Cambridge Audio P40

[Radio Fixer]

A mechanical eng friend, not local, has one and its fine apart from what sounds to me as an intermittent Vol control.

Well I never do it but he has nothing to lose by squirting in some WD40 if there is access.  Some say it works  

Its a long time since I worked on Amps but I imagine R3 is the control although not working to ground but to the low Z HT line (maybe for hum reasons).  If so I guess its still Log law and to be certain that the control is actually the problem can he just reverse the outer connections and try the control the other way around?

If that fixes it he would just need to buy a new control which he believes Mouser has.

   

https://www.radiomuseum.org/r/cambridgea...r_p40.html

thanks for advice   Gary

[Mark Hennessy]

R3 isn't the volume control - it sets the gain of the phono pre-amp, so has no effect on the other inputs. It provides a negative feedback path around the input stage - the overall gain of those first two transistors is roughly R3/R1. I've no idea if it's a user control or a pre-set on the PCB.

I haven't immediately found a full schematic - just the phono pre and power amp seems to show up. The volume control will be located somewhere between these two sections.

[Murphyv310]

Looks like a preset to me. It's doubtful that's the control with the problem.
As Mark says the VC is elsewhere on another part of the circuit.

[Radio Fixer]

I see that quite clearly now you have explained it Mark.

Thanks to both Gary

[Radio Fixer]

Well the AMP has come to me.
Actually the Vol pot seems OK to carry on.  There is a dead area right at the bottom, tiny rotation but I wouldnt consider that abnormal.
Only fault I have found is an intermittent channel on one AUX Phono socket.
   

Here is a clip from Craig back in 2018.  I will PM him and see if he has a schematic.
   

Would folks on here change all the electros?  Are the little brown jobs electros? they are marked 220/16 but have no polarity indications.
     

Its working fine with no hum and very little clicks / pops when changing inputs 
There is a problem with the power ones as they are only 35mm high and the best I can find are 41mm which might just go in.

My feeling is to leave things alone apart from fixing the AUX phono.

thanks Gary

[Radio Fixer]

Cant see how to PM Craig?

The little brown caps could be non polarised 220 mF at 6V. Read closely what looks like a 1 is more like a /.

Gary

[ppppenguin]

Its working fine with no hum and very little clicks / pops when changing inputs 
There is a problem with the power ones as they are only 35mm high and the best I can find are 41mm which might just go in.

I had a problem replacing the main reservoir caps in some powered speakers a couple of years ago. The originals were obviously duff, the amount of ripple on the supply was horrible and you could hear it. The space was liimited and I couldn't find replacements that would fit. I think I came down from 4700uF to 3300uF to make it all work. I tried it on the bench before ordering spares to make sure there wasn't a hum problem. I suspect it may have affected the minimum mains voltage but I didn't test with a variac.

[Radio Fixer]

Well I have played the AMP for hours now and the power caps dont change temperature at all. No bulges so not like yours Jeffrey. Its simple when you have a clear fault.

Electros from even earlier can be alright if the item is used. Repaired a 60's Ekco radio that was used every day and the power caps were still good. With wired stuff its easy to measure leakage current and there was virtually none. Left those and its still working fine.

Gary

[Mark Hennessy]

Cant see how to PM Craig?

It depends on which theme you're using. Start by finding him in the Member List, and there will be a link to PM him there. If you're using the MyBB Default theme, that link is obvious, but if you're still using Vienna, you have to click his name (above his avatar) to bring up a menu with the contact options.

Alternatively, you can go to your PMs, click on "compose", and start typing his name - it will auto-suggest as you type.

Even easier, this direct link should get you started:

https://golbornevintageradio.co.uk/forum...d&uid=1296

Hopefully he'll be happy to post the schematics in this thread? Though it wouldn't be difficult to reverse-engineer them. But schematics probably aren't necessary.

It's one of the later models, and does look to be fairly straightforward - an op-amp based phono pre-amp, and no line pre-amp. So the line level sources go straight into the power amp via the input switching and volume/balance pots. The schematics posted earlier are for the earlier models, so we can disregard those.

Yes, I think it's worth changing the electrolytics, given the age, and the fact it's in pieces on your bench now. After all, there's not too many of them. The reddy/brown round ones don't have the best reputation, although I can't say they've given me too many problems over the years. The schematic would be helpful to determine (a) if they're polarised or not, and (b) if they actually have to be. Here is an example replacement in non-polarised form:

https://uk.rs-online.com/web/p/aluminium...rs/1763786

Shame they come in packs of 20. I'm sure other suppliers would sell in smaller quantities. But there's a good chance it might be in the lower "limb" of the negative feedback network - if so, it shouldn't have much DC bias on it (should be well under a volt - perhaps only 10s of millivolts), so a polarised one might be suitable. But that needs verifying before proceeding.

The pots look to be fairly standard types that should be available today (although there's a chance the balance control could be a special type). The input selector switch might be harder to find today. Musical Fidelity used them in the A1 and related amps back in the day, and they were quite unreliable thanks to a poor pre-amp design that put a small DC current through both the switches and the volume control. A replacement today involves a bit of metalwork - not pretty, but perfectly safe and reliable. See the bottom of this page: https://www.markhennessy.co.uk/mf_a1/mods.htm - something similar could probably be done here if necessary, but hopefully this switch is OK.

Don't overlook the Tape switch. If it's a "tape monitor" switch - as was the norm at this time - then it interrupts the signal from the rotary switch. So when in the out position, the signal from the rotary switch is passing through the NC contacts. In the MF amp, these always need replacing because of the DC issue...

Even if it's behaving for you now, I'd say that it's definitely worth doing a bit of preventative maintenance to keep it alive for another 40 years

[Radio Fixer]

Thanks for a comprehensive reply Mark

This one is from the 1980's I'm told.

The early ones apparently had no power transistor heatsink other than the metal case.  This now has a  large slab of solid aluminium which the power transistors are bolted to as are the bias diodes.  Neatly done.

I have run it for hours (no hum at any volume), never played so much music for a long time, and the power electrolytics run cold and have no swelling so inclined to leave them.  I find these fairly 'modern' ones last if the device is regularly used.  I repaired a 60's radio that was used on FM every day.  As it was wired it was easy to break the common earth connection and measure the currents with a true RMS voltmeter... virtually DC leakage free.

All input stages seem fine and no rustles and crackles so again inclined to leave the few small electros alone.  If it were easy to remove the board I would change them but it isnt.  If it gets problems in the next few years it can come back.  In 40 years someone else can fix it.

Pots and switches seem perfect and the only fault was a broken track to the centre pin of the L  Aux Phono connector so one channel dead.  Fixed with a piece of fine wire and solder.

He actually uses the Aux sockets for the turntable via a Pre-Amp that also give a USB output to his laptop.
Rega Fono Mini A2D Phono Pre Amplifier Black (New Version) (hifix.co.uk)

thanks again  Gary