Cambridge Audio A300 - help needed to troubleshoot dead channel

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Greetings,

I have a Cambridge Audio A300 integrated amp for repair. It has a dead left channel and a working right channel.

I have measured the voltages on the output Darlingtons (the infamous SAP10N/P), left vs. right channel. You can see the voltages attached, in red. I have also checked the Darlingtons with the resistor tester (for shorts), left vs. right channel, with identical results, except that the emitter resistor on the SAP10P on the dead channel measures 5 ohms, instead of .22 ohms. I have bypassed it with an external resistor of about .70 ohms (the lowest I had), but it didn't seem to make any difference...

Can anyone make sense of the voltages I am seeing on the dead channel and advise if the Darlingtons need changing, or else if there is a fault in the driving circuitry before the Darlingtons?

Thanks in advance for any advice :)

Kind regards,
Bogdan
 

Attachments

  • Voltages on output Darlingtons.pdf
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The LED lights on both channels. Should I measure the voltage on R208?

Forgot to mention in my previous post that I went with a diode tester over all the transistors on the left channel and they measured the same as on the right channel. Also that visually everything looked OK, apart from a bulged C203, which I changed.
 
The output is biased towards the negative rail. Either a break in the positive, or excessive conductivity in the negative. Any items. Measure all voltages across all transistors. U201 is either locked or faulty. Check separately. Base-collector resistor 1m and collector current control (multimetr 10a) at 12 V momentarily.
 
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Thanks OldDIY!

Could you please detail how to check base-collector resistor 1m and collector current control (multimeter 10a) at 12 V?

I assume it means measuring the resistance between the base and collector of U201 with a multimeter, correct? What about 1m? Using the megohm scale? JFYI, I measured the B-C junction with a diode tester and it measured the same as on the working channel...

For the second part, I assume that means measuring the collector current of U201. But what about 12V? Should I supply U201 with just +12V instead of +38V???

Regarding voltages on transistors, I tried that a couple of days ago, and measured all of them, again left vs. right channel. Most of these voltages on the left channel were so far out, that they didn't make any sense to me! So I decided to ask for help on the forum. Also comparing these voltages with the working channel isn't that obvious, since the voltages on the working channel move around quite a bit, some by about 1-2V, over the span of a few minutes...
 
Hi OldDIY!

I managed to turn on the SAP10N, but not the SAP10P. Looking at the voltages I measured earlier, with the darlingtons in circuit, I would have thought it would be the other way around...

However, to turn on the NPN, I had to drop the value of the B-C resistor, because with 1 megohm I could only get .6V between base and emitter, not enough to turn on a darlington. I used an 120 kohm resistor instead and got the following values: Vce= 12V, Vbe=1.27V and Ic=.22A. I did a further measurement at Vce=15V and got Vbe=1.46V and Ic=1.07A, increasing as the transistor (on the radiator) heated up.

However, no luck with the PNP. At Vce=-1.5V, I already get Vbe=-1.33V, but Ic=.00A.

Has the PNP gone bad, or am I still missing something?
 
Hey :) I confirm the measurements were done at Vce=-1.5V (not -15V).

Anyway, I confirmed the PNP darlington is bad by hooking it up to the Peak atlas DCA. It reads as a diode, while the NPN as a proper darlington. Didn't know the Peak DCA also does darlingtons :D

Any ideas about what else I should check, with the darlingtons out of the circuit? I am worried something else might have gone bad, with all these DC coupled transistors...
 
Hey :) I confirm the measurements were done at Vce=-1.5V (not -15V).

Anyway, I confirmed the PNP darlington is bad by hooking it up to the Peak atlas DCA. It reads as a diode, while the NPN as a proper darlington. Didn't know the Peak DCA also does darlingtons :D

Any ideas about what else I should check, with the darlingtons out of the circuit? I am worried something else might have gone bad, with all these DC coupled transistors...

You can check the base-to- emitter voltages of the transistor complement in circuit under power to see if these have same voltages - comparing the working and non-working channels.

To avoid accidents connect one of the probes on your meter to earth and use the other to touch spots of the base and emitter separately and note the individual measurements.

This is a lot of work so it would not be silly to prepare a spreadsheet to record these and to do PNP and NPN testing in separate runs for each channel in separate columns to allow side-by-side comparisons.

If the layout is tight you could strip out a suitable length of insulated wire and push the measuring probe through the vacant space leaving just the tip of this exposed. If the speaker output terminals are screw fastening types - I put the black probe through the black terminal.

If you look at the emitter in a transistor as the point of origin of the electrical charges it may be easier to identify where series components may have failed.

PNP power transistors are more prone to failure than NPN types - simply this could have been bad luck.
 
Hi mjona, and thanks! Sorry for the late reply, some other things got in the way...

You can check the base-to- emitter voltages of the transistor complement in circuit under power to see if these have same voltages - comparing the working and non-working channels.

To avoid accidents connect one of the probes on your meter to earth and use the other to touch spots of the base and emitter separately and note the individual measurements.

This is a lot of work so it would not be silly to prepare a spreadsheet to record these and to do PNP and NPN testing in separate runs for each channel in separate columns to allow side-by-side comparisons.

I have begun doing that and posted the voltages at the begging of the thread, on the schematic (see attachment). Used GND at the speaker terminals as reference.

If the layout is tight you could strip out a suitable length of insulated wire and push the measuring probe through the vacant space leaving just the tip of this exposed. If the speaker output terminals are screw fastening types - I put the black probe through the black terminal.

I have a couple of suitable multimeter lead pairs, with short pointy tips, that I can have a poke around with into tighter places. Really needed them for this amp...

If you look at the emitter in a transistor as the point of origin of the electrical charges it may be easier to identify where series components may have failed.

This is very interesting. Would you be able to detail, to a noobie like me, how you would go about doing that?

PNP power transistors are more prone to failure than NPN types - simply this could have been bad luck.

Good to know ;)
 
Hi!

I had 15 ohm power resistors on each output (lowest value I have).

U202 is kaput, however, I was able to turn U201 on, as instructed by OldDIY, and it also reads OK on the transistor tester...

But I will change both anyway and I think I will upgrade from SAP10N/Ps to 15s or 16s.
 
Hey :)

I have used a Peak atlas DCA to measure the darlingtons. U201 reads as an NPN darlington transistor (with its various parameters), while U202 as a diode.

It is worth to try with 100R base to emmiter to see if previous stages works good, could save new transistors. Don´t forget to remove load resistors in that case.
(and U201/202 out ofcourse)

Let me check if I got this right. With the darlingtons removed from the board, on each channel I should connect a 100 ohm resistor from where the base connects on the board to where the emitter connects, so I could check the drivers. Can you please confirm?
 
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