Author Topic: Marantz 1072 speaker output voltage spike when switching on  (Read 742 times)

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Offline frozenfrogz

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Marantz 1072 speaker output voltage spike when switching on
« on: March 15, 2018, 11:26:34 am »
Dear all,

I fixed an old Marantz 1072 hifi amplifier that was not working and blowing fuses.
I could work out almost all of the clunky bits someone put in it in an effort to "fix" it. That dude somehow thought it was a good idea to connect the -39V and +39V rails together with a thick jumper cable :palm:

The amp is now working, I get amplified audio on both channels and everything is fine, except when starting up the amp.
There is a nasty voltage spike and DC offset on the speaker outputs when switching on the amp. It settles after a couple of seconds, so switching of the outputs, power cycling the amp and then switching on the outputs after some time does not get the speakers killed :)

However, I want to get rid of that issue.

Attached is part of the schematic and a scope capture of what I get on the speaker output.
Edit: Nothing connected to the input of course!

Also, here is a link to a scan of the service manual.

If one of the power caps (C808, C809) was leaking, could this result in the observed behavior or should I be looking some place else?

Thanks and kind regards,
Frederik
« Last Edit: March 15, 2018, 11:39:16 am by frozenfrogz »
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Offline oldway

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Re: Marantz 1072 speaker output voltage spike when switching on
« Reply #1 on: March 15, 2018, 01:44:05 pm »
Generally most dc-coupled amplifiers have a relay that protects the speakers against a dc component that could damage them

The relay will only switch on if both amplifiers have an output voltage of less than 500 mV.

It's strange that this amplifier does not have such a relay.

During power on and power off, imbalance and asymmetry may occur if the capacitors have lost their capacitance.

This causes the presence of a DC component that prevents or delays the switch on of the protection relay.

But here there is no relay .... !!!!!!

I advise you to replace all the electrolytic capacitors of power supplies (main power supply and auxiliary power supply) as well as all the electrolytic capacitors of the power amplifiers.
 

Offline frozenfrogz

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Re: Marantz 1072 speaker output voltage spike when switching on
« Reply #2 on: March 15, 2018, 02:04:46 pm »
Of course it was the power caps. It’s always the power caps xD

Here is the transient I get after replacing the old Nichicons with new Rubycon 6800uF, 63V. That, I can live with.

Bzw. No, this amp does not have relays.
« Last Edit: March 15, 2018, 02:06:50 pm by frozenfrogz »
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Online coromonadalix

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Re: Marantz 1072 speaker output voltage spike when switching on
« Reply #3 on: March 17, 2018, 01:32:41 pm »
yeah  its a good spike,   i would check all psu caps, as oldway says :   the supply rails when starting are'nt equal and the amp will output dc component,  personally i would add a time delayed relay in the speaker outputs.


If everything in the amp is okay, the biais adjustments are okay etc ... i would change all electrolytics in your amp section c704 708 720 722  etc...    and the other channel too, wont cost a fortune.

In the past i had an biais transistor who was going nuts after a while, the amp started fine but after an hour, dc component began to appear ???
 


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