JCM2000 DSL401 Buzz

[Pentode707]

Hi,

Got some issue which I am not able to figure out what is causing it. I notice the amp got a buzz in the clean channel which intensity depends on the adjustment of the bias pot. The higher the bias, the higher the buzz. It really sound as ripple and I thought about a power supply filtering but the funny thing is that the noise is not noticeable in the other 2 channels.
I have replaced main poewer caps, interchanged preamp tubes, some bypass caps and even try to increase bias filtering......no result. The buzz seems to be higher when no guitar is plugged.
Did find the schematic in this link: //musiciansroadhouse.com/download/file.php?id=4667&sid=7ef0567644fcddd452c04dac87eb3281

Just wondering if anyone experience alike issue iin which buzz is only found in one channel and varies with the bias.

Regards!

 

[Pete Farrington]

Any noise will tend to track bias, because hotter bias = higher gain.
Buzz is often due to the heater circuit losing its balance / reference.
R34&35 ok?
What resistance do you measure between any node on the heater circuit and chassis common?

 

[Pentode707]

Hi Pete,

Perfect hint! Iwas focusing on the power supply for that evil buzz and I did not think about the heaters supply. Actually balance is OK but the 3 preamp tubes are heated by a raw DC supply. It uses an encapsulated power diode bridge and two filtering caps (C73 & C78). One of them was in short and I was getting a nice 8Vpp 100Hz ripple. Replace those caps and the sky got sunny!!
Your suggestion brought me to the solution and I do appreciate, thank you!!

 

[Pete Farrington]

I was getting a nice 8Vpp 100Hz ripple. Replace those caps and the sky got sunny!!

Well done tracking that gremlin down!