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Marantz 2245 left channel puzzle

Just a rewind question, says you did some work on the 2245?
Did it work before you did the work, or are you starting on something that did not work?

FYI, I have worked on many 2245/2270 and they are going on 50 years old, from time to time I do get units with mixed boards.
 
So, what I have done so far based on your recommendations:
- I reflowed the board, all solder joints
- put pots in middle position
- checked pins for the output transistors
- checked serial number of device, board and compared to schematic
- r788 and r789 are new

...and still no positive outcome...

Boards have identical numbers (yd2819007-0)
Left channel looks a bit older colour wise.

The only difference is two missing jumper wires and a capacitor. These I plan to implement.
IMG_20220729_104427.jpg IMG_20220729_104706.jpg IMG_20220729_104754.jpg IMG_20220729_104857.jpg
 
This is how the story goes: I got the device as defective, left channel very low. Replaced blown outputs (also reversed, if I remember correctly) replaced h758 and 759, some resistors and some more transistors. Also the pots.
Device started and worked for some time and then developed the current condition.
All other boards I recapped and checked after each rework.
New lamps, velum, etc...
 
Things to check/test:
-board has original diff. prs.
-and as mentioned by someone else original round bias transistor

Need to pull and test these!
Sometimes they will test good on a tester but act up when powered.

*Did you note trimmer positions-pull measure and set new ones to around the same?
-for bias I would have as full off for now.
 
patfont said:
Things to check/test:
-board has original diff. prs.
-and as mentioned by someone else original round bias transistor

Need to pull and test these!
Sometimes they will test good on a tester but act up when powered.

*Did you note trimmer positions-pull measure and set new ones to around the same?
-for bias I would have as full off for now.
Click to expand...
Patfont,
You are right! The bias transistor is gone! I replaced it with the one from the right channel (measures ok) and now there is no problem.
I measured the resistance of the old pots and transferred these as close as possible, but when I tried to bias it is all gone.
I will now order MJE243G or 2SC495 or 2SD 612 and replace both boards. Would you recommend any of these or whichever I manage to find?
Should I check shth else around H670?
Thanks!
 
I forgot to mention that I replaced diff. pair with matched ksa992s. These on the pictures are the new ones
 
Thank you, patfont!

I was looking for substitutes and first substitute I find is bc337. And I happen to have some of these, so I will check and come back with info.
Has some of you used these as replacement?
 
About the capacitors at the bottom of the board: I believe that these are 500 pF, but the closest I have is 680 pF. I think at these values it will not be a big difference. What do you think?
 
Thanks Steven!
This is useful. He also speaks about the wire jumpers. I have jumpers on the right channel and also 3,6 Ohm which I replaced with 3.9 but have not removed the wire jumpers. Will do it the same way on the left and see what happens...

What do you think about replacing the 500pF with 680pF?
 
AngelAT said:
Thanks Steven!
This is useful. He also speaks about the wire jumpers. I have jumpers on the right channel and also 3,6 Ohm which I replaced with 3.9 but have not removed the wire jumpers. Will do it the same way on the left and see what happens...

What do you think about replacing the 500pF with 680pF?
Click to expand...
For the ceramic caps, I tend to use Leestereo's recommendation in his great 2270 restoration thread here. The same caps are in the 2245. As for the jumpers or resistors at R784/785, there is a trick that has worked for me when changing the bias transistor. Those resistors would normally be 3.9 ohms. But if, after changing the bias transistor, the bias is too low regardless of trimmer setting, I replace R784/785 with a straight wire jumper. That usually raises the bias range for the trimmer so that you can get your 10mV. If the bias is too high after changing the transistor, and won't adjust into the 10mV range, I change R784/785 to 5.6 ohms. This will usually lower the range where the value can be set to 10mV.
 
Thanks Steven!

I have not been posting, because this board is driving me crazy!!!
It is shorting somewhere again....
Even with the original H760 (checked and working ok).
I have reflowed all solder joints, checked H761 (ok). Used original and new output trans with same result.
Will now check the new trim pots.
...and continue digging...
 
OK, we have a breakthrough!
It was R763. And no short.
Now it looks that things are on the right track. Checked both channels for dc current and bias and both look ok.
Will reassemble and test again.
BTW for bias trans I used bc337.
Will come with updates.
 
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