Sugden a21a power amp repair

[adamus]

I have a sugden power amp. It is loosing its left channel intermittantly once warmed up. For the first 2 mins its fine, then it starts loosing and regaining the left channel.

Anyone got any ideas of where to start? I checked the input wiring and joints - its fine.

 

[Jan Dupont]

According to your description it's most likely a bad solder joint, as it happens when the amp warms up............

 

[adamus]

that was my thought. Time for some checking.

 

[Mooly]

What vintage is it out of interest.

 

[adamus]

i am not sure. ITs a black poweramp, with a horrible yellow 'neon' switch. a21a. Looks latte 80's. I checked the caps, they all seem fine (nothing dry etc). time to looks at joints. the sound distorts pre cutting out. could this potentially be bad output trannies, i know sugden bias the hell out of them.

 

[poynton]

i am not sure. ITs a black poweramp, with a horrible yellow 'neon' switch. a21a. Looks latte 80's. I checked the caps, they all seem fine (nothing dry etc). time to looks at joints. the sound distorts pre cutting out. could this potentially be bad output trannies, i know sugden bias the hell out of them.

Probably right at the front end.
If it was in the power section, something would have blown !!

Sounds like one of the supply rails in dropping out.

Andy

 

[Mooly]

Do you have a meter etc. You are going to have to take some measurements, possible comparing left/right. The output midpoint is as good a place as any to measure first as any power amp problem will probably show up here first.
Does it seem heat related, going duff after say 30 mins from cold.
Is the speaker output AC coupled on this one --- single rail supply.
When it goes faulty is it "tapable" if you poke the PCB with it on.
When it goes faulty does it stay that way until you switch off.

 

[adamus]

its must be heat related. Its fine for 2-3 mins, then left channel goes slightly fuzzy / crackles, then drops, then returns intermitantly.

Nothing looks burned. No bulging caps. Anyone know where i can get a schematic?

 

[adamus]

Think i have fixed it!!!

Bad solder joint on the left out channel on the relay board. Reflowed adn fingers crossed i have enjoyed 10 minutes of music.

Coud be fluke though, will have to leave it running for a bit.

 

[adamus]

30 mins in and it is still playing sweet music.

Think i have sorted it. Thank the lord for the 'tap test'.

 

[Mooly]

Amazing what a good whack will fix.

 

[nigelwright7557]

Amazing what a good whack will fix.

I had a similar problem on a new build.

The amp would power up but the output would then very slowly go to B- ! The amp sounded fine with music through it to start with.
Turned out to be a dry solder joint on an input resistor that tied the input to ground.

 

[Extreme_Boky]

A21a single ended class A

Had this one sitting for a while waiting its turn... one channel was dropping out. Turned out to be a faulty bridge rectifier B80 C3200/2200; replaced it with B500 C7000/4000. Tricky / intermittent. The amp appears to be soldered & put together in a back-shed. Unbelievable...

Anyway, if there's a drop-out -> check bridge rectifiers as well.

Nick