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Old 8th February 2008, 10:56 AM   #1
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Default ETI 5000 problem

Way back in the 80's I built a power amp, I think it is an ETI5000. It has worked like a dream all this time except for (rare) blown fuses when I played the Telarc recording of Tchaikovski's 1812 at full blast (the cannon shots did it).

Recently the amp started playing-up (after 20 yrs of good service). One channel drops out after about 5 min of music. I need to troubleshoot but I can't find the original ETI articles with the circuit diagram and circuit description (I think there were two articles).

Can anyone help?
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Old 8th February 2008, 03:43 PM   #2
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i think you should email silicon chip and see if they can get a hold of the article, i think they have archives of all the old EA and ETI stuff. at the most i think they chare 8.80 for a back issue article.

however a google image search for eti 5000 found this image
looks like a schematic for the amplifier in question....
Click the image to open in full size.
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Old 9th February 2008, 08:53 AM   #3
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Default ETI 5000 amp problem

Thanks heaps for the advice and the schematic. I should be able to fix it now, especially since I have one channel working so I can make comparisons.

My (not very well educated) guess is that one of the transistors driving the FETs has an age-related thermal runaway problem. Thank goodness my age related problems do not include that!

If anyone out there can offer advice on what to look for when troubleshooting, it would be much appreciated.

Alex
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Old 9th February 2008, 04:49 PM   #4
jaycee is offline jaycee  United Kingdom
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It's probably worth changing all electrolytic capacitors as well, there could be a problem with the input coupling cap (C1) causing this. Go for a polyester unit if you can. I would guess the power supply capacitors could also do with replacing.

I would have thought getting BF469/470 would be a problem but no - Jaycar has them!
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Old 9th February 2008, 11:11 PM   #5
Fanuc is offline Fanuc  United Kingdom
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You need at least four pairs of outputs to deal with low impedance loads presented by the speaker.

If it is not the load Z dipping at certain points, the load phase angle at certain impedances (even moderate) could destroy the amp.

Kevin
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Old 10th February 2008, 03:25 AM   #6
WayneK is offline WayneK  Australia
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I have repaired mine a few times but it has not been used for some time now. It is often easier to replace all the low level and driver transistors, saves trying to find the exact transistor that is causing the problem.

Also the heat sink for the bf469 bf470 transistors is very small in the original design. I have used much larger aluminium angles and connected that to a horizontal piece of aluminium that spans both boards with the cross piece of aluminium extending beyond the driver transistors.

This amp can also oscillate due to the mosfet 5w resistors being slightly inductive. One solution was to connect a capacitor across to join the 2 mosfets together (220nf from memory) or replce with parallel 1w carbon films to make them up to 5w or 6w power dissipation.

Regards,

Wayne
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Old 10th February 2008, 08:43 AM   #7
sandyK is offline sandyK  Australia
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Default ETI 5000 Problem

Alex
Replace C5,C12 and C13 in both channels. They dry out after a while and may cause instability problems and increased distortion.
Wayne is correct about the need for extra heatsinking.
Also, check R29 for discolouration due to overheating caused by instability. The thread at the link below may be of interest.

SandyK (another Alex !)


http://www.diyaudio.com/forums/showthread/t-103649.html
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Old 10th February 2008, 10:51 AM   #8
Pulse-R is offline Pulse-R  Australia
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I repaired my dad's one a couple of months ago.

The symptom was left channel very small output, only 1/2-wave driven, very quiet and distorted. on turn-on, the output swung momentarily towards -ve rail, then stabilised at zero. Right channel was fine.

one of the driver transistors had died (Q6 IIRC) causing the driver stage to go fully -ve, and the dc offset corrected it back to zero, leaving only a little bit of rail swing for the output stage.

anyways, I replaced all the transistors in the driver stage (Q3 to Q8) cost me about $20, and replaced all the insulating washers and thermal paste at the same time. I thought I felt a burr on the heatsink for the 6 transistors, so I made the sink all flat too - just to be sure.

reassembled and test - very nice sounding amp.
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Old 10th February 2008, 11:10 AM   #9
sandyK is offline sandyK  Australia
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Default ETI 5000 Problem

Pulse-R
Are you talking about the ETI 5000 (AKA ETI477) or AEM6000 ?
David Tillbrook designed both, but only the AEM6000 series had
an offset corrector.
SandyK
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Old 11th February 2008, 12:06 PM   #10
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Default ETI 5000 amp problem

Wow, thanks you guys (sorry, people). Good to know the transistors can still be sourced (thanks, Jaycee. Pulse-R). I will replace the low power transistors and if that doesn't work, turn to the capacitors (thanks WayneK, SandyK and Pulse-R).

I really do appreciate your help. It's great to know that there are people out there willing to help! Kinda restores my faith in humanity.

The ETI 5000 is a great amplifier. Mine is connected to four speaker boxes (two channels, each two boxes). Boxes are 8 Ohm so the load is 4 Ohms per channel for 100W per channel output from the ETI 5000.

Speakers are Philips 12 inch woofers (20 Hz cone resonance) and 5 inch midrange in each box with four Audax tweeters (series/parallel to get 8 Ohm from 4x8 Ohm). The boxes are big (each 4x2x1 foot approx) and totally sealed so the low freq response is superb! Clean but able to make the whole (weatherboard) house shake.
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