Happy Holidays! AudioKarma,
I own a non-working Acurus A250 that I hope to rebuild soon. After reading this entire thread multiple times, developing a plan to rebuild my amp, and annotating multiple circuit diagrams for the Acurus A250, I wanted to summarize some major recurring themes, and share them for reference by future owners/builders in the spirit of DIY. Whenever possible, I’ve listed the post number, or added a link. The schematics used as a reference are located here
#19 #163 #774
I’ve also attached a
tab-delimited TXT file (exported from Excel) listing these items and some preliminary pricing (sometimes using what’s in-stock today vs the best recommendations on this thread)
If I am missing or providing inaccurate information, please let me know so I can correct or post a new summary. Thanks to all who have shared, asked, or answered questions raised in this thread.
A) PSU mods
1) replace the two big Power Supply (Filter) capacitors, increase quantity from 2 to 4, 6, or 8; increase total uf from 46k uf, up to 100uf or more. Reduce ESR and Ripple Voltage as much as possible, triple-check size and proposed location of capacitors fits in the case securely.
#54, #376 through
#379, #547, #611, Cap Sizing at
#204
2) Create a star-ground point at the center of a copper or aluminum plate or heavy-duty wire connecting 2 or more filter capacitors. See photos at
#7, #382, and
#608 through
#611.
3) Provide bleeder resistors between each filter capacitor + and - leads. Optional - add bypass capacitors at the PSU, or post-fuse on the amp boards (see also note C5) for details.
#9, #36
4)Replace rectifier with One (or Two rectifiers - Two allows for potential dual-mono style channel separation)
Part # GBPC3506-E4/51, notes at
#5 Photo at
#7,
5) Add Turn-on delay/ softstart circuit to reduce inrush current, either via thermistor, or more complex custom inrush control circuit.
#28 through
#32; #376
6) Potentially add a CRC snubber between the transformer and rectifier(s) to reduce transformer ringing (can be calculated via Quasimodo jig from DIYAudio.com) See
PDF link to document by Mark Johnson.
B) Rail conditioning
1) Add post-fuse parallel capacitor network on both rails per board to reduce noise/ripple from PSU. One 47 uf to 100 uf (100v) Plus 1 uf 250v ECWF in parallel, wired to board.. See posts
#17, #18, #128 through
#130 ;Photos at
#177 through
#180.
C) Capacitor Refresh on main circuit boards
1) Polypropylene Caps -
#704 through
#708
2) Late Production Change at C13, C14 from 39pf to 200pf, but in other comments on this thread, the recommendation is to use Mica caps at C12, C13, C14 – 220pf
#106
3) Late Production change: Add 15 pf cap near Q4 & R18 - shown on reference schematic
#19
4) Input coupling caps: C1 (.47 uf ECWF) and C2 (10 uf, 35v Nichicon Muse BP)
#5
5) Nichicon Muse 220 uf, 50v at C7 ;
#156
6) C16 Removed - shown on reference schematic
#19
7) PSU Capacitors can be bypassed individually, (
#159) though potentially, most of the benefits from this mod are already gained by adding the post-fuse capacitor network discussed at note B1. (
#8 - #9, and
#35 through
#37)
8) Add bypass capacitors to Zener Diodes D3 and D6 for lower noise…
#11 through
#14.
D) Output Zobel Circuit
1) Relocate zobel network on Left-hand board away from thermal switch to reduce noise on left channel.
#516
2) C15 was changed from .10 uf to .22uf in late production, - shown on reference schematic
#19 see also resistor change at
note G2.
E) Speaker terminal Ground
1) Disconnect ground wire running between Input RCA and Speaker terminals on each channel, run wire directly from Input RCA to Star-ground.
#761 through
#765
2) Diagram of Star Ground with above connection removed in red:
#151
3) Add speaker protection board with DC detect; turn-on and turn-off thump protection. (Multiple diy or ready-built solutions available)
#737 : #738; #790.
F) Switch replacement or repair
1) Repair
#298, #594 through
#599, #743, photo at
#698
2) Replacement
#643
3) Arc suppression caps
#33
G) Board Resistor Refresh
1) General replacement recommendations
#2
2) Late production change: R34 from 10r to 1.5r
#322 use wirewound resistor, - shown on reference schematic
#19 see also Zobel change at
note D2.
3) R7 and R14 – increase to 1-watt.
#141
4) Update Bias Potentiometer to 1K r multi-turn, measure value of existing Potentiometer and pre-set new to similar value prior to installing (somewhere in the middle?) Photo
#41, #703
H) Transistor Replacement and Matching
1) Replacement tips and hints:
#333,
#482 through
#510;
#661 through
#698
2) Matching Transistor
PDF doc from DIY Audio Hafler DH200 Mod forum.
I) Bias checking and setting
1) How to check bias
#280
2) Bias resistor / potentiometer – See
note G4.
3) Bias recommendations 6-13Mv depending on rail voltage and speaker impedance:
#69, #715
4) Bias and distortion
#679
J) Powering-up after mods or repairs
1) Variac and Dim-Bulb tester -
#57
