ricothetroll

Repairing my Marantz 2270 amp
« on: June 27, 2009, 01:17:11 PM »
Hi everyone !

I have a Marantz 2270 that i got from my granfather. The problem is that maybe i got it too soon, like 10 years ago when i was 16 y o. At that time i had no idea about anything on electronics and plugged my bass guitar directly into the "dubbing in" input. That worked during a while but then some smoke when out from the amp and now the amp doesn't work anymore. I finally decided to open it and try to fix it !

The problem is that i'm not competent enough in electronics to find by myself where the problem comes from. When i switch it on, only the panel's light works, and i do not here the speaker protection relay's familiar click anymore. The headphones output doesn't work neither. I do not see any component that looks burnt.

I have scanned the schematic and the functionnal block diagram :
http://insidecircus.free.fr/Marantz_2270.jpg
http://insidecircus.free.fr/Marantz_2270_Block_Diagram.jpg

Thanx in advance for your eventual help.

Have a nice we !


rodabod

Re: Repairing my Marantz 2270 amp
« Reply #1 on: June 27, 2009, 01:40:58 PM »
Without actually looking at the schematic, have you checked if the (larger) output transistors are scorched?
Quote from: tv
punchy fat bastard chip

ricothetroll

Re: Repairing my Marantz 2270 amp
« Reply #2 on: June 28, 2009, 07:46:06 AM »
i checked them once again, they seem to be ok... i know something actually burnt because there can't be no smoke without fire but i really cannot see what i is... :(

s2udio

Re: Repairing my Marantz 2270 amp
« Reply #3 on: June 28, 2009, 04:10:58 PM »
Have you checked the Mains transformer ?
Disconnect the secondary and check the output V
Shorts on the dc power rails ?
On the end of a Rural Twisted Pair.

chriss

Re: Repairing my Marantz 2270 amp
« Reply #4 on: June 29, 2009, 04:06:36 AM »
You should take a look at the copper-traces of the PCB. Maybe you can see an area that has turned very dark. That could be a hint then.

ricothetroll

Re: Repairing my Marantz 2270 amp
« Reply #5 on: September 10, 2009, 05:42:17 AM »
Hi again !

So, after lots of researches I first though i finally succeeded fixing the unit...

- I tested every transistors of both power amp boards and changed the bad ones with NTE equivalents (2 power transistors were blown and a few others).

- I changed the relay of the speaker protection circuit, that was actually soldered inside for having heated too much. That circuit detects the DC amount on the speaker o/p, if it is low enough the relay opens ("click").

- I also changed every electrolytics on those 3 boards (2x amp + protection).

I worked along with Rob from http://www.irebuildmarantz.com/ that helped me a lot by emails. He's a specialist of those oldies. Take a look at "restoration" on the menu, you'll find a tutorial for restoring your amp by yourself. Or you can give him for complete professional restoration. He is really helpful, competent and nice so the least I can do is a little bit of advertisement ;)

I switched it on and everything worked fine. After a few minutes of use, I turned it off. The day after, when I switched it on again, the relay refused to open ! After a few times trying, I opened the beast again and checked again every transistors (out of the board) and solders. Everything Ok.

When I then powered it on again, same thing as before :at first powering it works ok, then the relay refused to wake up. Crap !!!

So I measured the DC amount on J757 (o/p of power board) and on one side I have about 30V (!!!), and that value seems to be decreasing with time. I think of some DC filter condenser problem but I don't really know which one could be in cause.

here is a close-up on the pwr amp board shematic :
http://insidecircus.free.fr/Marantz_2270_ampboard.gif

J753 and J761 are the alimentation of the board, J757 is the o/p, J751 the i/p, J752 i/p shield.

Has anyone an idea for solving that problem ?

Thanks in advance, and have a nice day !

ricothetroll

Re: Repairing my Marantz 2270 amp
« Reply #6 on: September 19, 2009, 05:34:11 PM »
Hi,

While playing around with the bias Pot I noticed that the DC amount on the spkr output comes back to near 0V when the pot is at its very maximum : all the way to the right, 1° to the left and it's 30V again ! I guess that one is to be replaced...

But whatever, that one has a solution ! :)

What doesn't have a solution for the moment is that the right channel, where I changed most transistors, tends to saturate already at low levels and at low frequencies.

I traced the signal by oscilloscope and the saturation appears already at the output of the differential amplifier and looks like that :



That's with a triangle signal. At every next stages the signal looks the same. Raising the frequency, the saturation disappears progressively (with equal peak-to-peak voltage signals). At 400 Hz there's no oscilloscope-visible saturation anymore.

Those two transistors are originals, I did't replace them, but they are checked OK.

This looks like some polarity problem, but I checked the polarity of all those first transistors by comparing it to the functioning channel : all the same. I checked those transistors : all ok.

I must admit I'm kinda lost...

It's always sad to see a sick old folk... :-[


 

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