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cambridge p60 is alive
cambridge p60 is alive
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Old 14th October 2007, 12:05 AM   #1
dukep is offline dukep  United States
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Cool cambridge p60 is alive

Well it has been years waiting patiently in a box. the pre-amp section is crippled due to corrosion of the pots, so I've decided to be an amp only. My vintage soundcraftman as the preamp.

I rebuilt the power supply (new caps/ rectifier/ bypass caps)
Then some flashy banana plugs and phono inputs. test.. hmmm-- sweet!

I had forgotten how well this amp performs. This cute little amp has a new life.

Anyone out there know the bias setting? I used the standard 5mv across the emitter resistor or ? (note:I don't have a distortion analyzer- but a good ear and 0-scope.)

Heatsink is a bit small and disjointed (4th transistor is on a 2nd section). It runs hot (after jamming 30min) and the 4th transistor runs cooler than its complement. Anyone know what the original specs were?
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Old 14th October 2007, 09:15 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
output stage is what?
Integrated darlington, or two stage EF, or two stage CFP, or somethng else?
5mV across the emitter resistor sounds like it may be CFP.
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Old 14th October 2007, 12:34 PM   #3
dukep is offline dukep  United States
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Default to-3 outputs

This has a pair per channel, mj802 & mj4502; bjt's
100v 30 amp outputs. Rail voltage runs +/-39v from a toroid xformer.

I slowly turned it up from 2mv to 5mv. the sound and crossover distortion improved.
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Old 15th October 2007, 05:15 AM   #4
Leolabs is offline Leolabs  Malaysia
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Hi,anyone here got any information on the P50 MKII???
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Old 15th October 2007, 03:21 PM   #5
dukep is offline dukep  United States
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Default found the schematics

This helped me, maybe others too. The bias is set at 7ma q-current.
measure current with + fuse out (~20), turn until x+7ma.

Then do the other channel.

and maybe applicable to the p50.
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Old 16th October 2007, 05:15 PM   #6
Rolf Zetterberg is offline Rolf Zetterberg  Sweden
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How do you know the current(which is what is the issue here) without knowing/telling the value of the resistor?
The P50 has TIP35/TIP36 output em followers,but with a little unusual arrangement of the resistors(0.15ohms).They are placed in the collectors and directly connected to + and -
I wish there was a standard using a 1ohm resistor to measure the current,and then the resistor is shorted in normal use.Would create a lot less errors,I believe.
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Old 18th October 2007, 10:58 PM   #7
dukep is offline dukep  United States
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Default bias never easy

I agree.

I usually measure the voltage across the emitter resistors which is much safer than pulling a fuse.
When you know that 7ma should flow through the emitter resistor, and it is 0.15 ohm ( this amp is a bit weird, with collector and emitter resistors). Run the calculation,, v=ir carry the zeros .....

I read the current by placing the multimeter on the amp output and emitter pin of the transistor, Mr. Mj802. .. but higher is not necessarily bad. Just closer to class A, but check the heat!
I'll be changing the heatsink anyway because my speakers are 4ohm.

I was once instructed to use a distortion analylizer ($$$) for a SAE amp. Don't have one. JUst tweek it and test, listen, and feel the heat. As we know, it is highly subjective. That's why it has us dIY's working so hard.

pwd
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Old 3rd December 2007, 04:36 AM   #8
frankwm is offline frankwm
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Exclamation New Vintage Cambridge Audio site!!

Hello.
I've, in the past seen some requests for info on the original Cambridge Audio gear - have P50 MkII / P60 / T55 / R50 myself - not all working (...).
Just over a week back I created a Google Groups site.

At present I've donated my Cambridge Audio P50 Service Manual - 80pp - contains all the info on the 3 types of that amp - there's a P60 circuit diagram - info/reviews on these + the T55 & R50 loudspeakers.
Stan Curtis is a member too!!

Hope that the site will be useful for info - also maybe some discussion on Cambridge Audio gear!

http://groups.google.com/group/vintage-cambridge-audio

All best
Frank
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Old 1st March 2018, 09:25 PM   #9
spandrel is offline spandrel  United Kingdom
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Wake up the Cambridge thread.
I've just got a P60, a design classic, in lovely condition. Has anyone got one of these? I need the colours of the transformer primary, it came with the voltage selector missing. Failing that what's the best way to find the sense of the two windings?

It needs a new volume control with a mains switch, I can't find one anywhere. It's a 47K log with a flatted shaft. Any ideas?

Maybe I'll just put it on a shelf and gaze at it.
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Old 2nd March 2018, 01:33 PM   #10
Ian Finch is offline Ian Finch  Australia
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Quote:
Originally Posted by spandrel View Post
.....I need the colours of the transformer primary, it came with the voltage selector missing. Failing that what's the best way to find the sense of the two windings?....
Having found each primary winding's leads, you could simply series connect them. The worst that could happen with series connected primaries, is that you get little or no AC from the secondary windings if the sense is wrong - hence no DC at the smoothing caps but maybe a bit of hum and buzz. Turn off promptly at the mains socket in that case.

There is a freehand schematic for the P60 and printed manuals for the P50, P70 and other similar models at Hifiengine which you can check for any wiring similarity but there's no guarantee that the transformer wire colour code will be the same for all models, nor that all products of the same model will have the same transformer supplier and colour codes. Cambridge Audio P60 - Manual - Stereo Integrated Amplifier - HiFi Engine

If you're the cautious type though, remove the secondary AC connections to the bridge rectifier and just measure the AC volts between the wires - securely connected to the meter probes by insulated clips etc. You should find about 24VAC between either wire and the centre tap or about 48VAC between them. Just pulling the 2.5A DC fuses there only isolates the power amplifier, leaving other circuits still powered, though I wouldn't worry too much in the event, since no voltage shouldn't be a problem and the other possibility is the right voltage
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