SX-450 Bando Transformer - D.O.A.

[Watthour]

A SX-4½ arrived FEDEX this morning. "Won't power up..." was the stated problem. As is customary, "No other testing was done..." and no one had any clue what might be wrong. Those are the kind that are right up my street.

Opened from the Kleenex box it was shipped in. pulled the sides and top, blew out the cat hair, cigar ashes, and 40 years of dust. Tossed it on the bench, connected a 40W in the DBT, and powered on. Nichto. Removed from power, metered switch, main fuse, and transformer primary, where I decided it was time to stop. The primary was as open as a California border.

About eight screws later and a little coercing the wires, and the thermal limiter was exposed. Of course, it was open as well. Fortunately, it had done its job and protected the transformer from the external abuse to which it had been subjected:

Time for surgery:

With the dead fuse exposed and pulled, it was time to hunt down the replacement. The fuse was rated at 169°C / 15A:

Fortunately, I keep a some spares on hand for just such an occasion. It's one of the few things NTE is good for:

With the new fuse buried into the winding, it was time to make the connections:

A little fiberglass sleeve over the connections, and sealing up the firecloth with enamel:

After that, I re-canned the transformer and put it all back in place. After a few minutes of assembly and allowing the enamel to flash off a bit, it went back on the DBT at 40W.

I swear it lit up my whole office. It wasn't totally unexpected, but was actually a GOOD sign. Upon further review, the cascade of problems became apparent. I found all the fuses in the unit had been replaced with brand-new Bussmann 10A/250V replacements. Well, it makes sense - If you're going to buy a whole box of 10A fuses you might as well use them all, right?

I pulled the fuses from the main bus/rail supply and tried it again on the DBT. It lit, dimmed, and I had dial lights. It actually powered up the 13V supply so the tuner even reacted. Amazing.

After a bit more poking around, you might have thought that the charcoal on the AF board would have been a clue. I'm not certain if it was before or after the 10A fuses were installed, but I am certain it was before the transformer fuse left the building. One, just one of the outputs had gone E-C shorted, and started the cascade of errors until the owner could no longer plug in fuses or parts enough to even make it light up.

And the auction listing simply stated "Won't power up..."

 

[zebulon1]

I love it!
What made you feel for the little 41/2?
It probably couldn't wait to see you. It was saying "Get me away from this guy".

Peel back the layers from your delightful description, and you can see the thrashing about the previous owner was doing. Funny.

 

[rcs16]

Nice surgery Dr.

 

[markthefixer]

Not only is that the first transformer I have seen that popped it's internal thermal safety,
but it's ALSO the first transformer I have seen that has had it's internal thermal safety REPAIRED!!!

Congratulations and KUDO's to You!!

With the fried 150 ohm R229, I'm surprised that the green emitter resistors (R235, 0.5 ohm only 2 watts) look ok.

 

[Watthour]

That surprised me too, Dr. J. What really shocked me was that the drivers were not hurt.

I had a spare 2SD313R pulled from another unit some time ago for upgrad to MJE devices. It tested O.K. and seems to be just fine.

FWIW, given the fact that you've routinely surgically altered the STV diode stacks, I'm a little surprised that you've never done this. Maybe you've never found one that someone tried so willfully to destroy. I did the same thing in a 780 some time ago, but didn't document it at the time. It was quite a bit easier, too, since the transformer construction is a bit different.

 

[markthefixer]

Maybe you've never found one that someone tried so willfully to destroy. .

Haven't personally run into a popped or destroyed transformer (not even way back in my bench tech days), and that was the first one I had READ about from a believable source. I had started to regard power transformers as invincible.

 

[rcs16]

I have a Sansui G-7500 transformer with a open primary. It took it apart but I could not find a thermal fuse if one exists.

 

[Watthour]

RCS - Some of them are much smaller, and flat.

Many appliance transformers don't have them at all. It's a little surprising that Pioneer did in this vintage, but they obviously did their homework when engineering their product (and it is evident in many other places in their designs, too). They are not on the schematics, either. I doubt that many manufacturers are doing this today.

 

[Markoneswift]

I had a Pioneer active sub a whole back and the Bendo transformer on that unit blew. No thermal on it unfortunately so I pulled the amp chassis to use for parts and scrapped the rest - turned out the woofer was blown too so couldnt even keep that.

 

[EastPoint]

I actually have a parts 450 that wouldn't power up either. Wonder if it's got a bad transformer too...

 

[Tom B]

Working on units in the 70's, I saw a number of open primary transformer windings as I worked the bench. These cases were usually the result of someone putting in a 20a or 30a car fuse in place of a much smaller fuse. Typically outputs shorted, blew the external line fuse, and the owner replaced the "BAD" fuse. Output were actually what was bad, not the fuse, which performed its job.
So we usually would open up the transformer to see if it was salvagable. Upon seperation of the covers, if we smelled "burnt winding", it usually meant the transformer was toast. Similar if you found the primary winding wire melted where it connected to the external wire. But if it smelled OK and had an internal thermal fuse, we would usually jump it with a small piece of wire, and fire it up to see if it would work. If it did, we would reassemble, and usually put another fuse on the primary winding, slightly lower value than the rated line fuse (Like 2.5a instead of 3a).
Understand that the work we did was "Production". We had to complete as many units as possible, as quickly as possible, and shortcuts such as this was necessary, because we usually didn't know what value the thermal fuse was, and where to get one. Its not like today with the internet and ebay or mouser.
Now as a hobby, I can spend as much time on a unit as I want.