Marantz PM-4

[Guest]

Here she is with her top off ready for work.

Next up off came the sides, which have hundreds of screws. Lots of old TIM that got removed, almost missed the plastic spacers. Which could have been fun!

Close up of one of the chips

Naked!

Back panel coming off, took this to remember the position of the wires.

Transformer out

Pics of the wire wrapping so I could remember it location. Look at all that dust!

More of the above

And again

Her undercrackers

Part way through shot with most of the new bits on and some of the board cleaned.

Look at how shiny that pcb is now Lovely attention to detail on it with really good screen printing; component drawing, component number and orientation for each part.

I did take a video of turn on, just in case it blew up in a ball of flame. Since it didn't the video is pretty shite.

 

[murray johnson]

good work Dom, it looks quite an involved operation!

 

[russ abbott - royds broth]

just took mine apart...looks similar re the dust. ;-)

 

[Guest]

Feck!

Only one side gets warm and the second relay doesn't click on after power up.

Oh well. Will open her up tomorrow night and re-check all connections and polarities.

 

[Guest]

murray johnson wrote:

good work Dom, it looks quite an involved operation!

It is!

Right pain to get at stuff without taking everything apart.

 

[Guest]

Well so far polarity is correct on everything I have replaced, off to check the wires I de-soldered next.

Edit:Wires are fine and in the right places.

Made sure nothing was touching anything else and that I hadn't flowed solder over pcb traces.

Couldn't see anything wrong. So looks like I will have to get the multimeter out and check every component!

Bollocks.

 

[Guest]

Hmmm just noticed Q715 on the schematic has got a mark on the side of it and the body (not the leg) of a resistor was touching it (R757). It is an NPN tranny (2SC2823 toshiba).

Could this be causing it? Q715 is on the left side of the main amp circuit which is the side the doesn't get warm.

Actually no, red herring. Looked at the pics before I touched anything and it was already like this.

 

[Guest]

Will retouch em all then.

:upgrade:

 

[Guest]

Still re-touching. Stopped part way through and fired her up again.

One thing I have noticed is that the side the warms up also humms. Not audibly but it does give the heat sink a bit a a vibration. Any clues on what this could be?

Will try and finish re-touching the joints on Friday as I have the day off

 

[reddish75]

just a thought but have you set the dc offset and correctly?, measure the dc voltage on the amp side of the relay thats not firing, if the voltage is only minimal i.e millivolts the offset trimmermay needsetting properly or replacing.

 

[Guest]

Found the two trimmers for offset, but not sure which relay to take the reading from.

Ho hum.

Think I'm over my head with this one now. Need to get someone with more knowledge to have a butchers at it.

Anyone fancy it? I will of course pay.

 

[reddish75]

does the schematic that you have give any indication on what the idle current and dc offset should be? and which pot controls which? If you could get hold of a service manual i could be interested in getting this running for you.

regards

chris

 

[Guest]

No service manual around.

The board clearly marks which trimmers are for offset and which are for bias. But no indication of idle current/offset.

Could possibly get this off another PM-4 user though as a there are a couple on this forum.

 

[reddish75]

From the copy of the schematic it looks like the idle voltage is marked at the test points, just need to find out if anything needs shorting when its adjusted so maybe asking the other members is a good idea. Also i'm up in manchester so you would have to send this to me, if you would like me to have alook at it.

chris

 

[Guest]

Was really looking for someone a bit more local. But will be in touch if I don't get sorted.

:^

 

[reddish75]

ok no worries, a couple of things to try would be to look at the schematic and see if the voltages comming off the power supply tally up with what you have, make sure that its in class b mode first. check r793, 794, 795, 796 to see if these measure ok.

also check the voltage on the coil, it should only measure low millivolts on both sides, it this measures in the voltage range there's a good chance that your output transistors and more are toast.

all of the voltage measurements are in reference to ground.

also building one of these http://blog.earthshod.co.uk/?page_id=120is a must when working on an amp, i'd recomend building one before you switch the amp back on.

regards

chris

 

[Guest]

j: j: j: j: j: j: j: j: j: j: j:

Amp is fixed

j:

Just got the call. Looks like a trace had lifted. All sorted now



Top bloke at http://www.jsaudiorepairs.co.uk highly recommend him. Plus he only lives few miles down the road from me so easy peasy dropping it off and picking it up.

Picking it up tomorrow, can't wait to hear how it sounds with the new capacitors :upgrade:

 

[i_should_coco]

Good result.

You got someone lined up to fix the Tannoys when you feck those up as well?

 

[SCIDB]

Hi Dom,

Glad you got your PM4 up and running, well done! As a member of the PM4 owners club, it's good to see these amps in action.

SCIDB

 

[russ abbott - royds broth]

:^ :^ :^