Marantz 2245 idle current won't adjust

Steven Tate

CEO Flat Forehead Soc.
.I know I'm not the first one to have this problem, but haven't found the answer in the search function. Just finished recapping a 2245. It sounds great. Before recapping, I measured DC Offset and idle current to make sure they adjusted out OK. They did, but I probably only ran it about five minutes per channel to check idle current. I recapped all boards, including replacing the diff pair with gain matched transistors. After completing the project, adjusted power supply and DC offset. All set well. Adjusted right channel idle current and let run for 15 minutes to settle at 5 mV. When I did the left channel, all looked well for about five minutes, but after 15 minutes, the voltage kept slowly climbing until the trimmer was completely counter clockwise with voltage at about 6.5 mV. Nothing looks hot and the sound is good. Think I might try a different matched diff pair? Where should I start? Thanks!
Steve
 
I doubt that the differential pair is part of the problem. If it worked before you recapped it, then I would look for something that you did, a solder bridge, a poor solder joint, a backwards capacitor, etc.

The 2245 is very similar to the 2270 (sn3901 and above). Here is an old thread from ecluser that contains much information about 2270 bias. Be sure to read post #11 and look at the diagram and the math.
ecluser has some of the best circuit analysis skills I have ever seen, I wish he would still stop by the Marantz forum from time to time.

Tom
 
I'll never live down the shame of that post! At least it's helped many. I've run into it a couple of times more and replacing the bias transistor with an MJE243G has cured it every time. I believe a pair of legs' connections have to cross, meaning the pinout order on the new part is not the same as the original.
 
OK. After a couple of times through it, because of my lack of background, I only have a vague understanding of ecluser's analysis. I mostly understood the part that showed that the temperature of the driver transistors was more important than the temperature of the power transistors. He discussed moving the bias transistor H760 for that reason. Randy, you mentioned above that you replaced the bias transistor with MJE243G. Did you put it back on the big heat sink or did you move it? Am I moving in the right direction? Thanks!
 
Steven,
If it worked before you started but doesn't work now then rather and try to "make" it work by changing the bias transistor I would go back as Tom suggests and check for a solder bridge or more likely a broken foil trace/eyelet.
Possibly even one of the legs of the bias pot has a solder joint bad from jostling the board around.

Bob
 
The problem I see here is that we don't know if this was a problem before the recap as it was only warming for about 5 mins, originally. This could've been an issue beforehand as well. For future reference, monitor these conditions for at least 30 mins, it takes a while for things to warm up.

I'd swap in the new bias trans and see how things go, what do you have to loose other than 10 mins of time. While you're in there also check out H761 and r763.
 
Bob, when I get the board back out, I will definitely go over it with a fine tooth comb. I only replaced three caps and the diff pair, but there was quite a bit of jostling, so it needs a thorough examination. And I will give a good look at H761 and R763. However, I thought that diode at H761 was basically unobtainium. :dunno:
 
Bob, when I get the board back out, I will definitely go over it with a fine tooth comb. I only replaced three caps and the diff pair, but there was quite a bit of jostling, so it needs a thorough examination. And I will give a good look at H761 and R763. However, I thought that diode at H761 was basically unobtainium. :dunno:

Exact replacement are but it doesn't hurt to check it out and make sure a lead hadn't cracked off. They are very fragile, try not to move the leads as they are prone to separating.
 
Thanks. I'll be careful. I have seen plenty of threads about broken leads on those. Something tells me they break without warning.:thumbsup:
 
I had the same exact problem yesterday with my 300dc!
I , like Tom said, Had a solder? bridge on a pre driver pin on the transistor side of the board that eventually flaired up as it found its way to the heatsink (collector of driver mounted next to it)
It left a track mark like a dead body chalk line.:idea:

The amp worked for an hour then lit up the relay as it smoked the points. real exciting:yikes:
It was after replacing the relay that i experienced the dc cycling, then the transistor flair up that took out both pre-drivers.
I spent the morning going over it with a fine tooth comb. it could have been a wayward cut off pin that i missed.
i am just about to put some juice to it to see if it passes my dbm. wish me luck..

it started ramping up dc in a cycling fashion, increasing transformer hum as it went. i'm glad it burn't cause it saved me time trying to figure it out.
 
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To understand ecluser's write up you should have, at least, a clear understanding of Ohm's and Kirchhoff's circuit laws.

Tom
Well, I go into it with a basic understanding of Ohm's law and never heard of Kirchhoff's Law. Looks like I have more research to do. :D
 
I had the same exact problem yesterday with my 300dc!
I , like Tom said, Had a solder? bridge on a pre driver pin on the transistor side of the board that eventually flaired up as it found its way to the heatsink (collector of driver mounted next to it)
It left a track mark like a dead body chalk line.:idea:

The amp worked for an hour then lit up the relay as it smoked the points. real exciting:yikes:
It was after replacing the relay that i experienced the dc cycling, then the transistor flair up that took out both pre-drivers.
I spent the morning going over it with a fine tooth comb. it could have been a wayward cut off pin that i missed.
i am just about to put some juice to it to see if it passes my dbm. wish me luck..

it started ramping up dc in a cycling fashion, increasing transformer hum as it went. i'm glad it burn't cause it saved me time trying to figure it out.
So far, mine operates just fine for the short time I listen to it for testing. But my next step will be some time with magnification studying the board and checking my cap polarities. Then some research to try to understand ecluser's explanation a little better. I've ordered the MJE243G's, and MBuras sent some pics of his upgrade using ecluser's information. That's probably the way I will go if I don't find a board or trace problem.
 
Some light reading for you....:)

Tom
Hah! Already have that open in another window on my computer. And in another window I have the Ohm's Law description to refresh my memory. I understand the conservation of energy concept, so sort of understand where Kirchoff's is coming from. But I'm going to have to go back to ecluser's post to try to see how it applies. Thanks! You're going to teach me something yet. Many men have tried and failed. :rflmao:
 
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