Almost there with the Akai AS-980 I thought, but.........

[m1keC]

All checked out on headphones, great sound, all 4 channels working fine. I changed all the bulbs for leds, that's 9 fuse lamps, 12 indicator bulbs and even the 2 in 2/4 channel display (that was really fiddly). By the way, the fuse lamps were working off 10v so I used standard 12v ones that are sold for car courtesy lights and they work perfectly - just the right brightness. Now I was ready to check it with speakers so first I checked the voltage on the speaker terminals - on stereo it was a perfect .02v on L and R front but showed .75v on the both rear channels. When I switched to 4 channel... suddenly it became .75v, on both front L & R and also rear L & R. I'm guessing there is something wrong with the amplifier for the rear channels. Does that sound reasonable? I checked the power transistors but they don't show any shorts...... Any suggestions welcomed. Photo attached.

 

[Soundfield]

Do you have the circuit diagram? Difficult to fault find without. I would have thought that the headphone output is simply tapped of from the speaker output which, if you say is working, implies there is nothing seriously wrong with the amps – but under most circumstances I would be very worried by any speaker DC offset (assuming it was DC you were measuring) when the selected inputs are shorted. Does the offset vary with volume setting?

 

[m1keC]

I'm attaching a circuit diagram of the amplifier board (there are 2 of them, 1 front L and FR, the other back L and BR). Also a schematic from which I have worked out something. Switch 12 is the selection between 4 channels and stereo boost. In the latter the inverted signals are fed to the -ve speaker terminals to give increadsd power. In 4-channel mode the 4 channels go to the 4 front and rear speakers. In my tests, in stereo boost mode I measured no dc offset on FL and FR and I now thing that is because the -ve terminals were also fed with the same dc offset as the _+ve. The dc offset must be coming from both amplifier boards - I wondered if R20 had become open but it measured ok if a bit lower than the spec. All the 8 power transistors test ok as well. Hope you get some ideas!

 

[Soundfield]

DC offset in a bridged amplifier is usually caused by either mismatched transistors, incorrect biasing or leaky decoupling or series DC blocking capacitors. You’re probably stuffed if it’s the transistors but the biasing may well have drifted over the decades due to ageing components although I’m not sure if there’s sufficient info in the service manual to allow you to adjust it. If you’re really lucky it’ll just be a leaky capacitor or two. I wouldn’t want to connect any speakers to it until you’ve got the offset down to 150mV at the most.

 

[m1keC]

I have to say that I was baffled that all 4 channels were showing the same problem but when I changed the final coupling capacitors and checked the old ones I discovered they had all lost half of their rated capacitance. This seemed to resolve the problem and all is now working great. The AS-980 is now for sale on Ebay!