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Chasing buzz in Pilot 232

multichamp12

multichamp12

AK Subscriber
Subscriber
Hey everyone, I recently picked up another Pilot 232, and rebuilt it since the previous owner changed parts and it was an absolute mess. Long story short, I brought it back to life, and the circuit is completely stock, but I am getting a very faint buzz/hum in channel B (Right channel). It can only be heard when I stick my ear up to the speaker, but its enough that I want to figure out whats causing it. Channel A is perfect, no noise. This is without a preamp or anything connected, and the inputs shorted, so I know its coming from the amp.

When I got the amp, someone mixed the caps up, and had the 80uf coming straight off the rectifier, instead of the 40uf, so that has been changed back to the original form.


Heres what I have done so far:
-Replaced all caps
-Cleaned sockets
-Replaced PIO capacitors in channel B (although I know they were most likely good, I just wanted to make sure)
-Swapped 12ax7s between channels, no change, then replaced 12ax7s with new ones, still no change.
-Replaced 12au7, no change
-Swapped EL84s between channels, no change.
-Checked grounds in channel B, they all seem fine

Note: This is the regular 232 version, not the SA-232, so there are some very minor differences.
Schematic:
http://www.vintagevacuumaudio.com/schematics-manuals/pilot/pilot-sa-232-schematics.pdf

Pics:
https://s17.postimg.cc/x0sfh5ewv/PILOT2.jpg
https://s17.postimg.cc/yfu05wikv/PILOT3.jpg
https://s17.postimg.cc/iucolz97j/PILOT4.jpg

Anyone have any ideas of what to try next?
 
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Check where the filter caps are grounded to. If its not the same point as original, that may be it. Generally you want the first filter cap at least to be grounded to the same place as the transformer HV center tap, the red/yellow in this one. I like to have as many of the caps as possible grounded to that point, but the first one is the most important.

Not sure if this one uses isolated input jacks, but the SA-260 does, and it wants the grounds of the input jacks tied at the first tube.
 
The first filter cap (can capacitor) has the HV center tap grounded to it. I dont believe its the power supply, mostly because I am only getting buzz out of the right channel.

The small 100uf sprague atom capacitors are not grounded in their original place though. Maybe thats causing an issue? The input jacks are grounded at the actual jack, riveted to the chassis, and the shielded cable is shielded at both ends.
 
I cleaned and re-tensioned all the tube sockets today, and double checked my voltages and part values against the schematic. Everything is spot on. If I pull the 12ax7 from that channel, the buzz disappears. Would this buzz most likely be caused by a bad ground somewhere in channel B? The majority of the grounds points are on those little tabs coming off the tube sockets, which I can see getting worn over time since the material is thin.
 
As stated in my first post, I have swapped 12ax7s between channels and tried 2 different NOS pairs with no change
 
if it stops with the 12au7 out, its at the very first stage then. Not much there, if the grounding for the input jacks is right, about all that comes to mind is AC lead routing possibly. Maybe try temporarily sticking a grounded shield over the 12au7 to see if anything changes. Maybe proximity to a transformer or whatever is inducing hum.
 
Okay that helps isolating to issue to the 12au7, I'll try a shield tomorrow. If that fails, I will re-route some components that might be too close to the heaters.
 
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