Threshold s300 Amp Power on Thud

[nellson]

Has anyone heard of this problem occuring?

On power-on a loud thud is now heard in my loudspeakers which concerns me greatly as it may cause damage.

The amp is working and sounding fine apart from that?


any known parts that fail or should be replaced for this problem?

Thanks in Advance

Richard (nellson)

[Zen Mod]

electrolityc caps ,no doubt;
time to replace all small ones on main PCBs

[nellson]

thanks on the reply.

I had a quick visual before posting to see if any leaky caps were present, they look ok but are definitely old.

I have had a look and notice only a 47uf electrolytic, tantalum and mylar cap on the board. whilst having the amp apart is there anything else that should be done?

It looks easiest to drop the bottom panel out(with toroidial and filter caps on it) so the 2 side boards are accessible to work on at this stage.( comments?)

I guess i'll try replacing the 47uf cap and see if it helps the problem.


thanks

Richard

[georgehifi]

While your at it replace that tantalum bias circuit C5 I believe with a good quality plastic film , it's been known to clap out and give trouble. Here's the circuit if you need.

Cheers George

[georgehifi]

Oops, that's the S300 mkII opto bias this is the S300 mkI just in case.

[Nelson Pass]

The only phenomenon I recall is that the feedback cap could
store a charge on turn-off and if you did not wait a significant
amount of time before turn on, that charge would create a thump
as the amplifier removed it. That is why you will see a resistor
across feedback caps in these designs - to bleed off a turn-off
charge.

You could try placing a large electrolytic cap across the bias
circuit - the Vbe multiplier, or in the case of the optical circuit,
across the opto-transistor.

[nellson]

Have replaced the 0.15 tant and 470uf electrolytic on both channels and thud is still present.

Does anyone else have a turn on thud on there power amp and what is considered acceptable?

I have attached s300 power supply schematic.

Has anyone had problems with the switch/breaker and what is the criteria for selection of the 2 capacitors 0.01uf off the switch?

[wayle45]

Nellson: Hi, It seems that you have schematics for the Threshold S300 ll. I'd like to set the bias and the DC offset but I'm new to this procedure. Are the blue pots the bias adjusters? Are there DC offset pots anywhere too? Please help- I'm a "Nervous Nelly" about this. Thanks, Wayne

[georgehifi]

Originally posted by wayle45
Nellson: Hi, It seems that you have schematics for the Threshold S300 ll. I'd like to set the bias and the DC offset but I'm new to this procedure. Are the blue pots the bias adjusters? Are there DC offset pots anywhere too? Please help- I'm a "Nervous Nelly" about this. Thanks, Wayne

Here ya go.

Cheers George

[wayle45]

High five!, George - Thanks, Wayne