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Old 11th January 2007, 07:03 PM   #1
ash_dac is offline ash_dac  United Kingdom
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Join Date: Jan 2005
Location: UK
Default 2004 Vox AC30 : LM3886 hack

http://www.divdev.fsnet.co.uk/repair2.htm


Quote:
Went pop between songs and then no more.
Retro look of the 1960s Vox AC30 amp made probably 2004 with lots of SM
digital chippery for effects undreamt of in the 60s.
One 12AX7 , but is it for show only ?
Heaters have an AC supply but the anodes don't seem have any HV components
attached to them. The speaker o/p is a non-retro National Semi LM3886T.
Poking around there are no
capacitors with greater than 50V dc working anywhere on the ps and o/p
board.
Damn hybrid O/P devices , -V rail to o/p short on the LM3886T,
burnt voice coil.
Tried a 100W TDA7294 as I've a couple at hand, pinning swapped to match
the 68W LM3886 and inverted to give some space and less wires crossing over.
Original TDA pinning
1+5 13+8
2 NC
3 14
4 15+8
5+1 -
6 -
7 4+1
8 9+10
9 2
10 3
Ignored the bootstrap cap.
Monitoring phones outlet all effects and classic amp colourations are functioning
so the valve is presumably an expensive and highly inefficient indicator,
for show only, the maximum DC on any of its pins wrt ground is -15V.
Other info
AC supplies 33,0,33 / 6 / 6.8V
DC +-43V
DC reg V 15,-15, 3.3
Can anyone make any sense of the hack ?

Does LM3886 fail in this way ?
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Old 11th January 2007, 07:16 PM   #2
Fenris is offline Fenris  United States
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Join Date: Jun 2004
Location: The last frontier
The tech merely replaced one chipamp with another of comparable specifications. Since the pinouts were different, he had to do some re-wiring from the old pins to the new ones. The numbers should be read as:

Original 1 and 5 pins on the LM3886 were rewired to the 13 and 8 pins on the TDA, pin 2 on the LM3886 is Not Connected, pin 3 on the LM3886 is changed to pin 14 on the TDA, etc.

Guitar amps usually have lots of distortion and are pretty heavily abused. This isn't good for solid state electronics. Also, if the manufacturer was using a tube for looks and not for any function, then this amp probably had lots of style and little substance. It probably used lower tolerance and lower quality components (and too small of a heatsink) to cut costs and that usually doesn't help reliability.
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