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
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
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.
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.
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
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
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 !!!
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 !!!
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
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
DC check on output is key. After the protection goes off, DC check at the input of the protection relay. >100 mv you have a failed transistor.
As said, replace the caps, they are cheap. Especially any near the 7317 or on the output. Getting the box of parts shipped from distributor to your home is 20 times the cost of any individual part except the output transistor or main amp module/IC. If somebody has had problems with TO126 driver transistors, BD139/140 are 3 for a $ and available everywhere. Hint the transistors from farnell mouser digikey or RS (of UK, not radio shack of USA) are real, the e-bay and corner parts store transistors probably fake . If replacing transistors don't forget heat sink grease and new heat sink insulators, which could have been the problem in the first place. Heat sink grease is poisonous, wash your hands before touching mouth nose or eyes.
If you are protective about saving your old parts, you can isolate somewhat to which one is causing the problem by hitting the circuit board with a hair dryer. If you limit the flow to one area, the part that causes the failure is where you are pointing the hot air.
For complete newbies, shotgun replacing the entire power output stage with a couple of kit amps based on LM3886 IC involves no debugging. It is probably harder to figure out how to disconnect the power from the old amp boards, than it is to get the new amp board to work. Lots of kits available down in vendor parts bazaar at the bottom of the forums page. 68 W and under for LM3886, 4 ohms speakers and up. If using 4 ohm speakers PS limit seems to be +-28 vdc power supply. For 8 ohm speakers the IC will go +-50 v power supply. Make sure your kit includes a substantial aluminum heat sink, some e-bay suppliers will make this an extra to be lowest cost.
As said, replace the caps, they are cheap. Especially any near the 7317 or on the output. Getting the box of parts shipped from distributor to your home is 20 times the cost of any individual part except the output transistor or main amp module/IC. If somebody has had problems with TO126 driver transistors, BD139/140 are 3 for a $ and available everywhere. Hint the transistors from farnell mouser digikey or RS (of UK, not radio shack of USA) are real, the e-bay and corner parts store transistors probably fake . If replacing transistors don't forget heat sink grease and new heat sink insulators, which could have been the problem in the first place. Heat sink grease is poisonous, wash your hands before touching mouth nose or eyes.
If you are protective about saving your old parts, you can isolate somewhat to which one is causing the problem by hitting the circuit board with a hair dryer. If you limit the flow to one area, the part that causes the failure is where you are pointing the hot air.
For complete newbies, shotgun replacing the entire power output stage with a couple of kit amps based on LM3886 IC involves no debugging. It is probably harder to figure out how to disconnect the power from the old amp boards, than it is to get the new amp board to work. Lots of kits available down in vendor parts bazaar at the bottom of the forums page. 68 W and under for LM3886, 4 ohms speakers and up. If using 4 ohm speakers PS limit seems to be +-28 vdc power supply. For 8 ohm speakers the IC will go +-50 v power supply. Make sure your kit includes a substantial aluminum heat sink, some e-bay suppliers will make this an extra to be lowest cost.
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I'm not familiar with this model but many Marantz amps seem to have very over sensitive protection.
I've had a few that work for hours no issues then click and mute kicks in forcing a re-start after which everythings fine for hours / days / weeks .
They do seem very fussy about impedance load so with a speaker that drops a lot all it takes is a few clicks more on the volume and the increase in stress triggers the protection .
From what i've read even they're new stuff suffers the same issue.
I've had a few that work for hours no issues then click and mute kicks in forcing a re-start after which everythings fine for hours / days / weeks .
They do seem very fussy about impedance load so with a speaker that drops a lot all it takes is a few clicks more on the volume and the increase in stress triggers the protection .
From what i've read even they're new stuff suffers the same issue.
Check relay driver transistor and electrolytic capacitors around it. If any of these go bad, the relay can go on and off intermittently. These kinds of faults certainly don't have to be related to the protection circuit actually tripping due to DC offset. Also, in rare cases, the relay itself can go bad and trip randomly, but this is very uncommon. Relays should always be replaced regularly anyway, as the contacts wear out.
Speaker Protection Circuits: Most have the modes that are
1. DELAY TURN ON, this stops any turn on noise.
2. FAST OFF, this stops any turn off noise.
3. FAILURE, it checks if the DC OFFSET is HI (>100mv) or LOW (10-20mv). This is set by the Designer.
4. OVER CURRENT, overload, short and or two low of speaker Z.
5. OTHER some amplifiers designers may add other features.
From your problems I would look for a bad solder joint or a bad / leaking cap 20-100uf cap in the protection board.
Good luck
Duke
1. DELAY TURN ON, this stops any turn on noise.
2. FAST OFF, this stops any turn off noise.
3. FAILURE, it checks if the DC OFFSET is HI (>100mv) or LOW (10-20mv). This is set by the Designer.
4. OVER CURRENT, overload, short and or two low of speaker Z.
5. OTHER some amplifiers designers may add other features.
From your problems I would look for a bad solder joint or a bad / leaking cap 20-100uf cap in the protection board.
Good luck
Duke
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