Just bought five Marantz MA500 amps, did I just get screwed? - diyAudio
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Old 16th February 2011, 06:16 AM   #1
bhays is offline bhays  United States
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Default Just bought five Marantz MA500 amps, did I just get screwed?

I purchased five Marantz MA500 monoblock amps on Craigslist tonight. I left them in the car while we ate, etc. It's about 40 degrees here and they were cold to the touch, but not below 40 degrees obviously.

When I got home and plugged them all in to ac power; two came on, then relay clicked a second or two as one would expect. One began clicking about twice per second (relay) and the other two made no sound at all (as far as relay clicking on, no signal connected yet). After about five minutes one of the silent ones started buzzing the relay so I turned it off.

I went ahead and installed them and two are working fine from the beginning. The one that was clicking twice per second or so started working normally. Of the other two, one just never seems to switch on at all. Power led lights but relay never clicks and no sound output. The other one starts buzzing the relay after about twenty seconds on and no output. The seller, of course, represented all five as working perfectly.

Could this be a temperature thing and these other two may start working normally once they reach room temperature or do you think I have just been burned?

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Old 16th February 2011, 06:47 AM   #2
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40* should not hurt those amps.
The most likely cause of problems (especially the one that came good) is that transportation has cracked/moved poor and marginal solder joints.
The cure is close inspection of every solder joint (including the good amplifiers) and resolder all the crook/marginal ones.

Eric.
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Old 16th February 2011, 06:54 AM   #3
bhays is offline bhays  United States
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Well, now the one that was buzzing has begun working normally. I let it buzz for a minute then it changed to a clicking and now stays on. I turned it off then on about ten times and it seems to be working normally. So I have gone from two of five working
to four. The one that has never clicked still is not working, but I have never left it on more than a minute or two. Tonight when I have time I will leave it on and see if anything ever happens. Do the relays sometimes 'stick' from extended storage?


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Old 16th February 2011, 07:39 AM   #4
jerryo is offline jerryo  Isle of Man
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Could be condensation? Hair dryer through vents?
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Old 16th February 2011, 08:11 AM   #5
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I have used boggins of these in installs and found them extremely reliable.
Give them a chance to normalise temp (overnight, inside) and try again. If one doesn't go, don't panic. They are not that hard to fix and minor transport damage is probably to blame - see message above.

Or, you could just have been screwed....
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Old 16th February 2011, 12:49 PM   #6
bhays is offline bhays  United States
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While I am waiting to rule out condensation; just out of curiosity, what sort of problem would cause relay buzzing or failure for a turn on relay to engage of the sort I am seeing with these?


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Old 16th February 2011, 01:14 PM   #7
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The relay(s) close and connect the speakers when DC conditions are correct - ie the output point of the output amplifier is at 0V +/- 100mV or so.
If a final output transistor has failed the output point of the amplifier will be at rail voltage .... 60V or so.
If the output point voltage is more than +/-100mV and less than rail voltage the cause could be failed transistor or capacitor in an earlier stage but more likely will be duff solder joint(s).
The sevice manual is here - MARANTZ MA500 Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics
Can you solder well ???.

Eric.
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Old 16th February 2011, 01:47 PM   #8
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It is quite possible they need capacitor renewal. These use a TA7317 IC for protection, and chattering relays is usually a sign of marginal capacitors around this circuit.

Additionally as stated by others here, there might be cracked solder joints that can be easily reflowed with a soldering iron. I wouldn't say you've been stiffed - these are nice little amps that should be easily and fairly cheaply brought back to full working order.
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Old 16th February 2011, 02:18 PM   #9
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Yes good point, the actual protection stage can suffer faults also.
I was fishing for a couple of measurement before I added this fault possibility.
Between measurements and us guys we can fix these amps at a distance.

Eric.

You have a fine set of amplifiers....what did you pay ???.
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Old 16th February 2011, 04:00 PM   #10
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Default Electrolytic capacitors

The capacitors Jaycee was referring to are the electrolytic capacitors. These have a water electrolyte that is sealed in by rubber, and great ones have a life of 25 years. Cheezy ones can go bad in three months. Cold accentuates the lack of water problem; sometimes at end of life electrolytics will start working after the device warms up.
These are aluminum cans with a plastic wrap, and a plus on one end or a minus near the other lead.
When I buy a new old bargain that I intend to keep, I replace the electrolytics 100% before I try to start debugging with a scope and schematic.
Check all metal with a meter under 25 V to frame before you touch it. Wear safety glasses soldering, it can splash. I remove old solder with stripped wire dipped on Oatey #5 flux paste from the hardware store. Wash off with a damp paper towel, this flux leaves an acid behind. I like tin/lead solder going back, and a WP25 or 35 iron with a chisel tip, not the pointy one that comes free with it. If you buy capacitors rated 2000 hours up, they might last 5 years or more. 105 degree C rated 2000 hour + capacitors are even better, especially stored in a hot car. Newark.com (farnell.com outside US) has the hours life in the selector tables, so you don't have to look up a lot of datasheets, or buy some expensive brand because you know it is good.
Anybody have a link to an app note on the TA7317? I downloaded the datasheet from datasheetcatalog.com, it is naked. I downloaded the MA500 manual, my computer can't display the .rar format. Thanks.
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Last edited by indianajo; 16th February 2011 at 04:21 PM.
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