SONY STR-DE635 NO POWER UP

Check main transformer primary and secondary.

Relay click indicates the standby transformer and system control is probably ok.

Are you sure you do not get an overload message on display and then shutdown? If yes, then main power supply has a problem.

If you did not have enough ventilation, it is very possible that the thermal protect cutoff inside the transformer on the primary has gone open (pretty common). Transformer is very expensive to replace.

Getting a hold of the smaller thermal protect fuses that go inside the transformers will be difficult at best as no one seems to carry them in single quantities.

If anyone has a supplier for those 4A thermal cutoffs used inside the transformers please post.

THANK YOU

Yes, no overload message present - all power display functionality is dead (for example, muting/bass button light are dead, nothing appears on LED display at any time).

This will be a junker amp; can you give any tips as to how to get inside the transformer to check the fuse? I cant see any way to get in it.

Thanks again

Matt

Typically you will see three wire connections on the primary, two going to the ac line, one all by itself. The two that look like a thick solid wire are usually the thermal fuse. You may have to remove the plastic guard to get at it.

Simply measure the primary of the transformer to see if it is open before going any further to see if there is a thermal fuse open.

No, you won't see it, it is embedded inside the wrapping inside the transformer. Sometimes it can be carefully removed and replaced, but rarely.

thanks!

Will let ya know what happens,

Matt

Yep, primary wide open.

Operation destroy transformer (in all likeliehood) underway. Oh well. Not as oif I can really make it any worse.

Anyone got any good supply house reccs for transformers?

Thanks Matt

Can you post some decent digital pictures of the transformer somewhere? I can then repost the picture with a marking showing where the thermal cutoff is hiding and how to get at it, if it is possible on yours.

I don't have a camera here, Ill see if my sis will let me borrow hers.

BUT - wouldn't do any good anyway, as ALTHOUGH THE TRANSFORMER IS NOW WORKING (Notice, I Didn't say "REPAIRED") - and all system functions appear normal...... I still have NO IDEA where the thermal fuse is.

I'll describe my approach as best as possible:

I followed the primary leads in to the transformer. In this case, Orange and white.

Covering the top of the primary windings was a plastic layer; cant really describe it but it sort of looked like a row of widely spaced teeth.

Ripped all that out off both top and bottom. A screwdriver and some needlenose pliers, and then the plastic was gone.

Now, where the primary feed leads go into the transformer, I noticed what I believe you called "the third lead" tucked down in front of the primary windings.

Using a small screwdriver, I "untucked it, and it appeared to somply be

Now I flipped the transformer over and just started digging in the same space that the above wires went into. I had no idea what I was looking for, what it looked like, where it was, or how I could possibly get to it even if I knew any of the above.

In the end, the screwdriver I was using easily passed _through_ the gap between the primary and secondary windings, and the orange and white leads had long since been ripped from wherever they were connected to.

All that was left were 2 leads. 1 on the 'inside' of the primary winding, on the bottom of the transformer, and 1 on the 'outside' of the primary winding, on the top of the transformer. My hope was that these were simply the beginning and ending of the primary winding.

So, to answer your question about the thermal fuse.... I don't think it exists anymore. There is a large pile of what looks like very fine sand on my work table though.

I got a measurement of 1.5 ohms across the leads, and figured that should make a nice big spark or a blackened outlet when I plugged it in. But hey, I know NOTHING of what primary windings should ohm out at, and so long as it wasn't at ZERO, I was willing to give it a shot.

Placed everything on top of a box (its in a few zillion pieces), and plugged the 3 or 4 seconday connectors in. Plugged the front panel in, gritted my teeth, stood back as far from the outlet as I could and still get the plug in, and .... ZOWIE, all the indicators lit up, relays clicked, LEDs sparkled to life, and apparantly other then a transformer thats a 'bit' more dangerous now then it was before, I apparantly have a working amp again.

So I guess my answer is pry around in there with a small screwdriver, you'll dig some paper out, eventually you will find the beginning and end of the primary, and there ya go.

Personally I'm quite pissed that the fuse was there in the first place. I can see the need for monitoring an overheating transsformer. I can also see an easy way to do it wothout burying a fuse inside of it. Is this part of the built in obsolescence program?

Other then maybe unplugging it when not in use, anyone have any suggestions on how to return a bit of safety to the xformer?

Ill still try to take some pics to give ya an idea of all hte hackery I talked about tonight.

Finally, when looking for info on this problem, I ran across a source that said another cause of this problem is shorted or failing output power transistors. Bear with me guys, I don't know the proper term, but the big huge honking transistors mounted to the massive heat sinks.

Looking at their solder connections, it does indeed they have spent quite a warm life.

What do ya say, should I go ahead and replace them as well? And if so, anything else?

Thanks much, Matt

The thermal fuse is there due to safety regulations to prevent a fire. An overheating transformer is one of the easiest ways a device like that can start a fire.

Well sure, I agree - but certainly, a design could be made where it is also replaceable.

Some main transformers can be "refused" from the outside. There is sometimes three legs, instead of two. You could hotglue the transformer as you put the layers back together. I have done quite a few over the years, & none have caused any problems. Rono.

Here's what this type of trasnformer looks like (schematic)

Some units have different line input voltages 120/220 etc.

A blown thermal fuse might mean a compromised enamel coating on the winding. Hence the regulating authorities would rather have us replace the entire transformer. Psst , just between u n me, i've bypassed a couple of thermal fuses and billed the customers. Guilty as sin ! Jango