[ksakai]

Hello all! first time posting here! I hope you dont mind me asking some technical problems 

I just bought 8B-ST THX '95 (I guess?) from my close friend. It has 2 broken channels (1 and 4) and missing 2 power xformer (he took them since i want lower price lol). one is clearly blown current gain stage while one remains unknown (he said it doesnt blow fuse but no output). before, the 1st channel LED would gone red after a while. I noticed the LDO regs were not tied to the heatsinks that makes them burning hot (and missing screws!).

long story short, digging my parts box and found some plastic washers and the screws complete with its nuts and metal washers. cleaned out the ceramic pads and added some high perf thermal compound for better thermal conductivity (there's no way those LDO pads completely flat. there should be ultra small pits). aanndd it solved my red light issue! one problem down 

I wanted to take apart the broken modules, fix it, and re-cap the entire modules to make it back to factory performance. BUT, the robertson screws prevent me to take it apart. idk how my friend un-screw the top lid 
what size is the screws? there's like 4 types from 0 to 3 here. i never get involved w/ this kind of screw before.

also, i'd like to know "DOA2c" module schematic. I have DOA33 but not DOA2c. I wanted to know if I could improve the schematic performance by simulating the original sch first

last, if I want to add a choke to the PSU, where could I tap the rail voltages? I noticed there's 2 jumpers on each modules, could I tap from there? it said "+55v" and "-55v". i'd like to improve the ripple as good as possible (measured 100mVp-p each rails)

i'll update the progress here if i'm allowed 




-add-

I noticed the schematic uses TL074 for the clipping indicator but mine has TL072 on each channels for them. any updated schematics?
my board revision is 2.5 something, made on July '95 if i'm not wrong.

sorry if i asked way too much 

[ksakai]

after taking another look, I realized the boards are pretty much mixed.
amplifier board : 7B-MB 2.58 July 95
Balanced input board : 8B-PSB Jan 94 using DOA2c op-amp
mainboard : 8B-PSB Apr 94

main rail caps were 8200u instead of 7500u (8BSCH24 Dec 96)
clip indicator uses TL072 instead of TL074
LM317/337 directly converts 55V to 17.5V without any transistor in series

this makes me harder to diagnose and thinking the possible upgrade path

at least now i know why the LDO regs were all toasty. regulating 55V (measured 51V on 220V mains) down to 17.5V. it means 33.5V drops! also, discrete input module were receiving 17.5V instead of 28V as stated on the schematic. I'm not sure whether there's changes on DOA2c components value between R2.0 and R2.4 module. But if there's no change, it means the bias points were different since the rail voltage were changed.

James, or anyone else who ever repaired this baby, help me.. I don't want to give up on this cutie

[James Tanner]

after taking another look, I realized the boards are pretty much mixed.
amplifier board : 7B-MB 2.58 July 95
Balanced input board : 8B-PSB Jan 94 using DOA2c op-amp
mainboard : 8B-PSB Apr 94

main rail caps were 8200u instead of 7500u (8BSCH24 Dec 96)
clip indicator uses TL072 instead of TL074
LM317/337 directly converts 55V to 17.5V without any transistor in series

this makes me harder to diagnose and thinking the possible upgrade path

at least now i know why the LDO regs were all toasty. regulating 55V (measured 51V on 220V mains) down to 17.5V. it means 33.5V drops! also, discrete input module were receiving 17.5V instead of 28V as stated on the schematic. I'm not sure whether there's changes on DOA2c components value between R2.0 and R2.4 module. But if there's no change, it means the bias points were different since the rail voltage were changed.

James, or anyone else who ever repaired this baby, help me.. I don't want to give up on this cutie

Hi

I would contact Mike at Bryston and he should be able to help.  mpickett@bryston.com

james

[ksakai]

Hi

I would contact Mike at Bryston and he should be able to help.  mpickett@bryston.com

james

Thanks, James. E-mailed him already and waiting for the reply. I'll keep updating the progress if i'm allowed 

[ksakai]

so, i was able to restore the faulty modules. thanks to another faulty channel, i forced to restore this thing 

my initial problem were high transient DC offset (3V for a secs then turns down to 0) and massive hiss on 2nd channel (ignoring the 1st and 4th faulty channels).

I replaced every electrolytic capacitors on the board and 1.5uF 35V tantalums on the input stage supply using a well matched generic parts. tantalums were replaced by 2x 1uF MKP10, hoping a better filtering. the final stages were replaced by MJ21193/21194 even though they seems to be fine. I want to keep the original transistors for the future. they're not that matched but pretty much close on each other.

both channels were biased to somewhere around 117mA. a lot higher than Bryston specification but i'm sure it's safe. it's still on the SOA range.
Yesterday, I re-biased the final stages to even higher, around 250mA. it's hella hot but eh.. that's fine. i'll add some silent 120mm fans.

but this morning, i turned it on, the pops and hiss is back!. i was like "wth is going on?". i can't see any more than 500mV offset on the 2nd try but the hiss is there! i wonder why?

this is just an update, anyway. channel 1 and 4 still left unused but at least i was able to recover those channels (i confirm this by turning it on without final stages. there's good output from the driver stages