Ampeg V4.com

forum

You are not logged in.

#1 2014-03-19 17:04:16

bogner100b
New member
Registered: 2014-03-19
Posts: 6

V4B Buzz /hum

Hi Guys, I'm new to the board and glad to meet any of you reading.  I've got my hands on a V4 that was blowing fuses.  I pull the chassis, take my meter to the Flyback diodes, and they are fried. 

I replace the diodes and viola - we are up and no blowing fuses.  BUT, loud buzz / hum so I pull the hum balance pot to test and it tests perfect.

I check the 100K resistors across the caps - ALL GOOD

I check the Tubes (pulling one at a time) and find out that the noise will only occur when the power tubes are in.

I checked each pre-amp tube swapping new 12ax7's and the noise is still present with each swap out.

I tested all the Caps (40/100's) and the )40/40/40/) and all test good.

I install known good power tubes, cleaned sockets and still buzzing / Humming.

Anyone have any insights you'd like to share with me?

UGH!!!!!!   Frustrated yet Determined.

Last edited by bogner100b (2014-03-20 09:02:32)

Offline

 

#2 2014-03-20 08:10:16

ROBO
V.I.P.
From: na
Registered: 2010-01-25
Posts: 134
Website

Re: V4B Buzz /hum

Welcome aboard!
It sounds like you are on the right track,
Try pluggin into the ext amp jack and into a mixer to see if the hum follows the signal into the mixer.

That will tell you for sure if the noise is coming from the pre-amp section or the power amp since the jack is wired in right in between the pre and power amp sections.

There are a bunch of coupling caps in the amp that will get dry and noisy after years and years.

If it still has its orignal filter caps it would be a good time to go ahead and re-cap the amp while you are in there, If they are not gone yet then they prob. will be soon, they are 30-40+ years old at this point.

Good luck and keep at it, this is the place to get it figured out.

Last edited by ROBO (2014-03-20 08:11:07)


Less talk, More rock

Offline

 

#3 2014-03-20 17:33:16

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: V4B Buzz /hum

the hum balance pot could be dead.   if you wanna check either replace it, or remove it and put 2 100 ohm resistors from each of the heater taps to ground.  make them 1 watt flame proofs.

Offline

 

#4 2014-03-20 18:11:53

bogner100b
New member
Registered: 2014-03-19
Posts: 6

Re: V4B Buzz /hum

Hangman -  Thanks for replying to my post.  Do you have pics as to what your recommendation looks like?

I can solder circuits SUPER WELL but I am not technical when it comes to terms like heater taps.  The resistors I understand though.

Thanks!

Offline

 

#5 2014-03-20 21:22:35

bogner100b
New member
Registered: 2014-03-19
Posts: 6

Re: V4B Buzz /hum

I soldered (2) 100ohm Resistors to the outer lugs of the Hum Balance Pot - Pulled the Ground (center wire)

tied the other non soldered ends of the (2) resistors and attached the ground wire to it. 

I still have hum......

Did I get this right or did I fail?

Thanks GUys

Offline

 

#6 2014-03-22 03:13:46

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: V4B Buzz /hum

hmm,  that should've taken care of it if it was the hum balance pot.   

what did you replace the flyback diodes with?   if you remove them entirely does the hum stay?   do you have an oscilloscope?

Offline

 

#7 2014-03-22 09:23:21

bogner100b
New member
Registered: 2014-03-19
Posts: 6

Re: V4B Buzz /hum

Hangman -  Can I pull them and its safe to fire up the amp without them?

Last edited by bogner100b (2014-03-24 15:21:06)

Offline

 

#8 2014-03-22 11:50:03

bogner100b
New member
Registered: 2014-03-19
Posts: 6

Re: V4B Buzz /hum

I lifted the legs of the diodes off the board and still humming???    (scratching me head)??

That leaves my only suspected item left which would be the OT!!!

R

Offline

 

#9 2014-03-23 10:00:32

hangman
Banned
From: Seattle Washington
Registered: 2006-09-03
Posts: 1848

Re: V4B Buzz /hum

Have you disconnected the OT primary and measured the doc resistance?

Offline

 

#10 2014-03-23 10:19:12

bogner100b
New member
Registered: 2014-03-19
Posts: 6

Re: V4B Buzz /hum

Will Do

Last edited by bogner100b (2014-03-24 15:20:22)

Offline

 

#11 2014-03-23 20:33:11

ROBO
V.I.P.
From: na
Registered: 2010-01-25
Posts: 134
Website

Re: V4B Buzz /hum

There are 3 wires that make up the primary side of the OT.

1 is the start of the primary widing, the other is the end of the primary winding, and the 3rd is the center tap.

The center tap is where voltage is applied, It leads back to the main filter caps.

The 2 "legs" of the primary widing go to each pair of output tubes. I believe they are blue and purple if I recall correctly.

You can use and OHM metter to check the resistance between the 2 legs of the primary winding of the OT.

You can then check the resistance from the center tap to each leg and it should be right around half of what you gat measuring between the 2 legs.

If you amp works at all and makes sound I would say its not you OT. they usually either work or dont, They are basically just a giant lump of iron with copper wrapped around it like any transformer.

Again I would check from your external amp jack. It can help you isolate the problem by basically cutting the 2 sections of the amp in half.

You can plug your guitar into the front like normal, then go out of the EXT jack into the poweramp in of a known good amp and see how it sounds.

IF it sounds fine then you know your problem is in your power amp somewhere BUT, If you still get them hum when you slave into another poweramp then you know it must be originating from your preamp.

That will tell you without a doubt where you need to start looking, It helped me a bunch when I was try to track down noisy coupling caps that ended up being in my phase invert circuit.

If you have a DMM and decent soldering skills you should be able to run the sound gremlins off, sometimes it just takes a little patience.

Good luck and keep at it.


Less talk, More rock

Offline

 

Board footer

Powered by PunBB
© Copyright 2002–2005 Rickard Andersson