Garrard 401 Driver - Trouble

[IslandPink]

Don't know if anyone can help , here goes -

I've got the power supply for the Garrard 401 cluttering up my kitchen table at the moment, because I can't get it to work . I need someone with a bit of SS knowledge to help de-bug the power amp chip part of the circuit .

The circuit is Dr. Imbabi's regenerated PS using an oscillator board feeding a low-voltage power amp, thet is supplied by stepped down mains, subsequently stepped-up to provide output . Link is :

http://www.eng.abdn.ac.uk/~eng289/401/401ps.jpg

I had several minor problems initially, mainly due to poor veroboard technique on the oscillator section, but now I'm looking good on most bits . The power supply rails on both boards are looking correct and are connected to their respective chips . I have a nice sine wave coming out of the oscillator , about 0.5V p-p from memory . Where it goes wrong is the 100k pot and input to the LM3886 chip .
The sine wave comes into the 100k pot (top) and looks good . If I then put the probe onto the WIPER of the 100k pot, it shows a CLIPPED sine wave - and the clipping level varies depending on the position of the wiper . The bottom of the pot is at ground as it should be .
This seemed amazing at first, but eventually I realised the lm3886 chip may have some sort of 'zener' effect that drags the input signal down for part of each cycle . I had first changed the pot , thinking it was duff, but got exactly the same effect with another pot .

So, I've checked all the connections to and from the LM3886 pins and I can't see any faults or shorts . I've checked that pins 4 and 5 are receiving their PS voltages ( +/-30 V , I forget which is which , but it was correct ) .

Anybody got any ideas, or any suggestions for further tests ?

Thanks in advance,

Mark

[NickG]

Other than checking the mute pin is connected, the clipping makes me wonder if its worth trying another amp chip, it could be dead, looking at the internal circuit. its just a transistor with a CCS load on the input pin.

[Clive]

Mark,

Have you tried connecting the oscillator to a hi-fi SS power amp via the level control?

The continuous power level the amp is asked to put out is quite high, about 12W? I know I mentioned this before but, is there any possibility that for some reason it can't cope with this? ie too small heatsink or the ps isn't delivering enough current?

Get it working, I need to know what it's like!!!

Cheers,

Clive

[IslandPink]

Thanks guys , really quick replies !

Nick :
"other than checking the mute pin is connected ....."
That sounds interesting . I may have clipped it off, thinking it wasn't needed. I'll check which one it is - is it shown connected on the circuit, or is it not shown ?

Clive :
I have thought of connecting it to another amp, but it would probably work , given the sine wave is there ! The 12W is quite practical from this chip . Yes, it is heatsinked, and it is not getting hot because it isn't doing very much yet !

Let's see if Nick has come up with something there .

Mark

[Clive]

Hi Mark,

Hopefully it is the mute pin and there's enough of it left to solder to.

My comment about the continuous power was just an observation of how different this is to an hi-fi amp. Many SS amps would cook if they put out 12W continuous.

You've obviously got a scope hooked up so you're ahead of where I'd be.

Cheers,

Clive

[IslandPink]

Hmmm,

Mute is pin 8 and it's on the circuit and almost certainly hooked up correctly . Must think again .

The heatsink could be a problem , but I have one about 2 inches square with 1/2" deep pegs all over it, bolted to a substantial piece of ali angle bracket which is stood off the box floor . I even routed some slots in the box top ( yes routed ) to give some air flow . Will have to suck it and see , maybe do a Mk.2 if it gets too hot . .. if it eventually works .

Mark

[IslandPink]

I'm still here -
feeling lonely ...

It'll be really good, when it's done , honest ..

MJ

[Clive]

I wish I could help.

[IslandPink]

Thanks Clive,

I'll smuggle it into work if I get desperate . We have an elec design group here .

On the circuit, I have assumed that the V+ line, going to pin 1, 5 means it can go to either pin 1 or pin 5 , and the other can be left redundant - is that OK do you think ?

Mark

[alnewall]

I would read that as being pins 1 & 5 but can't be sure. It might be that one of the pins has been omitted from the diagram for clarity.