Marantz 1200 New Member Question

[ejman]

This is my first post and after lurking for a while I have to say this forum is absolutely awesome!

After reading about vintage equipment for a while I decided to purchase a Marantz 1200 Integrated amplifier @ ebay. It came kind of bashed up with a bad hum in one channel and weak signal (I tested the amp on cheap headphones at a distance first!). Fortunately I had read a good part of EchoWars terrific posts about DC offset and checked this first before hooking up to my magneplanars - R ch 35.2 mv; L ch started out at over 1 v but after fiddling with the controls and generous deoxit use I finally got it to eliminate the hum and the reading settled to 67.8 mv - not good.

Nonetheless, based on that thread's info that the 67.8 mv although not good was not dangerous I decied to hook it up to my maggies. Long story short I was so impressed with the sound of this amp - great bass, great imaging, a definite performers are there feeling I've been listening to it ever since (I've been using a Creek amp before which is perhaps more detailed than the Marantz but is nowhere near in the quality of the bass , the warmth and the sense of magical music being there in front of me).

Of course, this made me wonder what would the amp really sound like if I could get the dc offset near 0?

Being very mindful about all the warnings not to release the "magic smoke", I ordered the service manual before touching 'nothin.

It came in today and I promply read in the balance test section to adjust potentiometer R504. Trouble is (see attached schematic and board pictures). R 504 is a 100K 1/4 w resistor with no visible (to me anyway) means of adjustment. Directly below R 504 is a potentiometer called R502 could this be the dc adjustment? I know better that to touch R526 in the center of the board which is the bias adjustment.

But I did screw up in that I fiddled with R541 on the right hand side of the boards because at first I thought that was the DC adjustment ( It's at the right "height" on the board but on the wrong side since it is above L501 which is a toroid) R541 and its companion R542 at the bottom of the board adjust maximum power output so that the pos and neg clip at the same level. I set them back to where they were physically but knowing how fiddly those potentiometers are I'm sure its only approximately right (It sounds ok though).

Anyhow, sorry about the rambling and long first post but is it possible that the instructions are wrong and actually R502 is the adjustment? if not, How do I do it any ideas?

Thanks all for your help

[EchoWars]

I'd like to help, but those attachments are wayyy too small for me to make out what's up.

Try again with a 800 x 600 pic (compressed to about 125K). The schematic showing the front end of the amp section would be a lot more useful than the foil pattern you posted.

Welcome!

...and the next time you buy gear from someone you can't trust to pack properly, send them the box and materials with instructions. People are idiots...you have to spell it out for them. :rolleyes:

[bully]

Just wanted to add some info from the Orion, 1961-73, 100 wpc, and an msrp $595. Some serious coinage for the time.
Worth having it checked over by a good vintage tech.
While I'n no fan of Marantz gear, i do know they made so good gear. :banana:

[Punker X]

There are 4 adjustments on the amp board. The 1200 shares the same amp board as a 250/240. The there are two metal cased pots. The one in the center of the board is the bias adjustment, the one the edge of the board is DC balance. The two plastic trimmers are to adjust clipping.

X

[SB Marantz]

There are 4 adjustments on the amp board. The 1200 shares the same amp board as a 250/240. The there are two metal cased pots. The one in the center of the board is the bias adjustment, the one the edge of the board is DC balance. The two plastic trimmers are to adjust clipping.

X

Yes, you are right about the pot's but don't forget that the 1200/1200B/240 and 250 came in 2 different design's.

The one showed here is the ''old'' one.

Be very carefull with the 2 clipping pot's because it can cost you the power transistor's if adjusted in the wrong way!!

If you want to enjoy such an old amplifier as this one; recap it inside out.

Set bias at about 10 to 12 mV's.

If the DC-offset is 1V at start-up that's allso an indication that the amp needs a little TLC.

Fiddling with control pot's and deoxit wil not have any result on bias or DC-balance.

Give the 2 pot's on the amplifier board some attention with deoxit and recap both board's + the pre-amp board and then set the bias and DC-balance.

Next step; power up and enjoy!

[ejman]

I'd like to help, but those attachments are wayyy too small for me to make out what's up.

Try again with a 800 x 600 pic (compressed to about 125K). The schematic showing the front end of the amp section would be a lot more useful than the foil pattern you posted.

Welcome!

...and the next time you buy gear from someone you can't trust to pack properly, send them the box and materials with instructions. People are idiots...you have to spell it out for them. :rolleyes:


EchoWars,

Thank you very much for your reply. I'm having trouble getting the schematic to be legible when reducing the file to a size that will fit audiokarma's upload requirements. What I ended up doing is uploading full size files to my wife's website at:

http://www.grayjranch.com/stereo.htm

Please take a look in there and let me know if this works if not, we'll come up with something

As to E-bay I hear you - second time this has happened to me first time it was a Dual 1019 that came in little pieces after that I decided no more turntables but I thought amplifiers would fare better oh well...

[ejman]

Just wanted to add some info from the Orion, 1961-73, 100 wpc, and an msrp $595. Some serious coinage for the time.
Worth having it checked over by a good vintage tech.
While I'n no fan of Marantz gear, i do know they made so good gear. :banana:


Thanks. Tha's one thing that caught my attention $595 from 1970 is what about $3,000 in today's $?

[ejman]

Yes, you are right about the pot's but don't forget that the 1200/1200B/240 and 250 came in 2 different design's.

The one showed here is the ''old'' one.

Be very carefull with the 2 clipping pot's because it can cost you the power transistor's if adjusted in the wrong way!!

If you want to enjoy such an old amplifier as this one; recap it inside out.

Set bias at about 10 to 12 mV's.

If the DC-offset is 1V at start-up that's allso an indication that the amp needs a little TLC.

Fiddling with control pot's and deoxit wil not have any result on bias or DC-balance.

Give the 2 pot's on the amplifier board some attention with deoxit and recap both board's + the pre-amp board and then set the bias and DC-balance.

Next step; power up and enjoy!

I guess I'm in trouble already because I did fiddle with R 541 in both channels when I tought these were the DC adjustment. I put them back to where they were before but I wonder if the adjustment is fiddly to the point where the same physical point would result in different resistance value when fiddled? I don't have a distortion analyzer or an oscilloscope as the manual calls for setting the maximum power output ugh!!! Would there be any audible sign to this adjustment being out other than dead silence or "magic smoke"?

Second question, the manual calls for bias adjustment as follows:

1) Rotate R526 fully CCW
2) Slowly advance variac until line voltage reaches 105 and speaker relay energizes - wattmeter should indicate less than 40 watts
3) adjust R526 until the wattmeter indicates 10 watts above the initial reading
4) repeat for other channel

There is no mention of setting bias to 10 to 12 Mv's - how would I do this alternate procedure? i.e. where to measure etc?

As to the DC offset starting out at 1 v and the comming down I don't know what did it but it happened- Is it possible that just being on for a while would "clear the crud" if the amp had been sitting for a long time? it sound oK now.

[Punker X]

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.

The clipping adjustment should be fine, unless you plan on it to full power. Which I wouldn't thing of doing to a vintage amp unless you recap and have it aligned with the proper test equipment. If they are off you will here pretty bad distortion. If you push it in that condiditon you will blow the amp or your speakers.

X

[SB Marantz]

I guess I'm in trouble already because I did fiddle with R 541 in both channels when I tought these were the DC adjustment. I put them back to where they were before but I wonder if the adjustment is fiddly to the point where the same physical point would result in different resistance value when fiddled? I don't have a distortion analyzer or an oscilloscope as the manual calls for setting the maximum power output ugh!!! Would there be any audible sign to this adjustment being out other than dead silence or "magic smoke"?

Second question, the manual calls for bias adjustment as follows:

1) Rotate R526 fully CCW
2) Slowly advance variac until line voltage reaches 105 and speaker relay energizes - wattmeter should indicate less than 40 watts
3) adjust R526 until the wattmeter indicates 10 watts above the initial reading
4) repeat for other channel

There is no mention of setting bias to 10 to 12 Mv's - how would I do this alternate procedure? i.e. where to measure etc?

As to the DC offset starting out at 1 v and the comming down I don't know what did it but it happened- Is it possible that just being on for a while would "clear the crud" if the amp had been sitting for a long time? it sound oK now.


There is a much easier way to adjust the bias.
Look at the photo.
Hook the multi meter on both ''spring's'' + at the + side = red on the board and so on.

Leave the amp on for about 10 min to assure stabilaty and repeat procedure for the other channel.

DC-offset; probes on the speaker terminals and set them as close to zero as possible.

http://img62.imageshack.us/img62/3206/img0925rw6.jpg

[ejman]

Bingo! - sounds wonderful - thanks to all of you for all your help this board is great!!!