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ETI 477 resurrected but with problems.
ETI 477 resurrected but with problems.
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Old 8th December 2008, 04:58 PM   #1
jonnoh is offline jonnoh  Australia
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Question ETI 477 resurrected but with problems.

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.
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Old 9th December 2008, 11:47 AM   #2
Keith Taylor is offline Keith Taylor  Australia
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ETI 477 resurrected but with problems.
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
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Old 9th December 2008, 12:27 PM   #3
jonnoh is offline jonnoh  Australia
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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.
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Old 9th December 2008, 12:27 PM   #4
jonnoh is offline jonnoh  Australia
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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...
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Old 16th November 2018, 02:15 AM   #5
Leinster Lad is offline Leinster Lad  Australia
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Quote:
Originally Posted by Keith Taylor View Post
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.

Hi Keith, old thread I know, but stumbled across it as I too am resurrecting my old series 5000.


I am interested to know what these modifications are that you speak of.


Dave
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Old 16th November 2018, 03:53 PM   #6
Keith Taylor is offline Keith Taylor  Australia
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ETI 477 resurrected but with problems.
Hi Dave, I also just stumbled across your post.

My 477 modules have been out of service for quite a few years and I am trying to recall what I did to them. I think Alex Kethel goes by the forum name sandyk on this forum. He suggested adding an 18 volt Zener diode in series with one of the BF470 transistors to reduce the collector dissipation. He told me he had discussed it with the designer who was in agreement with the idea.

I have also been in email contact with janusz who has a thread on this same amplifier. I think he is from Western Australia, and seems more interested in component upgrades rather than basic reliability issues.

The board design of the 477 modules seems to be in contravention of a practice that Douglas Self has written about. The feedback resistor R19 needs to be connected to the output trace further downstream than the board design intended it to be.

I could probably find my modules and tell you where the Zener diode goes in the absence of documentation.

Keith
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Old 17th November 2018, 06:46 AM   #7
suzyj is offline suzyj  Australia
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The zener goes between the collectors of Q7 and Q8, making the balance better between sides of the second stage diffamp.

Alternatively insert another 200Ω trimmer and adjust it so it so the two sides are equal.
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Old 17th November 2018, 07:27 AM   #8
googlyone is offline googlyone  Australia
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ETI 477 resurrected but with problems.
Interesting, if historical thread.

I used to use these, and he AEM6500 amplifier which was identical. I suspect that if I dig in my cupboard I will find some PCBs in the back of it somewhere.

To describe these amplifiers as "useless" is foolish - the performance was pretty good even for today, with distortion in the 0.0X% region and lower. Once working these were stable and well behaved. They did use FETS which some people seem overly perturbed by.

The heatsinks on the drivers did run very hot indeed. I added extra aluminium to mine to keep temperatures to a reasonable level.

There was a mod to the output devices, adding a small ceramic cap. If you don't have this in your mod I can go looking for the old articles.
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Old 17th November 2018, 08:31 AM   #9
JMFahey is offline JMFahey  Argentina
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S-C-H-E-M-A-T-I-C ??????

What are we talking about?
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Old 17th November 2018, 03:03 PM   #10
Leinster Lad is offline Leinster Lad  Australia
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Quote:
Originally Posted by suzyj View Post
The zener goes between the collectors of Q7 and Q8, making the balance better between sides of the second stage diffamp.

Alternatively insert another 200Ω trimmer and adjust it so it so the two sides are equal.

Thanks for the suggestion Suzy.
Any chance you could elaborate on the details ? What needs to be equal ?

I am only a humble hobbyist with a good soldering iron.
A rough sketch of that part of the circuit would be all I need !


Thanks heaps
David
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