MODIFICATIONS TO PRINTED CIRCUIT BOARDS.
Amplifier Board M12368 iss.5 originally fitted. Circuit diagram iss. 2.
1. Amplifier board M12368 iss.6
Copper track layout modified - component layout ünchanged.
2. Amplifier board M12368 iss.7 Circuit diagram iss. 3.
R4 changed from 10k to 22k
R5 changed from 10k to 4k7
R9 changed from 180„ to 220„
R19 (3k3) removed (combined with R23)
R23 changed from 3k3 to 1k2
C9 (330pF) removed (would be in parallel with C11)
C18 47nF fitted to -ve supply after FS2 - see circuit diagram
FS1 and FS2 effectively changed places
R2 changed from 2.2„ to 10„
Copper track width reduced
3.(a) Amplifier board M12368 iss.9 introduced ¤ serial number 9000. Circuit diagram iss. 4.
R41 22„ added
L3 6.9µH added
C15 0.1µF added
C16 0.1µF added
C18 (47nF) removed
C19 1nF fitted between base and collector of Tr10 (not recommended if stable without)
Copper track width reduced
Also at s/n 9000 a clamp circuit, on PCB M12400, was mounted on the output terminal (Fig. 15).
This detects excessive d.c. offset at the output and short circuits, blowing the internal 4A fuses
FS1 and/or FS2 to protect the loudspeaker.
3.(b) The following component changes were made ¤ serial number 29000. Circuit diagram iss. 5.
R10 changed from 1k to 1k8
R27 changed from 8k2 to 15k
R29 changed from 8k2 to 15k
R35 changed from 0.08„ to 0.091„
R36 changed from 0.08„ to 0.091„
D1 changed from LR120C to LR150C (op-amp voltage increased from 12V to 15V)
D2 changed from LR120C to LR150C (op-amp voltage increased from 12V to 15V)
4. Amplifier board M12368 iss.10
Identical to M12368 iss. 9 except copper pads for power transistors modified for production.
5. Amplifier board M12565 iss.3 Introduced at serial number 59001. Circuit diagram iss. 6.
Other QUAD 405s with this PCB fitted were serial numbers 57301 to 57600 inc.
This board incorporates the clamp circuit and the ESL voltage limiter is now a link
6. Amplifier board M12565 iss.5 (Π405-2 PCB). Circuit diagram iss. 7.
Was fitted at serial number 62500 but with a 405 name plate until serial number 65000.
See page 12 for 405-2 PCB changes.
Alternatives
Transistors - on PCB M12368 iss. 5, 6 & 7 BDY77 or BDY74 may have been used for Tr9 and Tr10.
BDY77 is a suitable replacement for both but beware - faster transistors may cause instabiliity.
On M12368 iss. 9 &10 and M12565 iss. 3 Transistors Tr9 and Tr10 may be 2SD424, 17556 or
2SD676 and are interchangeable.
Tr2 - BC682, ZTX304, BCX32 and BC546B are interchangeable.
Tr3, Tr4 - E5458, ZTX504 and BC556B are interchangeable.
Tr7, Tr8 - 40872 or 2SA740 are interchangeable.
LED - LP1 - HP5082-4850, Exciton XC5053, Toshiba TLR114A (or any modern LED with R40 adjusted).
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