Thermal Shutdown Question

[badmike]

My dad has an old "MCS" (Says JC Penny on the back) stereo from sometime around the 80s. The problem is when it warms up the sound goes out. All the motors spin and the lights still glow, so it's not turning completely off. This happens on any input. It's worse in summer. When it goes off you hear a relay click. Turning it off then on again will get it working until it warms back up then click! I tried warming up areas with a hairdryer and found that the power amplifier ICs are VERY sensitive to heat. They're more sensitive than I expected. They're mounted on a pretty big heatsink so it looks like they're made to get hot. I checked the screws holding them to this heatsink they're fine. I also looked for obvious bad solder joints and cracks in the circuit board. Didn't see any. Prodding with a plastic pen didn't cause misbehavior either.

I checked Google for the datasheet and found these ICs have thermal shutdown, but it doesn't say at what temperature they shut down at. It says the operating substrate temperature is 125 degrees Celsius. Does this mean the power amp ICs can work at 125 degrees Celsius? I'm not getting them anywhere NEAR that hot when they click off. In fact when they go off I can put my finger on them and barely feel any heat coming off them. Shouldn't it get REAL warm before clicking off?

Model is 683-2252



I checked for bulging caps, didn't see any.

I looked on Eserviceinfo.com and didn't find anything.

I think the power amplifier ICs have defective thermal shutdown, but I'm not sure. Anyone got any other ideas? These parts seem to be discontinued.

The speakers are the same ones that came with it, but he got it at an estate sale. They look like the original speakers. They don't say "Sony" or "Toshiba" on them, but they don't mention JC Penny or MCS either.

The ICs are the stk4044 II.

http://www.google.com/search?hl=en&q=stk4044+II&btnG=Google+Search

http://www.ampslab.com/trans_stk4044.htm

Thanks in advance.

[kc8adu]

Re: Thermal Shutdown Question

i am not sure if the circut has bias and dc offset adjustments but i would start there.
if not most likely bad stk's
the click is the protection relay.
its to keep the amp from destroying the speakers if something fails.

[badmike]

Re: Thermal Shutdown Question

Thanks, I'll look for some variable resistors tonight. It's an old cheap stereo, but it sounds better than a lot of MP3s out there and I don't have to worry if I accidentally release the "magic smoke".

[999999999]

Re: Thermal Shutdown Question

Be very careful putting your finger on them, they may have a dangerously high voltage at their pins. Spec sheet says +-73V max, that's 146V(!) rail to rail. An output protection relay wouldn't necessarily mean there is no longer power to these parts.

I suspect there is a thermal sensor affixed to the heatsink or elsewhere triggering this relay, it is not the chips' thermal shutdown you are encountering but a second shutdown circuit. Chips with shutdown generally do it internally, no relay needs triggered for that to happen nor would it have a purpose in conjunction with that shutdown method.

In other words, I think it was just coincidence that heating them seemed to have this effect, that actually heating them did also mildly heat the other shutdown circuit sensor which is prematurely activating.

IMO, you need to trace back the circuit from the relay to the driving sense circuit and look at the solder joints and parts susceptible to aging in that area like capacitors.

I hate to write it, but if all else fails and you really don't care about letting the smoke out, you could just pull the relay and wire directly the relay contacts that it closed, being sure you have the right ones to the right positions... but really it is fixable if only an understanding of the parts activating it is achieved.

[hardwareguy]

Re: Thermal Shutdown Question

The STK 4044 is actually a nice IC, it came in three versions. The most common is the II version and it is rated at 100W @ 0.4%THD. Upgrading it to a V or XI version will yield lower (V) or MUCH lower (XI) distortion. They are pin compatible, just drop it in and enjoy.

They have NO protection facility whatsoever. They are hybrid ICs, the internal circuit is essentially a very tightly packed amp stage made from discreet transistors. It is not a monolithic chip at all. Oh and yeah, don't go poking the pins...... the rails run at around 50V, 100VDC is available across Vss and Vdd!

My guess is the transistor that drives the protect relay, the thermistor or the protection IC (usually a 9 pin Toshiba device) has gone off the deep end. The STK could be bad as well (output transistor goes leaky when hot is a possible failure), but the 40XX series was one of the tougher ones. It is discontinued but fairly easy to find.

There was a closely related 200W @ 0.08THD that has three extra pins, IIRC...... it's BIG, 78mm across! Retrofitting it in would require a different PSU and some extra leads. Fan cooling may be needed with heatsinks designed for 100W modules. A 120W module can be popped in place of a 100W module just fine, you get a little more headroom if the reserves are adequate.

Long live STK modules!

[999999999]

Re: Thermal Shutdown Question

They have NO protection facility whatsoever. They are hybrid ICs, the internal circuit is essentially a very tightly packed amp stage made from discreet transistors. It is not a monolithic chip at all.

Then why does the linked datasheet clearly state on page 1: "load short-circuit protection, thermal shutdown..."

If you drew your conclusion about it not being a monolithic chip amp from the equivalent schematic, you might take a look at the equivalent schematics for other known chipamps. Yes there are a bunch of transistors inside, but so it is for any of them.

[hardwareguy]

Re: Thermal Shutdown Question

Then why does the linked datasheet clearly state on page 1: "load short-circuit protection, thermal shutdown..."

If you drew your conclusion about it not being a monolithic chip amp from the equivalent schematic, you might take a look at the equivalent schematics for other known chipamps. Yes there are a bunch of transistors inside, but so it is for any of them.

I have actually sawed the top off of a dead one to see inside. Look up Sanyo IMST to see how it is made...very interesting. And the datasheet is merely saying that said protection can be added. Short one out and see if it survives without an external protection circuit....... it wont!

Note the spec: available load shorted time. This allows you to use a fast blow fuse as the protection. Panasonic/Technics did this on some of their early amps up until around 1980, very effective IF the owner puts in the proper fuse!

The low value resistors in the output leads on the example schematic in the datasheet make sure the amp never gets a dead short, they buy the amp some time so the relay can cut out or the fuse can blow. Bad for the damping factor, but speaker cables are too so it doesn't really matter. If designed right and given a proper PSU it sounds quite good.

STK module based amps can sound really good if properly designed, Technics had several with THD figures lower than 0.01%! S/N ratios were in the 90s too.

[999999999]

Re: Thermal Shutdown Question

If you don't wanna believe me don't but it's a chipamp not just a few transistors in a big blob of epoxy.

[Brian C]

Re: Thermal Shutdown Question

Hi BADMIKE....... The click sound is actually a relay contact (make/release) sound. Usually there are 2 types of protection circuit in Power amplifier: eg. DC offset protection and over-current protection. Some amplifier has thermal protection by using thermistor. Apart from this, there is a DC supply fault protection. All these protection circuit generate a trigger signal to micro-controller (during fault) so that micro-controller can turn off the unit or speaker... or shutdown the power amplifier IC (protection).

If ur amplifier sound goes off after the "click" sound, that means the speaker is being disconnected from the amplifier output. The tiny micro-controller in the set turn off the output relay so that it protects ur speakers.

U can post the picture of ur amplifier unit here so that we can roughly guess what is wrong. If u want to troubleshoot...:
(1) Check the STK supply voltage, (both pre-amp and current amp stage) during normal operation and during fault.
(2) Check DC level at the STK IC output.

Troubleshooting will be easier with the help of digital oscilloscope.