C45 Tone Bypass Issue?

mawgui

New Member
Recently bought an M45 Amp and C45 Preamp at auction. Amp works great. Preamp works great so long as Tone Bypass is engaged. If I disengage the Tone Bypass to use bass and treble controls, volume drops off and distortion increases. Any ideas would be appreciated!
 
Recently bought an M45 Amp and C45 Preamp at auction. Amp works great. Preamp works great so long as Tone Bypass is engaged. If I disengage the Tone Bypass to use bass and treble controls, volume drops off and distortion increases. Any ideas would be appreciated!

Welcome to AK..
First thing to try is cleaning the T bypass switch as well as the tone controls with deoxit.

http://www.hifiengine.com/library/yamaha/c-45.shtml
 
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Thanks but... Problem is Still There

Thanks for the suggestion. And taking the faceplate off gave me a chance to get the cosmetics all spic and span. However, the problem is unresolved. What's next?
 
Yes, both channels. Very quiet, with distortion if I turn it up. Volume with tone controls engaged never reaches the level with Tone Bypass engaged. Frankly, with Tone Bypass engaged, it sounds very good indeed. All other functions work as intended.

So, what do you think?
 
First thing I'd check is the supply voltages for the tone EQ flat amp. Disreguard ! ( I just looked over the schematic)
 
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Alright. I can do that... but what am I looking for and how do I find it? I have a multimeter and I am happy to learn something new (that's how I repaired my Apogees) but can you provide me a bit more guidance? I've had the case open and everything is pretty clearly labeled (actually, it's wonderfully organized!): what do I want to poke my leads onto and what values am I looking for?

Thanks!
 
http://www.hifiengine.com/library/yamaha/c-45.shtml

Ok..It almost has to be ...
1. dirty or broken tone control(s) ie. Bass,mid or treble.
2. dirty or broken tone by-pass switch.
3. cold,cracked solder connection in the tone circuit.
4. cracked circuit board trace in the tone control circuit.(Main circuit boards 1 and 3.)

No active components involved ie. transistors/opamps/diodes

Might have to disassemble and inspect/clean the by-pass switch. Will require removal from the C-45 to disassemble.If its the type of switch I think it is.Just spraying deoxit into any opening usually doesn't get the cleaner where it needs to go.
 
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Thanks, Avionic! I'll try to take a look at the circuit boards without making a mess (I'm still haunted by my childhood misadventures disassembling my Mom's Instamatic camera to "fix" it... up until that point I had thought that cameras were mechanical and optical devices...). If it's a solder connection or circuit board trace I should be able to find that.

You correctly guessed how I tried to get the spray into that Tone Bypass switch. I have come to realize that contact cleaner is rather analogous to duct tape in its range of uses. In the last month I "restored" two sets of speakers with little more than a couple blasts of spray and some vigorous manipulation!

Thank you for the tips. Any other insights as to what to look for will be very much appreciated. I hope to get a chance to investigate further this weekend.

Thanks again!
 
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