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FDP Forum / Amp Mods, Repairs, and Projects / Loud Humming Noise

WEBZIGHT

USA

If It Ain't Broke, Don't Fix It!
Feb 10th, 2017 01:50 PM   Edit   Profile   Print Topic   Search Topic

Greetings All,

I have a Fender Hot Rod Deluxe that I purchased new back in 1996 or 97 that presents a very loud humming noise after coming off STANDBY!
I have replaced ALL the tubes with matched set (finals) new ones and the same thing occurs! (Thought it might be a microphonic tube)
I suspect the power supply filter caps are the issue as I've seen this before in other consumer tube equipment I worked on back in the day when everything was tubes (telling my age);-)
I have checked the Signal Caps and they do not appear shorted!
My next step is disassemble the chassis circuit board and look at the power supply filter caps!
If anyone has had this issue and fixed same please pass on any helpful advice!
I started my electronics career working on tube type TeeVees, Radios, and Stereo/Audio Equipment! ;-)

Hammond101
Contributing Member
**********

So. Cal. USA

Feb 10th, 2017 02:08 PM   Edit   Profile   Print Topic   Search Topic

20 year old filter caps certainly could be the cause and since you are capable of doing the work they should be changed just because of age.

The other thing to check is the shorting jacks most likely on the FX loop. Connect a guitar cable between the in and out jack and see what happens. Input jacks on these also fail as do their solder pins to the pads on the main board.

I'd recommend an ESR meter for testing the caps in circuit. At least you know what your in for before pulling the board loose. Best $100 you'd ever spend on test gear short of a DVOM. Search ebay for "Blue ESR Meter".

I've also see the solder joints on the power tube sockets fail, hum, noise, ugly sounds. I always re-float these with a HRD on the bench.

WEBZIGHT

USA

If It Ain't Broke, Don't Fix It!
Feb 10th, 2017 10:42 PM   Edit   Profile   Print Topic   Search Topic

Thanks Hammond 101 for the feedback ;-)

Don't have an FX loop but I did a physical inspection and found cold solder joints on the two 5W resistors and associated diode (+/-16 VDC)

The first 200 µf filter cap was puking it's guts so I have ordered a whole new high performance set and will replace them ALL including the bypass caps ;-)

Remarkably the the tube solder joints all still look new ;-) (Inspected with 10X glass)

Fingers crossed that the caps are all there is to this problem ;-)

FDP Forum / Amp Mods, Repairs, and Projects / Loud Humming Noise




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