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Old 7th February 2005, 07:29 AM   #1
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Default eti 480 build problems

I recently bought an amp kit (a 480) from Dick Smith, and finished making it yesterday. It was my first try, so please excuse any excessive noobieness ...

Ok, so I am using the suggested transformer and power supply, and they work fine to the bit where I'm supposed to calibrate the trimpot with 2 resistors in place of the fuses. Turning it on though causes it to burn out the 2 resistors.
I noticed the trimpot was currently at 0, and tried again setting it up much higher, but had the same problem.

Basic stuff I've tried - double & triple checking components are all correctly placed/oriented, and checking anything I can with a multimeter. Some results i got that seemed somewhat strange:
With nothing across the fuses (I can't really put anything there atm cause it'll blow up :/), i get +-40V across both fuse holders, ~5V across R5, R4, R13, 0V across R3.

I took photos in the hope they may point out some glaring error, but I'm not much better with the camera than soldering, so try to ignore the blurriness.
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Old 7th February 2005, 09:13 AM   #2
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Those voltages would be about right. Double check the orientation of Q6 in the heatsink hole - the orientation is opposite to what the leads and pad layout would suggest ie. the base (centre lead) comes from the opposite side of the transistor to join to the pad.

If that is ok, solder a short wire across C & E of Q6, and try the power up again. This wire should reduce output current to zero. If that is ok, double check all components around Q6.

Also make sure there is no connection between the heatsink bracket and any components (case of output transistors, collector of Q7 & Q8 etc).

Cheers
Graeme
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Old 7th February 2005, 10:24 AM   #3
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hi aquasync,

Have you checked with a multimeter than none of the bolts holding the output transistors in place are shorting to the heatsink. I can't see any insulators on the mounting bolts in the picture.

Also, the speakers shouldn't be connected when testing. I did that once and blew the resistors.

You should really set the bias once you have a proper heatsinks attached. Be very careful just using the aluminium angle.

This is the best site I have seen for further information on the eti 480.

http://www.alphalink.com.au/~cambie/

Good luck
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Old 7th February 2005, 12:35 PM   #4
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Thanks for the ideas centauri, I'll try that with the Q6. Do I need to have something crossing the fuse holders (my resistors are well and truly trashed now, suppose I need to by some more) for this to work?

As for shorting to the heatsink bracket, I was concerned about this. The kit I got came with these "plastic bushes", which I was supposed to cut in half and put in the holes. I did that, but it didn't seem very adequate. However, I tried to check if they were shorting by putting the multimeter in resistance check mode, and putting a probe on the bracket and on the transistors individually, without getting a reading.

As for not connecting the speakers yet - don't worry, I haven't done that, in fact I'm not entirely sure I know how to connect the output, but I'll figure that out if I can ever get the rest working.
Thanks for the website too, some info there I hadn't seen.

Oh, and here's the bottom picture, (couldn't tack it on before), which may help explain the insulation better than I...
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Old 7th February 2005, 01:16 PM   #5
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Quote:
Originally posted by aquasync
Do I need to have something crossing the fuse holders (my resistors are well and truly trashed now, suppose I need to by some more) for this to work?

Without the fuses / resistors, the voltage across Q6 (collector - emitter) should be somewhere around 1.2 to 1.5V. To test the Q6 shorted trick, then yes, the resistors should be back in.

Was just blowing up and trying to enhance your photo of the board underside - not real easy to make out, but Q6 around wrong way could be real possibilty.

Pic shows section of your photo, with proper Q6 orientation.

Cheers
Graeme
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Old 7th February 2005, 02:45 PM   #6
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Just checked the Q6 voltage then, and got 1.6, so thats probably pretty good. Its in the right way, although not easy to tell now with all the heatsink compound, but I definetly had the flat part of the transitor facing towards the pcb.
I'll have to wait a bit to get resistors before I can test out shorting the Q6 (or is it feasible to just link them?).
I can probably coax a better photo out of my camera whn the lighting is better.
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Old 8th February 2005, 03:38 AM   #7
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Ok, tried with the shorted Q6, and I got ~0.1 volts over the resistors. I checked the components again, and remain quite confused .
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Old 8th February 2005, 12:51 PM   #8
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Good - that means the outputs and drivers are all ok - it is the voltage across Q6 that is the problem.

With the resistors AND the short link disconnected, measure the voltage across Q6 again (you said 1.6V before), and vary RV1 full one way then then other and see if the voltage will get below this, say 0.5V or so. Only IF you can get the voltage lower, then retry with the resistors in place. If you cannot get the voltage lower, let us know what range you can get.

Cheers
Graeme
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Old 10th February 2005, 01:32 PM   #9
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Ok, tried that, and couldn't get it lower than 1.4v, this was at the trimpot's max of around 500 ohms.

I decided to try taking the screw out of Q8, because I noticed that it was conducting to the bracket (Q7 wasn't). No insulation for those 2 is provided, nor any mention of any importance, and further, none of the three leads conducted. I didn't think it would change anything, because it didn't seem that the shell conducted anyway, but after removing the screw, I have since been able to get the voltage over the temporary resistors down to the target range, albeit somehow burning R15 & 17 now .
Maybe something else was responsible.
Retrying the above test gives similar results still.
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Old 10th February 2005, 01:54 PM   #10
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The metal on the back of Q7 & Q8 should be connected to collector (centre pin) and MUST have insulating washers under them.

If R15 and R17 are overheating without the same occurring to R14 & R18, then there is possibly still a short to the heatsink via Q7 or Q8 mounting.

Cheers
Graeme
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