MArantz PM47 - repair help. - diyAudio
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Old 5th January 2013, 09:55 PM   #1
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Default MArantz PM47 - repair help.

Hello DIYAudio forum members,

I wish to say firstly that I have been reading through some of the threads to see if I could find a solution to my problem and I have been in awe of the collective knowledge out there. I have a basic (very basic) understanding of electronics but the levels of some of the discussions in incredible, I take my hat off to you all.
As you may have deduced I did not find a solution to my problem and so have resorted to start a thread on the subject in the hope some one may have advice.
I have a Marantz PM47, not too expensive admittedly, but I bought it many years ago and it has served me well. Recently though it gave up the ghost and I am of the mind that I should at least see if I can repair it before throwing money at the problem and buying another amp.
The symptoms...
It may have started with one of the outputs going down over a period of time. I only noticed when the sound would suddenly become fuller and the volume would go up a little and I suspected that one of the speakers had not been functioning until that moment. (My speakers are close together so it was not more obvious that one may have been down) I just assumed it had been a loose connection between amp and speaker. Then one day there was no sound at all, a few minutes later it returned. This happened a few more times until one day the sound never returned. If I turn up the volume of the amp fully I can hear the music faintly from the speaker. I have looked inside and there appears to be some leakage from the two things in the power amp section of the circuit board.
My question, is this a simple problem and is it worth fixing?
A very decent person on Fixya tracked down the manual for me.
Thanks for taking the time to read this,
W.
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Old 5th January 2013, 11:40 PM   #2
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The usual suspects are the speaker relay, the volume pot, input and tape monitor switches.
Test the relay first, the one that clicks a few seconds after switch on. Tap its contacts gently to see if it gives poor contact. Be careful not to make any short circuits.
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Old 6th January 2013, 07:54 PM   #3
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Thank you very much for your reply grommeteer, and thanks to the mods for putting the thread up. I had a look for the speaker relay, and whilst educating my self as I go, I found they are a sealed plastic box. So I gave the box a tap, plugged in the two speakers and it seems to work fine with an MP3 player going to the aux input..
Then I connected the MP3 player into the CD input and no sound , ditto with the tape 1 and 2 inputs and the tuner input . I ran it through the phono input and could hear sound but with interference, (has this something to do with the earthing of this input?) The head phones are working fine as well.
So I am now guessing the problem lies in the these 4 input. Is it odd for all 4 to be malfunctioning or do they share a common component the the aux and phono do not?
2 steps forward, 1 step back.
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Old 6th January 2013, 08:09 PM   #4
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Without a schematic, sounds like dirty switch contacts to me. Disassemble and clean with a rubber eraser, then wash with alcohol. If not possible, rotate a lot of times, if that doesn't help, replace. It could be different electrolytic caps on the different inputs, they tend to crack the seal and lose the water after 5-20 years, depending on initial quality. Once I get a problem with audio gear over 15 years old, I replace all the e-caps in the first order after diagnosis. I use ones rated service life over 3000 hours, 10000 if I can get them no sense installing 5 year garbage.
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Old 6th January 2013, 08:35 PM   #5
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Thank you indianajo for you interest, I have to go and research what you suggested as I have a very basic knowledge of what is inside the amp.
I did just notice that while the MP3 player is connected through the aux input and plays fine, it does so when any of the inputs are selected on the front panel. So for example the sound does not change when it is connected via the aux and the CD or tuner or tape 1 or tape2 or phono button is pressed on the front panel. The same goes for the phono input at the back, the sound comes through the speakers regardless of what is selected
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Old 7th January 2013, 08:31 PM   #6
AmpPM57 is offline AmpPM57  United Kingdom
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Take the opportunity to upgrade some of it's components, I have the PM57 and like you "cheap but has served me well for years" I want to upgrade it's components but I don't know what to upgrade for best results, if you do, go for it they're great little amps, I put a post up a while ago about upgrade options for it but no-one replied :-(
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Old 8th January 2013, 08:14 PM   #7
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Hi AmpPM57, I would love to upgrade the components of the PM47, its a great idea and as indianajoe suggested the electrolytic capacitors should be overhauled, but I am a total novice (stone age compared to some of the work on display here) I looked around the internet for a few hours to try and familiarise myself with the possibilities and could not even begin to pick out what would be a suitable replacement. My first goal is to get it running again, grommeteer has suggested following my last symptom post that the issue lies in the input switches, I am currently looking to see if I can get my head around that, which actual part is responsible for this action. By coincidence I have just bought a new record player, and the amp went down before I could hook it up.
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Old 8th January 2013, 08:28 PM   #8
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I had a marantz amp once that would play for a random lenght of time and then give off a "buhwummf" kinda sound and then go silent.

This was before i had any knowlege in amplifiers but after scrapping it i found out if was a voltage regulator circuit for the driver ic that was failing and cutting out after a certain amount of time.

Perhaps your issue is the same thing.
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Old 8th January 2013, 10:47 PM   #9
jaycee is offline jaycee  United Kingdom
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This amp is from the period when Philips owned Marantz.

Having looked at an awful copy of the schematic from www.eserviceinfo.com, this sounds like one of the LC78212 input selector IC's has been damaged. They were vulnerable to ESD damage which can cause the symptom you describe.

I suspect you will not have much luck in getting a replacement part as they are long obsolete.

Using the phono input will have been distorted as this input is for use with a moving magnet cartridge so 1) it has RIAA correction 2) it has a lot of gain which will be overdriven using an MP3 player.
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Old 9th January 2013, 09:07 PM   #10
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Hello Tekko, thank you for your reply, the gradual decline of the amp didn't really follow the course you described, having said that it may indeed be the voltage regulator for the driver ic, I really don't know enough one way or the other.

Hi jaycee, that is not good news. (Never had a product from Philips that was anything near quality). I have the same schematic and was looking at, strangely I thought something similar, no real reason for my conclusion other than it is in the input selection section of the circuit board and looks like where all the inputs converge.
I did find many electrolytic capacitors with the values of the ones in the amp, 6800uf 50v 85degrees, on ebay, many from China and Taiwan, I was surprised that they were relatively inexpensive.
Whilst googling LC78212 I did find this, could it work? DNX500 Lc78212 I/C Denon
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