Replacement capacitors for Arcam Alpha plus

[AndrewB]

Can anyone give me some advice on which capacitors to replace & what type I should use?

My 21 year-old Arcam Alpha plus amplifier is sounding a bit tired – general lack of clarity and the bass has all but disappeared (it never was ‘bass-heavy’, but what there was has faded). It still basically works OK though – so I’m guessing the capacitors are getting life-expired, even though there is no sign of any bulges, etc in the cases. If I’m totally wrong about this, please tell me!
I’m about to get a new pre & power combination, but would like to keep the Arcam in my workroom, and return it to its former sound.

I have a copy of the service manual and schematic which I can follow, but I don’t know enough about amplifier circuit design to know which capacitors will have the greatest effect. Having said that, there are basically only 3 electrolytics per channel (all 22 microF) in the power amp circuit, a couple of 10 microF per channel in the pre-amp, and the 2 6800 microF (filters?).
Currently they are all Rubycon CEW M – but I can’t find any results googling for these.
Suggestions please for alternatives that are easily available in small quantities.

Thanks, Andy

[AndrewB]

Where are all the passive component experts?
Sure would appreciate your help.

[AndrewT]

c13, c210, c211 are the problems. The other two 22uF are in the protection circuit and allow transient currents to pass that are needed to make speakers sound right.

C13 would explain the lack of bass when new, but this is also limited by the smoothing values of 6800uF.

Is there room to change the 6800uF to 15000uF (15mF)?

If so then change C13 to 100uF, if you keep 6800uF then c13=47uF, not higher. For 10mF use C13=68uF.

If I were changing this I would try to squeeze +-40mF onto each supply rail and change C13 to 150uF or 220uF.

The phono input is cheap crap. Forget it.
The rest of the pre-amp stages are probably interfering with the quality of signal passing to the power amp side. Bypassing as much as possible would be worth experimenting with.
Look at the NAD thread on bypassing as many of the extra stages as possible.

[jaycee]

Agree fully with AndrewT. Forget the phono input, its very basic and if you want good turntable performance, buy a seperate phono amp.

The PSU capacitors C210/211 will almost certanly be knackered. Personally I also feel that the voltage rating of those caps is too slim - I would try and fit 50V ones that will fit into the same space. Again, if you can increase the capacitance value and maintain the size, it will improve things.

Indeed, for max quality the preamp is best bypassed and a better alternative used, but if you want to keep it then I would replace C8 and C11 with some high quality parts. For C8 you could use a 1U 63V MKS2 capacitor (Rapid has these), for C11 i'd go with a good quality bipolar electrolytic capacitor.

It might be worth pulling the muting FET Q1, but watch out in case there's a large turn on thump. You might also consider removing the VI limiter stage by pulling Q9 and Q10, but you would have to provide an alternate protection method for your speakers (which IMO is a good idea anyway - the VI limiter wouldnt prevent damage in the case of a failed output transistor)

All in all, for it's age the Alpha is a great little integrated amp. My only gripe would be the bias stage.

[AndrewB]

Thanks for your replies.

Finally found some caps for the PSU that will fit (63V) - height is no problem, but the originals were only 22mm dia, and there is only just enough space to fit 30mm dia (without the hassle of drilling new holes, etc.).
Most commonly available seem to be 35mm dia.

I'm going to stick with the original capacitor values to start with - will update thread when they are fitted.
Then possibly experiment with the changes you suggested.

Cheers,

Andy

[AndrewB]

Finally got all the caps I needed and have fitted them.

A couple of hours burining in, and the old amp has got its soul back!

Thanks for your help.

Andy