Technics SA-310 Receiver Fixed!

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If the "outboard speaker protection device " is a "crowbar" how do you know it was not a failure in that which knocked out the amp ?

Boy does THAT have a familiar ring.

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Live Fast, Die Young and Leave a Pretty Corpse

Hi!

The device in question opens the circuit by tripping a relay (one per channel).

The left channel of the amplifier was already dying before I was using it anyway. At first it got weak and distorted when I turned up the volume. It would come back to working order if I turned it back down.

Then it stayed broken, with the weak and distorted audio on the left channel. Around that time, I put the good speakers away, got some crappy ones and decided that I'd rather not smell them burning up.

It played for a while beyond that and then "click" ... the relay tripped.

William

Hi!

I don't think I know what you are getting at.

William

Some years ago an audiophile reviewer complained that an amplifier's heatsinks got unduly hot. The manufacturer replied that that was the purpose of the heatsink, to remove heat from the output stage.

Although this makes a sort of sense, the fact is that an extremely hot heatsink -- except when the amplifier is driven to the point of maximum dissipation, which I believe is about 35% of clipping power for a weakly biased class-AB amp -- suggests that the heatsink isn't big enough.

The STK subbing. That actually went on a lot with so many different models of those receivers and amps.

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Live Fast, Die Young and Leave a Pretty Corpse

Hi!

I'd certainly agree with that.

It may very well not be big enough. When I removed the old module, I noticed that the factory's application of heatsink compound was basically "glob it on there, the thicker the better".

I cleaned all of that mess off, followed by rubbing down the backplate of the new module and the mating surface on the heatsink with 99% isopropyl alcohol to assure that both were very clean. The old compound was dried up anyway. Might as well do it right. Then I did it up with a *thin* layer of Radio Shack's basic heatsink compound, applied to the back of the module with a credit card slice. Just a very little of the compound squirted out from the top when I screwed the new module back into place...a lot less than the excess "blob" that had been resting on the old module's top.

I have no reason to believe that the hybrid module was ever replaced prior to this. My other, similar Technics receivers also show signs of "too much heatsink compound".

This seems to be doing a better job. With the old module, I remember its plastic casing getting hot along with the heatsink. The new module gets the heatsink hotter while its plastic casing stays much cooler. This could mean a lot of things. I take it to mean the heatsink is being more effectively utilized now.

The receiver itself is a slimline unit:

which limits any possibility for modification. I definitely do think I could take two small DC fans, borrow a little power from the 28.5 volt tap on the transformer, run it through a rectifier and greatly reduce the heat problem. As each of the 28.5 volt taps power the audio amplifier, one easily ought to have enough power to spare for some little fans.

I also have an SA-210 and SA-120. These have similar power output to the SA-310, use the "Series I" Sanyo STK-XXXX modules and seem to run in conventional Class AB mode (no Computer Drive or "synchro bias"). Both of them put out a LOT less heat when idling or working hard.

William

Hi!

I read somewhere that some manufacturers even did this when building a new receiver, using a bigger module where a smaller one had been called for.

This new module went into place very well. I did a better job of cleaning out the through-holes than I first thought, although I did have one pin miss its hole and get a little bit bent up in the process. It was an easy fix. Soldering the new module in also proved to be pretty easy.

William

I've replaced several dozen of those back in the 80s when I worked for a person who was factory authorized by 10 major brands. I remember writing down what I needed and getting replacements with more pins or less pins. Then having to look through Panasonic tech literature and choosing replacements by physical/electrical description and ignoring the STK numbers to some extent. Finally had hand written my own cross reference.

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Live Fast, Die Young and Leave a Pretty Corpse

A follow up to my follow up...one thing that I've noticed is how hot the various "Computer Drive" receivers run even when operating normally. So I found my IR non-contact thermometer and had a look at the newly repaired SA-310.

At idle for around 20 minutes, the temperature of the heatsink had risen to

130 degrees Fahrenheit (54 degrees Celsius)! And the unit was just *idling*. The hybrid's internals are probably a good ten degrees hotter still. Call me picky, but that is not what I'd call the recipe for semiconductor longevity! Ouch! (Just how did the last hybrid--presumably the original--manage to live so long?)

By comparison, the Class AB SA-120 (with its identical power rating) idles around 88-90 degrees F after being on for a while. Its STK module also has a much smaller heatsink.

For now I have made a "contraption" consisting of a DC muffin fan affixed to the bottom of the SA-310 using bubble wrap on the corners to stop the case from vibrating. With this setup, and two books at each end of the receiver to give the fan clearance, the temperature never gets above 105 degrees F even with the cover on. Most areas of the heatsink are slightly cooler than that (in the high 80s or lower-middle 90s).

Later, when I get some free time, I plan to install two small fans on the heatsink, helping to pull air over it and hopefully keep that hybrid alive and comfortable.

It has been suggested that minorly violent and mostly non-destructive disassembly of an STK-series hybrid amplifier is possible. Some people even make the claim of having repaired the bad parts within. I may just have to try it if the old hybrid hasn't walked off yet.

The DC offset as measured at the speaker terminals (no source playing, no speakers attached of course) is between 8-12mV for both channels now. I believe the unit to be fixed. :-)

William

Do you have the temp specs of the replacement STK?

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Live Fast, Die Young and Leave a Pretty Corpse

Hi!

No, I don't. I've never even been able to find a datasheet for either of the STK-2038 II or STK-2048 II modules, nor anything that seemed closely related.

Of those I did find (for the Series I modules), I don't recall seeing any temperature information. But it's been a while and those copies were pretty illegible.

I was only a little bit surprised to find that something like this wasn't included with the replacement. In this case, the replacement module is actually a new old stock genuine Panasonic replacement.

William

A PDF at alldatasheet.com shows a 2040 as having a max junction temp of 150 C. That's 302 F. I would guess most are the same.

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Live Fast, Die Young and Leave a Pretty Corpse

Hi!

I don't think the module would live very long at that temperature. The behavior of Computer Drive seems to keep it hot, too. I've always been of the school of thought that said "keep your silicon devices cool and they will live longer".

Then again, I have no reason to believe that the old module was ever replaced, and it did survive 27 years worth of usage...

I don't know about the relative toughness of STK-XXXX amplifier modules. I've always been of the impression that they are not always robust. Yet it seems that people have abused them and gotten away with it. And outside of this one receiver, I've never lost one...

William

I've always thought the PCDPP to be robust. Your right they wouldn't last too long at a max Jt of 150C but most survive well at a working temp of

105C.

Panasonic, Sanyo, Sansui, Pioneer Yamaha etc used them. Rear Projection TV sets used them for convergence amps which I consider the hardest working part of the set. So there must be some reason these were chosen other than a convenient package. If you use the correct amount of heat sink compound spread evenly and torque them down properly provided the heat sink has enough surface area there is no reason for these to be problematic. Like you said the originals in your SA lasted 27 years even biased class A.

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Live Fast, Die Young and Leave a Pretty Corpse