Yamaha B-2 restoration journey

Discussion in 'Yamaha' started by Mr. Yamaha, Nov 16, 2016.

  1. Oilmaster

    Oilmaster Ozzy Subscriber

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    First reply would why these bypass caps are needed.....

    Following several reference books on power amps (Bob Cordell, Douglas Self), the caps are merely to account for resonance frequency cancellation of the big reservoir caps beyond the 500 kHz region, and less for RFI/EMI HF decoupling as often mentioned. As such 1uf to 2.2 uF is often more than appropriate, and you can keep using polypropylene capacitors as Mr Yamaha did. Polyester caps are not distortion free, and starting with a distortion source at the very beginning of the power supply chain....

    Secondly, the whole resonance topic of the large reservoir caps in the higher kHz region is sensitive (worsened) by inductive elements, and as such soldering them as close as possible to the pins of the reservoir caps is key.
    Soldering those caps on the VFET boards is adding a lot of inductance in the 'system' due to the wiring and PCB track between reservoir caps and the bypass caps..... not good, potentially making things worse than not having them at all.

    As for HF decoupling, on the contrary.... decouple as close as possible where needed, i.e. on the VFET boards themselves, using SMALL value bypass caps in the order of 47nf to 100nF (max, like for the +/-B2 rails on the driver board); or else you may start coupling instead of decoupling.....

    So it's a dual job to do.

    PS: the B+/B- power rails on the VFET boards continue to the driver boards via de connecting PCB below the heatsink, and have no further HF decoupling on the driver boards (which is a shame), so adding those 47~100nF bypass caps on the VFET board incoming power rails (to E) is no luxury at all in the era of wireless communication.
     
    Last edited: Feb 13, 2017
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  2. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    While I've heard all these theories before, I always thought that the main reason for such bypass caps was because electrolytic caps are limited in their response speed and faster caps like polypropylene are quicker and can fill in the gaps. Of course, that is just one effect of many.

    You are a wealth of knowledge, Oilmaster. Thank you, Sir.
     
  3. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    So, I am in process of replacing all resistors in signal path on driver boards with shiny new ones. I knew this board had fusible resistors on it, but I didn't expect to see so many in the signal path. I replaced the first six or so at the front end of the signal path, but the last six at the tail end all appear to be fusible. (R151, R152, R163, R164, R165, R166)

    I understand what fusible resistors are and why you use them, but why so many in the signal path?

    These absolutely should not be replaced with regular resistors. Right?

    Are these there to protect the Holy VFETS, or would their improper replacement present a less drastic risk?
     
    Last edited: Feb 17, 2017
  4. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    I am also curious about R237 and R238 on the power board, which seem to be both in the signal path and part of a power supply - or am I reading this wrong?

    They are 4.7 Ohm metal oxide film "fire proof" resistors - wattage not given. Comment about exactly what purpose these serve and could they be substituted by something better?
     
  5. zaibatsu

    zaibatsu Active Member

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    I know Vishay's CMF metal film line comes in a fusible/flameproof version, and are highly regarded for audio applications.
    http://mouser.com/Passive-Components/Resistors/_/N-5g9n?Keyword=71-CMF+Vishay&FS=True
    I'm not sure if all these 71-CMF are flameproof, though I did follow the flameproof sub-category to get to that link.

    I'd be interested to hear Oilmaster's suggestions too.

    I feel it's important to confirm the wattage and match that for your subs.
     
  6. Mr. Yamaha

    Mr. Yamaha With Kenwood Subscriber

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    I replaced R237 and R238 with 3 Watt Vishay CPF Metal Films and placed them underneath the board to get less heat under that board. I also replaced alle those orange PCB scorging resistors with Vishay CPF's with the proper air clearance and sometimes underneath the board (diffent varieties of Wattage).

    And the same Vishays will be used for R104, R110, R118, R119, R122, R126, R129, R167 and R168 (diffent varieties of Wattage).
     
    Last edited: Aug 1, 2017
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  7. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    Nice work!

    I found some Jantzen Audio MOX 10W flame proof resistors in my stash that I can use for R237 and R238. Great idea to put them on the other side of the board, because those Jantzens are quite big. Incidentally R237 and R238 that I just took out were way off spec at 3.6 ohms.

    I'm REALLY curious if anyone can comment on why (R151, R152, R163, R164, R165, R166) need to be fusible. Of course the default position is to keep them fusible since most of us won't understand exactly why the designers made those fusible.

    BUT, if anyone can comment from experience - not having those fusible - will that pose a danger to the VFETS?

    Has anyone ever seen a B2 with any of R151, R152, R163, R164, R165, R166 fusible resistors blown?
     
    Last edited: Feb 18, 2017
  8. rottalpha

    rottalpha Yamaholic Subscriber

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    What I was told is that the primary reason was the need for a flame-proof resistor at those locations in the circuit.
     
  9. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    O
    Interesting. Does it get that hot next to those transistors? I wonder, if one were to mount those resistors on the other side of the circuit board, wouldn't the need for the flame proof resistors disappear?

    Of course that can also mean to avoid fire in the case of circuit failure that overdrives those resistors.:idea:
     
    Last edited: Feb 18, 2017
  10. rottalpha

    rottalpha Yamaholic Subscriber

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    actually that is exactly what it means.
    Have you ever seen a carbon resistor catch on fire??
     
  11. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    No.

    Carbon film or carbon composite?

    I am using carbon films in my rework.
     
  12. Mr. Yamaha

    Mr. Yamaha With Kenwood Subscriber

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    So today I finished the VFET boards. I replaced all jumpers with 1mm silver plated copper wire and I replaced the C701's. I also resoldered all connections on the boards. After that the normal deep clean process.

    After I finished the boards I also replaced all jumpers of the power board with the same wire. Also a deep clean (again) as well for the power board.

    I hope to install the VFET boards, the power board, the heatsinks and the VFET's tomorrow.

    IMG_3346.JPG IMG_3354.JPG
    IMG_3355.JPG

    IMG_3353.JPG
     
    Last edited: Aug 1, 2017
  13. rottalpha

    rottalpha Yamaholic Subscriber

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    really good lookin' :thumbsup:
     
  14. rottalpha

    rottalpha Yamaholic Subscriber

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    if it has carbon in it, it'll burn. Try using metal films where the manual calls for flame-proof, or fusible (if you do not end up using fusibles)
     
  15. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    I am using Takman Rex, which has a ceramic core. I am going to try burning one of them to see what happens. :D
     
  16. Oilmaster

    Oilmaster Ozzy Subscriber

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    That is IMO the purpose in most cases.

    Shortread: printed circuit boards combust pretty well, with a risk of fire propagation to the surrounding furniture, and then your residence

    Longread.....

    Manufacturers must by legislation and associated electrical code ensure that domestic electronic/electric equipment is safe in case of an internal failure.
    Hence, an amplifier must be able to fail (a shortage within for whatever reason) without setting itself into fire.
    A simple igniting resistor might (with some help) set a PCB into fire, then there is no stopping it anymore.
    PCB's are pretty much what aluminium is for airplanes.... in fact the last material you should have in case of fire...

    In case of a VFET failure, or any other transistor failure, that will be in 99% of the cases a plain short, creating an overcurrent in the transistor's associated circuit

    When a B-2 VFET fails, it will pull a very high amount of current out of the associated driver stage, which would set regular emitter resistors into fire (at least the 70's regular resistors).
    As such, all the driver's emitter resistors are "fusible". In this case the word fusible is ambiguous: the function should be non-flammable, but the white resistors that Yamaha used are indeed containing a sort of fuse in series with a resistor (note that Sony did not use those, they used truly non-flammable resistors in similar situations). And hence this is where "fusible" and "non-flammable" are swapped/misunderstood.

    Proof of those fusible resistors blowing? Oh yes...... I have a friend's B-2 on my bench here, in which the 2SK and 2SJ VFETs where swapped in one channel during a simple revamp job (not by me).
    When the unit was powered on, the VFETs in that channel fried in a millisecond and blew 3/4 of all fusible resistors in the driver stage.
    Noting that.... fuses are by far the slowest failing element in a circuit, and the circuit short(s) caused a couple driver and VAS transistors to blow as well (prior to the fusible resistors....)
    That's the reason behind the saying : "components are there to protect the fuses", but we are drifting off here....
    Anyway, the "exposed" resistors blew without setting off any fire; exactly what ought to happen.

    In various other cases I have experienced fusible resistors to act as a true circuit protection, mostly in the Sony TA-N7B.

    Example one:
    One AKer used an OTL tube pre-amp to drive the N7B.... in direct coupled mode.
    The pre-amp being a semi-professional built (or semi-amateur depending how you see it)
    OTL with bad coupling caps and no start-up/shutdown muting circuit.... can cause nasty DC currents or spikes...
    And one day.... poof it went....
    As a consequence the TA-N7B's input stage saw some high DC voltage levels and went into overload, which resulted in high currents in the input and notably the VAS part.
    Luckily Sony has put fusible resistors in the power rails to the input/VAS parts, which blew and paralysed the amp.
    Simple replacement of fusible resistors was enough to get it working again.
    Conclusion: be VERY careful what you connect on DC coupled input, and certainly never OTL tube pre-amps.
    The B-2 has DC coupled input option as well, and when a DC is present (or nasty transients), some transistor may fail and consequently overload some resistors again. Those resistors are made 'fusible" again in the B-2.

    Example two.
    I was finishing a TA-N7B rebuilt and making final checks when a probe slipped in the driver stage area.
    poof poof poof.... and a few cute little white smoke plumes.
    Various fusible resistors from VAS to driver stage / inside the driver stage / and one VFET gate resistors blew....
    So yeah, those (new, made in 2016) non-flammable resistors did make a very little bit of smoke when blowing, but that was it. No fire.
    And no VFET dead...

    So, it doesn't matter on which side of the PCB you put your fusible/non-flammable resistors: it's function remains the same, and the risk of fire using regular resistors as well.
    As much as I like Takman resistors, they are not fusible/non-flammable.
    In such case, the excellent Vishay Dale CMF comes in (excellent audio resistors); they are "flame retardant", which means they won't keep burning.

    Vishay Dale CPF is "Flameproof, high temperature coating" The advantage of the CPF series is compact size for the rated power: VERY useful in the very dense B-2 unit with small leg spacing. So, besides for "fusible" function, they are also usefull for the places where high wattage is required in the B-2, such as the original red-colored power resistors in the power supply section.
    That's why I recommended the CPF series to Mr.Yamaha for the power supply, and a few more on the driver boards.

    The large CPF in the PSU used are 3 watt !
    More than enough to replace the original 1 watt; no need to go to exotic and larger body resistors.

    Other options are (for "fusible" functions):
    TE Neohm FRN series (Flame-proof fusible metal film resistors)
    Vishay BC NFR series (Fusible Power Metal Film resistors)
     
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  17. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    Oilmaster, we should be paying you for advice of this quality.

    I tried connecting one of those 47 Ohm Takmans to a 22VDC source as an experiment, doing it under the exhaust fan in my bathroom. It smoked for a short while, then threw a 1cm flame for about 2 seconds, and then flamed out. It currently measures about 500 Ohms. (photos attached)

    I think I will order some CMF or CPF dale resistors for those fusible resistors.

    What about R237 and R238 on the power supply boards. What would you recommend for those two on the output?
     

    Attached Files:

  18. rottalpha

    rottalpha Yamaholic Subscriber

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    Now u know how carbon burns :D
     
  19. belgianbrain

    belgianbrain AK Subscriber Subscriber

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    :thumbsup:
     
  20. Mr. Yamaha

    Mr. Yamaha With Kenwood Subscriber

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    Yesterday I installed the VFET boards, the power boards and the heat sinks. I also used the same v/d Hul speaker wire for the signal path from the VFET boards to the power board and back from speaker - terminals to the VFET boards. I also learned once again that triple checking is mandatory. Even though I labeled (correct I thought) every wire, I apparently swapped two labels and thus 2 wires were swapped. A 12V + with a ground E, so that may have caused some fireworks. During the last check (before pics and the electrical scheme) I noticed the swapped wires :eek:

    I also think the solder connections of the new speaker wires on the power board need some silicone around and between them, they are very close tho each other.

    I did not have the guts to reinstall the VFETs with the sil pads, because I do not know exactly how tight the bolts need to be.

    Do you guys have any idea?

    IMG_3378.JPG

    IMG_3390.JPG
     
    Last edited: Aug 1, 2017

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