Seeking STK8050 (Technics SU-V4)

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I orded an STK8050 from Dial Electronics and unfortunately I think it's a counterfeit.

The replacement device failed instantly upon application of power.

I have to do more checking to be absolutely sure that nothing in the amplifier caused this to happen but I'm at least reasonably certain in was the IC itself.

In the original failure the power IC had shorted to the positive rail and one of the synchro diodes had opened.

In the replacement, the IC shorted to ground.

The only commonality I could find in these failures regarding possible faults on the amp board itself would be with the quiescent current circuit. Too much current would cause the output stage to overload and short. The problem with this conclusion is that the components of this circuit appear to be alright.

Will write back with more later when I feel like doing more checking. For now I'm highly annoyed and don't want to look at the amplifier.

I am interested in hearing if anyone else has run into this problem (I know these IC based amps are prone to failure on overload).
Mmm that's an unfortunate set of circumstances.

I first looked at these ICs last year when I was offered a similar Technics amp which was faulty. I was tossing up whether to purchase the amp when I read a couple of reports from people who said that the ICs were somewhat unreliable.
That, coupled with the cost of them and the need to source them from overseas sealed my decision to find a different project.

There is a big issue with knock-off parts. They look identical to the OEM factory parts, so there's often no way to tell if they are the real deal or not. This is not a new problem - it's been happening for years. It started in the 90s off memory, when I was repairing TVs. We would order in replacement line output (horizontal output) transistors from the local supplier that we'd dealt with for years.
Upon fitting these devices, they'd work for a couple of hours then let the smoke out. At first we thought we'd missed something else in the circuit that was causing these things to fail - but it turned out we'd got a batch of china knock-offs.

Dial Electronics are a reputable outfit - been around for a while. I'd go back to them and see if they can shed any light on the source of that part. If they can verify that it's a genuine item, then something is still wrong in the amp.
With the IC out of the board, measure all connection points where it mounts and see if there are any voltages present which shouldn't be.

Also check the supply rails against those shown in the schematic.
Indeed I'm quite familiar with the counterfeit semiconductor issue.

This came up a lot with the prized 2SC3281/2SA1302 from Toshiba. I have a batch of originals but of course ordering new replacements is almost impossible without getting fake ones.

As for the Technics amp; I've sat back down in a calmer moment to look it all over. So far I cannot see anything wrong with the rest of the amplifier. The bias circuit, synchro diodes, diffamp, etc. are working properly. There is no reason the circuit should have popped an STK8050. The VAS/current source are operating properly and pulling about 8mA (as they should). The Icq setting potentiometer is working properly and there are no failed resistors or transistors.

I've messaged Dial Electronics to see what they can do. I await their response.
I've gotten another chip from Dial; this time I'm doing rigorous tests to see what went wrong.

I've been powering the amp at lower supply voltages to prevent pushing the output stage of the STK8050 out of its safe operating area.

What I've determined is that the chip from Dial does work; it biases properly and reaches essentially the correct operating voltages. The problem lies in how its bias resistors were selected though. It seems that the current into the internal drivers via the synchro circuit is much greater than it should normally be. On taking external measurements I've also found that the "new" STK8050 is different from the original. The Vbe multiplier resistors are both 2.4k in the new one, while they are different/smaller values in the original. Also, the bias network around the drivers and synchro circuit shows different resistors (in such a way that would tend to agree with the increased operating current seen in circuit).

What I find is that by modifying circuit values on the board around the STK8050, I can make the new chip work. (Modification includes adjustment of the bias and synchro clamp voltage pots but almost to the end of their travel).

Unless Technics revised the STK8050 at some point along the way (I find this incredibly unlikely since there is no change in its designation), this chip must have been made differently and obviously not to the original specification.

I'm still looking for any malfunctions in the amplifier PCB which could cause this but so far I've found none. My next course of action will be to put the original chip from the other channel in the channel I'm working on. If everything works then I have proof that it is the new IC and not a problem on the board.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.