In that case, my next step would be to inject a signal at the input and try to trace is through the board to the output, find where the signal is lost.
Audio amplifier Cambridge Audio Topaz AM10 no output
- Thread starter Zjho
- Start date
In that case, my next step would be to inject a signal at the input and try to trace is through the board to the output, find where the signal is lost.
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Yes i was trying to do that earlier today with playing mp3 tune but thought my meter won't pick up typical song freqencies, i need to get hold of an mp3 recording of a set frequency such as 3khz to have a chance. I have a service manual for the Azur 340 amp whose schematic might assist.
Very good next step
YouTube, and your phone is your friend here!
Plenty available
Did you check if there was a DC offset on the outputs?
Did you check if the speaker protection relay was operating?
Use your phone and (as suggested above) an app to generate a set frequency tone. For tracing, use an earphone attached to a pair of probes. An old crystal earpiece is more sensitive, but I don't think you will need that much sensitivity.
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Very good next step
YouTube, and your phone is your friend here!
Plenty available
...it's also worth double checking on other known faults.
Did you check if there was a DC offset on the outputs?
Did you check if the speaker protection relay was operating?
Thanks for that 3khz link.
Checking output shows no DC offset to ground.
The speaker relay is not working, there was a small hole covered by a tiny slip of transparent plastic on the top, i've poked a sewing needle into it and besides stabbing my finger have not managed to resuscitate said relay. Hope have not made things worse.
A great plan, shall be busy pursuing this amongst other things suggested !
Not the best fault finding technique I've come across!
Next step - is there power to switch the relay.
If there is, the relays at fault, if there isn't, the output protection may be cutting in, suggesting an issue with the amp IC ...or, a short on the output (does the relay operate if the speakers are disconnected?)!
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Not the best fault finding technique I've come across!
Next step - is there power to switch the relay.
If there is, the relays at fault, if there isn't, the output protection may be cutting in, suggesting an issue with the amp IC ...or, a short on the output (does the relay operate if the speakers are disconnected?)!
Yes i should have consulted before thrusting sewing needle into relay in hindesight.
There is power to the relay, 33 volt identical reading on two end pins, presumably for energizing the internal coil. Have yet to learn the mechanism, i would have guessed a pusle is sent such as a square wave on one of the pins to move the relay switch but don't understand why there would be a diode accross those pins as would mean pulse might only work one way unless relay has a spring inside or voltage required is less than forward .6v of diode ? Am in process of constructing a probe to earphone testpiece as kindly reccommended by Harry Bloomfield above.
Going to be so useful having all this test gear for future troubleshooting in years to come even if this box is toast.
The relay coil needs current continuously, in order to operate.
The diode is placed across the coil to protect against back emf.
When the current is removed from the coil, the magnetic field collapses, inducing a current; this current could easily kill devices operating the relay, were it not for the diode.
The diode is placed across the coil to protect against back emf.
When the current is removed from the coil, the magnetic field collapses, inducing a current; this current could easily kill devices operating the relay, were it not for the diode.
The diode is there to reduce/discharge the back emf from the coil of the relay's switching.
Did you mean these pins?
The relay has a 24v DC rated coil - if 33V is correct across those pins, the relay could well be damaged.
Assuming the fault was with the protection circuit, you could briefly bridge the relay outputs, and see if there is an output?
Although there is an element of risk to this!
Thanks for both of you reminding me about diode protecting against back emf spikes.
Yes it is those pins ringed in red, there is no voltage across them under power, the 33v identical reading mentioned in post 37 was relative to ground, i should have been a bit clearer.
Perhaps there was a moment when the amp worked that an upstream component failed and let 33v through and relay coil and back emf diode blew.
Resistance between the pins is 1087 ohms in both directions, taking about 8 seconds in time to get that figure from a few ohms higher prior, perhaps suggesting both diode and relay are broken ?
As you may be aware 33v is one of the main board rails from the smoothing caps.
Have been tempted to bridge the outputs, volume down and perhaps into a light bulb for starters, then light bulb/earphone/speaker in parallel or series. However perhaps should go upstream of the relay coil circuit first ?
The photo of main pcb traces will have to be redone/upgraded at some point as can't easily refer to them even by playing with their contrast in GIMP image editor now that all has been screwed back down !
Glad to see the main power IC is the same model as in the Azur 340 whose service manual shows pin layout voltages.
1087 seems a higher value than I would really expect, so the coil is possibly suspect. You need a diode test feature on your meter, to be able to test the diode - you need to apply enough voltage to forward bias it to conduct and the display should show the forward voltage drop in one direction and a different higher value in the other.
One testing technique to avoid removing components is to simply cut tracks - using a Stanley blade, to cut a thin break to isolate a component. You could do that easily, to test the coil resistance in isolation. You can repair the breaks by simply scrapping the solder resist away from the copper and solder tinning across the break. Any value of resistance less than infinity across the coil would then suggest the coil is usually OK.
1100Ω is perfect for that relay:
https://www.conrad.com/p/afe-bpm2-ss-224lm-pcb-relay-24-v-dc-5-a-2-makers-1-pcs-1425189
Next time you have the board off (and with power off!), connect a 9V battery across the coil and see if the contacts make or break. An easy enough task to rule out the relay itself.
Further probing would be a good idea before attempting to bridge the relay!
The battery will have to be connected the correct way, or it might blow the diode.
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