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Model 1200B Amplifier Issue

slayer44

slayer44

Well-Known Member
Greetings All,
I was gifted this amp and was told it had issues. I put it on the DBT first and no problems there. Powered it up and relay energizes quickly. I then checked for DC at the speaker terminals and discovered that the left channel was reporting rail voltage! (51V). I then checked all the voltages at the important points (differential was high. Around 15 V)
I then followed the troubleshooting instructions from the 1200 manual and measured normal range at the speaker terms BEFORE the relay clicked in and get about 118mV. Once it clicks, it goes back to 51 volts. I hold the relay open and get do not get 51V at the terms. Does anybody know what in the rectifier board can cause this? (I am assuming it's the rectifier board) I just got this this amp 3 days ago and it is all I have checked so far, but any insight is welcome. Without an answer from my Marantz gurus, I will pull the rectifier board and check all those components. I can deal with the amp board once I get the high DC figured out.
Also, does anyone have a BOM list to repop these amps? Modern transistors would be especially appreciated.
Thanks for reading.
 
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slayer44 said:
Greetings All,
I was gifted this amp and was told it had issues. I put it on the DBT first and no problems there. Powered it up and relay energizes quickly. I then checked for DC at the speaker terminals and discovered that the left channel was reporting rail voltage! (51V). I then checked all the voltages at the important points (differential was high. Around 15 V)
I then followed the troubleshooting instructions from the 1200 manual and measured normal range at the speaker terms BEFORE the relay clicked in and get about 118mV. Once it clicks, it goes back to 51 volts. I hold the relay open and get do not get 51V at the terms. Does anybody know what in the rectifier board can cause this? (I am assuming it's the rectifier board) I just got this this amp 3 days ago and it is all I have checked so far, but any insight is welcome. Without an answer from my Marantz gurus, I will pull the rectifier board and check all those components. I can deal with the amp board once I get the high DC figured out.
Also, does anyone have a BOM list to repop these amps? Modern transistors would be especially appreciated.
Thanks for reading.
Click to expand...
Check the output transistors for a short. A short here will send that voltage to the relay.
 
Any suggestions to replace the diff pair? They might be compromised and I usually replace them anyway. I have a ton of KSA992's already matched (I am a Pioneer guy).
 
Check the outputs, drivers and pre driver transistors for sure they need to be gain matched
What is the code of the current differentials fitted? I seem to recall this is similar to the Marantz 250 and I have a good sub list
 
Hello Quad,
I dug into it a little last night and my suspicions were confirmed. Somebody that had no business being there, was indeed. I tested all outputs on left side amp (they were fine) and then discovered the right amp had three of the same outputs in a row, plus an unknown that didn't match anything. Last I checked, these need to be 2 PNP and 2 NPN. They all tested fine, which is surprising. So that means my pre-drivers and others down the line are probably toast. I also removed my main filters to get a better look at the relay board. C702 is blown for sure. The rubber cap is squeezed out of the capacitor body. I would definitely appreciate your 250 parts list, since I am going to discover more problems. I can't trust anything this previous guy did. I will download the 250 service manual today and compare the two.
I also powered the unit with ALL 8 outputs removed and am still getting 51V at right speaker terminal. I have a lot of fish to fry with this unit. One step at a time.
Thanks so much in advance.
 

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Here is a list I saved from a 250 restoration I did a few years ago for q510-11 I ended up finding to-66 subs instead which were 2n3583 and 2n6211 if you can find suitable to-66 subs here it’s better.
In addition to gain matching follow your serial number relevant service manual if you need to try keep hfe ranges in a certain range in certain locations
Also look for the service bulletins that may be relevant for this model
Hope that helps
 

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Also check the high watt resistors I found several that were out of spec or burnt on the bottom as too closely mounted to the board
 
Thank you quadmatrix82!
I did a more extensive dive into the relay board last night. Since I was getting 51V at the left speaker with all the outputs removed, it wouldn't have done any good to work on the amps until that was fixed. I found two transistors were replaced and one leg each wasn't soldered in! I replaced that bulging cap (reported .01 picos, not 47 mf). pulled a leg of each diode and those checked out. Plugged the unit back in and now I don't have 51V at the terms. So I am gaining. I am going to buy new MJ outputs and go through your list next. Thanks for all the help. I can't believe I had one working channel with all these problems.
Take care.
 
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