you are carrying out at the moment, is that it
applies high frequency AC to the cap, which is what is required to arrive at
a true ESR value. It is this use of low voltage AC, which also makes the
meter able to perform its test with the component still in-circuit, which is
a huge advantage, particularly when a board is hard to get out of equipment.
This assumes of course, that you can get to the cap's leadouts on the top
side of the board.
Arfa
Cydrome Leader
a good description of shotgunning:
http://www.troubleshooters.com/tpromag/200510/200510.htm#_Appropriate_Shotgunning