Marantz PM66 se problems, information wanted

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I have been doing some research on this amp as my own has been having some problems, in that the right side output channel is at a lower volume than the left, which wasn't the case about 2 months ago.

Whenever I put in this amp into google it comes up with this website, and from what I have read I assume that this site is full of people who know far more than I ever will about this subject, so, like a moth to light it drew me in.

I am not totally sure what the problem actually is yet, I have ruled out the CD player (Marantz CD-48), the speakers (Kef Q35) and the cables (QED) and narrowed it down to the amp right hand speaker channel.

I have been told that Leaky electrolytic filter caps will result in audible hum, as there is no hum that rules that out. The thing is, the sound is good on both channels, but the left channel is quite a bit louder than the right, meaning that the balance has to be almost all the way to the right to actually balance it.

Does anyone have any ideas/hints/tips? Where can I get a schematic from so that I can start detailed diagnostics?

If it helps, I took some photos of the insides of the amp, mainly to document, but also incase it showed anything obvious up.

Thanks for your help, I appreciate it.
 
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Electronic faults causing gain problems are very rare, the causes are usually simpler.

First steps would be to use a scope and known signal source which would find the problem area in seconds but I'm guessing you haven't got one :)

So you need a test CD with a test tone of around 100 to 1Khz and a DVM that can read AC voltage reasonably well. It doesn't need be accurate so much as have good resolution at low levels. It should be able to resolve a voltage of say 1.22 volts and not just show 1 or 2 volts.

You would then check (no speakers connected) at the amp outputs first with the test tone playing and confirm that there is a difference between channels. Set the volume to give around 10 volts AC.

Assuming there was a difference then you next check the AC voltage at the input to the volume control to "split" the amp in half and determine where the problem is.

Most faults like this are caused by faulty controls such as balance, volume etc and by cracked (damaged) print.
 
Check the input selector, if it is mechanical problem is there. Probably oxidation and contacts of gold are full of black film over it. Use contact spray to clean the input switch, or if you have knowledge disasemble switch and clean the contacts. 10 years is enough to make that film on gold contacts.
 
Dry Joints Galore!!!

Just repaired a Marantz PM66-se audio amp with no speaker relay operation and discovered -38 volts on L751 output coil (Left Channel). Traced fault back to input of channel and found R719 68ohms O/C emitter resistor for Q701, to find Speaker relay intermittently working and sometimes clicking off... dry joints galore on main board was the cause,also the "source direct button cut out left channel....you guessed it, dry joints on Source relay board!!

:drink:
 
PM66SE KI Sig also humming

Hi all,

I have a Marantz PM66SE KI Sig that also has hum on the R channel. Until recently it spent 10 years in its box in my attic and now want to use it again.

I took the top off just now and noticed that the two large blue Elna capacitors have leaked onto the circuit board. Can anyone tell me if this is likely to be the cause of the hum, and give me an idea if it is repairable please? I don't want to throw good money after bad getting it assessed by the Marantz repair shop if it is already junk.

Many thanks

Mark Fear
 
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Is it leakage or is it glue ? Many manufacturers glue caps down to prevent mechanical stress and movement.

If the caps were the cause I would have expected both channels to hum. Does it hum with just speakers connected ? (no other connection to any other sockets)
 
It definitely looks like leakage, but I guess it could be glue. It looks like leakage to me as it looks like it has 'run' rather than been pasted on, and it is only on one side of the cap.

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An externally hosted image should be here but it was not working when we last tested it.


The right channel hums with just speakers connected, or with just headphones connected. It does it on all sources - I've tried all the inputs apart from Phono. When I first unboxed it after 10 years in the loft the right channel didn't work at all, then after about 10 mins it gradually came up to the same level as the L channel. Apart from the hum on the R channel it's fine otherwise, but the hum is quite annoying, particularly when listening to classical music or speech.
 
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Bit difficult to be 100% sure but I'm going to say that doesn't just look like a leaky cap. It seems to partially cover one of the wire links as if its been flowed from above.

Those caps are common to both channels as they are the main reservoir capacitors. The one channel also not working at all and then gradually recovering also doesn't really tie in with those caps although the fault itself could well be a faulty cap.

This is more like a leaky cap,

leaky electrolytic - Bing Images

Is the hum affected by the volume control setting ?

Is the hum a very pure deep tone or is it a bit 'harsh and buzzy' ?

Both the above checks must be done with NO source components connected. That eliminates any issue of ground loops. You want just the amp plugged in plus speakers/headphones.
 
Thanks for the help Mooly.

Now I understand why these caps are connected together in the PM6010 Service Manual I found (apparently the board layout is the same). And as you say it wouldn't make sense to be affecting one channel only.

The hum is not affected by the volume control, or anything actually. I have checked without any sources connected to any of the inputs and it still does it. It sounds like a low level, but still intrusive, buzzing sound and is completely constant whatever you do. It reminds me of the sort of audio interference you used to hear from old TVs when other electrical items were brought near to them. I have also noticed that you can actually hear the power supply buzzing as well if you put your ear close to it.

One thing I have remembered is that I was once connecting a VCR to the amp (over 10 years ago) and must have shorted something, because a very loud noise came out of the speakers and the audio output of the VCR never worked again after that, and as I recall it was the R channel I was connecting at the time. I thought the amp had been unaffected as I don't recall hearing this hum before but perhaps it did do some sort of damage after all. Is there anything I can look at to test that?

As you can tell, I'm no electronics engineer, but I did do a fair bit of electronics theory as part of my degree 20 years ago. Just not familiar with how to turn that into practice fault finding real devices. I'd like to use this as an opportunity to learn, but not that brave yet (and very aware that there is exposed mains AC under the hood).
 
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Mechanical noise from the transformer is something that almost certainly isn't related to the signal buzz. Trannys buzz for a variety reasons, the actual purity of the mains (harmonics present) being a big factor. You might find it better at certain times of the day.

The audio buzz... difficult to say really. I've looked at the circuit and the power amplifier stages are all fed from the same shared supply. Buzzing with the volume turned down rules out the low voltage supplies to the preamp stages.

Without a scope to see what's going on I think all you can do is try and isolate where the problem. It might be worth shorting the power amplifier input to ground and removing the feed into that channel from the volume control.

There are two resistors here. You could short out R704 (47k) and remove R702 (100ohm). That would both ground and isolate the feed into the right channel. Without test equipment I can't think of any easy ways to fix this.

Going back to the caps... if you suspect leakage (I suspect glue) then both those possibilities could cause problems because leakage is conductive and unfortunately so to the glue as it ages. That could conceivably couple ripple from the power supply into the amplifier stages. The only to be sure would be to remove the caps and clean it all up. Remember the link I said had 'glue' on it ? If it were conductive and if that link (as an example) were part of the signal stages then that could well cause something like this.

I'd perhaps go down this route before isolating and shorting things out.
 

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Reviving most recent thread on PM66.

I've got an PM66SE which had one channel louder than the other, I also found that moving the DIRECT cwitch made one channel disappear totally and then reappear if I moved it a little.
I tried the following:
- checked all the PCB and found many cold solder joints and resoldered the all the PCBs
- checked all the switches - INPUT/DIRECT/TAPE and after desoldering opened and gave them hot bath with aluminium foil and soda. that cleaned all the oxidation and made all the switches work correctly. No moving of swithc made any change to sound. But levels still were different in middle position of balance pot or with DIRECT switch engaged It was the same on all inputs (except phono of course).
- I took the iphone with signal generator and set it to 1000Hz, max volume and set it to AUX first and started tracing the AC voltage difference from input board to front botom board (the one with balance switch and headphone jack connector) and found that from input PCB to front PCB and then to Volume pot the Voltage were equal around 0,28V. If I made volume level to the maximum - the input voltage of both chanells after volume pot was also equal, but if I set the volume pot to 75% - I fit a difference between channels - 0,16V - 1,14V or something similar. and difference was present fron 100% to 0% of volume.

Then I measured the output AC on speaker terminals - it also had similar difference but "amplified" to 22V - 20,5V. On maximum volume the difference was not present.

So the questions are
- Is my idea about difference in Volume pot resitance making the channels sound different - correct?
- should I replace the Volume Pot with a new one or it's possible to repair the original?
- which one to get?
- Is the motorised Alps Blue the only adequate choice?

Main problem here is that I got this amp as a present. And spending around $60 for a motorised blue alps seems a little bit too much. Maybe ther is a way to resurrect the existing pot? maybe with parts from other pot or some cleaning?

Or maybe I'm wrong and I should look in other area on the amp and check somwhere else?
 
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The first checks if you can measure AC voltage accurately are to measure the AC voltage applied to the 'top' of the volume control. That should be near identical for both channels. Then measure the AC voltage on the wiper of the control. Both channels should track equally... give or take.

You can also determine if the problem is before or after the volume pot by linking the control 'top' terminals. That forces an equal signal to be applied to both channels.
 
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