Repairs to my New M-1.0t

[Vintageaudiorevival 9]

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 October 17 by Vintageaudiorevival

[Receiver2000 4,077]

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

[Half Life 638]

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 2,032]

Welcome!

[timmyjim 460]

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. 

[Zenith4me 1,485]

Welcome to the TCS!

[Rob 6,005]

Welcome to the site!!!

[AndrewJohn 10,495]

Welcome to the site - lots of good reading here in the archives, on the m1.0t amps, and fixing them.

[Vintageaudiorevival 9]

Thank you everyone! Going to just do it all. Just ordered all the parts. Anything worth doing is worth overdoing. I only Paid $100 for the stack, so throwing another $300 into it is a no brainer! Thank you again!

 

[Vintageaudiorevival 9]

Got all finished up.

 

With new Electrolytic capacitors, the thump is nearly non-existent!  

 

It is there ever so slightly on Power off, moreover, when I power it on, There is a loud "Buzz" Type "Inrush" Sound in the RIGHT speaker Only. It happens quickly and then goes quiet, Powering down there is nearly nothing. Interesting enough, when I tried to apply the "No Thump modification" (Or whatever it is that you're all calling it) It was near silent when powering on and off, however, when the circuit modification was in place, there a lot of white noise (High noise floor) on the RIGHT channel. I moved the "Mod" components from right to left channel and the problem stays in the right channel. Which tells me that there must be something else wrong in that channel.

 

I am guessing that it is related to the "Surge" sound when powering up in the right channel. It has done that since I have gotten it.

 

Any Ideas. I tried searching but have not found much. Added a Video so you can hear it. it's an odd one. Its only in One channel

 

Thank you! 

Edited Wednesday at 02:33 AM by Vintageaudiorevival

[Nahash5150 10,747]

Hello,

 

That's definitely not normal, and not the kind of anomaly the 'thump fix' was designed to correct.

 

One could swap the input board connectors between left and right to see if the problem is in the input section or in the amp stage. I have a suspicion it's the muting circuit in the input board.

[Vintageaudiorevival 9]

10 hours ago, Nahash5150 said:

Hello,

 

That's definitely not normal, and not the kind of anomaly the 'thump fix' was designed to correct.

 

One could swap the input board connectors between left and right to see if the problem is in the input section or in the amp stage. I have a suspicion it's the muting circuit in the input board.

That’s a good idea. When I have the input board disconnected, it is silent. So that may be a clue. I’ll try it and report back. Thank you!

[Vintageaudiorevival 9]

No dice. Swapped the input board plugs from left to right channel. Buzz stayed in right channel. Got to be somewhere else. 
 

I’ve checked all the pre drivers. drivers. Input transistors, all the capacitors, everything is good. All voltages and rails come up and down evenly. Everything is within spec. 

Edited 19 hours ago by Vintageaudiorevival

[Nahash5150 10,747]

11 hours ago, Vintageaudiorevival said:

No dice. Swapped the input board plugs from left to right channel. Buzz stayed in right channel. Got to be somewhere else. 
 

I’ve checked all the pre drivers. drivers. Input transistors, all the capacitors, everything is good. All voltages and rails come up and down evenly. Everything is within spec. 

 

Then we need to go to amplifier repair 101, especially after finding shorted pre-drivers.

 

Check these resistors since you didn't mention them:

 

R154, 156 43R FR VAS HR emitter
R146, 144 10R signal coupling
R168, 170 10R FR HR low current
R172, 174 10R signal coupling
R196, 198 22R, 27R CMR bias

 

Deviations more than 10% are BAD PARTS.