Cambridge audio P50 bias adjustment

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Hello all,

My second post here, since last time I received such good help I figured I'd try my luck once more.

I am currently working on a Cambridge Audio P50 amplifier, not the mk1 or the mk2 but the gunmetal enclosure one (there seem to be three versions). I bought this amp in non working condition, there seemed to be a problem with some PSU transistors. I traced the signal with my oscilloscope and all seemed well.
Before closing it up and calling it a day I decided to check the dc offset. My problem is, I am always getting -2 volt on D6 (where I am supposed to measure) no matter what the setting on the bias adjustment pot.

The schematic is included, can anyone guide me in the right direction?

Thank you very much in advance!

Roy Vorster


  • Cambridge audio P50 power amp.pdf
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Thank you very much! T117, i'll be checking for a low resistance on C4 soon. JonSnell, since I am observing the exact same voltage on both channels I presume this can't be the cause.

This observation also lead me to think the problem might be in the PSU. I noticed for example that V+ is exactly the same as V- (both +42.5V). I'll measure the voltages from the PSU and such and I'll get back to you. Too bad it's quite a pain to get the board out.
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There seems to be a second version of MKII with an improved protection circuit but that should not affect the problem. The schematic you linked though, doesn't seem to be for P50 because it shows 2 pairs of NPN output transistors as seen in the 1989 P70,C70,A70 series. I think the P50 uses a single pair of complementary TIP35/36 output transistors. If not, the correct schematic is'nt easily available.

Perhaps you have not used the right schematic, which you can check against the two assembly arrangement diagrams for P50 MKI, MKII and MKII(mod.) here: Cambridge Audio P50 Manual - Stereo Integrated Amplifier - HiFi Engine

Generally, if D6 is really fitted in parallel with one emitter resistor of the output transistor pair, you should still read a voltage of only 5mV (adjustable as specified). If not, then there is failure there in one of the output stage transistors and/or drivers, such as a C-E short.
Like I said in the opening post, there is a different one which is not an MK1 of MK2. It is, however, part of the same series as the a70 and the p70. I shall link in the manual for all of them down below.

This unit uses 4 npn power transistors(2SD1046).

Cambridge Audio P50 Manual - Stereo Integrated Amplifier - HiFi Engine
On this page, download "service manual (preliminary)" by meik65. That is the correct service manual and the manual I got the schematic from.
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Further questions-
With that 40volts measured at both rails, what is your common reference point for both measurements, using the same (black) probe ? Is it OV, chassis ground, loudspeaker -ve terminal?
What is the voltage across the output terminals (assuming the speaker relay engages)?
If not, then measure output at the amplifier side of the speaker relay (if fitted) or the output node on the PCB. i.e. The junction of OP transistor emitters, emitter resistors etc.

There were at least 3 different designs, not just versions of P50 available there but I understand now its as P70 with 2 pairs of output transistors. What a shambles of model ID confusion by Cambridge!
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Those voltages were measured with chassis ground, although chassis ground is connected to loudspeaker ground (multimeter). This amplifier seems to have no speaker relay. The voltage measured over the terminals is also -2 Volts.

As I stated earlier, I won't be able to make any more measurements till this afternoon.

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