STK404-140S Question

[The Dude]

I'm just curios if anyone has enough experience with these things to tell me whether this might be a counterfeit or not. It was purchased as "Original Sanyo". I'm having serious doubts if that's true. Looking at the actual output devices within the part (on the heat sinks), they look substantially smaller than the ones from an original part and it seems suspect to me. I could certainly be wrong. It's been a while since I cracked one of these open. The reason I ask is because this is the second failure in this amp- both times the STK was replaced. The IC's for each repair came from different vendors. I want to make sure I get a "real" part next time, if that's even possible. I'm thinking NOS versions of these are rare as hen's teeth. BTW, the amp is the GC Acoustic B200H.
For your viewing pleasure:

[nickb]

Wow! I'm really impressed that you got it open so cleanly and with no damage Do tell how you did it....

I did fit one of these to VOX about 6 months back. No reports of failure so far but I was highly sceptical about the originality. I bought a spare and now I'm tempted to do what you just did.

Do those output transistors bond directly to the heatsink?

[J M Fahey]

**Usually** fake transistor chips are covered in the infamous white epoxy.This is your presumed fake:


Maybe they think it cools them better because "itīs a white paste"? Only half kidding here ... or not at all
Tech removed one white blob showing a die too small fo intended power.
I get mad because if they used just a slightly larger die in an otherwise complex circuit, it would actually work.
Who cares who actually made it as long as it works?


Fake:


Original (no epoxy of any kind hiding power chips):


Fake:


Some go to heroic levels to repair them:

[J M Fahey]

Wow! I'm really impressed that you got it open so cleanly and with no damage Do tell how you did it....

They are not actually potted or gooped but just a real discrete power amp, built on a conventional PCB, with a plastic cover over them.

[nickb]

You could have played me for a bit there

I took a look at the ones I have and see that what you say is true. I kept the original so I could lift the lids on both it and the copy but I think you have already answered the OP , so Dude, yell if you want me to do it.

[The Dude]

Thanks for the comments, guys. I suspected it was fake, but.......
Nick, to get the thing cracked apart, I set it legs up on the bench, leaned it against something to keep it from tipping over, inserted a small screw driver next to the legs on each side and gently whacked it with a hammer until it started coming apart. I then pried it the rest of the way. If you could crack open a known original part, I'd love to see the difference. My recollection is that the output devices were larger last time I opened one. It would be nice to confirm and I no longer have the original part. That said, don't put yourself through too much trouble on my account.
My next problem is finding a "real" part to fix this thing once and for all. If I can't repair it with confidence, I might just refund and bail on the amp.

[nickb]

Here it is. BTW this was an STK404-130




Busted transistor:

[nickb]

Agee Juan. Also, if they were upfront and said something like "new design compatible replacement for STK4XX made with current techniques" then everyone would be happy.

[J M Fahey]

Just as a "Plan Z" , please post a couple gut pictures of that amp, a general view and the power amp/heatsink area.
Since they reproduced the aesthetics of the late 70`s ACOUSTIC amps,that chassis should be roomy and half empty.
Also post rail voltages.
Plan Z might involve mounting a cheap simple power module inside that chassis, attached to old heatsinks and fed from original supply.

[The Dude]

Oh, there's plenty of room in the chassis. Here's a picture. I'll get some voltage measurements as soon as I have time to get back into the thing. I had thought about mounting another power module in there. That may be an option. I'll have to see what the customer wants to do.



Edit: Forgot, thanks for posting the pics Nick!

[J M Fahey]

Just looked at your amp: you have lots of space BUT Power Amp PCB also contains main power supply (so far no big deal) but also low voltage supplies, Fan supply, DC protection, etc.

You might still add an external module, only pull strictly power amp parts, just to put them out of the way, and wire the 5 basic connections to new power module: IN / OUT / +V / -V / Ground
Amp will work (as in: amplify and drive speakers) but will lack some functions, specilly protections.

Keep looking at the STK module and itīs really a packaged (not even encapsulated) discrete power amp, and as such .... "repairable".

Given it uses power Darlingtons and resistors are external, I wouldnīt be surprised to find that only them are dead and other parts are functional.

If I had free time, and just out of curiosity, Iīd grab a couple power Darlingtons (even humble TIP142/147, but in that case use a lower supply, say +/-40V or less) and try this brain surgery:



If I ainīt wrong, left one should be PNP and right one NPN .

That said, I found a UK supplier who claims to have STK:
https://www.express-elect.com/about.php

[The Dude]

Yep. The power supply and power amp are indeed on the same board. That said, I think I could still install a power amp module and just use the existing supply. Leaving the STK out should, as you say, "pull strictly power amp parts" since the STK basically is the power amp. I actually checked out the STK after I popped it apart and it's not just the outputs that are shorted. Drivers are also toast and there are some burnt resistors. I'll update after I check to see what rail voltages are and thanks for your help!

[J M Fahey]

As you already know, I donīt care too much about tube (slightly bored about the same old 4 or 5 recipes on the menu, repeated over and over and over and over) and on the contrary am a sucker for SS design ... and that amp reeked of Rick Kukulisī hand (every designer has some pet ideas which he uses wherever he can) and on a hunch checked his designs: that "Acoustic" amp is actually a (modern) Kustom.
To be more precise, the KBA115C/210C .

[The Dude]

Thanks Juan! At least I now have a schematic that's close!

[glebert]

Dumb question, but would Acoustic (or perhaps Kustom) sell replacements for this IC? I know Kustom offered to sell me a reverb tank for my Kustom Defender V100 amp (which I still haven't gotten around to finishing..).