GRF quad amplifier

[Silvi]

Dear all I need a schematic for quad GRF class D amplifier model 406. This amp has its power supply dead and would greatly appreciate if someone could provide me with a schematic to aid me with repair.

[J M Fahey]

Think it will be hard to find.
I guess it´s a generic amplifier sold under many brands, but there is a GRF company on the Net, try writing them.

That said, lots of Techs have a generic approach to repair SMPS, with or without schematic, search for "Troubleshooting & Repairing Switch Mode Power Supplies Jestine Jong" , download and read it.

VERY practical book, helps you repair 90% of SMPS out there without schematic.

Focused on most common problems: shorted Mosfets/Diodes , blown capacitors, bad controllers.

It helped me a lot of times.

[Silvi]

My problem with this smps is that there are no shorted mosfets and diodes as I already searched for such faults. I also disconnected the amplifier and tried it alone but still it doesn't start. Some chip is withholding its start up for some reason. Now a days these have over and under voltage protection. They even have a sense for the presence of AC voltage so that they shut down in the event of a sudden power cut. This will protect the tump on the speaker output etc. Above everything else sometimes I also find the current control fitted with the same op amp maybe this being a quad type and will handle various tasks. Without a schematic it will be hard to find the cause. One time I had a similar repair and after a lot of searching and trying out I found a surface mount 1uF cap which lost a little value and was not capable of delivering enough voltage to the opamp thus this was keeping the output low and holding the start up of this smps.

About the trouble shooting book of Jesine Jong I already have it and have been through it a few times.

I wait a bit more maybe someone will come up with a schematic I can use.

Thanks and regards Silvi

[nickb]

Have you identified the controller chip?
Have you read its datasheet?
Do you have an isolating transformer and scope?
Have you then checked the pins to see what signals are incorrect?
Can you post a picture of the PSU?

[Silvi]

Have you identified the controller chip?
Have you read its datasheet?
Do you have an isolating transformer and scope?
Have you then checked the pins to see what signals are incorrect?
Can you post a picture of the PSU?

1) The control chips seems to be a couple of TL555C
2) Yes I have an isolating transformer capable of 1.5Kw. Yes I have a digital scope.
3) What signals everything is dead except the aux supply which is putting out 25v

I am finding difficulty uploading files (pics) as it seems that no matter how small the files are the website is not allowing me to upload more files.

[nickb]

There are some restrictions on posting files on new users. I'm not sure what the work around is. You could try putting them on 3rd party site like Google Drive or OneDrive, or send me a private message with them and I'll post them.

[nickb]

Here are the pics Silvi sent me

[nickb]

From SMD codes data book
A4W = BAV70 dual diode
A1W = BAW46 dual diode
A6 = not clear as not a unique code see alltransistors.com/smd-search.php?search=A6

I think your only hope it to trace the SMPS out. You can try approaching Guangzhou Dema Electronics and Sound Ltd. and ask for a schematic. You'll find them on Made-in China dot com

[Silvi]

Here are the pics Silvi sent me

[ATTACH=CONFIG]52671[/ATTACH]
[ATTACH=CONFIG]52672[/ATTACH]

[ATTACH=CONFIG]52674[/ATTACH]
[ATTACH=CONFIG]52673[/ATTACH]
[ATTACH=CONFIG]52675[/ATTACH]

As far as I can tell this is a flyback smps having a couple of parallel mosfets in series with the primary winding. The output is of a single voltage of 120v. A schematic will surely help out to localize some protection circuitry configuration. I checked the power side and all fets and diodes are still good. This amplifier is relatively new. The smps will not start even with the quad amplifiers disconnected.

[Silvi]

From SMD codes data book
A4W = BAV70 dual diode
A1W = BAW46 dual diode
A6 = not clear as not a unique code see alltransistors.com/smd-search.php?search=A6

I think your only hope it to trace the SMPS out. You can try approaching Guangzhou Dema Electronics and Sound Ltd. and ask for a schematic. You'll find them on Made-in China dot com

Thanks for making clear what are those un identified smds. I found out that the one marked A6 is a 4148 diode. I will go round the pin header again tomorrow and make sure that every component is good.

Regarding the schematic I will try to e-mail Guangzhou Dema Electronics and sound ltd and see if I get any feedback from them.

I thank you very much Nick for your support. I will be back with news If I manage to fix this amp.

[J M Fahey]

Just curious what does the actual amplifier need, supply wise?

What are amplifier specs, to estimate power needed to feed it?

Maybe you can steal similar power from a similar amp and feed the modules, even if at reduced voltages, to check whether they work, or are reasonably repairable.

It would be disappointing to repair SMPS with much trouble, expense and waste of time, to find modules are dead and again, complex without schematics.

As a last point: how much does that amp cost new? ... or can be bought for, used but working?
That usually puts a strong and low roof over what you are allowed to do.

[Silvi]

Just curious what does the actual amplifier need, supply wise?

What are amplifier specs, to estimate power needed to feed it?

Maybe you can steal similar power from a similar amp and feed the modules, even if at reduced voltages, to check whether they work, or are reasonably repairable.

It would be disappointing to repair SMPS with much trouble, expense and waste of time, to find modules are dead and again, complex without schematics.

As a last point: how much does that amp cost new? ... or can be bought for, used but working?
That usually puts a strong and low roof over what you are allowed to do.

This amp had actually A 100K 3 watt resistor which was open circuit with no signs of burning or whatever. I found it and hey presto the smps came to life. The quad modules had nothing at all and worked like a charm. They where tested with 1Khz tone up to the max with none of them having any ill effects.

Unfortunately the client had another similar amp this time (4X 120w RMS each) which apart from the same 100k resistor there seem to be other problems. Till now finding an opto and possibly a Zener or diode which I cannot make up what exactly the markings are( I need the schematic for this as otherwise I have to desolder the other pin header from the repaired amp) and I also found a pnp smd transistor which is burned. I am still waiting for some TL555c to arrive and to change them. Hopefully the pin header will work and the smps starts up. PS I can disconnect the smps from the amps and I can try it alone.

Lastly the amp cost around €650 so its worth fixing.
Thanks for your interest

Regards Silvi.

[J M Fahey]

Glad you repaired the first one.
100k and 3W rating hint at "starting" supply for the controller IC (or board).

Since the original died, I suggest you upgrade it, either 2 x 200k 3W in parallel or 2 x 47k 3W in series, pick your poison

Personally I´d pick the second option, fear most exceeding voltage rating than dissipation (which is accounted for in both cases).