VOX AD100VTH Power Transformer Problem?

[HotHead]

Pretty sure I stepped in it this time. Picked up said amp in a trade. Known problem was blowing 3.15A input fuse on power up.

Primary side of PT shows 3.3ohms. That's basically a dead short to 115VAC, yes?

Am I missing something?

[Enzo]

No, it doesn;t show that. It may or may not be shorted. It is a transformer, they don;t work on resistance.


Look up "light bulb limiter", make one, and use it.


Now, disconnect your power transformer secondary wires - all of them. Now power up the amp - we are now powering just the transformer - does it still blow the fuse? (with a bulb it won;t blow the fuse, just makes the bulb come on bright. If tghis still bugs it, then the tranny is probably toast. Usually it is the last thing on the list.


If the transformer passes the power to primary only test, then connect the secondaries back up, one at a time, until you find whioch one has the problems. My vote will be the main power rails supply.

[J M Fahey]

Fully agree and add: that amplifier has an STK404 power amp chip, probably a 130S and it´s probably blown.
You´ll have to replace it.

[HotHead]

Gawd am I rusty! That and my Navy training wasn't exactly focused on audio gear...

Okay, the "red" pair of secondary leads (90VAC) is the problem. Unfortunately, I am unable to find a usable schematic on-line and VOX doesn't want to share. :-(

I assume that "130S" refers to a 130watt power rating. If the STK404 monolithic amp is the problem, can I substitute a lower power version? Neither Mouser or Newark show the 130S variant.

[Jazz P Bass]

The STK130S is the latest version of the IC.

Pin to pin compatable.

I asked at Mouser & a tech indicated these facts.

I have used the S successfully in replacing the plain 130.

As to checking the installed IC, get the datasheet.
http://pdf.datasheetcatalog.com/data...wlpx9ha3wy.pdf
Usually when these 'brick' ic's blow, the output pin will be shorted to either (or both ) of the power rail pins.
If a short is apparent, it's best to remove the ic & make sure it's confined to the ic itself, and not the board.

[HotHead]

The part installed in the amp is an STK404-130S. Neither Mouser or Newark show a STK404-130 of any type. They show -70, -120 and -140.

Datasheets are pretty much meaningless to me. It's mostly engineering info. I am not an engineer. Just a poor musician trying to support the band's gear needs. I buy or trade for broken crap, fix it, and trade or sell it for a few bucks. I have to do most of the guitar/amp/PA maintenance as well.

That's good info if I knew which pins were the output and power rails. I still don't have a schematic...

[Jazz P Bass]

This is where I purchased a STK404-130S.
B+D Enterprises - Search results for: 'Stk404-130s'

[HotHead]

Thank-you sir! Looks like it's gonna cost more than I had hoped. Oh well, that's the gamble.

I took the trusty meter to the beast and found 5 pins shorted together. Pulled the damned thing out (gawd I hate desoldering multi-pin components), and I can now power up the amp without blowing the fuse. Yay !!!

Thank-you gentlemen for you're patience and advice.

[J M Fahey]

Ok, you travelled half the way, up to pulling the bad STK and apperently confirming ot´s the only bad part.
By the way, I HOPE you pampered the PCB, didn´t tear, pull or burn those pads and tracks.
Hope you used a good desoldering pump.
Now the good news: the 140 is the same physical size and pinout of the 130, so you can use it.
Remember to apply thermal grease, replace the mica if one was used, etc.
Turn the amp on first through a bulb limiter and check it´s fine.

[52 Bill]

Datasheets are pretty much meaningless to me. It's mostly engineering info. I am not an engineer.

I'm no engineer either, but the datasheet will show you which pins are supposed to be connected to the power supply and which ones will be connected to the output, etc.

Knowing the pinout will always help when you test for voltages and for shorts.

[HotHead]

I've been soldering/repairing for decades. Please don't assume I'm an idiot just because my troubleshooting skills have deteriorated over time.

IC's were almost unseen in the gear I trained on. Monolithic amps like this didn't even exist. Not that it matters. As one of my instructors said, "Everything is a toaster. Power goes in, makes heat, then goes to ground."

I did not see a pin-out diagram on the datasheet. At least not what I would call a pin-out. A schematic is much easier for me to work with.

[Jazz P Bass]

Here is the pinout diagram.

Most probably, Vox used this layout too.

[52 Bill]

I've been soldering/repairing for decades. Please don't assume I'm an idiot just because my troubleshooting skills have deteriorated over time.

IC's were almost unseen in the gear I trained on. Monolithic amps like this didn't even exist. Not that it matters. As one of my instructors said, "Everything is a toaster. Power goes in, makes heat, then goes to ground."

I did not see a pin-out diagram on the datasheet. At least not what I would call a pin-out. A schematic is much easier for me to work with.

Sorry if you thought I was being disrespectful, I was just trying to help. Have a great day.

[Jazz P Bass]

Here is the SM if that is needed.

The power block is on page 4.