Cambridge Audio A1 MK3 SE repair (TDA1514 chips burned)

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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.
 
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.
 
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.
 
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.
 
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.
 
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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.
 
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.)
 
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.
 
I was thinkin that I would measure look if the fuses wont burn with new capacitors and no TDA1514A:s in place. If all would be well and I could measure the voltage at the points where the TDA1514A:s are placed I'd solder them in place.

The bim bulbe tester seems interesting enough for future projects too, but I have no spare parts for it now.
 
check for possible short circuit of output.
allso, use a fast blow fuse, half of the original rating.
prior installing a new chip do mesure the voltage on the board, and see if everything is normal.
use a 10K resistor as load, and mesure output voltage.

I find it hard to burn a TDA chip. actualy i use them verry often to power all short of things.
 
Thanks for the tips! I replaced the caps and tried powering on. The fuse still blows on the left side. Started withouth chips, so no damage done. I measured the big power caps, the right one shows 25v as the left one is at zero.

Measured the output terminals for shorts but found nothing.
 
Check the transformer secondaries for continuity and the bridge. Working with one rail shorted is not going to do the chips any good.

I always have a look at the output network resistors, 3R3 if they followed the data sheet. Typically these are under-rated and fail open. This makes the chip oscillate and fry.
 
With no chips in, you still have a short. The main supply is also regulated down to +/-15V to supply opamps. You will see a pair of 7815/7915 on there - check those.

You may also find that the board itself is damaged. Are there any burns on the PCB ? Burnt PCB can become conductive.
 
Allright, I think Im done tinkering for today. I took out U3 and U6 transistors, plugged out the preamp board and fired up the power amp board.

This resulted in C29 and C31 caps exploding (or putting out smoke). A good sign maybe? Atleats the voltage is going on both rails, it didn't like being fired up without the TDA1514 in place and no preamp board plugged?
 
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