Help with mis-behaving Cambridge 540A

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azur 540a v.2

I'm always late to the party.
Gents I have a no sound situation with a solid red protection light and all input lights lit. It just stays there. No blink patterns to decode. Funny thing is next time it just displays a single blue standby. No apparent powering on, and pre out gives nothing.
Amp is clean. I think I have tried all the easy things.
So far I am unsure of any hard reset procedure.
 
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CAMBRIDGE AZUR-540A V2.0 SM Service Manual free download, schematics, eeprom, repair info for electronics
Assuming you are prepared with a little electronics experience and basic tools like a DMM, you can check out the service manual linked above for the schematic.

The outputs will be blocked by the speaker relay because the protection circuit senses a DC fault at the output stage of one or both amps - so it prevents you loading down the output with speakers which would otherwise make certain that any minor damage became bigtime damage. That's why "hard reset" is called stupid here.

What you can do is first check whether any significant (> +/- 50mV) DC level exists at the output terminals of each channel before the relay, marked J12 and J6 on the PCB, inside the case. The measurement is with respect to ground so use either terminal J16 or 17 for this. If you have significant DC there, it's likely due to blown output devices which are now obsolete but don't go looking to buy more trouble with Ebay shops - buy genuine parts from reputable sellers.

Take care not to short any wire leads or tracks by slipping with probes whilst looking at meters - they have a nasty habit of doing that and cause more damage, unless you clip-lead or tack-solder test leads to the terminals or use some care and patience. Post what you find though, it may even be a different problem.
 
azur 540a v.2 dmm testing

Ian. Thanks a million. I will get out the dmm and ground to j16 and check j12 carefully at 200mv setting and report back. I appreciate the service manual. I have not done much work with sms as of yet.
I'll go as far as I can with no variac or scope. Love the sound of these amps but I think they may have gotten a bit too fancy. KISS seems to be my favorite theory for amps. The cap5 system is not really helping right now it seems as from what I can tell it should be a blinking once protection light not solid and all lights lit.
 
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J12 is only the right channel but if that's 1.9mV and not 1.9V, its a remarkably low DC offset. Now check the left channel by measuring j6 to j16. Whilst you are in there, check that the measurement to j5 is the same and recheck the left channel by measuring that j11 measures the same as j12. These points do need to be connected in order to get output power to the relay and hence to the speakers, so I would expect them to measure the same.
 
540a v2

J6 measures -2.9 but I used j17 as ground. Hope that is ok.
All measurements using the lowest vdc setting on my old craftsman dmm at 200mv. While I was in there I heard a strange faint fizzling like water in a pipe. Then it just went out. Scary when you have your head in there. Sounded like it may have been coming from the bank of 3 transistors on a heat sink in the middle of the preamp board.
Still looking for the other 2.
Thank you for your help.
You must have done this a few times.
 
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So far, so good with the power amplifiers as they seem to be normal and operational with such small offsets. I haven't worked on this actual model but it has several design features in common with some models I have. Still, that doesn't mean I know the answers - just a few suggestions on how you might narrow down the problem.

The protection circuit is often blamed for no sound out, when it is simply doing what it's supposed to do, according to the input it gets - or not. We should check it's operational starting with the supply voltages to the protection board (+/15V, +5V) then removing the interconnecting lead plugs. The sockets usually have tiny catches to keep the plugs in place. You can release them a small blade, then refit them to ensure good connections (power off before pulling anything).

The transistors on the large heat sink are the SAP15 power output devices - there should be 2 for each channel or 4 in total. If they are emitting noise, they would likely be hot too! A finger on the plastic cases will tell you that. There are no unsafe voltages there, unless you touch the connecting leads. Perhaps the noise is actually coming from power transformer, close by. Whatever it is, you need to identify where it comes from.

Last of all, is this a recent purchase of a pre-loved amp?
 
dmm grounding

Ian et al. Sorry to be a pain. I see j11 tucked under the plaatic danger flap close to the mains. When testing j11 do need to ground back to j16 or can I use a closer ground. I think I see another j12 also a black lead beside it. Evidently you can have multiple j12 s as long as it's on another board? Need to make sure where I can put these probes without a shock. Sorry for my hesitation:mad:
 
540A

Would anyone be interested in taking this 540 here and get it working again in exchange for say a bit of cash and the use of it for a year or two?
A very nice sounding and looking amp that i am having trouble diagnosing on my own without better equipment. Too bad to have it go unused. Someone should be able to get as much joy out of this as i do from the 640.
nilest@gmail.com
 
You say all input lights are lit? This sounds like the microcontroller is dead. Check for +5V on pin 20 of U2 on the front panel PCB. it should be marked as a "PIC16F72".

If there is power there, check the resonator CRS1 which should be below the chip. This is a 3 pin 4MHz ceramic resonator. With no scope, the best way to test this is simply to replace it.
 
azur 540a

Thanks for the help.
I'll check that and post back.
The input lights all light as does the standby and protection when I attempt the reset or protect disable when holding power while plugging in. I doesn't seem to follow the typical noted #1 circuit fault listed on the back where upon dc present it I think is supposed to flash a series of times denoting the type of fault.
Thanks again.
 
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