My "new" C-70: Disc Direct not working

Dr. Ear

Super Member
Was working on my "new" C-70 this afternoon. Replaced some resistors and Zeners because of corroded legs (Sony glue), one Zener in the phono PS was almost completely gone. Replaced a lot of caps, reflowed a lot of bad solder joints, cleaned everything nicely afterwards and adjusted all parameters (never had a preamp before with such an extensive adjustment procedure). Hooked it on my M-2 and it worked :banana: . CD sounds great, the "normal" phono input works, too. But, :scratch2: when using the "Disc Direct" button the volume is much lower and the sound comes equally distorted in both channels, either with the MM or MC inputs. What could cause this problem and where to start?
 
No one?:tears:
The lousy copy of the schematics I have is not very useful because I cannot read the reference voltages properly. Does somebody here have a good copy?
 
No one?:tears:
The lousy copy of the schematics I have is not very useful because I cannot read the reference voltages properly. Does somebody here have a good copy?

Damn. I was going to ask you that! :scratch2:
 
C70 direct disc not working

Hello Doctor EAR

I have to look in my stock if I have the C70 service manual in high definition (around 40Mo ) ! :banana:
If not what section do you like to have for starting your investigations ?:scratch2:
 
Thank you all! In the meanwhile I could find a good copy of the schematics. There is another symptom maybe worth to mention. According to the user manual the light on the parametric tone control button should go off when "Disc Direct" is activated. In my case it does not go off but is dimmed only. Is this normal?
 
I have a working C-70 here. Tone LED dims down on this one as well.Others have had lots of problems with those little orange relays.
 
I have a working C-70 here. Tone LED dims down on this one as well.Others have had lots of problems with those little orange relays.
It's the relays! Operating the "Disc Direct" button involves a total of 4 relays, two of them are hidden under the control pcb. I opened one of them and the tone came back when I applied some pressure to the contacts using a toothpick:banana:
Is there a drop-in replacement available?
The C-70 is the most complex Yamaha preamp I ever had in my hands.
 
Can you take the covers off? You could try giving them a few cleaning cycles in your ultrasonic cleaner and then dry them out real well in the oven at low heat ...
This is exactly what I did! Desoldered 7 relays, gave them a 40 minutes bath in a pro ultrasound cleaner (deoxidizing agent added), rinsed them with aqua bidest. and IPA. Polished the contacts with paper strips. The relays are like new now, zero Ohm! See pics.
 

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$55 x 7 makes us do a bit of work to avoid the buy. Nice job on those relays.

Unit should sound better now, too. Most connections cleaned up along with the relays.
 
$55 x 7 makes us do a bit of work to avoid the buy. Nice job on those relays.

Unit should sound better now, too. Most connections cleaned up along with the relays.

The distortion level bad relay contacts can produce was something new to me. Before this experience I was thinking only defective semiconductors could do this.
Yes, indeed, the recapped and reflowed C-70 sounds very good. It gives a more opulent presentation than e.g. a C-4. It is a good match for the NS-2000.

I would recommend using a 3 mm strip of 1000-2000er grinding paper instead of paper strips for cleaning the tiny contacts. In my experience liquid contact cleaners, like Deoxit are not very helpful in such a case.
 
C70 direct disc not working

Hello Dr.ear

Nice to see that's working now !
I do agree with Avionic : try a C1 and you will see what a complicate preamplifier mean ! :yes:
I did 2 these last month and it was a long long job .
Take a look on my attached file to confirm that everything goes well and all adjustments are done as they should :thmbsp:
 
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Hello Dr.ear

Nice to see that's working now !
I do agree with Avionic : try a C1 and you will see what a complicate preamplifier mean ! :yes:
I did 2 these last month and it was a long long job .
Take a look on my attached file to confirm that everything goes well and all adjustments are done as they should :thmbsp:

Thank you for the docs. I have already done the adjustments . They were within specifications. But, why would I need a "frequency counter" and an "oscilloscope" for that:scratch2:
 
C70 direct disc not working

Why ?
I think that oscilloscope is to see the audio signal form
the audio oscillator is to explore and confirm values from 20 to 20K
Frequency counter to control frequency of the audio input signal
and Voltmeter to control value at test points when adjutments are made
I never adjust a C70 yet but it seems not complicate ...If you get the right values that's your unit is fine ! :music:
 
This is exactly what I did! Desoldered 7 relays, gave them a 40 minutes bath in a pro ultrasound cleaner (deoxidizing agent added), rinsed them with aqua bidest. and IPA. Polished the contacts with paper strips. The relays are like new now, zero Ohm! See pics.

Good save..:thmbsp:
 
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