Sherwood RX-5502 Receiver Protection Shutdown, Repair, thoug

[William R. Walsh]

Hello all...

This is part of a longer story which I won't get into now. Suffice it
to say that I've wanted to get my hands on a failed Sherwood RX-4105
or RX-4109 stereo receiver to see just what it is that kills
them...abuse, misuse, weak parts, bad engineering/quality control or
something else. I've never had any luck coming into a truly broken
one, but someone recently gave me an RX-5502 that would just shut down
right after power on. I've been very happy with all of the RX-4105 and
4109 units I own.

The RX-5502 is a so-called multi-zone receiver. That is to say it can
support up to eight connected pairs of speakers, with four of the
pairs playing a different ("room 2") source if that is desired. It has
two complete stereo amplifiers in place, each one claimed to have an
output power of 100 watts per channel. (Obviously they're dreaming if
they think that this receiver is ever going to output 400 total watts
of power without catching fire, but...) This example was manufactured
sometime in 2008.

As found, this set would indeed power on for a few seconds, and shut
down with a blinking standby LED. I started checking things out. In
this set, the amplifier board is separate from the main board, so this
was not terribly hard to do. Every power device tested good with a
simple ohmmeter check, and nothing looked burnt or distressed on the
amp board. This doesn't look like a case of a failed power transistor
to me.

I'm working without service literature or even a schematic as Sherwood
would not provide them, but there is printing on the board that
identifies what each conductor in the ribbon cable going to the amp
board is used for. This set has a "test mode", and unlike similar
models, the "test mode" allows the power to stay on indefinitely while
the display test is running. Testing for voltages is a lot nicer
without having to constantly turn the set back on again! Voltages are
what I'd expect for B+ and B-, but a twelve volt input to the board is
hovering around a few hundred millivolts at most. That could do it!

Removing the amplifier board from the system and running without it
was probably risky, but it seemed like a worthwhile thing to do. With
the amp board removed, there was still no voltage from the +12 volt
connection. It still hovered around 300mV with the set on.
Interestingly, every now and then, a good power up was possible with
the amp board out, and the set would come out of protection.
Okay...where is the +12 volt supply generated?

Over in the power supply section there are a few linear voltage
regulators--two heatsinked 7812s and one freestanding 7912. One 7812
and the 7912 are doing their jobs, but the other 7812 is cold to the
touch and does not seem to be doing anything. (In fact, it was putting
out 300mV when I later checked it.)

Replacing the failed 7812 with an LM340 solved the problem. The set
immediately came back to life with the amp board in place, and it
plays. It appears, based on simple observation, that one 7812 is
powering the coils leading up to the speaker selection/protection
relays and the other is powering the amp board itself. What other
loads might be powered by these regulators has not been determined.

While the set is working, I don't like the temperature at which the
new regulator is running. Within ten minutes, its heatsink is on the
verge of being too hot to touch for more than a few seconds. There is
evidence on the circuit board that these regulators have always run
very hot. I've been in touch with Sherwood America, who said "the
regulator may become too hot to touch and possibly fail". I strongly
suspect there are bad capacitors on the amp and main boards, which
will need to be replaced and may be stressing the regulator. Yet
Sherwood seems to be saying that the extremely hot operation is
*normal* here. (However, it should be said that there is something of
a language barrier with the folks I've been communicating with.)

I could install a fan above or larger heatsink on the regulator and
I'm not above doing it if that is just the way things will be. I
suspect that would force the regulator to operate more reliably.

What I really want to know, though, is whether or not a drop-in
replacement with more current delivering capability than the LM340 or
78xx series exists. I've looked halfheartedly over the years but never
found anything. I could always build a more capable regulator board
and hack it in there, but I don't really feel like doing that. A fan
would be easier and faster.

I'd also like to know if anyone has had an RX-5502 on their repair
bench, and if they could comment on just how hot its regulators were
running. Any thoughts would be very much appreciated!

Thank you.

William

 

[spamtrap1888]

On Oct 12, 10:05 am, "William R. Walsh" <wm_wa...@hotmail.com> wrote:

Hello all...

This is part of a longer story which I won't get into now. Suffice it
to say that I've wanted to get my hands on a failed Sherwood RX-4105
or RX-4109 stereo receiver to see just what it is that kills
them...abuse, misuse, weak parts, bad engineering/quality control or
something else. I've never had any luck coming into a truly broken
one, but someone recently gave me an RX-5502 that would just shut down
right after power on. I've been very happy with all of the RX-4105 and
4109 units I own.

Why would you care so much about some Korean POS? Sherwood was once a
fine Chicago company, from the era when Chicago was the consumer
electronics center of the world.

 

[Gareth Magennis]

I've been in touch with Sherwood America, who said "the

regulator may become too hot to touch and possibly fail".
William

Well there you go, the manufacturer has backed up your observations that
this regulator does indeed run very hot. Its not that uncommon to find this
kind of thing.

Stick a heatsink on it. Job done.



Gareth.

 

[William R. Walsh]

Hi!

Why would you care so much about some Korean POS?

Well, because I can? Because I like 'em? Because the internal design
is pretty clean and straightforward? Because I like their sound and
think they're a pretty damn good modern receiver? Maybe because it's
just one other thing that won't end up in the landfill? Has the
purpose of this group changed since I was last here?

That's not the question I asked, or the information I wanted. Pulling
out my IR thermometer reveals that the regulator really isn't getting
as hot as I thought it was...about 124F or so at its hottest point.
I'll not worry about it further, though I probably will add a fan
powered by the lesser loaded regulator.

William

 

[William R. Walsh]

Hi!

Stick a heatsink on it.  Job done.

It's already got a goodly sized heatsink on it. In fact, I couldn't
find anything bigger in the TO-220 category. Operating temps turn out
not to be as bad as I thought--about 124F at most. Perhaps the
original regulator was just defective. I'll never know.

I will probably just run a fan from the other, less loaded regulator
and suspend it over the circuit board. That would be a lot less work
than milling some scrap heatsinks from old PC power supplies down to
fit.

William

 

[Mark Zacharias]

"Gareth Magennis" <sound.service@btconnect.com> wrote in message
news:9Jnlq.3718$PP2.252@newsfe23.ams2...

I've been in touch with Sherwood America, who said "the
regulator may become too hot to touch and possibly fail".
William



Well there you go, the manufacturer has backed up your observations that
this regulator does indeed run very hot. Its not that uncommon to find
this kind of thing.

Stick a heatsink on it. Job done.



Gareth.

Sherwood makes receivers for many others, and there is a common problem the
past several years with cheap fixed regulators failing. Mostly 79 series 12
and 15 volt regs but also 78 series positive types. They are heatsinked, but
fail anyway.

Mark Z.

 

[spamtrap1888]

On Oct 12, 2:53 pm, "William R. Walsh" <wm_wa...@hotmail.com> wrote:

Hi!

Why would you care so much about some Korean POS?

Well, because I can? Because I like 'em? Because the internal design
is pretty clean and straightforward? Because I like their sound and
think they're a pretty damn good modern receiver? Maybe because it's
just one other thing that won't end up in the landfill? Has the
purpose of this group changed since I was last here?

Your purpose in coming here seemed quite quixotic, not to say
masochistic, and therefore provoked curiosity:

"Suffice it to say that I've wanted to get my hands on a failed
Sherwood RX-4105 or RX-4109 stereo receiver to see just what it is
that kills them...abuse, misuse, weak parts, bad engineering/quality
control or something else. "

Hard to imagine anything other than frequent failure would provoke
this quest. A likely parallel in the automotive world would be a
mission to isolate failure modes of the Yugo.

Regarding the purpose of the group: Generally people come here to get
information to help them repair various pieces of electronic
equipment, often because their livelihood depends on it. Instead, you
want advice so you can redesign this run of the mill receiver for
greater reliability.

That's not the question I asked, or the information I wanted. Pulling
out my IR thermometer reveals that the regulator really isn't getting
as hot as I thought it was...about 124F or so at its hottest point.
I'll not worry about it further, though I probably will add a fan
powered by the lesser loaded regulator.

Why clutter up such a clean and straightforward internal design with
adequate cooling?

 

[who where]

On Wed, 12 Oct 2011 10:05:18 -0700 (PDT), "William R. Walsh"
<wm_walsh@hotmail.com> wrote:

(snip)

What I really want to know, though, is whether or not a drop-in
replacement with more current delivering capability than the LM340 or
78xx series exists. I've looked halfheartedly over the years but never
found anything. I could always build a more capable regulator board
and hack it in there, but I don't really feel like doing that. A fan
would be easier and faster.

Alternative 3-terminal linear regs will still dissipate the same
amount of heat regardless of their current capability.

If the problem is in the load (i.e. faulty components) then that
should be where you focus your attention.

If the real problem is the load current and the consequent thermal
issues (i.e marginal/poor design vs SOA) then measure the actual load
current and consider a drop-in switcher such as the (TI) PT78T112V or
(RECOM) R-7812.

 

[Jeffrey Angus]

On 10/12/2011 12:05 PM, William R. Walsh wrote:

What I really want to know, though, is whether or not a drop-in
replacement with more current delivering capability than the LM340 or
78xx series exists.

Actually, it sounds more like they blew the design by having too
much input voltage for the output voltage and current.

As a modest example of what I mean:
18V input 12V output at 1 amp. This is 6 watts dissipation at
the regulator package.
24V input 12V output at 1 amp. This is 12 watts dissipation
at the regulator package.

<http://www.datasheetcatalog.org/datasheets2/52/529144_1.pdf>

Doesn't specifically give a voltage drop vs output current
derating curve. But I would think that adding a bit of series
resistance to the input side and letting some of the waste
heat go away there would go a long way towards extending the
life of the regulator package.

Jeff

--
"Everything from Crackers to Coffins"

 

[Gareth Magennis]

"William R. Walsh" <wm_walsh@hotmail.com> wrote in message
news:32e0bfb3-bb79-4af3-9866-caa4be3930e9@j19g2000vbn.googlegroups.com...

Hi!

Stick a heatsink on it. Job done.

It's already got a goodly sized heatsink on it. In fact, I couldn't
find anything bigger in the TO-220 category. Operating temps turn out
not to be as bad as I thought--about 124F at most. Perhaps the
original regulator was just defective. I'll never know.

I will probably just run a fan from the other, less loaded regulator
and suspend it over the circuit board. That would be a lot less work
than milling some scrap heatsinks from old PC power supplies down to
fit.

William

Oops, should have read your post properly, it was a long hard day yesterday
and today I have aches and pains to remind me.


Gareth.



Gareth.

 

[JW]

On Wed, 12 Oct 2011 10:05:18 -0700 (PDT) "William R. Walsh"
<wm_walsh@hotmail.com> wrote in Message id:
<947a0fdb-1d0c-43da-8c6e-5834b9c31498@k39g2000prl.googlegroups.com>:

What I really want to know, though, is whether or not a drop-in
replacement with more current delivering capability than the LM340 or
78xx series exists. I've looked halfheartedly over the years but never
found anything. I could always build a more capable regulator board
and hack it in there, but I don't really feel like doing that. A fan
would be easier and faster.

These will go up to 3A.
http://search.digikey.com/us/en/products/MIC29300-12WT/576-1118-ND/771587

Bottom line is that it's the heatsink and the voltage in that will
determine whether you'll actually get 3A.

 

[Adam Sampson]

"William R. Walsh" <wm_walsh@hotmail.com> writes:

Alternative 3-terminal linear regs will still dissipate the same
amount of heat regardless of their current capability.
Okay...if you don't mind my asking, why is that the case?

Because that's how linear regulators work -- they behave like a sort of
intelligent resistor. If you regulate 28V down to 12V with a linear
regulator, the regulator acts like the right value of resistor to get
rid of 16V as heat.

You can do better with more modern regulators, but you need a switching
regulator rather than a linear one. National's "Simple Switcher"
regulators are good for this sort of thing; you pretty much just add an
external capacitor and inductor.

--
Adam Sampson <ats@offog.org> <http://offog.org/>

 

[William R. Walsh]

Hi!

Your purpose in coming here seemed quite quixotic, not to
say masochistic, and therefore provoked curiosity:

I've never been described in quite that way before. ;-)

Hard to imagine anything other than frequent failure would
provoke this quest.

To the contrary...I have a number of these sets used in various
applications. All are well treated and used under reasonable
conditions. I've never had a failure, even when making heavy demands
from the set. The one time I accidentally shorted a pair of speaker
wires, the set's protection functioned just as it should and there was
no lasting damage.

Yet I look around on the 'net and see people saying "well, the
Sherwood unit blew up, blew sparks and smoke, shuts off past a certain
volume level or just shut down one day, never to turn on again". What
few repairs I've seen done suggested that blown final transistors or
bad filter caps were the cause. My experiences don't align with what
people were saying.

I have no particularly good reason for wanting to know other than to
say I am eternally curious. "Why?" is one of my favorite questions. As
proof that I have no shortage of obnoxious opinions, troubleshooting
and repairing this receiver was arguably more worthwhile than watching
what passes for television these days.

Regarding the purpose of the group: Generally people come here to get
information to help them repair various pieces of electronic
equipment, often because their livelihood depends on it. Instead, you
want advice so you can redesign this run of the mill receiver for
greater reliability.

Not totally. I wanted to share what I found, in the case that other
people here, who might someday be faced with the same situation, could
potentially find a solution. Though I've not been here for a while, I
am not new to the group or its purpose. Your initial response was
taken to mean more of a "why would I bother repairing that piece of
junk".

If I think that something I've repaired could break in the same way,
making a better repair or improvement to the design is something I'd
go ahead and do, provided it does not involve a massive reworking. As
I have never found any simple linear regulator better spec'd than the
78xx series, I thought I'd throw the question in while I was here.

I don't do this professionally, nor do I have the audacity to claim
that I am a professional. I'm just someone who knows a modest amount
and takes an interest from the sidelines.

Why clutter up such a clean and straightforward internal design with
adequate cooling?

No doubt the unit was designed to a price. Who could say what the
original designers had in mind...or maybe they just made a mistake.

It has to be said that Sherwood has been (mostly) helpful along the
course of this repair, much more so than the competion (Sony and
TEAC).

William

 

[William R. Walsh]

Hi!

Alternative 3-terminal linear regs will still dissipate the same
amount of heat regardless of their current capability.

Okay...if you don't mind my asking, why is that the case? It would
seem to me that a more capable part asked to do the same work as a
less capable part would have an easier time of it.

If the problem is in the load (i.e. faulty components) then that
should be where you focus your attention.

There does not appear to be any problem with anything that is powered
by the regulator's output (though I do have some suspicious caps to
replace elsewhere).

William

 

[William R. Walsh]

Hi!

Actually, it sounds more like they blew the design by having too
much input voltage for the output voltage and current.

The regulator is supplied with 25 volts. While I hadn't really looked
at the curves and figures, what they are doing did not sit well with
me from the outset. Why ask the regulator to do that much work?

But I would think that adding a bit of series
resistance to the input side and letting some of the waste
heat go away there would go a long way towards extending the
life of the regulator package.

I had not thought of this and might just sit down and do the math to
see what I'd need. What I do will depend upon whether or not there is
room to put something in place without going to extremes.

William

 

[William R. Walsh]

Hi!

They are heatsinked, but fail anyway.

The regulator I replaced had a fully plastic body, and I wonder if
this did not help to accelerate its demise. The replacement part is a
Motorola/OnSemi LM340 with a metal tab. It seems to me like this would
work better at shedding heat than a fully plastic body.

William

 

[Dave Platt]

In article <y2afwiwzvxl.fsf@cartman.at.offog.org>,
Adam Sampson <ats@offog.org> wrote:

Because that's how linear regulators work -- they behave like a sort of
intelligent resistor. If you regulate 28V down to 12V with a linear
regulator, the regulator acts like the right value of resistor to get
rid of 16V as heat.

You can do better with more modern regulators, but you need a switching
regulator rather than a linear one. National's "Simple Switcher"
regulators are good for this sort of thing; you pretty much just add an
external capacitor and inductor.

Murata now has some interesting little switching regulators which
"mimic" a 7805 - they have the same pinout, same current capacity, and
are built on a small PC board which is roughly the size and shape of a
TO-220.

I recently installed one in an old MFJ packet-terminal node controller
(TNC). THe original TNC design uses a 7805, with a small clip-on
heatsink, and a thoroughly-incompetent attempt to couple the heatsink
to the case for additional heat transfer (they gooped the heatsink
surface with silicone compound, but did *not* actually fasten the
heatsink to the case!). Even with the clip-on heatsink, the 7805 was
running quite hot... not too hot to touch, but warmer than I like.

The Murata part dropped right in, requires no heatsink, works nicely,
and it's barely warm to the touch.

As with any decision to substitute a switching regulator for a linear
regulator, you'll need to consider the output noise - it might be too
much for an audio amplifier to tolerate. Or, one or more additional
stages of filtering may be required.

These Murata parts are about $4.50 each in onesies, through Mouser.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

 

[who where]

On Thu, 13 Oct 2011 08:57:08 -0700 (PDT), "William R. Walsh"
<wm_walsh@hotmail.com> wrote:

Hi!

Alternative 3-terminal linear regs will still dissipate the same
amount of heat regardless of their current capability.

Okay...if you don't mind my asking, why is that the case? It would
seem to me that a more capable part asked to do the same work as a
less capable part would have an easier time of it.

It WILL have an easier time of it but OTOH it will still require
dissipation of the same amount of heat.

If the problem is in the load (i.e. faulty components) then that
should be where you focus your attention.

There does not appear to be any problem with anything that is powered
by the regulator's output (though I do have some suspicious caps to
replace elsewhere).

William

 

[who where]

On Thu, 13 Oct 2011 11:45:48 -0700, dplatt@radagast.org (Dave Platt)
wrote:

In article <y2afwiwzvxl.fsf@cartman.at.offog.org>,
Adam Sampson <ats@offog.org> wrote:

Because that's how linear regulators work -- they behave like a sort of
intelligent resistor. If you regulate 28V down to 12V with a linear
regulator, the regulator acts like the right value of resistor to get
rid of 16V as heat.

You can do better with more modern regulators, but you need a switching
regulator rather than a linear one. National's "Simple Switcher"
regulators are good for this sort of thing; you pretty much just add an
external capacitor and inductor.

Murata now has some interesting little switching regulators which
"mimic" a 7805 - they have the same pinout, same current capacity, and
are built on a small PC board which is roughly the size and shape of a
TO-220.

Although I didn't say so explicitly, the two devices I mentioned are
also TO-220 "drop-in" switchers. It is certainly an emerging market
area.

 

[Dave Platt]

In article <lvqe9797rhk78ufl87fknd7g1iqgoensto@4ax.com>,
who where <noone@home.net> wrote:

Murata now has some interesting little switching regulators which
"mimic" a 7805 - they have the same pinout, same current capacity, and
are built on a small PC board which is roughly the size and shape of a
TO-220.

Although I didn't say so explicitly, the two devices I mentioned are
also TO-220 "drop-in" switchers. It is certainly an emerging market
area.

The "Simple Switcher" (tm) parts I have seen up until now, have the
switcher controller, but you need to install external components
(inductor, etc.)., so their pinout isn't directly 7805-compatible.

The Murata parts have all of that on their own PCB - no additional
external components are required, if you can live with their output
levels.

I would expect to see other manufacturers come up with these sorts of
complete drop-in-ready modules.

--
Dave Platt <dplatt@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!