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-   -   Cambridge Audio A1 MK3 SE repair (TDA1514 chips burned) (http://www.diyaudio.com/forums/solid-state/213312-cambridge-audio-a1-mk3-se-repair-tda1514-chips-burned.html)

lehtinel 25th May 2012 03:21 PM

Cambridge Audio A1 MK3 SE repair (TDA1514 chips burned)
 
So, I found this amp from a trash lorry as our neighbouring house is being renovated and thus emptied.

It had one fuse blown and one TDA1514 chip with a hole in it. Replaced the fuse and the other channel TDA1514 also burned.

I have ordere replacement chips and power caps also. Any idea what could be wrong with the amp? What to measure before putting in new chips that could get burned.

jaycee 25th May 2012 04:06 PM

The chips have no overload protection and are probably under-heatsinked. The other TDA1514 was probably dead too. Make sure none of the associated parts have died. It's unlikely you'll find a schematic for the amp itself, but the datasheet for the TDA1514A should help somewhat.

Interesting that they used the 1514A in the SE edition. I picked up a similarly dead A1 Mk3, which used a single LM4765. It too had a hole burnt through it, an exploded cap, and the damage was so bad that a hole had been burnt through the PCB. I intend to reuse the case and transformer.

lehtinel 25th May 2012 04:27 PM

It had the default heatsinking, so it shouldn't get too hot (atleast what I think). No other parts show any damage, I've taken all parts out of the casing and cleaned out the boards. I will change the caps on the power amp board as soon as they arrive.

This is a really interesting amp - the case filled with emptyness ;) The NAD:s I have owned have really been in a different league.

jaycee 25th May 2012 04:53 PM

Cambridge have a nasty habit of using heatsinks with inadequate thermal rating. It's just a chip-based amp but for the price you cant really argue.

lehtinel 26th May 2012 12:28 PM

Oh, I haven't hear that CA are bad at heatsinking, but the heatsink in this one doesn't really look that good / big. I'll try just replacing the chips as they arrive, in it doens't work, I might use the transformer and casing for a DIY amp.

Any idea about the quality of the thoroidial transfromer, no info on it.

AndrewT 26th May 2012 01:55 PM

If a domestic duty amplifier is used for domestic duty with the recommended load impedance and at average levels that are typically 20dB to 40dB below the maximum power output then the sinks don't need massive dissipation capability.

If one takes the chipamp to near it's maximum supply voltage and uses lower impedance loadings and listens at average levels that are 6dB to 12dB below maximum output power then the sinks must be able to keep the chip at reliable temperatures. If one also adds in the requirement of cool Tc to minimise the Spike intervention (applicable to National chipamps only) then the National guideline for heatsink dissipation must be roughly doubled.

There are three scenarios in those previous paragraphs. The builder has to choose which is applicable to them.
If Cambridge choose the first scenario, then the designer/manufacturer is doing nothing wrong, nor dishonest.

jaycee 26th May 2012 08:57 PM

AndrewT, the heatsink in the A1Mk3 i had was 120mm wide, 50mm high with 45mm long fins. The power supply was ~ +/-28V using 10000uF capacitors, with the one LM4765.

Is that sufficient for domestic use ? Maybe, all I know was when I got it, the guy said "I think i turned it up too loud and the fuse blew". The chip had a hole in the middle and the PCB burnt through.

I'm looking to build an amp in it now as a weekend project. Dont know if I'll just rebuild my LM3886 based amp in it, or do something discrete yet.

AndrewT 26th May 2012 11:04 PM

The Manufacturer made for domestic duty.
If you don't abuse it it will perform to specification.

lehtinel 28th May 2012 03:15 PM

Allrightey, got my spare parts today. Now, what do you think, should i replace caps, and start up the amp withouth the TDA1514A:s in place, just to make sure the new one's don't blow out?

If the fuses don't burn out then I would puth the TDA:s in place and see if it works allright.

As I got the amp the right channel TDA was burned and the left channel fuse was gone, as in replaced the fuse the left channel TDA also blow out (it didn't seem blown befor it, although I cant be sure.)

jaycee 28th May 2012 05:41 PM

It should be safe to power up with no chips installed - obviously you wont get any output :)

I'd rig up a "dim bulb tester" at this point - that is a cable with a 60W incandescent (not energy saving!) light bulb in SERIES with the live wire. It will limit fault current and provide a visual indication of a problem if there is still a fault.


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