Cambridge Audio A3i repairs and mods

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#201
Have you read much of this thread? The opening post by the designer has a lot of useful information that covers fixing faults that were in the original product, such as the LED fault.

Perhaps your problem is among them and you can make repairs without further problems.
 
#203
There isn't always a direct connection between fault and fix. DIYaudio isn't a repair or fixit organisation and we are more concerned with designing and building our own, so we tend to take a circumspect, case by case evidence view of problems in commercial products rather than simply say: fault=no light, fix=replace bulb or such.

In your case you say the amplifier takes time, a few minutes, to shut down and that means it's most likely a heating problem - now, is the source of the problem becoming a bit clearer? X-Pro made this symptom fairly clear in the text:
....a common fault - as a result of a local board overheating one of the LED's (usually LED2) or Q8 transistor next to it (BC640) goes open circuit. Thankfully, protection circuit in this case just disables the output. It is useful, if you came across this amplifier, replace both LED2 and Q8, mounting them at least 10-12 mm from the PCB....
Click to expand...
....The fault it has developed is that after a few minutes it cuts off and no sound is delivered to the speakers although the cd or cassette player is still functioning and can be heard via headphones direct from device.....
Click to expand...
This may not be so obvious to you yet, but to others, the warning bell will be ringing here.

Bear in mind that there could be a lot more possibilities and there is no single cause for heating to shut down an amplifier but this has to be a strong chance, according to the designer and I think we can take his word for it, as the first probable fault to look at.

This is the only free manual download I could locate, apart from those mentioned in the thread. cambridge audio a3i Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

I assume you have some experience with electronics, have basic tools, a reasonable Digital Multimeter and can solder. You'll obviously need to purchase a few replacement parts according to the parts list in the service manual or any substitutes suggested in the thread.
Otherwise, it would be sensible to seek professional assistance.
 
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#204
Ian
Thank you do much for taking the time to help me understand the site and the members and for your assistance in finding the thread and pointing out that in it which should be my staying point.
I have done knowledge and skills albeit I am usually on the recording sound making rather than a creator of technology sound capturing. Willing to learn four like most things if one tried to learn and be creative one can perhaps achieve our acquire more of what one would like than is perhaps available from retailers.
Having said that I have been generally content if not always happy with certain high pitched loud singing quality.

Thanks again Ian for your helpful and positive welcome to the forum. I appreciate it!

Regards
John
 
#205
TRANSFORMER CAMBRIDGE AUDIO A3I

Hi all

I am new to the forum and I look for a replacement transformer for my amp A3I and complete schedules

Can anyone send me the amp AC voltages, and prior stages and complete schedules or at least the DC voltages.

You can help me please with links in UK or Germany for this replacement

Thank you
 
#206
Welcome to the forum, I hope you find some suggestions for your problem but will also enjoy the DIY projects and discussions here.

As stated earlier in this thread, the only free manual for the A3i available on the internet is the preview linked in #203 and there is no parts list or transformer specification in that. You could however, email the OP - see post #10. However, important voltages are shown on the schematics above and it becomes clear that this needs a transformer with dual 25VAC secondary windings with a total power rating of around 160-200VA. The maximum transformer size you can use is actually determined by the clear space available. - measure and compare.

An R-core transformer was used on most issues of the amplifier that I have seen but as this was a custom product over 15 years ago, I doubt you will find a replacement. A toroidal transformer may be the only affordable option but there will be plenty available to suit this amp.

Also check that there is room and clearance for the toroid's single mounting bolt assembly. Make certain to specify the correct mains voltage rating.
 
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#207
Hello,

I have a working one of these but am about to make the mods, there is some signs of heat damage to some components.

I've noticed that Q9 Q109 & Q201 are supposed to be tucked into holes in the large heatsink, well in mine only the Q201 is! 🙄 someone couldn't be bothered in production 🤐

The green and brown 100nf caps.. why the difference? green 5mm pitch brown 10mm are these best replaced? some have been toasted🙁
 
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#208
It's doubtful whether there will be much point trying to cool TO92 transistors this way but since Q9, 109 are the Vbe multiplier transistors (bias controllers), sensing the temperature of the output devices is integral to their function. Hence they do need to be inserted in or clamped to the sink.

Is this a second-hand unit that may have been messed with already?
 
#209
Ian Finch said:
It's doubtful whether there will be much point trying to cool TO92 transistors this way but since Q9, 109 are the Vbe multiplier transistors (bias controllers), sensing the temperature of the output devices is integral to their function. Hence they do need to be inserted in or clamped to the sink.

Is this a second-hand unit that may have been messed with already?
Click to expand...

It is second hand, don't think it's been "messed with" 😱 all the components seem original. the heatsink would have to be removed to dislodge the transistors, who knows !
 
#211
You'll find all the schematics with the answer linked in #203 and elsewhere in the thread. All supplies are derived from the dual 25VAC secondary windings and the supply for the protection circuits is fed from 50VAC appearing across both these via IN4001 diodes D205, 206. They may be close in PIV rating for the job but that's what's specified.
 
#212
as_audio said:
Just use any cheap 5mm green standard LED.
The temperature is no problem if you change the resistor,
see post 90. I use what I have on top of my toolbox, can
send you a few, but the local supplier can surely help.
Voltage is a little bit less than 2 volts at these currents.
A.
Click to expand...
I replaced Q8 and Q10, however, I need to replace the LED 2. Can anyone give me an idiotproof description what to order? I havn't found high temp LEDs, neither.

Color: green (I guess this does not have an effect on anything)
Diameter: 5mm
Voltage: ? a bit less than 2 Volts... 1.9 V

So this for example?
LED at conrad electronics

Currently there is some kind of protection covering the pins of the LED2. Do I need to replace is that way including this protection? Is that against heat damage?

Thanks for help. I miss my music at home, since it took me a long time to find this thread...

best wishes
raimund
 
#213
If you read the first part (2008) of the thread, you'd see it isn't critical but an old style 3 or 5mm dia round led should be fine. They may also be called "standard LEDs".
I guess you know what you're looking for, from that Conrad link. You could also try here: HLMP-3507 - AVAGO TECHNOLOGIES - LED, 5MM, GREEN | CPC
The colour of standard LEDs was important as the voltage range was different for each colour and fairly consistent from one manufacturer to another. Modern, high luminance LEDs are not consistent and vary between manufacturers quite a lot. There doesn't appear to be a fixed relationship of colour to voltage.

What sort of material is that you have, as I've not seen it. Is it insulating plastic?
 
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#214
Ian Finch said:
If you read the first part (2008) of the thread, you'd see it isn't critical but an old style 3 or 5mm dia round led should be fine. They may also be called "standard LEDs".
I guess you know what you're looking for, from that Conrad link. You could also try here: HLMP-3507 - AVAGO TECHNOLOGIES - LED, 5MM, GREEN | CPC
The colour of standard LEDs was important as the voltage range was different for each colour and fairly consistent from one manufacturer to another. Modern, high luminance LEDs are not consistent and vary between manufacturers quite a lot. There doesn't appear to be a fixed relationship of colour to voltage.
Click to expand...
Great. Have found one from Avago at conrad electronic as well. Just the luminous Intensity is marginally higher. Thanks.
Ian Finch said:
What sort of material is that you have, as I've not seen it. Is it insulating plastic?
Click to expand...
I guess it is a kind of fiber material, pulled over each pin before soldered to the pcb. The other parts on the pcb do not have this. Just the four LEDs.
 
#218
#219
My A3i developed an issue of quiet sound in one channel. Sounds similar to the one described here

Also , would appreciate if someone could mail me schematics so I can use them and keep them with amp for future needs. Mail is my username at gmail.

Thank you
 
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