Cambridge Audio Azur 540P Phono Preamplifier repair

R

rotco

Active Member
Hi all,
A friend of mine gave me this Cambridge Audio 540P, and say it's not working.
he doesn't have it's transformer adaptor also.

so, until I'll find a replacement transformer adaptor, I opened it's housing and looked inside.
I see one of the six electrolytic capacitors at the inner power supply circuit is swollen on the top, so that's issue #1.
then I notice that one of the (two) transistors which are mounted to the bracket (heat sink) looks burned.
it's 7818 positive voltage regulator. so that's issue #2. (attached picture, the left transistor.)

I order them both and will replace them, hope that's will bring it back to life.

my question is, related to the bad transistor.
I notice it doesn't have the insulator washer of the screw that attach it to the heat sink.
(like I know that how those transistors mounted usually, and like the other adjacent 7918 transistor has ).
through my google search for finding replacement transformer adaptor, I see additional units, same model, that also doesn't have an insulator washer over this transistor.
is it make sense? is it OK?

Thanks!

by the way, I believe that whats ruin this unit is that my friend couldn't find the original 12vac transformer adaptor, and used another transformer adaptor he found in its house, but probably 12vdc! the AC adaptors at the secondary are not common.

azur-540p-1-cambridge-audio.jpg azur-640p-full.jpg
 
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Those are voltage regulators - not transistors.

No washer may not be an issue - its probably OK that one of these touches ground, depending on what rail ground is referenced to (probably negative?)

The best way to test is to obtain a suitable second hand 12VDC of correct polarity & current and try the unit out. If the voltage regs are working OK, you will be able to measure input & output voltage carefully on their legs. Otherwise you are just guessing if these are faulty or not.
 
rotco said:
by the way, I believe that whats ruin this unit is that my friend couldn't find the original 12vac transformer adaptor, and used another transformer adaptor he found in its house, but probably 12vdc! the AC adaptors at the secondary are not common.
Click to expand...
That's a very distinct possibility.
 
Hi,

thank you both!
this soldering should be pretty easy, since not much needed, and everything so "roomy" and not crowded. only two suspected components.
is that really important? i have a "regular" iron and desoldering tool. (the regular "pen" with spring that sucks the melted tin)

please see the attached photos, originals that i took off the unit.
you can see that the left voltage regulator back insulator seems black and burn.
plus, the electrolytic cap on the top right corner is swollen on the top.

slimecity said:
you will be able to measure input & output voltage carefully on their legs. Otherwise you are just guessing if these are faulty or not.
Click to expand...
how to measure?
each of them have three legs.
which leg of which voltage regulator should be the positive and negative output voltage?
thanks
 

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Yes, 12 VAC sorry

Far left pin of both is input voltage. Centre pin is ground, right pin is output. Measure very carefully across first the centre & left pin, then across the centre and right pin of each one - ensuring the DMM probes dont short across the pins.

Pretty cheap and yucky caps in this too. Pretty typical Cambridge Audio Im afraid. A recap would be in order down the road sometime
 
rotco said:
...Cambridge Audio 540P, and doesn't have transformer adaptor...
Click to expand...

That's a potential opportunity to upgrade the power supply! *

Here's a link to the service manual.

rotco said:
...then I notice that one of the (two) transistors which are mounted to the bracket (heat sink) looks burned.
it's 7818 positive voltage regulator. so that's issue #2.
Click to expand...
Unless there were exceptional circumstances or your friend is extremely talented, it's not possible to damage the 7818 regulator without destroying R42. 78xx & 79xx regulators have built-in short circuit protection.
Your subsequently supplied picture shows that R42 is intact.

rotco said:
I notice it doesn't have the insulator washer of the screw that attach it to the heat sink.
...is it OK?
Click to expand...
It's OK on the 7818 (it's NOT OK on the 7918).
Bizarrely, Cambridge Audio install a mica washer between the 7818 and heatsink, but omit the insulating nylon washer/bush!!!

Your pictures also show that the AC power input socket is damaged.
slimecity said:
...Far left pin of both is input voltage. Centre pin is ground, right pin is output.
Click to expand...
That's true for the 78xx regulators.

For the 79xx regulators, right pin (3) is output, centre pin (2) is input (hence the requirement for an insulating bush and mica washer) and the left pin (1) is ground.

Good Luck!

* The PSU upgrade requires a 18V AC transformer, replacing C1 & C6 with wire links and removing D2 & D4.
And increasing the value of R1.
 
Last edited:
since you're already in there, list the parts (resistors, caps, regulators, etc) using
(parts list, schematic, board layout - all must agree) and rebuild the power supply.

and if there's any issues - post here.
 
All good observations from Hamish.

I note that it's C1 that is bulging. If you put a 12V DC input into this amp, you would permanently reverse bias C1 (assuming centre positive on the power jack), via a convoluted path to GND, through D3/R42/U1/load. It might not like that...

C1/C6/D2/D4 (with D3/C3, D8/C5) are a voltage doubler, required to get enough headroom from the 12V ac input to allow the 7x18 regulators to give 18V dc. Hamish's suggested mod simplifies it to a half-wave rectifier.

I've got a 640P, somewhere, bought secondhand for £5. Must try it out at some point...
 
I have a similar model (see signature). The only problem I had with it was that I was picking up a radio station with my tonearm wires because the ground wire was not making a good connection. I sanded off the silver paint under the ground screw on the preamp, and that solved the problem.
 
@Hamish119
you were right! the units works just fine! i obtained a 12vac transformer and gave it a try, and surprisingly everything worked just fine!
both channels sounds good.
so, im not sure how this bulging capacitor is affecting.

eitherway, i will probably give it some work.
not sure why many of you guys says this capacitors are so cheap and underrated.
is it really make an effect if i will change all the six of them with
220uf 50V KZ MUSE Series Nichicon
?

Hamish119 said:
* The PSU upgrade requires a 18V AC transformer, replacing C1 & C6 with wire links and removing D2 & D4.
And increasing the value of R1.
Click to expand...
what did you mean here?
this adaptaion sounds pretty simple to do. i don't mind to do it if you say its "upgrading" the PSU.
also, is it mean that i need to obtain a replacement for the 12vac external transformer to 18vac? ( i believe it will be hard to find, my country 220v btw)

last question please, regarding this voltage regulators.
in general, what are they doing at all?
if i understand correctly, this PSU board gets 12vac from the wall, then it have diode bridge to convert to dc, then it have capacitors to smooth the dc and stabilize it.
so why do the circuit need these voltage regulators?
thanks
 
rotco said:
is it really make an effect if i will change all the six of them with
220uf 50V KZ MUSE Series Nichicon
Click to expand...

Unlikely. They are in the power supply, prior to the regulator. They are not directly in the audio path, where higher quality caps can have a notable effect.

The voltage regulator regulates the rectified input voltage to a known, fixed voltage, and, in doing so, removes the ripple and voltage variation that is inevitable with a simple, capacitor-filtered, rectified mains supply. In doing so, they reject any audio influence that cheap capacitors may have (barring failure).

Low quality capacitors are likely to fail after a shorter life. That's the only thing you might have to worry about. But I wouldn't...

Now you have an AC supply, and the unit is working, I'd leave it alone. If you must, replace that bulging cap; as I mentioned, it may have been damaged by the DC supply.
 
Personally I would ditch the whole power supply. The 78xx/79xx regulators are actually quite noisy. I have used adjustable symmetrical power supplies using LM317/LM337 linear regulators which are extremely quiet. These can run about +/-5 to 30vdc from a suitable 24-0-24 AC transformer at up to 1amp with a heatsink.
 
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