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-   -   Help with NAD T750 (http://www.diyaudio.com/forums/solid-state/218971-help-nad-t750.html)

cocokuif 2nd September 2012 08:36 PM

Help with NAD T750
 
I have replaced the speaker relay of the front amplifier, but I must have done something wrong: the relay doesn't switch on the front speakers anymore.
Can anybody help? A service manual or schematics perhaps?

jitter 4th September 2012 06:11 PM

Why did you replace the speaker relay?

Ian Finch 4th September 2012 06:29 PM

Does the replacement have the same connections and similar specification to the original part and, as I guess Jitter refers, was the original relay operating before you replaced it?

The original may not have been working because of a power supply or error sensing circuit fault rather than relay fault.

cocokuif 4th September 2012 06:30 PM

No sound from speaker
 
Now and then one of the speakers went silent, which could be fixed by switching the speakers on and off or by turning up the volume. I had already cleaned the relay, which did help for some time. The T750 is a disaster for servicing: the whole unit has to be disassembled in order to get to the soldering side of the PCB. So I didn't have an ideal position for soldering the new relay and perhaps the solder touched some other part of the PCB. That's what I think has happened. I now have the old relay in, but no sound from the front Amp. I really need the service manual to figure out what happened and fixed the unit.

cocokuif 4th September 2012 06:36 PM

The coil resistance of the new relay is slightly higher than the original, but it has the same voltage (24V) and it can switch more power than the original.

jitter 4th September 2012 09:03 PM

I could not find the service manual, but that doesn't mean you can't do at least some things. First make sure that you didn't cause an accidental short with solder. Then check if you didn't damage the pcb traces. On professional pcbs these can easily get damaged, even more so on pcbs in consumer electronics. This may not always be visible to the naked eye, though.

Does the relay make a clicking sound or not? This may be hard to hear if there are other relays switching at the same time. Measure the voltage across the coil when switched on and again when switched off.

A slight difference between coil resistances should not matter.

One of the things you can do without a manual is measure if there's an audio signal going to the relay or not, preferably with a scope. Supply the input of the offending channel with a sinewave or music and measure the signal where it enters the relay. If there's a signal, then either the relay isn't being actuated or it doesn't make proper contact.
No signal may indicate a damaged amp, or damaged or shorted traces. See if you can measure the signal somewhat closer to the power amp to rule out damaged traces in the vicinity of the relay.

cocokuif 5th September 2012 11:32 AM

Thanks Jitter
 
Thanks Jitter for your suggestions. Yesterday I have completely disassembled the unit so I should be able to inspect the PCB . Following your suggestions I can also put my newly acquired function generator to work :).

jitter 6th September 2012 05:03 AM

Sounds like you have the right equipment. Let us know what you find...

cocokuif 7th September 2012 01:37 PM

SUCCESS!
 
After having verified that the audio signal was reaching the relay I decided to follow the PCB traces leading to the coil of the relay. It proved to be a simple circuit with 1 transistor , a cap and a diode. Measuring the voltages at the transistor lead to the conclusion that it was dead. Unfortunately it was a type with build-in resistors (DTC123):confused:. On the internet I could find the specifications of the DTC123, so I took an old 2N1613 and soldered 2 resistors to the base and emitter and tested it outside the unit: the relay clicked :). All I had to do was changing the old relay for the new one and getting the 2N1613 and its resistors in place. A bit of a delicate job, but I managed.

Many thanks Jitter for your advice.

Fred

jitter 7th September 2012 02:24 PM

You're welcome. Nice to know that you fixed your amp!


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