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Old 24th November 2006, 08:35 AM   #11
Ovation is offline Ovation  United Kingdom
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Hi Again Hugo,
Aah - So they're just pretty caps :-) I should have realised that I suppose.

I previously measured the power going into the FETs on either side of the large caps was about -86 Vdc and +86 Vdc (Naturally between those points I get 180 ish Vdc)

So far eveything I have tried to test seems to be working okay. Interestingly most caps seem to offer a resistance in Mega-Ohms , I was expecting less - But never the less they seem to discharge and charge as I reverse the meter leads.

There's no harm in asking obvious questions but, Yes I have checked all fuses :-)

I've got some reasonable equipment available - I've got:
- Tektronix DVM with all the usual functions including Diode, Capacitor testing, Various Holds/Averages and a Frequency Measurement (The cap test on these 4700uF caps tells me that they're 1.6mF, but the Symbol for the test is "-| (-" So I presume polarized caps are tested, but not sure if it coveres such large caps.
- AVO Meter
- Dual Channel 100MHz Oscilloscope
N.B. I've also got a Wayne Kerr LCR meter, but I don't have the leads for it, so not sure it's much use to me.

I presume you've suggested tightening the screws/nuts on the FETS because of Grounding? It looks to me like they're on a common ground - Could a dirty earth cause one of these to hum ?

Thanks for all your help so far - I feel pretty confident that we're getting closer to a diagnosis.

I will be out tonight (GMT) so the next opportunity to try anything will be over the weekend.

Many Thanks
Jonathan
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Old 24th November 2006, 09:59 AM   #12
Netlist is offline Netlist  Belgium
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The schematic attached is from an old CA4 but it gives an idea of how the mosfets are wired. Note that the metal cases (sources) are the output. If you measure DC on these, the output protection will kick in. Also, the gates should have +/- 0.9V with respect to ground.

/Hugo
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Old 24th November 2006, 04:48 PM   #13
Ovation is offline Ovation  United Kingdom
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Thanks for the schematic - that's helps a bit...
I've just realised that I've been a real dimwit ... Don't quite know what I was thinking, but somehow I've invented mosfets that have only a Gate and a Drain!!!
So I'd better test between Gate/Drain and the 'SOURCE'! Doh!

Given that I kinda missed the 'source' even though it's obvious, I reckon there's still a chance of a failed mosfet causing all my grief!

I'll check that, plus look for spurious DC and will report back!

Cheers
Jonathan
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Old 24th November 2006, 05:06 PM   #14
Netlist is offline Netlist  Belgium
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I any significant DC is present at the output, you will need to take out every mosfet to test properly.
At least temporarily remove the rail fuses. Any remaining DC -from the big caps- will influence the measurements.

/Hugo
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Old 25th November 2006, 09:46 PM   #15
Ovation is offline Ovation  United Kingdom
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Hi Hugo,
It looks like I've got + and - 0.4vdc on the Gates, and no DC (>0.01v) on the sources.

I'm still puzzled by this.. I don't want to leave the amp on to long at any one time as the Soft-Start Resistor starts to get quite hot...

I'll keep working on this - I need to look at the diagram in more detail and test some more points.

Thanks
Jonathan
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Old 25th November 2006, 09:58 PM   #16
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I wouldn't worry on the soft start R. It's made to get hot.
If it's an NTC of course.

Now, what if you apply a signal and look at the output signal with a scope? (before the output relais of course)

/Hugo
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Old 26th November 2006, 10:07 PM   #17
Ovation is offline Ovation  United Kingdom
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Hi Hugo,
It looks like I'm not going to get an easy fix for this :-(

The Power Amp section seems to be working just fine... I used a Burosch CD Test disc to put a 1kHz tone into the amp and hooked up my scope to the output stage (just before the relays). I get a perfect 1kHz on the scope, all be it with a large peak-peak voltage :-)
As expected, a music CD also produced a very normal looking output.

So the Amp's going into protect mode, but there's no DC on the output and the main power stage appears to be fine. Regardless of what source I feed into the Amp, the VU meters remain permanently with two leds lit per channel.
None of the relays ever switch over - I can understand the output relays, but what about the Soft-Start resistor relay? Surely once the amp has come on, that should still switch over to bypass the resistor? (I presume that soft start doens't actually reduce the voltage sent to caps/mosfets, but simply provides a bit of 'ballast' to help avoid spikes on start up?)

Do you have some other ideas because I'm definitely getting out of my depth with this amp.

Many Thanks
Jonathan
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Old 26th November 2006, 10:47 PM   #18
707dave is offline 707dave  United Kingdom
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Hopefully i have attached protect cct, hope you can read it!
The three relays are in series so they will all work together. Try checking the To220 transistor near the fan as this drives the relays. also check for an open cct relay coil.
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Old 27th November 2006, 01:07 PM   #19
Ovation is offline Ovation  United Kingdom
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Hi Dave,
Thanks for the input on this....

Interestingly enough I'm pretty certiain that the TO220 transistor (if it's Q45 in the following photo - http://dynamicdiscos.co.uk/AMP_Pics/Large/DSCF0203.JPG ) has already been worked on in the past as the soldering is clearly different.
(You can see flux left round the pins in this pic: http://dynamicdiscos.co.uk/AMP_Pics/Large/DSCF0184.JPG )
I did test it for shorts a while back and it seemed fine, I'll test it again though and I haven't checked the coils on the ouput relays yet...

Do you by any chance have a higher resolution picture of the circuit? The one you sent is useable enough, but none of the text is readable :-(
Do you have the entire schematic available for this Amp? It would be nice to get hold of it so I can perform future repairs on this amp, and perhaps help other people if they have the same series.

Many Thanks
Jonathan
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Old 27th November 2006, 08:09 PM   #20
Netlist is offline Netlist  Belgium
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On the schematic posted by Dave, I see a few electrolytes.
Don't trust them.
Next go measure the diodes, then the transistors.
You are close to a solution.

/Hugo
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