Cambridge 640a Repair

[P0tt3r]

Hey everyone,

Totally new here. Fairly new to the HI FI world as well
NVR done a mod. Nor a repair.

Picked up a Cambridge Azure 640a V2 for fairly cheap 110 CAD thought it worked fine until noticing the balance as off.

One speaker is louder than the other.
Tried other speakers.
Used the same wire.
Tried other channels etc.

It's come down to the amp not working properly.

What would cause this?
Capacitors? Should I buy a completly new set of quality caps and solder them in myself?

Would something else be the cause that I cannot fix at home?

Thanks!

 

[rayma]

the balance as off. Capacitors?

This is a fairly complex amp with lots of parts and connectors.
Make certain that the balance control is centered first.
Capacitors are less likely to be the cause of this problem.

Are all of the sources off balance? Perhaps the volume control is bad.
Does the tone defeat switch help (with the tone controls centered)?
An oscilloscope would make it much easier to localize the problem.

It's best (to be absolutely certain) to do this:
Note which side is too low in volume.
Turn it off, switch L and R input cables, and also switch L and R speaker cables.

Is the same side still too low in volume? Then it is not the amplifier.
Is the opposite side now too low in volume? Then it is the amplifier.

 

[P0tt3r]

Thanks ms for the reply rayma. There is only one source a 540p phono preamp. It has worked before and when tested with my old amp still works fine (balanced).

Volume control might be bad. But seems to work ok? I cleaned all the pots with electric cleaner hoping for an easy fix.

The tone defeat does NOT help.

 

[rayma]

I cleaned all the pots with electric cleaner hoping for an easy fix. The tone defeat does NOT help.

Did you swap the phono stage outputs, and also try different audio cables?
Also, did you clean the switches? Those can be worse than pots for giving trouble,
especially the tape monitor switch. Could be one of the many connectors.

Then probably you'll have to get a tech with test equipment
to trace down the problem.

To diagnose this, input the same test signal to both inputs, and trace it
through the circuit with a scope and schematic, to see where the bad channel
drops compared to the good channel. Do you know someone who can help you
with their test gear?

 

[jaycee]

If you have a 'scope, check the signal on the PREOUT outputs at the back. My guess is the fault is in the preamp. Probably an electrolytic capacitor used for signal coupling. Unfortunately while the amp is well designed, it is manufactured using cheap parts.

 

[P0tt3r]

Thanks everyone. I did a thorough cleaning at least I think I did.

Could it be the biasing? Thinking about testing that.
Would biasing variance effect channel output?

 

[sgrossklass]

Could it be the biasing? Thinking about testing that.
Would biasing variance effect channel output?

Nope. Bad components and solder joints in the feedback network may be more like it.

Check preamp output channel balance as suggested. A computer may be able to lend an input.

 

[P0tt3r]

Ok I think screwed up. Not totally sure. But...

I was reading the 640a V1 service manual and it said adjusting bias. But not realizing that the board is different on a v1 vs v2 I have adjusted the bias switch but am unable to locate the resistor to test the voltage.

Does anyone have the v2 service manual so I can correct my mistake?

Thanks.

Ps nothing is wrong yet. Bet I don't want to hook up any speakers in risk of damaging something now.

 

[P0tt3r]

NVM I figured it out.
Still haven't figured out the off balance issue. But the bias part I got adjusted.

 

[P0tt3r]

So I measured the pre outs with a multimeter and as suggested the balance is off 4.5 and 5.3 steady voltages.

What would be the next step in repair? Should I just give in and take it to a tech?

Thanks again.

 

[Ian Finch]

As already suggested, test the preamp output first, by connecting from the pre-out sockets to your old power amplifier/speaker system or any other working amplifier or stereo player you have to hand. Have you followed rayma's procedure in #2 fully?

 

[Ian Finch]

Sorry, I didn't notice your post #10 until after my post appeared - perhaps that was moderation. Anyway, you answered the question well enough so it's clear that the preamp already has some imbalance, assuming both channels are fed the same signal to begin with.

All you probably need is the schematic to track the signal path back through the preamp, comparing the signal levels between the channels as you go, right back to the input terminals, having first made sure that the levels read at the inputs are equal to begin with. This should locate the general area where any variation may be and narrows down the types of faults and the parts to check. One possibility is the C28,30 (and C14,C25 in the control section) electrolytic coupling capacitors which can deteriorate and restrict rather than pass the signal. Page 8 of the service manual has the analog section of the peamp, the digital control and protection section follows on Page 9.

Free download of the service manual etc. after registration: Cambridge Audio Azur 640A - Manual - Stereo Integrated Amplifier - HiFi Engine

 

[tellyrepair]

I am repairing this unit for the thread starter and found something very interesting:


A burnt R104 (15 ohm 0.5W resistor), that still measures 15 ohms in the right channel on the board, just before the speaker outputs on the back. I also found L2, a 1.2UH aircoil, in parallel with R104, with only one side soldered to the board.


Now this means that the right channel would have 15 ohms higher resistance, possibly causing R104 to get burnt. It could also explain the noticeable difference between the 2 channels. It might also explain distortion that present when the amp is first turned on. This also means that this amp was bad from the factory in China.


Is my reasoning correct?

 

[jaycee]

If it's the resistor which should be in parallel with the inductor, then yes that would explain the imbalance. The speaker current normally flows via the inductor, which at audio frequencies simply looks like a short circuit. If it was having to flow via the resistor then the output would be attenuated, and the current flow would burn the resistor.

 

[Ian Finch]

Without V2 service manual to refer to, I just assume that that this version still uses Sanken SAP series darlington output transistors and the output coil (1.2uH) and parallel resistor to be similar. This has been standard practice for decades and resistor values are much lower (around 2R2), usually with a higher power rating like 1-5W. V1 schematic shows this with R224,264.

15R seems like a useless high value for the duty. I don't know how it could have survived at the rated power output for even a moment without the coil. It would then need to dissipate at least twice the speaker power demand in normal use anyway eek:

 

[mjona]

So I measured the pre outs with a multimeter and as suggested the balance is off 4.5 and 5.3 steady voltages.

What would be the next step in repair? Should I just give in and take it to a tech?

Thanks again.

If you have downloaded the V2 version of the 640A from Hi-Fi Engine look at R34 and C17 values 1k and 100uF and the same values for R79 and C41.

These decouple the nfb to earth leg in the divider network that sets the closed loop gain in the power amplifier.

It is possible with the age of the equipment that a cold solder joint has developed over time and one of these networks is compromised by not connecting to earth or has increased the resistance in this leg.

There is a similar pair of networks that do likewise for U3A/U3B which have a dc connection to earth via R14 and R7 at 10k each. The nfb dividers for these decoupling sections sets the close loop gain for each of U3A/U3B at 2.

If one of these resistors does not connect to earth you will still get some sound from the affected channel - but the closed loop gain would reduce to 1.

I see reason to be more suspicious in the last regard.