D&B Audioteknik D12 - Power supply relay issue

[kdaudiorepair]

Hi,

Ive got a d&b d12 and when you switch it on the relays chatter like crazy on the power supply board. amp powers up fine and the fan spins but the relays chatter and then cut the amp cuts power.

When i opened it up it was full of dust and dirt so its been cleaned properly. Is it as simple as the relays have burned out due to heat over time or is there another issue i should look for.

I've got adequate soldering and electronics skills so have ordered another 4 relays but i'm wondering if anyone has seen this issue before?

Thanks

 

[kellys_eye]

Do you know what the relay actually does? Is it part of the PSU or the anti-thump (at switch-on)?

Either way there's an overload potentially a short - somewhere.

 

[Harald Kapp]

Either way there's an overload potentially a short - somewhere.

Or the electrolytic capacitors have lost their capacity. The aditional current from the relays could lower the voltage such that the relays turn off, which in turn leads to a rise in voltage across the capacitors. Then the relays turn on again and the cycle repeats. See e.g. here.

 

[kdaudiorepair]

I think it may be part of the anti thump as it usually takes a few seconds for the relays to click on and they usually stay on.

Ive taken a few picture of the board to see if this may help at all?

I'm currently awaiting a reply from d&b to see if i can source and service manual, schematic or further information to source the issue.

 

[Harald Kapp]

Visually the capacitors shown look o.k., but that doesn't exclude that they may have lost capacity. Also check the caps on the power supply board, post photos, if possible.
Would you have a capacitance meter to check the caps?

 

[kdaudiorepair]

I've got access to a Fluke 77 series II multimeter but sadly this doesnt have a dedicated capacitance testing feature.
The board above is the main power supply input.

hopefully i can attain some schematics to look further into the issue.

 

[Harald Kapp]

The electrolyticcapacitors don't visually look bad, but that is not to say that they are good. Loss of capacitance doesn't always show visually.
You could roughly test the capacitance of the electrolytics with a multimeter. You'll find lots of info on how to do this on the internet. In my opinion this is lost time. You'll have to remove the capacitors from the pcb for performing the measurements and while you're at it you can simply replace the main suspicious ones by new ones (marked red). Same effort for soldring, no effort for measuring, no risk.

Replacing the smaller ones won't hurt either, but in my experience these are less prone to deteriration than the big ones.

Of course, I can't exclude another reason for this behavior. It's simply the first place I'd be looking for.

 

[kellys_eye]

The board above is the main power supply input.

'Input' - meaning the mains filtering section (as per the grey capacitors and inductors) and, potentially, the switch-on thump protection circuitry but if the problem really IS in the power supply section - where it commonly is - we need to see that actual board.

A quick check of the output power transistors (for shorts) wouldn't go amiss either. I don't think your problems stem from this particular board rather than from elsewhere in the equipment.