I recently acquired a McIntosh C20 preamplifier. This example is in fair to poor cosmetic condition. None of the knobs are original, the front panel is a bit worn and the wood case is in need of refinishing or reveneering. My original thought was to install it in my music system, see just how good it sounded and then decide whether or not to keep it or pass it on.
When I bought the unit I was told that there were no tubes in it. Since it was in the case I was unable to verify that information until I got the unit home. Much to my surprise, after I removed it from the case, I discovered that there was a full complement of tubes. Removing the top and bottom cover revealed that all the capacitors (black body with colored stripes) and resistors (mostly ½ W, 10%) appeared to be original. The only components that appeared not be original were the diodes used in the filament supply.
Using a Variac, I slowly brought up the voltage over the course of an hour. I noticed that one of the power supply capacitors was getting warm, so, I started measuring voltages. Voltages were low all around and I determined that the input filter capacitor (C1 on the SAMS) was bad. The bad capacitor was a dual section twist-lock specified at 40uF, 40uF @ 350V. Rooting around my OOS capacitor supply I found a capacitor that would fit and after testing it on my Spargue Tel-Ohmike TO-6 and finding it good, I installed it in the preamp. Voltages were up, but, still below spec.
Further testing revealed that the last section of the second twist-lock capacitor (C2D on the SAMS) was bad (leaky). This time I could not find a suitable replacement, so to verify the operation I temporarily kluged in a single capacitor to replace that last section on the quad section. Voltages were better but still low in the phono stage. Further testing revealed that the root cause of the remaining low voltages were the result of leaky coupling capacitors (SAMS C11 and C51).
Here is my dilemma; if I am ever going to hear this preamplifier work as it should, I will most probably have to replace every single capacitor. This will take a little bit of money and a fair amount of time. On the other hand removing the original components will most probably reduce the resale value and alter the classic sound of the preamplifier.
I would really like to hear from other McIntosh C20 owners. What should I do? Have you replaced the paper capacitors? If so, did you use modern technology (polypropylene or polyester) capacitors or did you replace the capacitors with NOS or new paper capacitors? What is the best way to replace the capacitors? Does one desolder them from the terminal board lugs or does one cut them out and attach the replacement capacitors to the lead ends? Do you remove the terminal boards from the chassis or try to work on them in place? Do you recommend replacing all the resistors as well or not?
When I bought the unit I was told that there were no tubes in it. Since it was in the case I was unable to verify that information until I got the unit home. Much to my surprise, after I removed it from the case, I discovered that there was a full complement of tubes. Removing the top and bottom cover revealed that all the capacitors (black body with colored stripes) and resistors (mostly ½ W, 10%) appeared to be original. The only components that appeared not be original were the diodes used in the filament supply.
Using a Variac, I slowly brought up the voltage over the course of an hour. I noticed that one of the power supply capacitors was getting warm, so, I started measuring voltages. Voltages were low all around and I determined that the input filter capacitor (C1 on the SAMS) was bad. The bad capacitor was a dual section twist-lock specified at 40uF, 40uF @ 350V. Rooting around my OOS capacitor supply I found a capacitor that would fit and after testing it on my Spargue Tel-Ohmike TO-6 and finding it good, I installed it in the preamp. Voltages were up, but, still below spec.
Further testing revealed that the last section of the second twist-lock capacitor (C2D on the SAMS) was bad (leaky). This time I could not find a suitable replacement, so to verify the operation I temporarily kluged in a single capacitor to replace that last section on the quad section. Voltages were better but still low in the phono stage. Further testing revealed that the root cause of the remaining low voltages were the result of leaky coupling capacitors (SAMS C11 and C51).
Here is my dilemma; if I am ever going to hear this preamplifier work as it should, I will most probably have to replace every single capacitor. This will take a little bit of money and a fair amount of time. On the other hand removing the original components will most probably reduce the resale value and alter the classic sound of the preamplifier.
I would really like to hear from other McIntosh C20 owners. What should I do? Have you replaced the paper capacitors? If so, did you use modern technology (polypropylene or polyester) capacitors or did you replace the capacitors with NOS or new paper capacitors? What is the best way to replace the capacitors? Does one desolder them from the terminal board lugs or does one cut them out and attach the replacement capacitors to the lead ends? Do you remove the terminal boards from the chassis or try to work on them in place? Do you recommend replacing all the resistors as well or not?