Wharefedale EVP-X 15 smps / amp module repair.

[acemc]

Hi Im hoping someone can assist me with a repair I am busy with.
I have a Wharfedale EVP-X 15 amp module that has a problem with the power supply.
The mosfets (IRF460) & other discreet components were blown & have been replaced. The psu is using IR2153 half-bridge driver.
IR2153 Vcc is taken directly from the rectified ac via a divider comprised of 100K 2W & 6.8K 1/4W.
This is where Im stuck - this voltage goes low & prevents 2153 from operating.
If I feed 15v to vcc from an external power supply, the supply comes to life but only delivers half the voltage it should.
I should get +-70v, but only get +-40v. My 15v rails are also down at +-8v.
IR2153 frequency is measured at 70.5khz - according to the schematic (attached) it should be 71khz, could that make such a difference?
I feel like it's something simple Im overlooking, but I just can't see it. I'm clueless which direction to go now.
Any help will be greatly appreciated.
Thanks in advance

[Audiorepair]

First of all, fix the problem that the Vcc voltage goes low, it shouldn't be difficult.

The voltage divider drives a 16v zener via a resistor, the zener should have roughly 16v across it, and transistor Q8 then passes on this 16v at its base to the two capacitors and the Vcc pin on the IC, giving a slightly lower voltage.


THEN you can establish whether or not there are further problems with the circuit.


You really don't want to try and fault find when you know there is a problem with the circuit you are supplying your external 15v with, and you haven't isolated that problem first.

[acemc]

Thanks so much for your reply. I have replaced all the zener's and transistors. All the resistor values measure fine according to the schematic. Without the 2153 in circuit, I have 16v across the zener & about 15v on vcc. As soon as I put the 2153 back in circuit, the voltage on vcc drops to 9v which is UVLO. All the components pertaining to this part of the circuit test fine, so this really is where Im stuck. I can't figure out what could be causing the voltage to drop. if I inject power directly onto the board between R7 & R27 then vcc doesn't drop. I just don't know what I'm missing here. Thanks for your input, I really appreciate it.

[SeanB]

Have you tried disconnecting the power supply outputs from the amplifier, 40V is suspiciously like the amplifier has a fault, and is drawing excess current. The current sense resistors, were they replaced, as they can fail high resistance, slightly high in value, so there is always current limiting. Other than that D12. D13, D14 are all suspect, along with R17, R18 and R19, as they all might have had stress when the supply failed. If you have not replaced C17, D3 and R3, or at least checked out of circuit, do so, so that you can have a chip power supply once operating.

If it still is not working check secondary side diodes D6 to D10, out of circuit, and the capacitors C25 to C28. Capacitors not only for shorts, check capacitance and ESR, or simply replace with new if unable to test them.

[Audiorepair]

The blue capacitor, C16, is a known point of failure, but normally that means the PSU doesn't work at all.

It would help enormously if you could rig up a mains lightbulb in series with the mains input, that would tell you instantly if something was shorting/drawing excess current.

If the power amp is faulty, there will almost certainly be a dead short on the output transistors you can detect in circuit, but as Seanb says you should unplug this to see if the PSU works fine without it.

[Audiorepair]

The resistor divider source is only meant to get the IC started.  There is not enough current available via the divider to run the IC at full whack.

The IC goes through a startup procedure, powered from the resistor divider, which puts a few bursts onto the transformer to charge up the caps etc.  Eventually, a separate winding on the transformer primary supplies the right amount of power to run the IC, via D3/C17/R3, taking over from the resistor divider.

Check to see if you are getting any voltage across C17, if there is none then the IC will not run, it will just keep trying to start before running the 15v down to the point it stops starting, then keeps doing this forever.
So your 15v will read considerably less.

Likewise, if there is a short somewhere, the IC will shut down during startup, and will never get it's full 15v from the transformer winding.



Edit:  Just realised C17 will get charged up from the resistor divider via R3, so just check or replace these 3 components, it is unlikely but not impossible the winding is open circuit.

[acemc]

Guys thank you so much!!. Thanks to your help I found the problem.
SeanB - I have actually been working without the amplifier board connected, I'm (trying to) repair in steps.
All the components you mentioned (D12-14 & R17-19) were damaged, but I had already replaced them.
Audiorepair - Thanks for the explanation of the start-up circuit.
Now that I understand it, I realised I wasn't getting any voltage from the transformer connected to R3, D3, C17.
It turned out to be the blue cap C16. It should be 1uf but measured only 500pf.
I replaced the cap & now the psu powers up & I have +-68v rail voltage
My next step now will be to move on to the amplifier board.
Thanks once again for your help - Greatly appreciated.