Marantz pm48 schematic - dc protection kicking in

[pdf64]

The speaker dc protection relay in my Marantz pm48 hifi amp has been a problem for a while, but now won't stay in at all. Anyone got a schematic available / solved this problem? Thanks - Pete.

[bluesjunior]

Hi Pete, I have also had problems finding a schematic for my PM-48. I think that we have the same problem. For several months now my amps sound would cut out and in with an audible click. Then around two weeks ago it cut out completely. I opened it up but could find no visible fault and the fuse was sound so I took it in to a local electronics repair shop and said to him to call me with a price before repairing it. He called two days later and said that he had fixed it and it would cost £68 which was more than I was willing to pay and he said he would unfix it and I could come and collect it.

When I took it home I plugged it in and there was no power at all so I opened it up and found that the fuse was ok but the circuit board with the remote jacks behind the transformer looked different. On closer inspection I saw that the green circuit board with the black and red wire coming from it had been recently worked on as the silicon fixing it to the transformer had been recently applied. The two wires are attached to the outside connections of the green PCB board and go to the black and red connections on the brown board. On the brown board between the black and red wires there is a empty socket for a blue wire as there is also in the middle socket on the green board (see photos below).
I am thinking that he didn't unrepair it but removed the blue wire which would also make the amp unusable. Maybe I am wrong here but as I cannot find a wiring diagram online and came across your post I wonder if you could send me a photo of the corresponding part of your PM-48 in order that I could check my theory out. If not could you tell me if there is a blue wire going from the middle connection of the green board to the middle connection on the brown board. Sorry for the bad photos but my daughter has taken my camera on holiday and I had to use the web cam instead.
Regards, George.

[pdf64]

Oh no, I've just got around to dismantling the hifi pile (PM48 sits at the bottom) and opening the amp up, and the images have been deleted. Any chance of posting them again?

[Alex R]

£68 was not a bad price for fixing your amp. You're not living in the real world.

FYI a protection relay cutting in is a symptom not a fault. The most common problem that might be causing it is shorted power devices, generally with other issues caused by the short. Sorting that out would be a complex problem, take a few hours and not a few parts, all to be paid for, and you refused to pay, so a skilled person put in a few hours for you and you cheated on the deal. Now you want to cheat the guy good and proper by reversing his 'unfix'. I for one hope he unfixed it good! No-one who tries to make a living repairing electronic goods will lift a finger on this forum to help you, of that I am quite sure.

[bluesjunior]

Here you go,
http://i4.photobucket.com/albums/y14...wersupply1.jpg
http://i4.photobucket.com/albums/y14...wersupply2.jpg

[pdf64]

These are cheap but (previously) functional amps, only cost £50, so I can understand bluesjunior's reluctance to pay £68 for a repair that he didn't ask for (rather just a quote).
Check the thread, all I've done is ask for a schematic / known fix.
My investigations so far indicate that the dc protection circuit is dropping out the speaker protection relay, in response to about 50mV dc on the amp output.
Anyone got a schematic or aware of a fix for this?

[pdf64]

Thanks George. Unfortunately the missing blue wire is just to the 120V tap on the mains transformer primary, so I don't think that's the issue, it's not been installed on my amp either.

[pdf64]

It may have been that your amp just had a dry joint in the mains primary circuit - have you checked that there's continuity live to neutral (amp power switch to 'on', not connected to mains, obviously!).

[bluesjunior]

Thanks Pete, it was just an idea and in the meantime I have bought a used Technics SU-X933 for £40 in a local second hand shop which to me sounds better than the Marantz. I will have a check on the continuity tomorrow and see how it goes, if it works I will sell it.

Alex R. I think you are the one not living in the real world and before you get on your high horse I will say that before I left the repair shop I specifically told the owner that £25 was my max and not to repair it if it cost any more as like Pete says you can buy these amps used for around £50. When he told me he had fixed it without consulting me further and that it would cost £68 I was quite rightly annoyed and refused to pay.

[Jazz P Bass]

$37.50 US to repair an old Marantz.
That is incredible.
You will have 1 hour in it until you get it apart & give it a general "health" inspection.
I would not have taken the job.
Period.

[Alex R]

Yep. If the guy took on a hifi amp with the protection relay kicked in for a max of £25 on the repair then more fool him. I never met anyone knowledgeable who would agree an hour's work as a maximum on a job of unknown complexity with the most likely fault being a serious one, and what's more I wouldn't advise you to trust anyone who did that with your kit, as they would be underestimating the very likely complexity of the repair, which is not a good sign in terms of their knowledgeability.

Personally I don't do estimates, as by the time I know what is wrong and what it would take to fix it, I am pretty near to fixing it anyhow, which seems likely to have been the position this unfortunate repairer got into. Generally I note that is the more expensive shops that do estimates. Certainly I'd have to charge more if I did.

Whatever, I find distasteful this attempt to get the guy's work done for nothing by trying to get one of his colleagues on this forum to tell you an easy way to make his fix work again, unless you intended to go along and pay him for his work once the amp was running, in which case I eat my words.

[pdf64]

I thought this was 'shematic requests', not the moral maze?
To call someone a cheat, without any solid evidence - now that is very distasteful.
Unless you guys have anything useful to contribute (eg a schematic!), please demonstrate your distaste by desisting with the thread hijack and finding somewhere else to pontificate.

[Alex R]

Morality is such a bore when it gets in the way of a guy's attempts to get something for nothing at the expense of someone else, I do agree.

[pdf64]

Alex R, your contributions to this thread have been a complete diversion from the matter in hand.
I politely repeat my request that you let it rest unless you have something useful, ie a schematic, to contribute.
Pete.