Anyone good at fault finding with amplifiers here?

[Joseph Noci 1Participant @josephnoci1]

Posted by duncan webster on 22/08/2019 23:35:24:

My radio ham friend reckons that when you run out of heat sink compound you can use copperslip. Anyone got any thoughts?

Now that's an interesting idea! Why would it conduct heat? because it's loaded with copper particles? I just got my tin of the stuff out stirred it up and stuffed the ohm meter probes in – nothing at all! not even megohms! Put some between two sheets of copper clad PCB, with strips of thin plastic (maybe 0.1mm?) interspersed to prevent the copper layers from touching, connected to the ohm meter, and pressed the copper sides together squeezing the copperslip down to a very thin layer. I hoped to force metallic particle contact – still nothing!

So, are the particles actually metallic copper? The colour certainly indicates so. Is it the lubricant that so effectively coats the metal particles that they are electrically insulated? Or is my stuff all oxidised and now an insulator? Still copper colour though.

Joe

[Neil WyattModerator @neilwyatt]

I won't lose sleep worry about it

[duncan webster 1Participant @duncanwebster1]

Posted by Joseph Noci 1 on 22/08/2019 23:56:49:

Posted by duncan webster on 22/08/2019 23:35:24:

My radio ham friend reckons that when you run out of heat sink compound you can use copperslip. Anyone got any thoughts?

Now that's an interesting idea! Why would it conduct heat? because it's loaded with copper particles? I just got my tin of the stuff out stirred it up and stuffed the ohm meter probes in – nothing at all! not even megohms! Put some between two sheets of copper clad PCB, with strips of thin plastic (maybe 0.1mm?) interspersed to prevent the copper layers from touching, connected to the ohm meter, and pressed the copper sides together squeezing the copperslip down to a very thin layer. I hoped to force metallic particle contact – still nothing!

So, are the particles actually metallic copper? The colour certainly indicates so. Is it the lubricant that so effectively coats the metal particles that they are electrically insulated? Or is my stuff all oxidised and now an insulator? Still copper colour though.

Joe

Had a google and found this. **LINK** Looks as tho almost anything is better than nothing, and genuine thermal grease is not electrically conducting

[Joseph Noci 1Participant @josephnoci1]

Yep, I know the typical thermal compounds/greases are insulators – was just wondering why copperslip is non-conductive…

Anyhow, rather than greases which are really messy when you repair a lot of amps..I have for many years been using a sheet material – the on I use is called ChoTherm – comes in various thicknesses sheets up to around 400mm x 400mm. Just cut to shape and size and fit between the component and heat sink. Heat conductivity performance tables provided with each, and they rival or surpass thermal grease performance. I have even used some that are a 'sheet' of pliable 'jelly' – does not really flow, but used to fit between tops of components and a combined heat sink / lid – cut a sheet section and place on top of the parts and fit/press the lid on – squeezing the stuff into all the crevices and spaces. It works very well, conducts heat nicely, and if you need to work on the parts, just lift the lid and peel the stuff out – it all comes out in one piece and is re-usable. Made by RayChem.

Guess I'm drifting off topic here, but seeing as Neil's amp is all fixed, I suppose we can..

Joe

[Neil WyattModerator @neilwyatt]

Finishing up MEW 285 while waiting for Farnell to deliver my mounting kits!

I have a mono camera, very small, just webcam sized chip, that I'm planning to convert to peltier cooling. As supplied the aluminium case has a 'column' that the chip is mounted on, thermal contact is a blob of grey putty that has the appearance of being 'loaded' with something, but i know not what.

What is in thermpath type compounds? Zinc oxide, titanium dioxide or…

Neil

[AdrianRParticipant @adrianr18614]

Neil,

There are quite a few different types. Apart from the silicon rubber type that come pre cut for the package, there are the mastic types and paste types. Each can have a different thermal particles. Ranging from Diamond dust to Zinc Oxide.

Where you dont need electrical insolation e.g. for a CPU you can just use the mastic pads or paste. The commonest OEM type is a grey mastic, that on the first few thermal cycles softens and squidges out.

The best for us mortals is Silver paste, having said that I would happily use white paste too, just not on my gaming rig where I am trying to dump 120W through 1" sq.

The secret is to have the two surfaces flat and smooth. No scratching the old stuff off with a blade. Then put it on very thinly, just a little to thick for engineers blue.

Danger of silver paste is it can conduct, so be very clean when removing old.

If in doubt Artic is a good brand to buy.

If you need electrical isolation e.g. for a TO3 package, you need to use a solid insulation i.e the rubber (or olden days mica) Depending on the type you may also need to use paste too, I have never used paste on the silicon ones.

After a few thermal cycles if you dont have a sprung loaded clip check the bolts are tight.

Adrian

[Neil WyattModerator @neilwyatt]

I have Farnell paste, I just wondered what it was as it looks suspiciously like a thick version of nappy rash cream…

My mica washers came this afternoon.

Apparently mica is used for making optical etalons, because the two faces are (supposedly) perfectly parallel as they follow molecular cleavage plains.

Neil

[Neil WyattModerator @neilwyatt]

All now working and tested.

Blimey! It is much louder without distortion – I suppose that just getting the extra ~8 volts of headroom before distortion sets in equates to about a 15% increase in voltage but over a 30% increase in power, but then allow for the fact that the asymmetrically driven speakers would have been adding distortion as well and the reversed speaker would have been taking away bass, it's hardly surprising it plays louder. Also I'm sure the sound is better, with a better bottom end (it deals very well with my five-string which has a low B – about 31Hz – you need good speakers to reproduce the fundamental without a lot of bass boost).

So, I'm a very happy bunny. My understanding of how these amps works is much better now, and also I've discovered the benefits of really looking below the surface – I had noticed a few months ago that it was clipping the top of the waveform but not the bottom and didn't register that this was the sign of a fault.

Thanks to everyone who contributed ideas and thoughts to this thread, I've learned a lot from you (and also Agedhorse in 'another place' who actually worked on these beasts).

Also thanks to Horovitz and Hill who explain thermal runaway in FETs in language I could understand – after the primer of this thread I could actually understand what they were on about in detail.

Neil

[Anonymous]

Posted by Neil Wyatt on 24/08/2019 20:37:56:

Also thanks to Horovitz and Hill who explain thermal runaway in FETs in language I could understand

By far and away the best general book on real world electronics there is. I bought Edition 1 when it came out in 1980.Later some barsteward half inched it from my desk at work. The company replaced it with the then current Edition 2. I treated myself to a copy of Edition 3 a couple of years back.

Andrew

[Neil WyattModerator @neilwyatt]

Posted by Andrew Johnston on 24/08/2019 22:06:46:

Posted by Neil Wyatt on 24/08/2019 20:37:56:

Also thanks to Horovitz and Hill who explain thermal runaway in FETs in language I could understand

By far and away the best general book on real world electronics there is. I bought Edition 1 when it came out in 1980.Later some barsteward half inched it from my desk at work. The company replaced it with the then current Edition 2. I treated myself to a copy of Edition 3 a couple of years back.

Andrew

I have 2 and 3, they are quite different, I miss all the examples, especially the 'spot the deliberate mistake' tests! You have to buy a second book for them now

If only they weren't so cumbersome, or I would read it in the bath!

Neil

[Neil WyattModerator @neilwyatt]

[Joseph Noci 1Participant @josephnoci1]

Very Nice Neil! And with 'improvements'!

Am busy trying to repair a 4KW Phonic stereo amp at the moment…All transistors, some FET's, no IC's.

Uses split supply rails as usual, but dual rail, ±55volt and ±120volt. The output stage has 16 transistors – 8NPN and 8PNP in parallel. These receive the ±55 rail when running below 1KW / 4ohms and there are some big FETs that then switch the 55volt rail out and the 120volt rails smoothly in..gives almost 4KW into 2 ohms…

13 Transistors in the input stages, 4 long tailed pairs..and 12 transistors in the output stages, excluding the 16 power output tranny's.

No scoundrels way out on this one..

We also have tons of mica here – literally mountains of it. Maybe that's why it is often dull and grey here in Swakop…Any light entering is trapped inside sheets of Mica and can never leave – out own Hotel California for light..

Joe

[I.M. OUTAHEREParticipant @i-m-outahere]

So now that you have it fixed will Stub be playing a few licks on YouTube?

[Nicholas FarrParticipant @nicholasfarr14254]

Posted by Neil Wyatt on 24/08/2019 20:37:56:

All now working and tested.

 

Also thanks to Horovitz and Hill who explain thermal runaway in FETs in language I could understand – after the primer of this thread I could actually understand what they were on about in detail.

Neil

Hi Neil, tut! tut!, fancy getting Paul's name misspelt. Threw me there for a moment, good job you got Winfield's one right.

Glad you got it fixed anyway.

Regards Nick.

P. S. sorry I couldn't resist a little banter.

Edited By Nicholas Farr on 25/08/2019 12:49:15

[Neil WyattModerator @neilwyatt]

Posted by Joseph Noci 1 on 25/08/2019 07:28:16:

Very Nice Neil! And with 'improvements'!

Am busy trying to repair a 4KW Phonic stereo amp at the moment…All transistors, some FET's, no IC's.

Uses split supply rails as usual, but dual rail, ±55volt and ±120volt. The output stage has 16 transistors – 8NPN and 8PNP in parallel. These receive the ±55 rail when running below 1KW / 4ohms and there are some big FETs that then switch the 55volt rail out and the 120volt rails smoothly in..gives almost 4KW into 2 ohms…

13 Transistors in the input stages, 4 long tailed pairs..and 12 transistors in the output stages, excluding the 16 power output tranny's.

No scoundrels way out on this one..

We also have tons of mica here – literally mountains of it. Maybe that's why it is often dull and grey here in Swakop…Any light entering is trapped inside sheets of Mica and can never leave – out own Hotel California for light..

Joe

Sounds fun… and that's why you have such amazing night skies

Neil

[Neil WyattModerator @neilwyatt]

Posted by XD 351 on 25/08/2019 08:42:48:

So now that you have it fixed will Stub be playing a few licks on YouTube?

[duncan webster 1Participant @duncanwebster1]

Don't give up the day job!

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