The C2 has become unusable. The controls are noisy, unbalanced,
and at times do not pass the signal on at all. Its overall sound is
not what I think it should be. It is too flat, indistinct / muddy.
The 33 electrolytic capacitors are the originals from
the 1970s and need to be replaced.
This is just too nice a preamp to be in this condition. It has actually
become a sought-after legend of sorts in the audiophile community.
The volume and balance control unit alone looks like a real bear to me.
Although it's full of parts made from unobtainium. I say what the hey. . .
I'll give it a go.
Yep. It's going to be a real bear all right. Bummer.
After 14+ hours of research on the C2 itself and then on the DigiKey
website, the 53 capacitors (20 spares) are finally on order.
They are all Nichicon UFG audio grade gold capacitors. I also have
to wait for the DeoxIT red, green, gold and 3-tube-kit to get here
from Amazon. 1 to 2 weeks wait.
Oops again. I didn't notice 6 of the capacitors are "Bi-Polar".
(I use to call them "Non-Polarized" back in the day).
They are now on order.
While waiting for the green caps I decided to service the volume / balance control.
It's pretty much SHOT !
The carbon traces that the wipers contact have dead spots, etc.
There is no way to completely disassemble this unit without destroying it.
DeoxIt D5 and DeoxIt Fader spray helped but not enough.
The balance control was extremely bad. What to do? You cannot find
a replacement and repairing this one is all but impossible.
So, I built a bypass circuit for the balance control.
No wonder I could never get consistent L to R balance. It probably
was also causing the eratic and unpredictable L and R gain level changes.
(See below "More Intermittent Weirdness" for more gain change problems).
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I am hoping that the volume control section will work good enough
to allow me to use the unit again.
If not, this pre-amp is toast !
Time will tell.
Balance function will now be an internal preset.
While reassembling the rear chassis and connectors, I noticed the Pre-Amp
output board connector looked really dirty. I sprayed it with DeoxIt D5 and
out came all this green goo. (The DeoxIt spray is not green). I plugged the
Pre-Amp output board into the connector and found extremely bad continuity and
again horribly intermittent readings with the ohm meter. Readings varying from
K-ohms to totally open. So I unsoldered the board connector, took all the contacts
out, washed everything in alcohol, bent the connectors to gain proper tension, and
put it all back together.
Now I'm getting readings through a 6 foot RCA cable, through the rear RCA
connectors, through the pins into the edge connector, to the solder lands on the
circuit board of less than 0.23 ohms with no sign of being intermittent.
I really don't know how this pre-amp ever worked at all ?
Well, as it stands right now, I do not like how it sounds. Bummer.
It sounded better today after I got a good nights rest. Ear fatigue I guess.
It is sounding better the more it plays, so I have left it on and making music for 8 hours now.
Who knows? Maybe the new caps are bonding with the Vertical FETs? Whatever works.
I was now able to hear a phase distortion throughout the response curve that made
everything sound harsh, truncated and overly flat.
So I tried some tweaks to the Balance Control Bypass Circuit. It improved.
At that point it dawned on me what I had done wrong in the bypass circuit installtion.
I will have to reconfigure the connection points for better output levels and balance.
It now sounds A LOT better. It has great stage, sounds very open and musical now.
It is set to flat on both the trebble and bass controls. The C2s tone control "0" positions
are a "tone-defeat position which bypasses the tone-control circuits altogether for flat-amp operation".
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I have never heard my stereo sound this good, especially set for "flat" operation.
Time to button it up and call it good.
Actual problems found and corrected:
The balance control was erratic to the point of measuring open in places. This
section did not respond to cleaning. It MUST be be replaced. (See bypass above).
The 4 section volume control was very erratic but never measured open at any position.
It responded very well to cleaning and overall remained usable.
All the rotary switches were intermittent to the point you could not touch them.
They responded very well to DeoxIt D5 and DeoxIt Gold application.
The Pre-Amp output edge connector that connects the RCA output connector
board was VERY intermittent. It had to be disassembled. (See above photos).
2 caps that are in the direct signal path had borderline bad ESR.
All electrolytic capacitors, both polar and bi-polar were replaced.
Most existing caps checked OK. As for sound, who knows?
My Balance Control bypass circuit board was installed incorrectly at first.
Correction of the connections made a world of difference. (See above explanation).
The power supply was out of calibration. I set both positive and negative to spec.
The regulation was very good when varied with the Variac.
33 capacitors replaced.