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Old 8th September 2005, 09:44 PM   #1
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Default marantz 22xx - front panel to EQ board

hi folks,

i am working on a 2238B here which exhibits a DC bias distortion - the scope trace jumps up and down as much as 10mv, intermittently. i have isolated the problem to the equalizer board - input looks OK and output is jittery. it is unaffected by the tone controls. of course, the problem is audible or i wouldn't be asking in here! now, i need to look over the circuit in more detail and see if i can identify a failed component (though i don't know if it's more likely to be a cap, resistor, or transistor, which are the bits i can see there).

i am trying to reach this circuitboard but removing the front panel is proving difficult. there is a small brass (?) cup around the treble shaft which holds the panel in place. i have pried up its edges, broken it a bit, scratched the panel, and succeeded in rotating the thing, but i have not been able to remove it. i assume similar marantzes have the same construction, so perhaps someone can clue me in on how to get past this?

any help would be appreciated.
thanks,
aaron.
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Old 8th September 2005, 11:40 PM   #2
anatech is offline anatech  Canada
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Hi aaron,
It's a nut. It unscrews.

Look for 2SC458 signal transistors. Replace them. I'll need to try and look up the replacement # for you.

-Chris
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Old 8th September 2005, 11:45 PM   #3
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Default 2SC945 will fit great!

Regards,

Carlos
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Old 8th September 2005, 11:51 PM   #4
anatech is offline anatech  Canada
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Hi Carlos,
Yes, not bad but the gain is a little low (by spec.). I think Marantz suggested 2SC1815. 2SC1775A would be my first suggestion but the collector current is a bit low (50mA instead of 100mA like the original).

Your suggestion would work of course.

-Chris
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Old 9th September 2005, 08:01 AM   #5
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hello anatech and carlos: thanks for the tips! here is what i found: after removing that horrible nut, i got the front apart and the EQ circuitboard into my hands. it seems to have three each of A841 (GR-BL) and C1581 (GR-BK) transistors. unfortunately for my repair project, i don't have the schematic or datasheets. perhaps someone else on here could verify those IDs for me.

googling revealed little about these two transistors. i suppose one is NPN and the other PNP, but i'm not well enough versed in them to know if the ones you suggested would be suitable replacements. of course, i'm happy to spend a couple of bucks on new ones and a couple of minutes soldering, if that indeed seems to be a likely cause of my problem. anatech, you seemed relatively confident, so i'd like to follow up on that suspicion.

in disassembly i discovered a cockroach carcass nestled in the pins of one of the transistors, but despite my hopes to the contrary, it wasn't the source of the problem. :)

thanks guys, cheers!!
aaron.
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