HI,
Could you please provide your opinion/expertise on the following:
I recently bought a ca. 33 + year old, but fully functional Bryston-4B ST AMP (which apparently was updated mid 2004 by Bryston, Canada to ST specs from a mid 1990's Bryston-4B NBR) .
See attached the Bryston 4B ST schematic.
I plan to refurbish this Amp at some convenient time; I'm now happily using it.
I quickly checked the power supply capacitors (8 X 5600 uF 100V 105 degC Nichicon) and they are still OK (Capacitance, ESR).
However, there are some 2 Tantalum capacitors, C18/C19 (1.5uF 35V), per channel used on +/- 33V stabilized Power rails (stabilized by a R / zenerdiode circuit) .
There is hardly any margin for Voltage overruns, to which tantalums react catastrophically (total shortcircuit, sometimes fire).
There have indeed been reports of catastrophic failure (total short-circuit) of those tantalums in this very circuit, resulting in the blowing of the main power fuse of the affected channel (Full dual mono construction).
https://www.audiocircle.com/index.php?topic=184960.0
Personally I have experienced such short circuits of tantalum buffer capacitors in power rails in 1970's/1980' Tektronix Scopes and REVOX / STUDER tapedecks (also no WV safety factor was employed in those designs.)
My question:
Would (in my Bryston 4B-ST) the blowing of the main power fuse for that channel be the only consequence of a failed +/- 33V power rail ( caused by a short-circuited buffer tantalum capacitor (1.5 uF 35V) C18 or C19), ??
or
could it perhaps lead to much more severe consequential damage: e.g. destruction of some near unobtanium TO3 (BR6521/22) Power Transistors in the TotemPole circuit?
I have not enough electronic skills to assess what would happen if one 33V power rail would die, as a result of a totally short circuited tantalum buffer capacitor C18 and or C19).
I would welcome your advice!
Cheers,
Martin
BTW
I plan to replace the tantalums by 1.5 uF 63V WIMA MKS2 film capacitors, (7.5x5 mm), which (just) can be accommodated on the PCB.
(I have used those WIMA's with good results in similar applications: e.g. REVOX A700, AKAI GX-630D Tape Decks).
Could you please provide your opinion/expertise on the following:
I recently bought a ca. 33 + year old, but fully functional Bryston-4B ST AMP (which apparently was updated mid 2004 by Bryston, Canada to ST specs from a mid 1990's Bryston-4B NBR) .
See attached the Bryston 4B ST schematic.
I plan to refurbish this Amp at some convenient time; I'm now happily using it.
I quickly checked the power supply capacitors (8 X 5600 uF 100V 105 degC Nichicon) and they are still OK (Capacitance, ESR).
However, there are some 2 Tantalum capacitors, C18/C19 (1.5uF 35V), per channel used on +/- 33V stabilized Power rails (stabilized by a R / zenerdiode circuit) .
There is hardly any margin for Voltage overruns, to which tantalums react catastrophically (total shortcircuit, sometimes fire).
There have indeed been reports of catastrophic failure (total short-circuit) of those tantalums in this very circuit, resulting in the blowing of the main power fuse of the affected channel (Full dual mono construction).
https://www.audiocircle.com/index.php?topic=184960.0
Personally I have experienced such short circuits of tantalum buffer capacitors in power rails in 1970's/1980' Tektronix Scopes and REVOX / STUDER tapedecks (also no WV safety factor was employed in those designs.)
My question:
Would (in my Bryston 4B-ST) the blowing of the main power fuse for that channel be the only consequence of a failed +/- 33V power rail ( caused by a short-circuited buffer tantalum capacitor (1.5 uF 35V) C18 or C19), ??
or
could it perhaps lead to much more severe consequential damage: e.g. destruction of some near unobtanium TO3 (BR6521/22) Power Transistors in the TotemPole circuit?
I have not enough electronic skills to assess what would happen if one 33V power rail would die, as a result of a totally short circuited tantalum buffer capacitor C18 and or C19).
I would welcome your advice!
Cheers,
Martin
BTW
I plan to replace the tantalums by 1.5 uF 63V WIMA MKS2 film capacitors, (7.5x5 mm), which (just) can be accommodated on the PCB.
(I have used those WIMA's with good results in similar applications: e.g. REVOX A700, AKAI GX-630D Tape Decks).
With the 3K9 series resistors, the current is inherently limited, and a short couldn't by itself blow the mains fuse; however, the loss of one polarity could have catastrophic consequences on the rest of the amplifier and the speakers.
MKS2 caps is a perfect substitute; a MLCC would also be useable
MKS2 caps is a perfect substitute; a MLCC would also be useable
D3 and D6 are there to minimize the internal damage if one rail fuse blows - but you'll likely lose closed-loop control and put DC out on the speaker output - there's some sort of protection circuit in the top right, not sure what it protects against though.
The main role of D3 and D6 is to protect the amplifier against inductive kickbacks from the load (speaker) during clipping episodes; they might do other things, but I don't see clearly which or when
Dear Elvee , Mark,
Many thanks for your comments.
So, it indeed seems prudent to replace these tantalum caps in the not so distant future!
I have the other electrolytic capacitor replacements already at hand, so I probably only need some 2-3 days (guesstimate) to replace those all on the AMP and auxiliary boards.
I found nice instruction video's on YouTube:
Although applicable to an older Bryston-4B version, the basics remain the same as the Yr 2000 ST model.
This particular BRYSTON -4B ST AMP was not used for a very long time, but apparently has survived the "recommissioning" really well.
It was shelved because one internal fuse had blown, but after fuse replacement and the recommissioning with a VARIAC (slow "mains voltage" increase) showed no real problems, so possibly only a current surge to the big power supply capacitors caused the blowing of the fuse.
BTW
I currently feed the Amp with 210V 50Hz (via a big autotransformer).
My local mains (between 12:00 - 16:00 Hrs) is well over 230V ~ nominal these days (due to the many solar panels and the resultant mains overloading).
( Power specs in NL are nowadays 230V~ +/- 10% (so mains voltage can range between 207 and 253V AC !!)
So the slightly lower voltage of 210V ~ (the Amp was designed for 220V ~ +/- 5%) will hopefully help too with reliability in the intermediate time.
Many thanks for your comments.
So, it indeed seems prudent to replace these tantalum caps in the not so distant future!
I have the other electrolytic capacitor replacements already at hand, so I probably only need some 2-3 days (guesstimate) to replace those all on the AMP and auxiliary boards.
I found nice instruction video's on YouTube:
Although applicable to an older Bryston-4B version, the basics remain the same as the Yr 2000 ST model.
This particular BRYSTON -4B ST AMP was not used for a very long time, but apparently has survived the "recommissioning" really well.
It was shelved because one internal fuse had blown, but after fuse replacement and the recommissioning with a VARIAC (slow "mains voltage" increase) showed no real problems, so possibly only a current surge to the big power supply capacitors caused the blowing of the fuse.
BTW
I currently feed the Amp with 210V 50Hz (via a big autotransformer).
My local mains (between 12:00 - 16:00 Hrs) is well over 230V ~ nominal these days (due to the many solar panels and the resultant mains overloading).
( Power specs in NL are nowadays 230V~ +/- 10% (so mains voltage can range between 207 and 253V AC !!)
So the slightly lower voltage of 210V ~ (the Amp was designed for 220V ~ +/- 5%) will hopefully help too with reliability in the intermediate time.
I dont think tants are used much these days.
I have seen quite a few youtube repair video's where tants have gone short.
I have seen quite a few youtube repair video's where tants have gone short.
These polymer caps in 50V rating might also be a good sub for tantalums:
https://www.digikey.com/en/products...5/6196532?s=N4IgTCBcDa4HRgK4DEAEBWADANRAXQF8g
https://www.digikey.com/en/products...5/6196532?s=N4IgTCBcDa4HRgK4DEAEBWADANRAXQF8g
Tantalum capacitors offer very low high frequency impedance in a small package; however, @ low frequency 50/60 Hz ripple currents they are poor in MTBF with any ripple currents. They FAIL as a SHORT and burn up PCB, COPPER and deposit carbon.
In any design you should derate all components to improve your MTBF.
I service older Audio Precision products and we did use Tantalum capacitors. The DCV were full regulated and the components were derated. I replace them with 0.1uf ceramic and low ESR alum caps on the back side of the PCB.
Duke
In any design you should derate all components to improve your MTBF.
I service older Audio Precision products and we did use Tantalum capacitors. The DCV were full regulated and the components were derated. I replace them with 0.1uf ceramic and low ESR alum caps on the back side of the PCB.
Duke
Get rid of them now! When they go, the board will be ruined too.
I wonder which fool put 35V caps on a 33V rail...
As a thumb rule, use caps rated at least 1.5 x rail volts, or double preferably.
63V is about right, and you can choose the capacitor that suits the circuit.
Is the company still around, or went belly up long back after people found it was not as good as the advertising?
As a thumb rule, use caps rated at least 1.5 x rail volts, or double preferably.
63V is about right, and you can choose the capacitor that suits the circuit.
Is the company still around, or went belly up long back after people found it was not as good as the advertising?