Repairs to my New M-1.0t

[Vintageaudiorevival 3]

Hello everybody. I found an M–1.0t a at Goodwill a few years ago. Long before I had the confidence to work on it myself. It did not work so I sold it not working. Always wondering what I was missing out on. 
 

Well, last week I came across another one with a C1 preamp and tuner. It of course also didn’t work, but this time I am determined to fix it. 
 

So after some troubleshooting, I found that the PreDrivers were shorted on both channels preventing it from powering on all the way. 
 

I have since replaced those, it powered up all voltages were good. I set the Bias and have been listening all day. It sounds great and I’m glad I was able to find another one. 
 

However, I have a very loud pop in the speakers about 5 seconds after I switch off the amplifier. 

 

It seems that the longer it is powered on the louder the “pop”

 

I have read the service bulletin about the the “turn on / turn off thump” and I’ve ordered the parts to complete the repair however, my question is, is THAT noise THE thump? I have heard amplifiers thump before, but this was a very loud pop. I wouldn’t describe it as a low. 

 

I have a hard time believing this “thump fix“ is going to take care of this.

 

Also. this is a Non-inverting model

 

Any insight would be greatly appreciated. Thank you so much.

Edited yesterday at 02:29 AM by Vintageaudiorevival

[Receiver2000 4,045]

What a great "second opportunity" for you!!! Welcome, great to have you with us.

[Half Life 624]

Welcome from VA!  Enjoy your stay!

 

While it's possible it could be something else, it is very likely just the "thump" that is typically found in these amps.  Power supply caps can contribute so be sure to replace those with some high quality types.

[3M_Audio 2,029]

Welcome!

[timmyjim 451]

Thump is one thing, but should not be overly loud.

 

I would put a scope on the +/-rails, one set at a time, starting with the highest voltage rails.

 

Do they decay/bleed down at the same rate?  They should look about the same, but opposite polarity.

 

Whats likely happening is that the +/-12V decay at much different rates, this causes the input opamp to rail in one direction or the other, creating a large transition at the output.

 

Since the +/-12V are derived from the higher voltage rails, it's likely that one of them is the culprit.