Quad 405 Service History

[Canetoad]

I bought a Quad 405 amp a while ago and it came with all the original paperwork and also the receipts from Quad for a full service in 1991. The question I have is would they have upgraded it to 405-2 spec or just replaced of existing parts? Anybody have an idea of their policy in this area? :scratch:

[Arkless Electronics]

If they are 405-2 boards then they should have the safe operating area protection modules fitted. Two of them, very small circuit boards sticking straight up from the main board. They may look like circuit boards or could be dipped in a resin but either way they have 5 pins going into the board.

[prestonchipfryer]

I had my 405-1 serviced last year in the Quad service depot at Huntingdon and made into a 405-2, with rca inputs and 4mm binding posts. I specifically asked for the upgrade as otherwise they check the amp and replace only the necessary components.

John

[Barry]

If it is a 405-1, then what can be done to it is limited by the circuit boards fitted. As Jez has commented, with the 405-1 the protection clamp circuits were located off-board and fitted directly behind the speaker connectors. Unless requested otherwise, Quad will only replace what components they can on the existing boards.

[Arkless Electronics]

All sorts of improvements can be made to "soup up" either the original or the 405-2 but as standard the 405-2 is noticeably better IMHO.

[Barry]

All sorts of improvements can be made to "soup up" either the original or the 405-2 but as standard the 405-2 is noticeably better IMHO.

Agreed! The most obvious upgrade is to replace the awful LM301 or TL071 op-amps with something better, replace the 12V Zeners for 15V versions and upgrade the power rating of the power supply voltage-dropping resistors feeding the op-amp. Thereafter, one can upgrade the output transistors and their drivers for faster types, as well as replacing the components in the 'balance bridge' with closer tolerance components.

[Arkless Electronics]

Agreed! The most obvious upgrade is to replace the awful LM301 or TL071 op-amps with something better, replace the 12V Zeners for 15V versions and upgrade the power rating of the power supply voltage-dropping resistors feeding the op-amp. Thereafter, one can upgrade the output transistors and their drivers for faster types, as well as replacing the components in the 'balance bridge' with closer tolerance components.

Yes indeed. Even the TLO71 is much better than the LM301 though...

[Canetoad]

Agreed! The most obvious upgrade is to replace the awful LM301 or TL071 op-amps with something better, replace the 12V Zeners for 15V versions and upgrade the power rating of the power supply voltage-dropping resistors feeding the op-amp. Thereafter, one can upgrade the output transistors and their drivers for faster types, as well as replacing the components in the 'balance bridge' with closer tolerance components.

Barry, is it easier to go with an upgrade kit from somewhere like dada or put together a list of the required parts myself. Which would be the best and cheapest solution? Can you suggest the best source of information on upgrades.

[Oldpinkman]

Barry, is it easier to go with an upgrade kit from somewhere like dada or put together a list of the required parts myself. Which would be the best and cheapest solution? Can you suggest the best source of information on upgrades.

If it was last serviced in 1991 then in addition to any upgrades you are considering it probably wants re-capping. The main power supply capacitors probably don't need replacing (although I have just replaced mine) but the board capacitors, in particular C5 are likely to need doing. The amp is capable of being real quiet when in good nick. So quiet I am incapable of telling if it is on.

[Canetoad]

Hi Richard,

By upgrade I include replacing all capacitors including power supply. Is it worth going for larger values than 10000 uF?

[Barry]

Barry, is it easier to go with an upgrade kit from somewhere like dada or put together a list of the required parts myself. Which would be the best and cheapest solution? Can you suggest the best source of information on upgrades.

Dada or NET offer various upgrade kits. Unless you have access to RS, Farnell or any other component supplier to order the required parts, I would opt to use Dada (Belgium) or NET (UK).