Cambridge Audio amplifiers

absorbentgnome

Rotting Member
Just a little heads up about these nice little amplifiers, specifically the A3, A4, A5, A300 series etc. (They're all 60-70W amplifiers, and loads of people have them)

I've had several in to repair recently, and become accustomed to their simple design. (Which is probably why people think they sound good.) Unfortunately they suffer smoky death from a few problems that are easily avoided.

1) The amp is not designed for 4 ohm loads. That means no wiring up of 4 (8 ohm) speakers - you will kill it.

2) The transformer inside is under-rated in terms of power capability. While this is generally not a problem, avoid playing at high volume for extended periods. Like at parties. You *will* smoke it. You'll know if you've smoked it if you turn it on and you get no power light at the front, and there's a nice enamel burning smell. A replacement is only £15 but you have to take the *whole* thing to pieces to remove it. (High volume for an hour [or two, touch wood] should be fine though.)

3) There is only simple short circuit protection. Don't wire up or fiddle with the speakers or wires with the power on. It's a lottery whether the protection will fail. If you're lucky it'll be fine. If you're not, bye bye output transistors at £5 a pop and there's two of them for each channel. Plus a few burnt resistors and possible exploded capacitor. Not good.

:ismile:
 

absorbentgnome

Rotting Member
That last one has happened if you have one channel that doesn't work and a strangely plastic burny smell if you sniff the vents. Mmmmmm tasty.

Happy amping.
 

absorbentgnome

Rotting Member
If the rustling occurs if you poke or wobble the volume or tone controls then it's just the variable resistors that have gone a bit scratchy. This is easily solved either by replacement (pointless unless completely fucked) or by squirting a *small* amount of WD40 into the variable resistor itself. Use the straw. If there is no obvious way the WD40 can get into the variable resistor then carefully cover the body in WD40. It will gradually penetrate to the resistor surface and working the control will clean it. This will get it working fine for at least another year or two. Also poor solder joints on the resistors will cause this 'rustling' so if it is easy to get at resolder the joints.

Rustling will occur with other poor solder joints within the amp. You can locate them by poking the amp board and components with a (non-conductive!) stick while it is on.

Rustling will also occur with dried liquids spilt inside (ha ha usually followed by a nice puff of blue smoke. Beer is quite conductive... )

Most likely it is just the variable resistors that are scratchy or have poor joints on. Easily solved. The a300 I had here the other day had a scratchy bass control that rustled on one channel. Assuming that the a series may have the same amplifier chassis as yours means that it's easy to squirt a little wd40 into the side of the variable resistor as there is a small hole. But it is difficult to get at the solder joints without removing both the back and front panels and the two attached circuit boards.
 

absorbentgnome

Rotting Member
(For anyone else though *never* WD40 your mixer controls if you have similar problems. You can take them apart to clean them, but once they're knackered you have to replace. If you have no cash, then try a small amount of engine oil to lubricate and attempt to remove the grit, heh then pray :iwink: )
 

psyfi

Pie Fly
Good thread. I have an old Cambridge audio amp. With a similar sounding problem to Colin’s but the left channel just drops out some times. It doesn’t seem to be so much in volume but rather the low and mids disappear and the highs are left and it crackles whilst this happens. This isn't when the control is moved just randomly comes and goes. Any tips would be useful. Cheers.:ismile:
 

BeatNik

DJohn Mustard Project
Hmm.. I have a an A1 for my monitors and it serves me well...
Only annoying thing is that at low levels the output from the right channel rapidly drops out :Sad: annoying for listening to music past 11
 

absorbentgnome

Rotting Member
All of those problems are likely caused by the control pots (variable resistors). Psyfi's will be the bass pot (try wobbling the control and pulling it away or towards the amplifier, or knocking the pot with the handle of a screwdriver to confirm). BeatNik's will be the volume pot. Sqirt of WD40 inside, work the control for a minute or so, sorted.

Must be a common problem then. The a300 bass would drop out on one channel randomly and only would drop out if you gave the control a good *hard* push or twist in the wrong direction.

Glad I can help. :Smile3:
 

jimmy3000

One setting, total
Just a quick thought the dw40 does that really work. It sounds crazy could you say where you learnt it. Also do you know what a dry joint is? I have lost a Right chanell in one of my Denons any suggestions

Regarding amps in general, I think people make a mistake going for a "hi Fi" amp like Cam audio or Arcam or many other types, and expecting tham to really blast, they are just not designed for it, I am always hearing storys of overheating fuses blowing. If you want to have some fun buy a true 100w rms at 4 ohms, power block or pa amp. they don't cost much and can have a proper feel the music psy party if you want to. There is a good second hand market in these.
 

absorbentgnome

Rotting Member
jimmy3000 said:
Just a quick thought the dw40 does that really work. It sounds crazy could you say where you learnt it. Also do you know what a dry joint is? I have lost a Right chanell in one of my Denons any suggestions

Regarding amps in general, I think people make a mistake going for a "hi Fi" amp like Cam audio or Arcam or many other types, and expecting tham to really blast, they are just not designed for it, I am always hearing storys of overheating fuses blowing. If you want to have some fun buy a true 100w rms at 4 ohms, power block or pa amp. they don't cost much and can have a proper feel the music psy party if you want to. There is a good second hand market in these.

A dry joint is simply a poor solder joint. A good joint occurs when the two pieces to be joined are hot and clean enough for the solder to flow over them in a 'wet' fashion. Joints without enough solder, flux or heat when made will fail after a few years. TV's that work when you bash em > dry joints. Recent Sony AV surround sound amplifiers suffer from dry joints on their protection relays which make them go into 'Protect' mode soon after switch on.

I cannot say where I got the WD40 thing from. It's something I started doing years ago, putting the need to get inside the pot without taking it apart and clean the pot surface, despite the fact that there is often no hole to get inside. It is actually advised NOT to use WD40 on pots, because the penetrating action can lift the etched resistor pattern off the inside. However in my experience (some 10 years of component level fault finding) modern pots do not suffer from this and have only seen a few rare cases of very old pots being further damaged. In any case a pot that knackered should be replaced. So (almost) a no lose situation. You don't have to desolder the pot, you don't have to take the amp to pieces and you only have to do it every couple of years when it dries out and goes all crusty again.

Your Denon that has lost one channel - First check any internal fuses to see if they have blown. Replace with the *same* type - a higher rating one could toast more parts of your amp! If they blow again then you have some serious component failure. More than likely death of the output transistors, which can be replaced with some patience. I am not familar with the Denon amplifier so can only give general rather than specific advice.

PA amps with fans are definately the way forward!
 

Faction

Proto-col
absorbentgnome said:
PA amps with fans are definately the way forward!

Not if you're trying to keep the noise down in the studio, they're not :Wink3:

My 'rustling' noise is not correlated to the volume control; when it appears it is not affected in any obvious way by turning the volume knob, or even the pan control, ie. it stays there whatever happens.

It's been ok for the past couple of days though...
 

Ratty

Sticky Toffee Pudding
I've got an A500 as my main amp. Got it for £100 second hand and it's excellent. Remote control, phono stage, and it gets cranked on a regular basis, no problem.
The Chinchilla likes sitting on top of it when it's on cos it warms it's bum up too :Smile3:

Only problem is one of the pre outs is dead but I think it's a dry joint on the socket cos I can see it at the output stage on a scope, just not at the socket. It looks like a major strip down job just to get to the solder joint too, so it's on the back burner for now.

I've got an old Cambridge pre amp too... served me well for about 10 years but just gathering dust now..

time for an e bay clear out :Smile3:
 

absorbentgnome

Rotting Member
I had an old 90W RMS a channel Pioneer SA-960. No fans, didn't have a particularly large transformer and had a reasonbly sized heatsink. It looked like a piece of crap as well. But it has to be the best hi-fi amp I ever owned... Stupidly loud easily enough for ridiculous house parties and just wouldn't blow up. There were even burn marks in the circuit board where it overheated each time. But it still worked!

Then some $£%£^£$%^ muppet spilled beer in it. And the parts needed to fix it haven't been made for years. Grrrrrrr!
 
Dude - you shouldn't use WD40 with electrical contacts. It lubricates and coats, leaving a fine film on the contact. You should really be using an Electrical Contact cleaner like Servisol that is designed to clean the contacts but evaporate, leaving no residue.
 

jamez_23

Blah
I have a Cambridge Audio amp - not overloaded ..... only really really used at low volume these days.

Lovely .... pretty cheap as well.

:Smile3:
 

absorbentgnome

Rotting Member
SteveJ... said:
Dude - you shouldn't use WD40 with electrical contacts. It lubricates and coats, leaving a fine film on the contact. You should really be using an Electrical Contact cleaner like Servisol that is designed to clean the contacts but evaporate, leaving no residue.

That is completely true. :iyes:

But my gamble is that the film prevents further slow degredation of the pot surface by air, particularly after working the control. And you're more likely to have a can of WD40 or cheap substitute kicking around. :Wink3:
 

absorbentgnome

Rotting Member
A bit more info:

Apparently the A5 has a thermal cut-out in, so although it still suffers from death after a few hours of punishment it's not terminal and can be reset by unplugging the amp.
It seems that the Arcam amp that is based on the A5 also has this feature.
[This has not been confirmed by my own eyes however]

@Colin: You'll have to track down the poor solder joint causing your problem with a stick (safe!) or your finger (lol got images of divining sticks here) and some patience. The power resistors (the large ones) can sometimes become 'noisy' as well but is pretty rare. That's about as much as I can help you with because the amp is not in front of me.
 
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