Thursday, September 1, 2016

Exploring the Sanyo STK3122-III

As part of my routine management of perpetually failing appliances, I once again found myself repairing my Marantz TA170AV.  Fully suspecting the failure, and recalling that I'd mentioned it in passing once before, I figured I would approach the repair with a mind to be a bit more analytical.

This receiver came to me years ago after suffering a slow degradation of output power over the course of months.  The original owner endured the loss of one channel and then the next before gifting it to me in the traditional manner.  What better way to reduce the regret of discarding a failed investment in equipment than to let it disappear into the life of that techno-hobo friend of yours? At the time, I merely traced out the signal path and found that the STK3122 amplifier module was dead.  I replaced it with a part from a repair shop, and all was well.

The unit sat unused for nearly a decade before I dragged it out and put it into service.  After about a year or so of mild use, I began to get noise in one channel intermittently.  The noise would follow in the wake of salient bass excursions, almost like a bad volume pot, speaker switch or other poor connection.  I frustrated myself with trying to fix connections before I noticed the crossover distortion getting significantly worse.  At that point, I pulled the unit apart and checked the biasing.  Sure enough, the STK3122 was at fault again.

In the course of looking for a replacement, I began to question what I should expect.  I know that counterfeit parts exist, and I had bought the replacement from a domestic brick & mortar store, so I assumed I was in possession of two vintage modules which had failed in nominal operation.  Was there some issue with device degradation over time (metallurgical, thermomechanical, or environmental factors) which would make vintage parts inclined to be less reliable?  I bought two replacements: one was a used vintage part, and one appears to be a NOS vintage part.  Once more, I replaced the module and everything worked.

Out of curiosity, I decided to see if there was anything to be seen inside the module packaging.  The datasheet provides an internal schematic, so maybe if there were indications of a failure cause, there may be an indication of what to expect of replacement part reliability.

Note lead width and exposed substrate insulation near pin 16

Starting with the original part I'd stripped from the unit years ago, I simply used a spudger and a lacquer thinner bath to remove the nylon device cover and reveal a beautiful hybrid circuit consisting of wire-bonded bare dice and printed film resistors.  I traced out the circuit and matched it to the schematic.  It's interesting to note that this layout includes the 470 ohm resistors and pads for capacitors which would allow the external compensation networks described in the datasheet test circuit to be built internally.  I'm not sure if there was ever a family of part cousins which made use of these internal features. 

As can be seen, the bias resistors R9/R18 appear to be scratched.  On closer inspection, both resistors were open-circuit.  The top silkscreen layer over the resistive material has multiple fine cracks across it, and the conductive material is broken along two narrow cracks which show evidence of local heating.  Assuming no external circuit factors could induce this within the scope of its rated bias point, this suggests any number of things that could be at play.  Quiescent power dissipation of the package is on the order of 3W.  These things run fairly hot without a heat sink.  The observed cracks could merely be a cumulative effect of heat causing material shrinkage or degradation in a static sense.  The dynamic effect of substrate expansion could also stress these resistors.  Once cracks begin to form in the conductive printing, current crowding at the partial connection will only result in localization of resistor power dissipation and further localization of its degradation.

That's my theory anyway.  I still had a second part to dissect.  Unlike the first one which had two completely dead channels, the recent failure had one good channel and a partially failed channel.  I was hoping to be able to compare the results and see if I could determine the original value of R9 and R18.  Maybe I could even demonstrate a repair!

Note lead profiles and lack of substrate connection
After some soaking and picking and prying, I eventually got the second module apart.  Much to my surprise, it had been a counterfeit part all along.  The entire module is constructed from SMT chip resistors and SOT-23-3 parts.  Obviously, none of this will help us determine part values in a legitimate Sanyo part.  In the process of disassembly, I had noticed that the leads seemed terribly easy to bend.  When I had tested this module in a breakout board, I had similar issues. Before long, the leads started peeling off, revealing the dark copper underneath.  Most of the leads on the part show similar signs of plating/solder failure.  I am not sure that this isn't the only thing wrong with the part.  The part had demonstrated faulty output bias when in-circuit, but my tests with the part on a breakout board were varied.  If I weren't lazy and worried about abusing the receiver's PCB, I'd fix the leads and reinstall it for a test. 

More delaminated leads
I need to emphasize what pictures like this reveal.  What you're looking at are copper leads which are mechanically socketed into form-fitting solder fillets, but not necessarily electrically connected to them.  The electrical continuity between the soldered areas on the PCB and the component MCPCB may be made via intermittent semiconductive contact between copper oxides and/or intermetallics, or it may be relying on the cross sectional area of the plating alone.  Just because the leads don't wiggle around and fall off doesn't mean a reliable electrical connection exists.

The schematic of the fake is identical to that shown in the original datasheet, though all the values are different.  All PNP transistors are MMBT5401 parts; all NPN transistors are MMBT5551 parts.  These are rather common general purpose 160v transistors.  The diode pairs are BAV99 parts.  It's noteworthy that all the parts in the counterfeit could easily be replaced.

I had to question how this handful of 0805 chips and SOT23 packages can comfortably dissipate a total of 3W.  For the moment, let's disregard the MCPCB and try a sanity check.  Assuming a limiting dissipation of 200mW per part, the module could dissipate >7W.  Of course, power isn't dissipated across all components equally.  I decided to take a look at the circuit and see if I couldn't come up with a better idea of the distribution of power in steady state.

I recreated a single channel in LTSpice and set an approximate bias point by adjustment of a dummy load with null input.  When the dummy load sets the output differential voltage to match the application schematic, the total channel dissipation matches the observed dissipation of the module. Checking the estimated component dissipation, a few things of note stick out.  Q1, Q2, and their counterparts in the other channel are operating around 80mW.  Output transistors Q7, Q8 and the counterparts are operating at around 400mW.  R9 and R18 are dissipating about 300mW each.

Keep in mind that this is just a halfass simulation with a simplified equivalent external circuit.  I fully accept that this could be inaccurate, but it falls in line with what would be expected.  Q7, Q8, and their counterparts will be the transistors with the highest dissipation.  The physical construction of the genuine hybrid part suggests this, and it's not like we can expect transistors like Q3, Q4 to contribute any significant fraction to the observed module power.

Considering Rth(j-s) values published for 0805 chips on multilayer PCB's with ground planes, 300mW on R9 and R18 doesn't seem too terrible.  The MCPCB should give these parts a lot more headroom than the worst-case values (~125mW) on a datasheet.  Then again, we have no idea what sort of value drift will occur.  The parts on the disassembled module are well within tolerance still.

Assuming it's an accurate estimation, the high dissipation on the output transistors might be of more concern.  Nominal power rating for these devices is going to be somewhere around 350mW.  This assumes minimal traces and pcb thermal conductivity.  Without a thermal resistance model for devices on a MCPCB, I simply assumed the substrate temperature is constant at 70dC.  This is higher than the observed temperature when operating without a heat sink.  Using a figure of 130K/W for Rj-c and a handwavy 10K/W for Rc-b, maximum allowable power within a 150dC junction temperature limit is somewhere around 600mW.  This is only a bit more than the highest figures for allowable package dissipation on multilayer boards.  These parts may be fine operating at 400mW, especially if the substrate has a heat sink attached. 

Of course, the thermal analysis says nothing about whether the counterfeit circuit can meet its specs in terms of noise, distortion, etc.  Although I may revisit it, I haven't bothered with much component-level testing of the fake.  I could put together a power supply and see if the quiescent dissipation follows the estimation.  I could pull the transistors in question and test them off-board to make sure they haven't been destroyed.  I may update this if I do anything more, but I'm inclined to believe that the fake was simply suffering from contact issues at the lead terminations.

While dissecting the fake doesn't help us better understand the original Sanyo part's internals, it does offer some information that will help make counterfeits identifiable.  There may be more than one breed of counterfeit out there, but I have been able to identify current Ebay listings which match this particular type. Also, this forum thread features the dissection of a fake STK3102 which matches these indicators.
  • The counterfeit appears to have a facility to allow pin 8 to be bonded to the aluminum substrate, although in the example I have, there is no substrate connection at all.  Counterfeit parts may lack continuity between pin 8 and the aluminum substrate.
  • The leads of the original Sanyo part are an iron alloy (magnetic), whereas the counterfeit leads are nonmagnetic. 
  • The original leads also have a sheared shoulder near the package connection.  In the photos, it's easy to see that the leads are slightly wider near the bend.  The leads on the counterfeit are formed differently and have a uniform width.  
  • It's also worth noting that any legitimately NOS parts will likely have heavily tarnished lead plating.  It's not a sure thing, but shiny beautiful leads are an indicator.  If you're buying a pulled part, who knows what kind of shape the leads will be in.
  • The orange substrate insulation of the genuine part is visible through the lead apertures in the molded plastic shell.  This is particularly easy to see in the empty opening for pin 16.  The counterfeit part is just a MCPCB and simply shows bright green solder mask everywhere. 
  • Finally, the easy way to spot some fakes is as simple as looking at the date/lot code on the front of the part.  The counterfeit part I disassembled has a code 5302.  Checking Ebay, I can find at least two "Genuine Vintage NOS" parts which have green solder mask, shiny leads, and a 5302 date code.
Ultimately, it's up to you whether the counterfeit parts are worthwhile.  Like I've commented before, I'm not against inexpensive parts in general, but if the part can't be trusted to meet its implicit specifications, its utility is severely limited.  I'm wary of the allowable power dissipation of the MCPCBA-type counterfeits, but I'm also worried about thermomechanical stresses in genuine hybrid parts.  In either case, I do suggest using a mechanically-supported heat sink on these parts. 

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