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From: b on 29 Jun 2010 17:50
On 29 jun, 18:26, "William R. Walsh" <wm_wa...(a)hotmail.com> wrote:
> Hi!
>
> > 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

I'd retrace my steps a bti by desoldering that last cap and checking
it out.use junk speakers just in case.
-B
From: Arfa Daily on 29 Jun 2010 21:18


"William R. Walsh" <wm_walsh(a)hotmail.com> wrote in message
news:1de39763-9576-4ab5-99f0-6730bd9359b5(a)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



From: William R. Walsh on 30 Jun 2010 10:35
Hi!

> 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
From: Arfa Daily on 30 Jun 2010 12:38


"William R. Walsh" <wm_walsh(a)hotmail.com> wrote in message
news:3896589f-88b1-428b-b873-061f75f4685a(a)t10g2000yqg.googlegroups.com...
> Hi!
>
>> 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

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

From: William R. Walsh on 30 Jun 2010 13:44
Hi!

> 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
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