Akai AM-U1 amplifier - UK Vintage Radio Repair and Restoration Discussion Forum

112stereo · 10th Sep 2020, 4:00 pm

I am trying to repair my son in law's amp.
It was OK before they went on holiday, but on return its developed a fault.
The left channel is very low and distorted, but if you plug in head phones they sound ok?
I have done some voltage comparisons between channels and both sides appear the same.
Swopped the speaker leads over, just in case, still the same.
The only thing I have noticed if you turn the balance control to fully left speaker only, turn up the volume up and switch it off when the capacitors are near fully discharged the left speaker bursts into life just for a second or so.
I had this a while ago with a leak amplifier and you kindly helped me out, but that was on both channels and it was a cap in the current limiter at fault.
I am thinking it might be the STK chip at fault.
Does anyone have any ideas?

Marconi_MPT4 · 10th Sep 2020, 6:53 pm

Before condemning the STK-461 it might be worth checking headphone jack socket switch contacts for high resistance, and associated PCB connections for dry joints.

Rich

112stereo · 20th Sep 2020, 8:27 am

Sorry for the long delay, we've been away.
I temporarily shorted the headphone jack and is still the same.
Resoldered quite a few suspect joints still the same.
Bit confused about bursting into life on switch off ?

Silicon · 20th Sep 2020, 10:30 pm

Is there a mute facility on the STK chip?

A fault in a mute circuit might give these symptoms.

10th Sep 2020, 4:00 pm	#1
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Akai AM-U1 amplifier I am trying to repair my son in law's amp. It was OK before they went on holiday, but on return its developed a fault. The left channel is very low and distorted, but if you plug in head phones they sound ok? I have done some voltage comparisons between channels and both sides appear the same. Swopped the speaker leads over, just in case, still the same. The only thing I have noticed if you turn the balance control to fully left speaker only, turn up the volume up and switch it off when the capacitors are near fully discharged the left speaker bursts into life just for a second or so. I had this a while ago with a leak amplifier and you kindly helped me out, but that was on both channels and it was a cap in the current limiter at fault. I am thinking it might be the STK chip at fault. Does anyone have any ideas?

10th Sep 2020, 6:53 pm	#2
Marconi_MPT4 Heptode Join Date: Mar 2011 Location: Dorset, UK. Posts: 521	Re: Akai AM-U1 amplifier Before condemning the STK-461 it might be worth checking headphone jack socket switch contacts for high resistance, and associated PCB connections for dry joints. Rich __________________ To an optimist a glass is half full; a pessimist half empty; an engineer twice as big as need be!

20th Sep 2020, 8:27 am	#3
112stereo Pentode Join Date: Jan 2014 Location: St Albans, Hertfordshire, UK. Posts: 215	Re: Akai AM-U1 amplifier Sorry for the long delay, we've been away. I temporarily shorted the headphone jack and is still the same. Resoldered quite a few suspect joints still the same. Bit confused about bursting into life on switch off ?

20th Sep 2020, 10:30 pm	#4
Silicon Nonode Join Date: Jun 2007 Location: Coulsdon, London, UK. Posts: 2,167	Re: Akai AM-U1 amplifier Is there a mute facility on the STK chip? A fault in a mute circuit might give these symptoms.