Repairing an old amp

[GezH]

Hi all.

I have a lovely old amp, an Akai am-2250. It's one of those old huge shiney amps with large knobs and lots of switches from the late 70s, I think it looks great. But recently it has stopped outputting sound in one channel. I've checked both sets of speaker outputs, plus the headphone socket, and the problem persists throughout.

I'd love to get it working again, but I suppose the problem is I can get one cheaper off ebay that it would be to get it repaired. I like tinkering with things and can do the basics but am no expert. I've taken it apart and cleaned the switches which has got rid of the static sound but hasn't solved the main problem.

Is there anything obvious I can check to find out what the problem is, or from your experiences what would be the likely cause when the sound stops in one channel?

Thanks for any advice.

 

[eggontoast]

I will take a stab in the dark and guess that this amp probably uses an STK pack for the power stage, it has probably failed. If you look inside it will be the large black thing on the heatsink, there will be a number printed on it STK??? You should be able to find a replacement off of flea bay, but get someone to diagnose before splashing the cash as the STK pack will probably cost you £20 ish.

EDIT : I have just checked and it does indeed use an STK463 (double check that number with your unit though), 99% sure that will be your problem. South West London Electronics has them on eBay for £14, you just need to desolder and remove the old one, clean the heat compound off of the heatsink. Apply new heatsink compound to the new device, refit and solder.

 

[Anonymous]

Just a guess, but if the Akai has a speaker relay (e.g. if after powering it on it waits, 'clicks' and then starts playing) that might be the problem. I've experienced that with vintage Pioneer and Sony amps. Cleaning the relay and making sure it connects okay worked twice for me.

Hope this helps,

Jack

 

[GezH]

Thanks both for the help

I'd already had some vague advice about the STK unit, and yours seems to second this, so I've ordered the unit as suggested, it's worth a shot. I'm not too sure about the soldering though, it looks like I'll have to take the whole thing to bits. It will be fun though I suppose!

As for the relay unit... I have no idea what this is or where to find it, so if there's any way you can point me in the right direction, I'd appreciate that.

Cheers.

 

[Anonymous]

Okay, I'll give it a try, but as English is not my mother tongue I hope others feel free to chip in.

The relay basically consists of two small contacts for each audio channel. When the amp is powered off, they don't touch, so there is no signal going to the speakers. When you switch on the amp, the relay 'claps' together after a short while so the signal path is complete. This short delay protects your speakers from the sudden flow of current.

The contacts in the relay get dirty or oxidize after a while, which means the sound on one or both channels can become muted, intermittent or disappear alltogether. Cleaning and adjusting them might fix this problem. As I said it did for me on two occasions.

The relay is usually located in a small plastic 'container' on the PCB. Google 'speaker relay' to get an idea.

Hope this helps,

Jack

 

[eggontoast]

I haven't got a schematic to hand but looking at some internal pictures on Google I don't think it has relay protection.

 

[eggontoast]

How did you get on with this is it back up and running ?