ETI 477 resurrected but with problems.

[jonnoh]

Hi there, I have recently resurrected an ETI 5000 power amp which I made what must have been more than 20 years ago. I am not an expert at electronics, but have done some study in the field yonks ago. I see other threads relating to this amp which suggest it is pretty useless. I must have been deaf at the time as I can't recall it sounding that bad. My speakers weren't that good; perhaps that's why.

At any rate, turned it on with no load and first thing it did was blow one of the fuses to the preamp (which I was using). Changed that and tested voltages and so on and it seemed okay. The left channel driver transistor heatsink was a little hotter than the right. Tried it with a load and the first thing it did was trip the speaker protection and give a puff of smoke - left channel only. Turns out it cooked R7 R18 (270R) as well as both BF 469. I changed all the electrolytics, semiconductors except for the mosfets, as well as the cooked resistors.

However, quiescent current is high even using the adjustment, and the driver heatsink does get hot with no load. There are no shorts in the mosfets but I think there may be leakage or whatever going on...does anybody have any ideas regarding this? I guess the next step would be to change the mosfets; I have found some online for about $10. Having said this, i'd only do this for sentimental reasons...i think I will buy another kit when I have the money...I am sure that something else will go bang sooner rather than later.

thanks.

[Keith Taylor]

Hi Jonnoh, I am from Adelaide also, and own a pair of these amplifiers which are still in daily use. They have suffered exactly the same fate as yours, only worse. Parts of the board had been reduced to carbon, which presented quite a challenge to rescue.

Maybe your faulty amplifier is oscillating. Is R29 (output Zobel) getting hot? I would be inclined to change all the electrolytic capacitors on the 477 boards. I have implemented a modification suggested by a guy in Sydney called Alex Kethel who is a member of this forum. I also did a modification to the positioning of the feedback pick off point.

PS I was going to send you an email, but notice this is not enabled. Feel free to email me, as being a retired electronic fault fixer I can probably help you.
Keith

[jonnoh]

Hi Keith, thanks for that.
I don't believe it is oscillating. Output voltage on that channel is around 120mV; on the other (good) channel it is about what they say it should be, around 25mV. I hope this is consistent with not oscillating. Unless it oscillated as soon as I connected the load?

It seemed to be R7 and R18 which were the most badly burnt. R29 and 30 are okay.

I have changed all the electrolytics (except the power supply), as the one thing I managed to remember was that these can become faulty with age. Rail voltages were okay. I don't think there is much more left for me to change except for the mosfets.

If you can offer any advice it would be appreciated. If you can supply info regarding the mods I'd be very interested also...give me something to fiddle with. That's if it ever gets working again.

thanks again.

[jonnoh]

By the way Keith, you should now be able to send me email as I have just enabled this. Unfortunately it turns out I can't send you email as I am a newbie and seemingly on probation. Thanks...