mwl168

One vote for the CFA/GRLV.

May I suggest moving the build notes and related discussions to the build thread?

I did successfully build both GR78 and GR79 using the GB boards. First of all, you need some capacitance on the output for the regulators to work. That's the first thing I would try. I did have an issue with GR78 at first and finally figured out I used the wrong part (used HN4A51J instead of HN4C51J). But try adding some capacitance on the output ends first.

IIRC, significant voltage is dropped in the chain of R26 and VR1 and I am wondering, depending on where VR1 value is set, if one of them is overly stressed beyond its rating and causing some issues.

Also, I think the frequent point of failure of such device has been the contact points of the switch itself. If they are not of high quality, they'll develop noise in the long run.

Hmm, what’s the voltage drop and dissipation on R26 and VR1 when you adjusted the offset to 0V?

Ok, that makes sense. When you increase the output device bias current, you need to tweak the offset adjustment chain (R26 and VR1 - 2K and 1K respectively in the schematic) so you can adjust the offset close to 0V. The easiest thing to do is to decrease R26 but make sure the voltage drop is somewhat even between R26 and VR1 so they are not stressed. I posted earlier in this thread of a similar effort I made on my build: https://www.head-case.org/forums/topic/8327-im-on-a-roll-the-kgsshv/page/187/ The goal is to get the offset and balance close to 0V as possible. They will drift a bit and that's normal.

Thanks James. Corrected.

I've asked this question before - what's the lowest offset voltage you can adjust to with the higher bias current of the output devices (lower R5, R6 values)? I would first get the offset and balance to reasonable range and see if that affects the distortion issue you are experiencing. If the amp is set up properly you should be able to get both to below 1V easily. If the amp was originally set for lower bias, increasing the bias current may cause the amp to run out of offset adjustment range without making change to the offset adjustment circuit.

I was not able to PM you so had to message you here. Please PayPal me $8.4 US for the shipping cost. @Orso_Aquilano: please check PM for shipping of your package.

Does McIntosh continues to use output transformers in all their amps?

All GB packages have been shipped as of today except for two. One of the two is pending the participant's response to my shipping payment request. Most of you have PayPal'ed me the shipping cost. Thanks! For those who have not yet PayPal'ed me the shipping cost please check your PM or PM me. I have been keeping track but there were 50 packages so may have missed a few. Shipping cost for US address is a flat rate of $8.4 US. For non-US address it depends on the location and weight and I added $1 US for the bubble envelop used. This GB is now officially closed. Happy building!

Sounds like you and I are keeping BDent's business afloat!

I am not sure if there is a "target" biasing. I've tried between 150mA to over 200mA and settled on 180mA IIRC.

Before you start replacing the sands, I suggest you do a diode-test on each of the transistors on the board and compare the result between the working and malfunctioning boards.