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Sherwood RX-5502 Receiver Protection Shutdown, Repair, thoughts wanted...

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

Reply to
William R. Walsh
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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.

Reply to
spamtrap1888

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

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.

Reply to
Gareth Magennis

Hi!

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

Reply to
William R. Walsh

Hi!

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

Reply to
William R. Walsh

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.

Reply to
Mark Zacharias

(snip)

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.

Reply to
who where

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.

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"
Reply to
Jeffrey Angus

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.

Reply to
Gareth Magennis

These will go up to 3A.

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

Reply to
JW

Hi!

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.

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

Reply to
William R. Walsh

Hi!

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?

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

Reply to
William R. Walsh

Hi!

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

Reply to
William R. Walsh

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
Reply to
Adam Sampson

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                                    AE6EO
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior
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Reply to
Dave Platt

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

Reply to
who where

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.

Reply to
who where

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                                    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!
Reply to
Dave Platt

On Thu, 13 Oct 2011 09:03:39 -0700 (PDT), "William R. Walsh" put finger to keyboard and composed:

I agree that's the simplest solution. In fact I've done similar things in lots of gear, including a 586 motherboard.

- Franc Zabkar

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Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

Hi!

If I've found the right part, I reckon you are speaking of the 7812SR/ SRH-C regulators. These look like interesting devices but the low current rating may be a problem. I need to rig up a way to see what the current draw actually is.

I don't think noise in the circuit would be a problem, this regulator appears to be powering a protection circuit. I've not traced the circuit to know exactly what it's powering beyond that.

William

Reply to
William R. Walsh

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