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Allen and Heath PA CP power amp

Hi,

I'm a bit puzzled by this blown power amp, as the design of these things is not really my forte.

(I have posted the schematic over on alt.binaries.schematics.electronic)

The problem is around the two Lateral Mosfets Q16 and Q21 (EXC10N20 /

10P20). R36 from drain to -70v is complete toast, R35 from the N channel to +70v is OK. R33 on the P channel gate is also fried, as is R30 on the other gate, but not quite as drastically.

Now, R36 has been replaced before by someone, but I'm fairly sure they didn't replace either of the Mosfets or any other components. Also, none of the SCA / 2SC output devices or their emitter resistors appear to be faulty, though I have so far just done a cursory check on these and the usual suspects on a blown output stage.

I have removed both Mosfets, and on a (low voltage) Peak Component Tester, and they show as both working, which may or may not prove a great deal really.

Now, I had one of these in a few months ago, and came across a similar fault, but only R36 AND R33 were burnt to a crisp, indicating I thought, a blown P Mosfet. Again this checked out as working on my Peak tester, but I ordered another from Allen and Heath anyway, which actually took about 3 months to arrive. Replacing the two resistors and the Mosfet effected a cure, and the unit has not been brought back in again.

The question is, what failure mode might be causing these symptoms? I don't fully understand what these Mosfets are actually doing in this circuit, or whether something else in the circuit is causing these failures. Why is the gate resistor on the N channel burnt?

Another major problem is actually getting these Mosfets - you can't, as far as I can tell, and Allen and Heath no longer supply parts, instead having subbed out parts supply to various "service centres", who can sell you a complete amplifier PCB, but not individual components.

Sorry if this is a bit long winded.

Cheers,

Gareth

Reply to
Gareth Magennis
Loading thread data ...

"Gareth Magennis"

** Semelab BUZ901P and BUX906P are the same devices made in the same factory.

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Can you post that schem somewhere other than ABSE ??

Like most here, I have no access.

..... Phil

Reply to
Phil Allison

"Phil Allison"

** You did not look very hard ..

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The " R " suffix is of no consequence for your amp, just an improved version.

... Phil

Reply to
Phil Allison

is

is

appear

has

don't

the

far

Can you repost schematic as gif/pdf or whatever on a regular hosting site

Reply to
N_Cook

Schematic is here:

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Gareth.

Reply to
Gareth Magennis

Thanks, that's some of the info I was hoping for.

formatting link

Gareth.

Reply to
Gareth Magennis

"Gareth Magennis"

** Comes up blank.

.... Phil

Reply to
Phil Allison

Hmm, not much good at this, I think that was an HTML link, works on my PC though.

Try this, might actually be a picture.

formatting link

Gareth.

Reply to
Gareth Magennis

"Gareth Magennis

** Blank again.

... Phil

Reply to
Phil Allison

On Wed, 23 May 2012 18:19:37 +1000, "Phil Allison" put finger to keyboard and composed:

I can see it, but the resolution is crap.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

On Wed, 23 May 2012 18:38:53 +1000, Franc Zabkar put finger to keyboard and composed:

This is better:

formatting link

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

"Franc Zabkar"

** Blank for me.

Sure you do not have to join up with this site first ?

.... Phil

Reply to
Phil Allison

The resolution things a bit weird - if you click on my first post of the HTML link, it does look crap, but when you zoom in and zoom back out again (Windows 7 zooming), it magically reverts to full res, as I uploaded it.

The jpeg link seems permanently crippled at low res though.

I tried a few sites last night trying to post this schematic, but many were full of crap - links that took you where you didn't want to go etc.

Why is something so easy so difficult?

Cuh.

Gareth.

Reply to
Gareth Magennis

On Wed, 23 May 2012 18:44:58 +1000, "Phil Allison" put finger to keyboard and composed:

Nope. I use Opera.

However, if I use OffByOne (which doesn't support Javascript), I don't see anything and I get a "No connection to ..." message.

Also, Firefox starts to display the image box, but then reverts to a blank screen and a new URL of

formatting link

Anyway, I got sick of farting around and uploaded it to my own web space (why can't everybody do this?):

formatting link

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

On Wed, 23 May 2012 09:50:31 +0100, "Gareth Magennis" put finger to keyboard and composed:

Why not upload it to a site that specialises in hosting free service manuals, eg ElektroTanya or eserviceinfo.com, rather than some annoying, ad infested image/file hosting site?

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

"Franc Zabkar"

** OK, the original ( low res) link will not open but works if I save it first.

Same goes for the one you provided above - which is high quality.

Many thanks.

.... Phil

Reply to
Phil Allison

Didn't think of eserviceinfo, which I use a fair bit, but will do that next time. I thought this would just work, or most would be able to access ABSE instead.

Thanks for taking the trouble to put it on your site.

Cheers,

Gareth.

Reply to
Gareth Magennis

You can access ABSE at this URL, but someone flooded the group with a couple gigabytes of outdated software and the post may not be there. Also, it can take days for a message to appear, but it's better than nothing.

--
You can't have a sense of humor, if you have no sense.
Reply to
Michael A. Terrell

"Phil Allison"

** In the above schem, 2.7 ohm resistors R35 and R36 are each bridged by the B-E junctions of three power transistors and their associated 0.15 ohm emitter resistors. It is therefore not possible for the 2.7ohms to burn if the connections are good.

In normal operation the standing voltage across each 2.7 ohm is about 0.4 volts - ie not enough to bias the output transistors into conduction. So the two mosfets drive the load at low power and the other devices come in to assist above about 100mW into 8 ohms.

Is that schem really correct ???

Hard to believe there are no zeners protecting the gates of the mosfets - if the output is shorted, the gates can be overdriven and the mosfets destroyed.

Also hard to believe the rail fuses connect DC to the *whole* amplifier - cos if one rail fuse blows, the output looks bound to swing over to the other rail resulting in relay and speaker destruction.

If instead, the rail fuses were only in the supplies to the 6 power transistors and DC is applied permanently to the rest of the amplifier - then burnt R35 and R36 resistors and dead mosfets are easily explained. It happens whenever fuses blow and these parts then have to carry all the current to the load.

Finally, 1 kohm resistors R30 and R 33 can burn if the associated mosfet fails - cos the dead mosfet will have a low resistance from Gate to Source so Q15 and Q20 will drive current through them to the load.

.... Phil

Reply to
Phil Allison

Thanks very much for the analysis.

Changed Mosfets thursday, but has now blown 4 out of 6 output devices on power up. (no load)

Been away since then, will tackle this very carefully again wednesday.

The 2 fuses had not blown by the way, the front panel limit LED was lit though. They are blown now.

It looks from your analysis and the schematic, that the amp should power up into its quiescent state with no 2SC/2SA devices present at all. (with R38/41 in circuit) Is this correct? Is this wise or helpful?!

Gareth.

Reply to
Gareth Magennis

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