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Marantz 240 question

ocdecio

ocdecio

The proof of the restoring is in the listening.
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Just got a 240. It was sold as "working", and from the pictures I couldn't see anything unusual.

Is there anything you would recommend doing before actually plugging it directly to the wall and trying it out? I connected it to a dim-bulb tester, and the light dims but not totally, which I think it should be OK for a 100W lamp to a 240W amp. The relay clicks in about 2 seconds.

DC Offset: Right channel 76mv, Left channel 24mV

Here's some photos:

https://audiokarma.org/forums/index.php?media/albums/marantz-240.1652/

I am somewhat surprised by how original everything looks and at the same time I feel like this was worked on at some point. The outputs have fresh looking compound (not like factory) but the semis are original types.

OK after checking DC levels, even though 76mv may be on the high side I connected it. It sounds good, very crisp with good bass but I expected it to deliver more to the same speakers I use with my 1120 - initially I was afraid it would blow them up lol. The heatsinks get warm to the touch without any load after 10 minutes or so - I wonder if this could be the bias that wandered off?
 
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It looks all original. This amp runs a bit hot. You should adjust the DC offset and also check and set the bias at 12mV.
 
Thank you, @sanjib ! I have it connected to a 1120 preamp out, and boy this thing sounds amazing. It makes me think how much of a restoration I really want/need to do. I see you have a nice one on your avatar - would you have any "must do" suggestions (other than of course the adjustments above) ?
 
@sanjib - I should be getting a 33 soon if everything works out. It looked original to me as well so my plan is to get them both as restored as advisable. I am leery of swapping the silicon on it unless they are in bad shape.
 
I will suggest changing those electrolytic caps, not any transistors, and while in there I also changed the wire emitter resistors. There is no silkscreen printing on board so you have to be careful regarding the polarity of caps when you put new ones. In my 240 some of the resistors on the relay board were really in bad shape and I replaced those.
 
@sanjib awesome, I will do that. After all there aren't too many electrolytics in this thing and the others like the IMB look military grade so I won't mess with any of them (or the tantalums). I had to replace transistors in my 1120 because they were noisy and triggering the protection but I did it with a sense of regret I must say.
 
@sanjib one question if I may... would you recommend deoxing the trimpots before adjusting bias/dc offset? I heard some of these may "jump" to one extreme or the other due to age and sitting for so long. Or just an end to end movement should do?
 
I had bias at 60mv in the R channel, 72 in the left channel - no wonder this thing was hot in idle. Now both hovering around 12mv. DC offset I managed to get it to around 10mv each.
 
Where you are connecting your DMM during bias set up? If you connect across one resistor R531 or R532 it should be set to 12mV and if you are connecting across R531 and R532 it should be set to 24mV. This amp needs to be idle for quite a long time before setting up the bias.
 
@sanjib I was following this procedure:

(I am totally assuming that the 240 is a 250 without meters, please correct me if I am wrong!)

Punker X said: ↑
Some of the later manuals and additions to the 250 manual give a simplifed method of setting the bias. Take the covers off the output transistors and measure between the voltage between the two inside transistors case. Adjust for 10 to 12mV's. Be careful with the probes not to short any thing.

That seems to match the M-250-4 Service Bulletin:
 

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You are absolutely right, I was rechecking my pictures and found I set 12mV across the two resistors not one!
 
Cool, thank you for checking! One can never be too careful with these things! Also the bulleting said to begin cold and recheck after 1/2 hour. And DC offset I assume can be done with the unit warmed up.
 
What a difference it makes! It runs much cooler now.
 
sanjib said:
I have refreshed one a few months ago (https://audiokarma.org/forums/index.php?threads/marantz-240.930329/) and it is running with 33. The model 33 is recapped by someone else with UPW caps in the audio path! I need to change those caps. This amp sounds really good to me after replacing all the caps.
Click to expand...

I mean UPWs should be okay in the audio path, most of them test better than any UKZ or UFG series... Though for smaller caps it's sometimes worth going film.
 
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