[squizcat]

If you are building a component that requires TIP50 transistors as substitutes for 2SC3942, you may need to insulate the transistor from the heatsink as the TIP50 has a metal back and the heatsinks on the Chime are grounded to the circuit board.

I'm still learning to read the schematics, and having followed the instructions with recommended substitutes, I was confounded that my Chime build was measuring ~130v at the 295v test point next to R438. 

I was beginning to wonder if I had become the owner of an expensive, high tech brick. Poty came to the rescue and methodically helped me go through the circuit, looking for clues. Thanks to his patience and skill, it's sorted!

Probably standard stuff, but I didn't know it and it sure baffled me.. Would have been nice to have been given a hint. The insulation kit would be a heck of a lot easier to fit before the parts were soldered into place.

Put the insulation kits in, also swapped the 3ohm resistor at R423 for a 2.7ohm which lifted the heater voltage from ~5.9v to ~6.15v (will need to tweak a bit more to get it to 6.3v), plugged it in and then wondered where the music was coming from . Took a moment to realize it was the Chime
 
I used some insulation similar to the following -


Thank you so much Poty!!!   

[poty]

Thanks for good words, it is always a pleasure to be able to help somebody!
I think there were two problems, but one of them disappeared without any intervention. The second problem was revealed only by photo!

[marantzfan]

Would insulators be needed on the cornet2 with this part?

[Jerwardemy]

I've been wondering that myself...I didn't insulate the TIP50's on my Cornet 2.  After looking at the schematic, it looks like the collector pin goes through a 910 ohm resistor to ground.  So, if the heatsinks are connected to ground and the back of the TIP50 ( this is also the collector) is not insulated...that means the collector now has 2 paths to ground? One through the 910 ohm resistor and another through the heatsink straight to ground?

I wonder if this could be why my H+ voltage varies by the time of day and whether my A/C is running? 

[poty]

Yes.

[marantzfan]

Ok so I ordered the pad, whats the plastic washer / collar piece?  Might just use a couple of nylon washers?

[Jerwardemy]

You want to use that plastic washer shown in the picture posted above.  It keeps the bolt from coming in contact with the hole in the transistor that it goes through. I just bought a couple TO-220 Mounting kits from my local Radio Shack, it comes with everything you need...well, except for thermal grease.

I just insulated mine and it made a big improvement.  I can now turn my receiver up nearly all the way and barely hear any noise from the Cornet.  The HF's are much clearer, blacker backgrounds, better soundstage, and more details are coming through.  There must have been a lot of noise coming from the ground into my signal before. This is great!! Just make sure you check the bolt with a DMM to be sure its isolated from the transistor.

[hagtech]

The heat sinks are not grounded on the CORNET2.  I forgot they were on the CHIME.

jh

[poty]

I've found a photo of the back side of the Cornet 2 PCB and now know that the heatsink on the Cornet 2 is not grounded. Sorry - it means that my previous opinion is not right. You do not need to isolate the transistor from the heatsink.
I've built the Cornet 2 (with some Cornet 3 modifications and several my additions) on my own PCB, so I don't have the original PCB to check.

[Jerwardemy]

Hmmm...well I have no idea why,  but I DEFINATELY heard an improvement from insulating the transistors. It was honestly an "Oh, there was the problem!" kind of thing.

By the way....where's the info on the Cornet 3 mods?

[poty]

By the way....where's the info on the Cornet 3 mods?

For example - http://www.audiocircle.com/index.php?topic=74326.0

[hagtech]

Well, I do have the C3 circuit worked out.  I built another one and got it all tweaked in.  The EQ is excellent.  What I need to do is work on chassis design.  I want it to be an easier build than the C2. 

jh