Help with Technics SA-303: Thump when powering on

[gghodg]

Hi folks,
I recently purchased this receiver for parts (no audio output) at a local pc recycle store. Luckily cleaning out the switches brought the amp back to working order, except for a rather loud thump when I turn on the receiver. Would replacing the factory ceramic capacitor (C1001) with a polypropylene film snubber capacitor reduce the thump? Or is the thump more common on amps of this vintage? Photos of the factory capacitor, and schematic attached. The service manual is too large to upload here. It can be found on hifiengine. I’ve attached screenshots pertaining to C1001 from the service manual and schematic. Thanks.

 

[kapitiaudio]

It's more likely to be a problem with the amplifier, rather than the capacitor across the mains switch. You may have a largish DC offset on the output. Measurements are required.

 

[gghodg]

DC offset at the speaker terminals is around 20mV.

 

[BSST]

I’m not familiar with this amp, but my guess is that it’s the “nature of the beast”. Some amps have output relays to delay connecting the speakers until PA bias has settled. You could add such an accessory. If you’re not using remote speakers, you could use the REMOTE selection position for a few seconds during power on, then move selector switch to MAIN.

 

[gghodg]

Thanks, Switching to MAIN a few seconds after powering up is easy 🙂. This amp does not have speaker relays.

 

[gghodg]

Update: It turns out the power up thump reproduces only when AM/FM muting is switched ON. I’m troubleshooting the muting circuit for the cause.

 

[gghodg]

Correction: Thumping is independent of selected source and muting on/off.

 

[Vrystaat]

Just build a delayed connect / disconnect circuit on the speaker output,using a relay,
really easy circuit, 5 seconds or so will be fine for caps to charge up before connecting and immediate switchoff.
Something like this
https://www.circuits-diy.com/simple-speaker-delay-circuit/
😉