Boss OC3 cooked transistor!

[miketbass]

  Hey everyone, I have a Boss OC3 for repair that has an absolutely TOASTED transistor, to the point that I cannot make out what it was! I managed to remove what was left of the old one and clean up the traces so a repair is possible. I cannot find a schematic and all of the board pictures on Google are too fuzzy to make out.

The part was labeled "J B" something and is Q15 on the OC3 board. I have found a picture of a PH3 online that looks very similar to the layout of my board. The PS5 also looks similar. The part labeled Q7 on this picture looks identical to what would have been my part.

Anyone have one of these they can crack open and tell me what this Q15 part is? Im guessing it's a FET of some sort used for the switching circuit in these modern Boss digital pedals. Anyone know?

[miketbass]

Wait.... Is this a 7805 voltage regulator? Can anyone verify?

[Slowpoke101]

You will not like this but Q15 is part of the 3.3V power supply.
Generally if Q15 is toasted IC6 is usually toast as well.
D3 going short circuit is usually the cause.







It is repairable but very fiddly. The board under Q15 is usually damaged which can make removing the toasted unit very difficult without further damaging the board. But it is possible and only some screaming will be involved. The main screaming match will be getting hold of the parts.

[Rob Strand]

Does your board have the BJ5L / BJSL IC (IC7) and the 4R7 or 151 inductors (L1 and L2) nearby as well?

My guess is the IC is a switchmode IC and the Q7 part you seek could be a MOSFET.
When I look up the JB78 code I get a MOSFET but I can't see that package offered.

If someone has one it would be far more useful than my guess.

[Rob Strand]

You will not like this but Q15 is part of the 3.3V power supply.

Yes, like that!

[miketbass]

*sigh* well 2sj190 looks like the part, and it looks like theres a source for about $2.00 per. The IC6 however I have no idea where to get and will look more. This pedal came to me with the plastic board insulator missing so I am guessing that the board has shorted against the bottom plate. I was able to get the fried Q15 out and have clean solder pads to pop a new one in.

At this point Im weighing the cost/benefit of pursuing this. Finding the parts will be quite the task and they appear to be obsolete making the availability and price an issue. I am tempted to order the 2sj190 and grab a few. If the replacement goes up in smoke I figure I may have one more chance to attempt rework and then the board will likely be damaged beyond reasonable repair.

If anyone knows of any equivalent parts that could be subbed in it would be very helpful. I will post IC6 voltages (if there are any) when I get home and check the integrity of D3. Thank you guys so much for the prompt and helpful replies as I now feel there is at least a possibility of fixing this!

[Rob Strand]

It's extremely wise to replace the IC.  Be aware there are a number of switch speed options for that device and you need to use the same speed as on your board.
The problem I see is if the IC is fried and the new MOSFET is held in the on state
it will dump 9V into the 3V3 circuit and take out a lot of stuff.
The fault may have already done that!

A safe but painful option would be to remove the IC and wire it up  circuit off the PCB with a dummy load.  It'a a lot of work and there' some risk of breaking it in the process.

You can only do your best.

[miketbass]

Well, I have verified that D3 is indeed shorted. It appears that this is a somewhat common problem with a few pedals in the Boss lineup utilizing digital chipsets. Im currently sitting through information from previous threads about these parts failures. Ideally I would replace these parts with "better" ones (higher voltage rating) as at least one component is only rated for 10v.

It seems that there may be a sub for the 2sj190 with a 2sj278. Finding a sub for IC6 seems possible with some datasheet work. Supposedly these parts can be ordered from Boss directly, I wonder if there is truth to this or if they are even still manufacturing with these obsolete parts? Anyways, at this point I'm looking to find some more readily available parts to sub in and if all works out have a solution to report for the next guy trying to fix this.

[Rob Strand]

Well, I have verified that D3 is indeed shorted. It appears that this is a somewhat common problem with a few pedals in the Boss lineup utilizing digital chipsets. Im currently sitting through information from previous threads about these parts failures. Ideally I would replace these parts with "better" ones (higher voltage rating) as at least one component is only rated for 10v.

It seems that there may be a sub for the 2sj190 with a 2sj278. Finding a sub for IC6 seems possible with some datasheet work. Supposedly these parts can be ordered from Boss directly, I wonder if there is truth to this or if they are even still manufacturing with these obsolete parts? Anyways, at this point I'm looking to find some more readily available parts to sub in and if all works out have a solution to report for the next guy trying to fix this.

I wonder if it was the diode that failed first then the MOSFET?  That would go against the 10V exceeded theory.  Having the MOSFET or chip fail first would not take out the diode.

In the old days I had a lot of good results with Boss (and Roland) spares.  They really did support their products.   Not sure about nowadays.  I supposed you could contact them.   If you buy from Boss then you won't be able to improve the parts.

The IC apparently has an F version which is rated to 16V but you lose the overload protection.   None of the versions of the chip have current sensing for overload.  Overload is done much more crudely.  If the voltage drops to near zero it allows a certain time for the fault to clear then it shuts off (not sure exactly).  The non protected chip would simply drive the MOSFET on into a short.   There's a very small amount of current limit due to the inductor but the inductor is small so large currents will develop in a short space of time.

The MOSFET you mentioned looks OK to me.   (I didn't check the pinout)

Maybe the diode rating needs to be checked?

[miketbass]

Alright, I went to Mouser and pulled up the parts I think will work.

Diode SS14:
https://www.mouser.com/ProductDetail/ON-Semiconductor-Fairchild/SS14FL?qs=sGAEpiMZZMtQ8nqTKtFS%2fMv7GkUEUYtzzVo6Hh%252bDSqrSgJk0UuukOQ%3d%3d

IC6:
https://www.mouser.com/ProductDetail/628-8520E33MC-G

I cannot find a proper mosfet on there and will have to dig further on that one. Im sure there are subs in production but finding one with the correct specs will take time.

[Rob Strand]

Alright, I went to Mouser and pulled up the parts I think will work.

They look OK to me.

I cannot find a proper mosfet on there and will have to dig further on that one. Im sure there are subs in production but finding one with the correct specs will take time.

While all the obvious stuff like current & voltage ratings needs checking, don't overlook:
- Cirss ; Don't worry too much about the other capacitances for a low voltage circuit like this.
- Vgs(off);  in the 0.5A to 1A region
- Rds(on) at the Vgs used by the circuit.

[miketbass]

Alright, I have placed an order for parts to attempt this fix. All are available from Mouser and come in at a couple $$ all together..... plus shipping 😁

I'm going to keep a lid on exactly what I have selected but have no fear, all will be revealed. I will let the suspense build and report back in a week or so when I recieve the items. Fingers crossed ~

[Rob Strand]

Fingers crossed

Indeed!

[miketbass]

Alright, got the parts in yesterday and installed. It appears that the 3.3v PS is now working just fine, but there is another problem of note - Ive got a short between the 3.3v and ground. Any reading across the 100uf caps,D3, etc shows as a short. I removed and replaced D3 again along with the electrolytics and just testing across solder pads I am reading a short. So something is cooked down the road I would imagine and pulling the 3.3v down. If I plug in a battery I get a 3.3v that slowly drops down about 10mV a second and keeps going.

Unless there are any obvious places to check I am possibly out of wind trying to fix this.

The MOSFET used was PCP1302-TD-H
Https://www.mouser.com/ProductDetail/ON-Semiconductor/PCP1302-TD-H?qs=xGcJQ%252bnsJwtl9GRuovbZ6g%3d%3d

Along with the other two components from above. I would consider this a semi verified fix as it appears to function correctly but the circuit does not work still, so take that as you will. I will try and post a very detailed explanation of why this part was selected from my father whom I contacted for help in the hunt for the proper MOSFET.

[miketbass]

An extremely informative and thorough explanation of the function of the MOSFET in this circuit, and in general, as provided by my wonderful father who helped me select parts for this repair.

1.       I noted the Driver chip, the E and F are significant.  The E part is speced at 10v max, while the F is good to 16v.  Looking deeper though at the spec sheet, there are the 'suggested operating range'.  The lower-voltage E part has a MAX voltage of 12v.  This should be OK.  (Plus, the E parts have overload protection while the F do not.  So – ya, stick with the E part)

https://octopart.com/search?q=S-8520F33mc&start=0

https://octopart.com/search?q=S-8520E33mc&start=0





2.       FET:   Attached is original datasheet.   It is a not great part, old.

·         Mostly notice the Rd-ON of around 1 ohm!  Causes heat, which is always bad.  But also provided some level of current limit into the inductor.   I have selected a like part you can use that should work well.

·         Not also the driver chip in the spec sheet on Page 9:  "EXT pin output current".  FETs can be looked at like a capacitor, that is the Ciss value the other guy mentioned (Good advice overall from him, BTW).  Do when we turn on and off, we need to charge and then discharge that capacity.  The Output current spec is part of this.  We need to make sure we can fully turn on the FET and OFF while ti is switching.  This part runs at 300Khz, which is getting up there.  And the driver chip has only a few mA current available, so we want to watch things and not get to large of what is called the 'Gate Capacitance' on the FET.  Critical items to look at:

                                                               i.      Vgd Max  (60v  -- Higher = better)

                                                             ii.      Rdon  (1 Ohm  = Typicaly lower=better)

                                                            iii.      Id Max (1A, 4A peek = Higher = better)

                                                           iv.      Ciss (10pF – lower = better)

                                                             v.      Vg-th  (2v, need to make sure it matches the driver chip output).



So, when a FET is turned on, we apply a voltage separation between Gate and Source  (Know as Vgs).  As the voltage raises, it will start turning on.  How hard it turns on is a function of current through the FET (Id), as well as Vgs (and temperature of course).   What we want to make sure is we can get the FET turned on rather quickly (use it as a Switch, not a liner amp) to reduce the amount of heat generated during switching  (Known as Switching Loss).  To do that we have to look at the driver chip current capability vs. the input gate capacitance (Ciss).    These two form in effect an RC time constantan and define largely how quickly the FET will turn on or off.   Increate the driver capability, and we charge the Gate Capacitance faster – and we turn on faster.   Lower the gate capacitance, and the same.  But reduce the drive current, and/or increase gate capacitance, and switching time will increase:  Meaning we spend more time in the eval 'analog' region where the FET is sort of turned on, and as a result we end up dissipating heat in the device.



Once the device is turned on, Rdon comes into play.  It will direct the amount of head loss vs. current through the device.   Notice Rdon is impacted by the final Vgs we end up getting.  Use a higher Vgs results in the lower Rdon, to a limit.







So that is some of the things to consider.  The charge on the lower-right hand side of the 2nd page of the FET datasheet is a good one.  Notice how it is showing in effect Vgs-th (Threshold)  of around 2.2v  And that is really does not get full on until Vgs reaches around 4 or 6v.     Now, if we look back at the driver chip datasheet table 5 on page 9, we can see the EXT pin seems to run around 0.4v above Ground when on.  If we have a 9V battery(which is more likely around 7v in the real world) we can expect the FET to be around 0.85 Ohms Rd-On.   The final critical think to confirm is the Threshold voltage.  Vgs-th, this indicated when the FET starts turning on.  The original FET seems to have a Vgs-th of around 2.2v per the charts.   We can select a part which is lower Vgs-th, but we need to make sure the driver ship will allow Vgs to get below the threshold and let the FET turn off.  Again, going back to the driver chip datasheet I see Vext being specced at Vin-0.4v (again in table 5).  So, that is telling me the driver chip will typical switch Vext from Vin-0.4v to 0.4v, resuting in a Vgs of 0.4v while off, and Vbat-0.4v  (Approach 6.6v) when 'on'.   OK, Vgs-off of 0.4v should be simple to work with, but we need to take care as there are some special 'logic level' FETs which have a Vgs-th of 1v or so, and they might not fully turn off with that 0.4v.



The final issue the the packaging.  Sanyo calls this a "2062A" package, which refers not to any standard format, but a specific SANYO drawing.   Here I can only say experience can solve this.   This package is more commonly known as a TO-89-3





OK enough of all that.  I would say try this part – it would work just fine:  https://www.mouser.com/ProductDetail/ON-Semiconductor/PCP1302-TD-H?qs=xGcJQ%252bnsJwtl9GRuovbZ6g%3d%3d



Ciss is notable higher at 262pF, however looking at the driver table 13 on page 34 I can see a few example transistors being used in their demos with even higher Ciss, so I expect we will be OK.  Doing some VERY rough check cals:   Charge on a cap à V=Q/c    (Q = charge on the cap in coulombs. )     Q = I*t   (Current * time)  -à   V = (I * t) / c



So, if we cals the time it takes a 160pF cap to reach say Vgs = 4v  (make sure the FET is well turned on) when being supplied say 2mA  (Small value from driver table, and a little lower --- the really CORRECT way to do this is calculate the resistance of the driver chip + the gate resistance, but some of those values are missing – so we will just use this as a quick check to see if we are totally in trouble).   OK, 160pF for the original FET takes 320nS, while the 262pF part takes 525nS.   On and off, the old part takes around 600nS, the new one 1uS.  300Khz switching (the frequency the driver chip is running at)  takes 3.3uS  -- hum, so we are taking a bit more time to turn on and off and it is a not-insignificant part of the total switching time.   But here is my assessment:  The extra loss fo the switching time might well be offset by the better Rdon value while not switching.   



Bottom line:  By feeling is you will be OK with this replacement part.  Give it a try and if you really want try scoping the Vgs as it switched on, looking at Raise Time and Fall Time vs. the overall switching time period.   (Might be hard, you need a FAST scope, likely a storage one, and also need to have really low capacitance probes...  But give it a try.





OK I know this was a LOT – but though would take the time to pass on some of the thinking behind it vs just say:  Buy this:  https://www.mouser.com/ProductDetail/ON-Semiconductor/PCP1302-TD-H?qs=xGcJQ%252bnsJwtl9GRuovbZ6g%3d%3d

[Slowpoke101]

You have done some excellent research. Thank you.
I will have to order some of these soon as my stocks of the original chips are nearly exhausted.

It is disappointing that you seem to have a short circuit on the 3.3V rail. Finding that will not be easy and it's most likely that the processor is toasted - but it may not be. If you have a low ohms meter (that can measure down to less than 0.01 Ohms without using excessive voltages) then finding the short isn't too difficult but having such a piece of test gear is a bit unusual. So the other option is to isolate various parts of the circuit until the short disappears but this requires cutting of PCB tracks and is extremely tedious.
PM me if you would like a copy of the service manual - need an email address to send the PDF to.

[miketbass]

At this point I'm not too optimistic about finding the problem. If this was a through hole pcb I'd get a little more cutthroat about lifting components and cutting traces. Being SMD with a double sided pcb its likely more effort than what the results may yield.

This pedal was being run without the plastic insulating sheet and what shorted and caused what damage is anyones guess.  Worst case I have some spare parts to use.

Thank you for the service manual offer, pm sent. I hope that this thread can help others repair their units and be of use to the DIY community.

[Rob Strand]

An extremely informative and thorough explanation of the function of the MOSFET in this circuit, and in general, as provided by my wonderful father who helped me select parts for this repair.

Your dad did a good job of writing that up.   You don't need us you already have a personal consultant.

Ive got a short between the 3.3v and ground.

That's a bummer but it's was always a possibility.

It's possible it's a cap or a short on the PCB but I have a nasty feeling it could be a chip.

In the old days Hewlett-Packard has a shorts-tester which was helpful.

A kind way to find shorts is to lift pins to isolate the problem but there's a risk you will break-off the pin.  It is a high and real risk on some of those small packages.   

The next method is to cut tracks to help isolate the short.

If you are going to give up, there's a two methods which are good for identifying shorts.
- Get a high current power supply.
- Set it to 3.3V with a current limit.  You really need a bench supply with adjustable output current.
- Initially set the current to low like 1A
- Connect the meter to the circuit.
- Connect the PSU to the circuit
- Check the meter voltage is like a short say < 100mV
- How large the meter voltage is can sometimes help you guess where the fault is.

Method 1 is to probe across the parts/chips and try to find where the voltage is lowest.  Typically the lowest voltage  is close to the shorted area.  If you can't see much change you might have to crank up the current.

While you are doing method 1 feel the parts on the board and see if anything is getting hot.   If the DC voltage measurement is low it is unlikely.

So if you got nowhere or it is unclear, now proceed to method 2:
- crank up the current.
- keep feeling around and see if you can find a warm part.
- you might need to wait a bit.

I have recovered boards using these nastier methods.

The down side is if you find it's a "big chip" you probably aren't going to replace it.

EDIT:
On thing I forgot, these days you can use this idea with an infrared camera instead of feeling around on the PCB.

[madtheory]

You will not like this but Q15 is part of the 3.3V power supply.
Generally if Q15 is toasted IC6 is usually toast as well.
D3 going short circuit is usually the cause.






Is this from an OC3 schematic or one of the other Boss SMT pedals?

It is repairable but very fiddly. The board under Q15 is usually damaged which can make removing the toasted unit very difficult without further damaging the board. But it is possible and only some screaming will be involved. The main screaming match will be getting hold of the parts.

[cctsim]

I have repaired a BF-3 in the past which has a similar board by taking out Q15 and IC6 and installing a normal through hole 3.3V regulator as follows:

Input on pad 5 of IC6
Ground on pad 2 of IC6
Output on pad 3 of IC6

It may not be as power efficient but it works fine.