Need Help. Woofer gets pushed out and hum when I turn on amp

[djn]

Hi all, When I turn on my Yaqin VK2100 amp, the right channel woofers get pushed all the way out with a loud hum, then it slowly goes back to it resting position and the hum fades. After that, all seems normal. What's going on? TKS in advance.

[simon7000]

There is a large DC offset thump on turn on! Very bad. Is there supposed to be a protection relay?

[jitter]

Looking at a pic of the inside of your amp I see two white blocks close to the heatsinks. My guess is these are the speaker relays. At power up, they should keep the speakers disconnected until the circuit has stabilized which usually takes a few seconds. It seems that the delay after power on isn't working anymore for the right channel.

Edit: in Yaqin VK-2100 mods there's a schematic of this amp that confirms the presence of speaker relays. These relays are operated from the same circuit and should therefore work simultaneously.

If you switch on your amp, do you hear a click after a short time?

[djn]

Good point about the relays. I should have added that this does not happen until the relays close. So the normal 10 secs or so go by without anything happening, then the relays close and the woof gets pushed out with the hum. Then it slowly goes back in to normal position.

[jitter]

Whoops, not good! In that case I agree with simon7000 that there must be a large DC-offset thump that slowly bleeds off, but it may not be the normal power up thump that the delayed closing of the speaker relays normally prevents.

Strange, a well designed protection circuit should never allow the speakers to be connected with a large DC-offset.

Disconnect the speaker in the offending channel and connect a multimeter in DC voltage setting in a range suitable for measuring the +/- 44 V PSU of the power stage. What does it read shortly after the relays click?

[simon7000]

I would look for a cold solder joint or burned resistor that charges any capacitors except any on the main power supply rails.

[djn]

I set my dmm to vdc and hooked it to the speaker terminals

The right channel, right after the relay closes, jumps from 1v to - 249mv then goes to -133mv and stays there.

The left channel, right after the relay closes, goes to 247mv then goes down to -155mv and stays there.

[djn]

Originally Posted by simon7000

I would look for a cold solder joint or burned resistor that charges any capacitors except any on the main power supply rails.

I'll start looking for that Simon. Thanks.

[Tekko]

Above 100mV is very bad and could be an indicator that the amp is approacing end of life or just a bad design.

A good amp has less than 15mV dc offset.

A great amp has less than 5mV dc offset.

A excellent amp has 0.00000000000000000000000000000000001pV dc offset.

[djn]

Weigh that high off set, is the amp a gonner or is it reparble