AMC CVT3030

[Breezio]

Thanks, are you able to give me some more detail on what to look for?
Unfortunately I don’t have much experience and, as mentioned, I can’t find the point where the noise comes into the final stage. It’s like chasing a ghost...

 

[scottjoplin]

It means there is a loop being created when you connect the cable that is picking up hum like an aerial, or is completing a circuit for ground currents when it shouldn't be.

 

[stocktrader200]

Instead of plugging the rca jack into the offending input, see if the hum comes back when the outside ( ground ) of the jack makes contact with the outside of the plug. don't plug in in normally. if the noise returns, touch the outside of the jack to the chassis to see if the noise returns. if there is no noise on the chassis ( should always be grounded ) the grounding problem is with the female connector on the amp.

 

[Breezio]

Thanks, you refer to an RCA jack but my problem relates to the audio cable from the preamp to the power stage (see pic). I assume the procedure can be similar. Just to make sure...

 

[scottjoplin]

I'm willing to bet you have a lovely loop in the "ground" circuit and a transformer sitting slap bang in the middle

 

[Breezio]

Dear all,


Here'sa a recap, and a couple additional things I tried:

• When the right power module is connected to the preamp, through either left or right signal cables, 120Hz hum is present. I can't measure anything relevant with the oscilloscope at the signal connector (either GND or R/L pins).
• When the signal cable is disconnected the hum is gone

• NEW> if I connect only the GND of the signal cable between preamp and power module, hum IS NOT present (see picture)
• NEW> if I connect only the R pin of the signal cable between preamp and power module, hum IS present (see picture)

Is this helpful to understand what is going on?


Thanks again for your support


Fabrizio

 

[scottjoplin]

Is this with the other channel connected? Can you double check the polarities (sig/gnd)?

 

[Breezio]

Yes the left channel is connected to the left power module.
How do I check the polarities?
One more thing: I swapped the tube modules and the problem is the same.
Thanks!

 

[scottjoplin]

What happens if you disconnect the left channel?

 

[Breezio]

Nothing changes, hum persists

 

[stocktrader200]

looks like the problem is on the main front board rather then the tube output boards.
You can test this by reversing ( if the cables are long enough ) the inputs on the board to see if the buzz the goes to the opposite channel.

 

[Breezio]

I swapped power modules (L/R) as well as signal cables from preamp (L/R) and the hum is still on the right channel.

 

[huggygood]

Forgive me but this amp is already a calamity at the base but now that it is modified, it becomes almost impossible to find the origin of the problem.
Anyway, I will look for a leak or pollution in the bias circuit.

 

[stocktrader200]

looks like a bad capacitor or joint on the preamp board. I would replace any electrolytics on the right channel of the preamp board. Use a felt marker to mark the right side top / bottom so that you don't replace left side components by mistake.

 

[Breezio]

I checked all the electrolytics on the signal path of the preamp, resolderd them, but nothing changes. Note that the hum is present independent of the the preamp channel connected to the right tube module. I am starting to think that there might be something related to the position of the right tube power module with respect to the preamp and power supply modules.

 

[stocktrader200]

does the amp use DC heaters?
does it make any difference to exchange the right and left channel tubes?

 

[Breezio]

Yes it does use DC heaters and I did swap left and right tube modules but the result is always the same

 

[Breezio]

Slowly getting desperate

As the title of this post says...I am running out of ideas.


Here is a recap of what is happening and what I have tried so far:

• When the power module on the right is connected to the preamp, through either left or right signal cables, 120Hz hum is present. The tube module is connected to the preamp through the cable indicated in the pictures previously posted. I can't measure anything relevant with the oscilloscope at the signal connector (either GND or R/L pins).

• When the signal cable (left or right) is disconnected from tube module on the right, the hum is gone

• If I connect only the GND of the signal cable between preamp and power module, hum IS NOT present (see picture)

• If I connect only the R pin of the signal cable between preamp and power module, hum IS present (see picture)

• If I disconnect the signal cable from the left tube module (the right being connected) the hum is still present

• If I physically swap the right and left tube modules (boards with tubes attached) the problem persists on the module to the right.

• On the preamp I checked the signal electrolytics for leakage (none) and resoldered them too.

• I checked tube bias and everythig is fine

• I have been trying to check with the oscilloscope probe where the hum interjects on the tube module but it's not easy since everything is so crammed and I am afraid of short circuiting.

• Put lid on/off

• Exchanged heaters cables between L and R tube modules

• Exchanged B+/Bias/GND connectors between L and R tube modules

• Inspected and gently poked with a fine wood stab almost all components on all boards

What I still find unexplicable is the relationship between signal cables and hum on the right module, without being able to measure anything on the cables themselves but only at the tube module output.

I would appreciate any other suggestion though I am also cognizant of how difficult it is to diagnose such a problem without physically being on the device.

Thanks anyway for the help so far.
Fabrizio

 

[Breezio]

Dear all,

I am happy to report that I managed to get rid of the hum. The problem was still related to the mod which I wired accoring to the old, awful, execution. And ground wiring was the actual problem.

I made sure a ground wire follows the unregulated B+ to the regulator board and back. That way the hum was gone.

Thanks everybody for the help!
Fabrizio

 

[scottjoplin]

That's good news