Hi, all. Just bought a very clean and presentable Cambridge audio P40. Bought as spares repair. Plugged it into the lamp limiter and all was good so braved it at plugged it in and hooked up some speakers. Basically, it's working but there's an issue with the sound balance between left and right channel.
The first thing I noticed was that the volume pot seems way to sensitive. By this I mean that from 0, it goes really loud as soon as you turn it. It is graduated though. Second thing I noticed is that the right channel doesn't become audible until volume is turned up a bit. This is regardless of which position the mono/stereo selector switch is in.
Next thing to note is that's the balance pot doesn't seem to do much at all. I can hear a shift it left /right bias when I turn it fully either way but it's barely noticeable and when turned all the way, it doesn't mute the opposite channel.
Other than that, it actually sounds pretty good. Only paid £30 and it's in lovely cosmetic condition. In my opinion it's a lovely looking amp and it seems very well made. it's really heavy for the size. Looking inside it doesn't appear to have messed about with.
I believe it's from the late 60's/early 70's which would make this the mk1.
Any pointers would be greatly appreciated. Haven't been able to find a wiring diagram either so if anyone has one that might be handy.
Thanks.
The first thing I noticed was that the volume pot seems way to sensitive. By this I mean that from 0, it goes really loud as soon as you turn it. It is graduated though. Second thing I noticed is that the right channel doesn't become audible until volume is turned up a bit. This is regardless of which position the mono/stereo selector switch is in.
Next thing to note is that's the balance pot doesn't seem to do much at all. I can hear a shift it left /right bias when I turn it fully either way but it's barely noticeable and when turned all the way, it doesn't mute the opposite channel.
Other than that, it actually sounds pretty good. Only paid £30 and it's in lovely cosmetic condition. In my opinion it's a lovely looking amp and it seems very well made. it's really heavy for the size. Looking inside it doesn't appear to have messed about with.
I believe it's from the late 60's/early 70's which would make this the mk1.
Any pointers would be greatly appreciated. Haven't been able to find a wiring diagram either so if anyone has one that might be handy.
Thanks.
This one perhaps ?
Try cleaning the pots & switches with deoxit first.
CAMBRIDGE-AUDIO P-40 AUDIO PA 20W SCH Service Manual download, schematics, eeprom, repair info for electronics experts
Try cleaning the pots & switches with deoxit first.
CAMBRIDGE-AUDIO P-40 AUDIO PA 20W SCH Service Manual download, schematics, eeprom, repair info for electronics experts
Yeah I've used contact cleaner in everything I can access. Might need to dismantle and clean them more thoroughly though.
Leaky electrolytic caps allow DC to pass through some controls and this also results in noisy operation. As this model is quite old and lubricant can't fix electrical faults, I would be replacing all the electrolytics with standard, known quality parts, not expensive boutique types. If you haven't done this already, good luck with your restoration. Do it right first time and you'll be much happier than having to open it up again again to iron out such faults that have developed over time.
BTW, the narrow operation range of the balance balance pot was not unusual in 60's -70's British designs. Typical DIY designs were also like this until Japanese products swept all aside and showed how more flexible features were possible and at lower cost too.
BTW, the narrow operation range of the balance balance pot was not unusual in 60's -70's British designs. Typical DIY designs were also like this until Japanese products swept all aside and showed how more flexible features were possible and at lower cost too.
The volume control having little range might have a lot to do with the input sensitivity, especially if you are feeding it from a CD player or a PC. The line input is apparently 250mV sensitivity, whereas that would be more like 1-2V on a modern amp
edit: According to the manual, the Radio input is 250mV into 180 ohms impedance. The Aux input is 50mV! into 100K. If you were using the Aux input, that would certainly explain why it's so loud!
edit: According to the manual, the Radio input is 250mV into 180 ohms impedance. The Aux input is 50mV! into 100K. If you were using the Aux input, that would certainly explain why it's so loud!
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Thanks for the advice. That would definitely explain why it's so loud! I am planning on doing a recap on this amp when I get some time. In the meantime, is there anything I can do to make it a bit easier to live with with regards to the input sensitivity? At the moment the only thing I have plugged into it is a DAC (raspberry pi).
Thanks
Thanks
Hi, I have an old Sonab Cambridge P50 bought in the late 70s. It has the same "problem" with sensitivity. As mentioned earlier the output from other connected sources wasn´t as high then. You could put a buffer with volumecontrol between source and P40.
Here is an informative site regarding the Cambridge amps (and more from the 60s 70s) by Paul Kemble.
A Paul Kemble web page - the Cambridge Audio 'P' series amplifiers.
Here is an informative site regarding the Cambridge amps (and more from the 60s 70s) by Paul Kemble.
A Paul Kemble web page - the Cambridge Audio 'P' series amplifiers.
Just dusted this amp off and thinking about getting the stuff together to recap it in the near future. Listening to it again now with a pair of Technics SB-F3's and it's really not bad at all. It's not distorted but does sound very slightly muddy compared to the Aiyima A07 I've been using as my go to in the same room. It may just be the way it's always sounded or maybe it's the 50 old electrolytic caps? So, I'd be interested to compare it when recapped. The right channel is definitely louder than the left which doesn't even become audible until it's turned up a fair bit with the left chanel being heard as soon as the volume pot is moved away from 0.
There is a pot on the back which says "level adjust", this doesn't seem to do anything at all??
I read back over the previous comments and remember the info about the British design balance having a narrow Operational range. Messing with it again today it barely seems to do anything at all.
Where do people recommend to buy capacitors in small amounts? I'm in the UK. I may need some advice on finding suitible caps of the right (or close enough) value(s) as when I've tried matching new ones online to what's listed on the schematics of old amps I've struggled to find exact matches.
Thanks again.
There is a pot on the back which says "level adjust", this doesn't seem to do anything at all??
I read back over the previous comments and remember the info about the British design balance having a narrow Operational range. Messing with it again today it barely seems to do anything at all.
Where do people recommend to buy capacitors in small amounts? I'm in the UK. I may need some advice on finding suitible caps of the right (or close enough) value(s) as when I've tried matching new ones online to what's listed on the schematics of old amps I've struggled to find exact matches.
Thanks again.
By the way I've switched it off for now as I don't want to push my luck in case something goes pop!
The level adjust pot on the back matches the tape out level to the tape playback level,
so that the tape monitor switch has equal levels in both positions.
Sounds like the volume pot is the likely cause of the channel imbalance.
I'd look for a substitute.
Only buy new electrolytic capacitors from a reliable distributor like Farnell, etc.
You also want fresh date codes on them.
so that the tape monitor switch has equal levels in both positions.
Sounds like the volume pot is the likely cause of the channel imbalance.
I'd look for a substitute.
Only buy new electrolytic capacitors from a reliable distributor like Farnell, etc.
You also want fresh date codes on them.
I have just got myself a P100 which seems to be a variant of the P40 but with very little detail other than a pdf schematic.
It works fine from the "Tape Monitor" input but the left channel is very quiet on normal inputs so hopefully a problem with input module or pre amp.
Seems green in operation using under 20 watts and has a different PSU arrangement with just plus 75 Volts regulated and no negative voltage rail.
It works fine from the "Tape Monitor" input but the left channel is very quiet on normal inputs so hopefully a problem with input module or pre amp.
Seems green in operation using under 20 watts and has a different PSU arrangement with just plus 75 Volts regulated and no negative voltage rail.
