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Posted

Hi all, I've inherited a beautiful old Akai AM-U04 amp, early 80s and made in Japan. 

 

It's exhibiting some unusual behavior so thought I'd check if anyone has had a similar experience on older amps.


When I first turn it on for the day, there is a very long delay (3 minutes!) before a relay clicks and any sound can be heard. After that initial wait, if I turn it off then on again it only takes a few seconds. From what I can tell this is the on delay circuit, but seems to be struggling when first turned on. 

 

I thought it may be a sticky relay but once it's on if I toggle the speakers on and off the relay clicks without any issues at all so I assume it's a more likely an issue with the control circuit for the relay than the relay itself. 

Any pointers for what could be the cause much appreciated! Once it's on its an incredible amp, no strange noises, crackles or anything. Sound quality is shockingly good for an old amp and it drives my System Audio towers with ease. 

 

Posted

To me, it sounds like an electrolytic capacitor that has gone high ESR. They will often start to get lower ESR once warmed up.

 

First up I would check the ESR of the electros around IC5 (relay control/drive). But you should also probably check any other electrolytic caps in the audio path. I have marked some on the schematic, if I can upload it. You could do a quick check by removing the cover of the amp, and warming up the area closely around IC5, with a hair dryer, or something similar, before switching it on, when it is cold.

 

AM-U04-schem.png

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Posted

So after a lot of troubleshooting, I solved this one today. As suggested by Bob above, the issue went away if I warmed up the main PCB with a hairdryer. If the amp cooled down enough, it would then trigger the relays off again and kill output to speakers.

 

Taking this into account, I got a can of freeze spray from Jaycar and selectively chilled suspected components until the amp until I triggered the issue. Turns out it wasn't the capacitors right near IC5 (which controls the relays), it was further upstream at C50. When I hit that capacitor with freeze spray, the relays would click off, then when I warmed it up they would click back on. I replaced the cap and amp is working perfectly now! She's a beauty!

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Posted
23 hours ago, pluralses said:

So after a lot of troubleshooting, I solved this one today. As suggested by Bob above, the issue went away if I warmed up the main PCB with a hairdryer. If the amp cooled down enough, it would then trigger the relays off again and kill output to speakers.

 

Taking this into account, I got a can of freeze spray from Jaycar and selectively chilled suspected components until the amp until I triggered the issue. Turns out it wasn't the capacitors right near IC5 (which controls the relays), it was further upstream at C50. When I hit that capacitor with freeze spray, the relays would click off, then when I warmed it up they would click back on. I replaced the cap and amp is working perfectly now! She's a beauty!

That makes sense. C50 is a filter capacitor on the supply to pin 1 of IC5 (PWR ON/OFF & Overcurrent Detection) I dare say you'll have more electrolytic caps drying up and going high ESR. Might be worthwhile getting yourself an ESR meter, if you intend to do more of your own work on vintage gear.

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