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Old 23rd January 2013, 11:58 AM   #1
pdf64 is offline pdf64  United Kingdom
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Default Marantz PM-48 Amp; Protection Acting Up- Schematic?

My Marantz PM-48 hifi amp has a big problem with the speaker protection kicking in and dropping the speaker relay out.
Has anyone encountered / fixed this?
Has anyone got a schematic?
When it works it does everything that I want a hifi amp to do (or at least am happy to pay for), so it would be nice to get it running again.
It's got so bad that the amp's unusable, however I rigged up a pre-amp line out from it and use the pre-amp with my old Rogers tube power amp, so it's not all bad.
Thanks - Pete
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Old 23rd January 2013, 12:15 PM   #2
Mooly is offline Mooly  United Kingdom
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If it works some of time (even for a second or two) then its not a catastrophic or destructive failure.

I would be looking for something as simple as poor soldering particularly on the driver and output transistors. Next suspects could be intermitent failure of the driver transistors... a weird problem but quite common and particularly affecting Japanese TO126 type devices.
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Old 23rd January 2013, 12:18 PM   #3
jaycee is offline jaycee  United Kingdom
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is it actually clicking off, or do you just lose sound?
Can't find a schematic unfortunately but I'd imagine its pretty similar to the usual AN7062 + transistors + TA7317P protection circuit they used around the time this amp was made.
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Old 23rd January 2013, 01:01 PM   #4
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The relay clicks in and out.
I managed to get it under control a few years ago by modding the signal back to the protection chip, halving the signal level, but things must have got worse.
I measured the dc on the speakers at the time and it was under 100mV.
It used to just be bad in Summer, so I gave it extra ventilation spacer feet, vacuumed out the grilles / heatsinks.
It's highly integrated, just a chip each channel for the pre-amp; I'll check the chip markings and get back with the info.
Pete
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Old 23rd January 2013, 01:22 PM   #5
Mooly is offline Mooly  United Kingdom
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100mv is getting on the high side and might need investigating. Once the relay trips you must measure internally in the amp to check the offset, before the relay
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Old 23rd January 2013, 07:09 PM   #6
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There was a case where a bigger model behaved like that:

Put the music to play in a low power everything is just fine go a few clicks up then protection kicks in .

Some of the MArantz of that era feature a couple of small capacitors located on the binding post directly where the speakers are connected .

It is the second time that come to my attention that these capacitors fail and actually short and trigger the protection but only at signal conditions .

Measure them at no signal conditions and everything is fine ...apply some signal and the short is there .

Look at it also

Protection can be activated by DC in the output ( any of them and still 100mv i think is low to activate it ) then signal overload , may be thermals but most of all short circuit !!!
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Old 23rd January 2013, 07:47 PM   #7
jaycee is offline jaycee  United Kingdom
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i also think theres a capacitor in TA7317 type circuits which, when it goes down due to age, causes intermittent protection problems. I cant remember which one, though.
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Old 23rd January 2013, 07:51 PM   #8
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right there is 3 of them next to the 7317 you may also replace them as a precaution ..Still Both amp and caps are actually quite young to fail ,,,better safe than sorry !!!
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Old 24th January 2013, 09:41 AM   #9
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The problem doesn't seem to be signal related; it will sit there clicking in/out sporadically with no signal, after the cd finished etc.
It first manifested over 5 years ago by taking a prolonged time to bring the relay in after switching it on; so I left it on permanently.
Then it would click out/in during warm weather as above, both with signal and without, so I improved the ventilation around it. Then it got worse so I modded it to reduce the protection sensitivity. Now it's so bad I just use the pre-amp.
Hope that helps; I'll get inside it at the weekend to confirm the ICs, dc offset etc.
Pete
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