Help needed for a NOVANEX amp

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first I want to thank all members here for sharing great schem/layout/advices/tips...
Second, I need help to debbug a Novanex solid state guitar amp...A friend of mine gave me this old Dutch amp as is...
I've checked the power supply and found +35V and -35V for Vcc and Vee respectively. Then I left out the 2N3055 power transistor pair and tested them in diode mode with a multimeter : They seem OK
With an audioprobe I run through the preamp stage and didn't find any cut...

Now I plan to change all polarised capacitors and left out all active components to test them out of the circuit.
Some of power stage transistors have been yet changed or at least desoldered and soldered again...and before waste hours of working I want to be sure of the power stage schematic. I'm not into this kind of design and I need help.

here you'll find the whole amp layout and the power stage schematic :

http://natwlad.free.fr/images/PCB.ppt
http://natwlad.free.fr/images/schema.ppt

Any help, suggestion, advice... is more than welcome

Wlad

[analogguru]

If you could post the schematic as .gif, .jpg or .pdf it would be much easier to help you.

analogguru

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Ok, here are the links for jpg files :

http://natwlad.free.fr/images/pcb.jpg
http://natwlad.free.fr/images/schema.jpg

Thanks in advance

[Ronsonic]

I'm sorry, I don't understand the question. What is wrong with this amp?

Am I right that it was given to you and you assume it is broken in some way?

If I've got that right, I would suggest actually testing the thing. Use something like a 60W lightbulb in series with the mains power to limit current and power it up with no load connected. If the light bulb gets bright and stays bright you may have a problem. Does it pass signal? Is there a DC offset on the output? If it passes those tests, see if it will pass a signal into a load. You will see it brighten the bulb as the amp delivers current into the load, you may want a larger bulb.

Electrolytic caps do fail, but I don't see a reason to just replace all of them until you know what kind of problem you've got.

Ron

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Sorry Ron for the confusion, but I don't assume the amp is broken : I'm sure it is...
And I think the problem comes with the power stage because :

- Speakers are ok (they work well with another amp).
- Power supply gives me +35 and -35 V DC.
- Any cut found in the preamp stage with an audioprobe.

Am I right

I've just left out of the circuit Q9 ,Q15 (2N5323) and Q11 ,Q19 (2N5321) to check them.
Q9 and Q15 are ok but Q11 and Q19 are bad...
I also found something weird : these transistors are mounted with "plastic spacer" (I don't know the correct word, it's a piece of plastic between PCB and component) and the pin emitter of Q11 was bent and connected in fact to +V while it goes to the base of Q10 on my schematic / PCB...
Someone can tell me which connection makes sense please

By the way, I don't know the lightbulb technic to test an amp. Is there any article or something about that Many thanks

Wlad

[R.G.]

By the way, I don't know the lightbulb technic to test an amp. Is there any article or something about that

Go to the hardware store and get a metal electrical junction box big enough to hold two outlet plugs and an AC power switch to go in the junction box. Wire the box with a power cord to plug it into the wall, the switch to interrupt the AC voltage, and the power outlets **in series** not in parallel. Plug a household lamp into one outlet and the amp under question in the other. The light bulb limits current. If the amp is shorted, no more than the bulb's rated current flows.

Of course, only do this if you already know how to wire AC power safely. Otherwise you're at risk of electrocuting yourself.

This is a very oddly designed power amp. It has some odd protective circuits in it that I think you should defeat until you get it running.

First, test or just remove that SCR over on the left hand side. That could be one of the problems if it's damaged.

Q19 being bad will prevent any audio from coming through - that's the voltage amplification stage. Q11 is the driver transistor for Q10, and likewise, no signal will pass if it's bad. It is possible that someone tried to change this setup from quasi-complementary to full complementary but did a bad job of it. Put it back the way the schematic says to.

It seems like Q13 and Q14 are current limiters. The amp might work with these removed. Likewise, I think that Q12 is part of the SCR firing circuit, and could be removed.

Q8 and Q9 are a constant-current load for the voltage amp, Q19. I would short out that resistor at the emitter of Q9 to force a pure class B operation until I got the rest of the amp working. This will make for bad crossover distortion, but will prevent thermal runaway until you get it working.

The circuit design flies in the face of several of my opinions on how to design solid state amps. If it were mine, I'd rip out all of the power amp, build a new on on perfboard to drive the power transistors and forget the problems with this design. Using the power supply, heat sink, and output transistors and junking the rest is good practice; these are the most expensive parts and you can put the low power stuff for a really good amp together much more easily than debugging this one.

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Thanks R.G., but I'd like to debbug this original power amp if it's possible before changing the whole power amp.

Anyway, I found Q11 & Q19 bad...I can try to replace them with good ones and see what will happen...
But where to connect the emitter of Q11? to the base of Q10 like on the schem or to V+ like it was until I left it out from PCB?

Another question, if I decide to finally change the whole power amp, which design can I use ? Any schematic prefered ?

Thanks for advices

Wlad

[R.G.]

But where to connect the emitter of Q11? to the base of Q10 like on the schem or to V+ like it was until I left it out from PCB?

Definitely like the schematic. It won't work any other way.

I redrew the schemo. Actually, I would only change a couple of things on it. Get rid of Q12 and the SCR. Put in a Vbe multiplier for the bias voltage instead of a resistor. Maybe change to a real current source for the input diffamp.

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Hey, I've changed those Q11 & Q19 that were found bad, and the amp is working !!!
However it still remains some bugs :

I got sound only if the reverb pot is at 4 or more, at 3 I got a little distorted sound and under 3 nothing...
Tremolo is only 2 or 3 seconds when I turn Intensity to 10...

I have to check the components of those 2 part, but someone could suggest something...
Any idea is welcome...

R.G., I'd be curious to take an eye on your modified power stage schemo...may be you can post it...

Wlad

[R.G.]

I got sound only if the reverb pot is at 4 or more, at 3 I got a little distorted sound and under 3 nothing...

Most likely your reverb pot is getting fed DC by one of the caps leading to it, and is then misbiasing the stage that comes after it. Check the reverb pot for DC voltage from the ground lug on the pot to the other two. I think you may find DC. If so, fix the caps or shorts that is letting the DC come over.

Tremolo is only 2 or 3 seconds when I turn Intensity to 10...

If by that you mean that you only get a couple of seconds of tremolo then it fades out, this is a classical problem in tremolos. Your tremolo oscillator is started up by the sudden kick to release it from being held off, then it does not have enough gain to sustain its oscillation.

I think you may need to examine every single electrolytic cap in the amp, or just replace them. You may also need to replace Q4 and Q5 if they have been damaged and are now low gain.

[sonic66]

I've also got a novanex amplifier i need help with.

i replaced all the electo capacitors as i guessed they might need replacing .....  but  it then  just blew the fuse.

i put the put the original power filter caps back and  it works ....

i'd bought 40v 10000mfd single section caps ....  cause the cap case and on of the pins were both connected to gound

are they infact multi-section 






thanks