Cambridge Audio Azur 540 A

Harykcz

Member
2019-12-14 6:12 pm
Hi, please, help me. I have Azur amplifier, one chanell is wrong. I send two pictures . Input signal goes to CN2. More input signal give more amplitude down but no up. The two tranzistors on the end are slightly warm. There is no speaker protection. (removed). Thank
 

Attachments

  • IMG_00059.jpg
    IMG_00059.jpg
    376.1 KB · Views: 244
  • IMG_00079.jpg
    IMG_00079.jpg
    943.1 KB · Views: 243

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Most faults on these amplifiers are down to the output transistors.

1. You need to tell us what the DC offset is (DC voltage at the output).

2. Are the 0.22 ohm resistors OK.

3. Is R99 OK.

4. What voltage do measure between pins 1 and 5 of the upper NPN transistor.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
The Sanken SAP15N and SAP15P darlington output transistors have been obsolete for many years now, replaced by simpler devices (STD02, 03) that eliminate the troublesome internal emitter resistors. DIY replacement may still prove difficult and expensive unless you have some experience with power amplifier repairs and troubleshooting. Note, if mica insulators are fitted, they may be irreplaceable so take care with them.

Needless to say, this may have been inevitable when the protection circuit was removed. Unfortunately, Sanken power transitors around on Ebay etc. are mostly fakes. Don't deal with unauthorized sellers and don't take risks when they look like an easy option. The photos may be of genuine parts but yours will likely be different, if only slightly. Unless you have the time and cash to waste on fakes and the expertise to spot them before they cause more damage through repeated mounting and soldering etc, don't bother.
 
Last edited:

Harykcz

Member
2019-12-14 6:12 pm
Measurement:
DCoffset: wrong channel 0,002 V, OK channel 0,056 Volt (multimeter, DC)

R 0,22 OK,,,R99 OK
pin 1 to 5 : 1,14 Volt both channels
Note1: On the oscilloscope, the upper line stay on one position 0,5 Volt above default 0,0.

More input signal, more sinus Volt down.

Note2 : I buyed both tranzistors for repair previously, when it would be necessary. From England. Perhaps they are no fakes. But for me, can i hear the differences ? I need repair my amplifier. One trouble : I am not enought brave to rotate the main desk bottom up.
 

Harykcz

Member
2019-12-14 6:12 pm
Any pictures : Note: R70 (good channel) is the same R99. Input signal both channels identical Volt and Frequenz.
From left: Wrong speaker out, R99 pin1, R99 pin2, OK speaker out, R70 pin1, R70 pin2
 

Attachments

  • wrong speaker output.jpg
    wrong speaker output.jpg
    411 KB · Views: 153
  • R99 pin1.jpg
    R99 pin1.jpg
    361 KB · Views: 126
  • R99 pin 2.jpg
    R99 pin 2.jpg
    399.7 KB · Views: 127
  • OK speaker output.jpg
    OK speaker output.jpg
    426 KB · Views: 16
  • R 70 pin1.jpg
    R 70 pin1.jpg
    343.1 KB · Views: 10
  • R 70 pin2.jpg
    R 70 pin2.jpg
    295.7 KB · Views: 5

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Measurement:
DCoffset: wrong channel 0,002 V, OK channel 0,056 Volt (multimeter, DC)

R 0,22 OK,,,R99 OK
pin 1 to 5 : 1,14 Volt both channels
Note1: On the oscilloscope, the upper line stay on one position 0,5 Volt above default 0,0.

More input signal, more sinus Volt down.

Note2 : I buyed both tranzistors for repair previously, when it would be necessary. From England. Perhaps they are no fakes. But for me, can i hear the differences ? I need repair my amplifier. One trouble : I am not enought brave to rotate the main desk bottom up.

I'm afraid nothing very conclusive shows up there. The offset is OK and while the 1.14V between base and emitter of the Darlington sounds a little low (and it is possible the problem is here) it is in the right ballpark.

I have marked some expected voltages on the diagram. If these are OK then I think it would be worth swapping that upper NPN power Darlington transistor and seeing if the problem is fixed. The 0.6V marked is the voltage between base and emitter.
 

Attachments

  • Annotation 2019-12-15 190713.jpg
    Annotation 2019-12-15 190713.jpg
    266 KB · Views: 56

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
We would normally recommend you to use a DBT (dim bulb tester) while working on something like this as it can save damage to parts if there are other problems.

It would also be a good idea to turn the bias preset (the 47 ohm) to give minimum bias before you power up with new transistors. Minimum bias is achieved when the preset is at minimum resistance i.e. when it is shorting pins 2 and 4 of the NPN and PNP outputs. If you look at the circuit it will become clear.