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Power consumption, bias, DC offset question.


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Hi all, just looking to tap into the broad knowledge of you all and thanks in advance. 

I have a pair of 30w class A maranz mono blocks, one was running intermittently cool for awhile then eventually it just ran cold. I borrowed a power consumption meter and one amp was drawing 110watts and the cold one only 9. (It still had pretty good sound btw) anyway I took the lid off and contact cleaned the only pot I could see which were the DC offset potentiometer. The amp warmed up straight away but still is only now drawing about 75 to 90 watts.. seems unstable and although much closer and sounds much better not quite up to what it should be and while warm again still less than the other one.

I've set both DC offsets to 0.

But I guess the bias is still out on one? Any suggestions where to go from here?

 

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What model number (MA-5?) need to track down the service manual. Did you measure the bias? Not real happy that tinkering with dc offset trimmer brought the amp back to life, maybe the trimmer is intermittent, generally marantz use good trimmers unlike vintage NAD (garbage...). If the amp is unstable (seen by power consumption) then I'd refrain from using it until you can check the bias. Maybe post a few pix of the boards/components of the problematic unit.

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Posted (edited)

Hi, it's a ma7a, Japanese, runs on 105 volts.. it's basically the same as the European ma24a. I'll attach a schematic.

Not sure how to check the bias?

It's been running ok, without getting too hot but I'll try to pinpoint the problems asap.

 

 

Screenshot_2021-03-09-20-28-34-133_cn.wps.xiaomi.abroad.lite.jpg

Screenshot_2021-03-09-20-44-24-602_cn.wps.xiaomi.abroad.lite.jpg

Screenshot_2021-03-09-20-44-24-602_cn.wps.xiaomi.abroad.lite.jpg

Edited by JoelG
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Yes, sorry I couldn't work out how to send a clear copy. Yes, I can only see a pot for DC adjustment inside as well. I guess being a class A bias isn't usually s problem or something that needs adjusting? 

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I guess bias is set with a resistor then, maybe component drift is the issue.

 

Or maybe not.

 

@mbz would have a better idea.

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Bias adj shown as RN51 in the ma-24 schematic,

image.png.dc44af6af1587221e82f255abd6b79f4.png

 

J705 contains test points for bias voltage measurements, need to locate service manual for voltage spec

image.png.5531566860ad189933ebb1c1b7f08ed1.png

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2 hours ago, mbz said:

Bias adj shown as RN51 in the ma-24 schematic,

image.png.dc44af6af1587221e82f255abd6b79f4.png

 

J705 contains test points for bias voltage measurements, need to locate service manual for voltage spec

image.png.5531566860ad189933ebb1c1b7f08ed1.png

Great, thanks! I'll I'll try to find out the specs and then give it a go.

Much appreciate the info guys!

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On 10/03/2021 at 7:35 AM, mbz said:

Bias adj shown as RN51 in the ma-24 schematic,

image.png.dc44af6af1587221e82f255abd6b79f4.png

 

J705 contains test points for bias voltage measurements, need to locate service manual for voltage spec

image.png.5531566860ad189933ebb1c1b7f08ed1.png

Just thought I'd let you know how it went. Couldn't track down a service manual, not many of these made.  But I took a reading off the "good amp" and adjusted the bias/idle to match then readjusted the DC offset and double checked the power consumption was equal. Both running at the same temperature now and sound much more present and responsive.

Thanks again 👍

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