| 200B 5 PA head hum/hiss issue [message #28094] |
Tue, 09 March 2021 22:47  |
Drjones1106
Messages: 19
Registered: March 2017
Location: Seattle
|
Junior Member |
|
|
Hey all, I'm back again for more info on a project. I have a nice white sparkle tuck and roll model 200 b-5, 4 channel PA head that is having some serious hum/hiss issues. I can't quite pin it down as 120Hz, but that sounds pretty close. I read elsewhere on the site that some hiss in normal in these amps, but this seem excessive. I've tried it on every cab I have, at friends houses, etc. It is there with no input, with input, it is unaffected by any of the preamp controls (vol, bass, treble), and is still there with the reverb tank disconnected. I have already replaced the cord with a 3 prong grounded. Using my crappy oscilloscope I've narrowed down the hiss to where the 50V positive and negative supply hits the output transistors. To me that points to the filter caps. I don't have a multimeter that can measure the 4500uf to see if they are still on value (I just can't bring myself to spring for a fluke). Is there another test I could to do see if they are leaky? I have already ordered some 4700uf 50V radial EL caps to test with, if that fixes it I'll spring for some higher voltage big honkin' can caps, but while I'm waiting for them to ship I thought I would ask if there is anywhere else in the amp I should take a look at. The one weird thing is if I lift the tip of the grounded reverb footswitch jack, the volume of the noise decrease by about half, but there is still noise. Which is very weird, because the noise as seen on the scope isn't in the signal path on that circuit board.
Can anybody point me in the direction of other things to test?
|
|
|
|
VintageKustom.com
Search
Help
Members
Register
Login
Home
]