Author Topic: WTX 500 Help! High frequency Noise (9 Khz)  (Read 253 times)

Offline metallifan

  • Newbie
  • *
  • Posts: 4
WTX 500 Help! High frequency Noise (9 Khz)
« on: October 08, 2018, 11:24:12 PM »
I'm helping a bandmate of mine, his amplifier makes a high pitched abnormal noise. It is usable in it's current state, mostly because bass speakers filter out most of the noise. It does become a problem when we want to plug it into a PA though (full range speakers). It didn't make that noise when it was new.

As far as to what might have caused it: Once, he used a line cable (thin gauge wires) to connect the amp and the cabinet, said cable also had a defective plug. Halfway into the concert, a spark originated from the plug where it was connected in the amp, which prompted him to switch the cable. Everything went normal after that.
However, we can't really say if that incident was the one that caused the malfunction because, as i said, the cabinet speaker filters out most of the noise, so he didn't noticed it until much later.

With the help of a friend, we hooked the amp to a speaker and an oscilloscope, and were able to measure the frequency of the noise, it is 9 Khz. A 512 Khz noise can also be measured, but we think it's simply the switching of the power supply
Additional info: The noise can be heard even without pluggin in the bass guitar, ALL outputs are affected, it doesn't seem to be affected by the twist of the knobs (other than master volume of course, and the High equalizer parameter).

We're scouting the circuit board in search of defective capacitors, resistances, signs of failure, etc. Any help will be appreciated!
« Last Edit: October 08, 2018, 11:34:36 PM by metallifan »

Offline Bassmann1968

  • Administrator
  • Hero Member
  • *****
  • Posts: 11851
Re: WTX 500 Help! High frequency Noise (9 Khz)
« Reply #1 on: October 08, 2018, 11:43:52 PM »
Hi metallifan,
at first I would isolate the power section with a plug in the fx return.
Maybe even a short cut this plug, to avoid introducing additional noise.
Noise still there?
Rock on!
Armin

PLEASE LIKE EDEN ON FACEBOOK!!!
No schematics available!!! Don't ask for it.
;)

WTP-Pre,1350,WTX260,WT300/400,WT1000,WTX1000N,NAV,WTDI
D112XLT,D210XST,D410XLT,D810RP,DNS410,DNS112

Offline metallifan

  • Newbie
  • *
  • Posts: 4
Re: WTX 500 Help! High frequency Noise (9 Khz)
« Reply #2 on: October 09, 2018, 03:52:59 PM »
Hello and thanks for the quick reply!

We tried plugging in the fx return and the noise is still there, although greatly diminished. We had to increase master volume in order to be able to hear it.

Offline Bassmann1968

  • Administrator
  • Hero Member
  • *****
  • Posts: 11851
Re: WTX 500 Help! High frequency Noise (9 Khz)
« Reply #3 on: October 10, 2018, 03:43:23 AM »
So, it sounds like it is coming either from the preamp board or the power supply.
Check the power supply with osci.
Also check the signal from the fx send, but I guess you got the full 9kHz blast there.
Armin

PLEASE LIKE EDEN ON FACEBOOK!!!
No schematics available!!! Don't ask for it.
;)

WTP-Pre,1350,WTX260,WT300/400,WT1000,WTX1000N,NAV,WTDI
D112XLT,D210XST,D410XLT,D810RP,DNS410,DNS112

Offline metallifan

  • Newbie
  • *
  • Posts: 4
Re: WTX 500 Help! High frequency Noise (9 Khz)
« Reply #4 on: October 11, 2018, 08:46:28 AM »
Thanks for the tip! We measured the power supply to the preamp board with the oscillator and the noise was indeed coming from there. A 10uf capacitor was soldered between the black and blue cables (negative and ground i believe), this solved the problem!

Unfortunately, another issue popped out after: the amp was picking up radio signals whenever a cable was plugged in the input. Solved by replacing the OP amps, luckily they are all socketed so it was an easy job. Removing the potentiometer knobs for taking the board apart was a PITA though, pulled the shaft out of two of them (MIDS and HIGH).
So now we're looking for ways to repair them, we'll try reassembly, if that fails, we'll order a couple of push-pull guitar pots online and see if we can replace the resistor in them (online pots are 250K and amp original pots are 10K and 100K for MIDS and HIGH). The MIDS pot value of 10K is suspect, and it looks like it was resoldered at some point. Can anyone check if their MIDS is indeed 10K?

Any advice on the pot repair will be much appreciated. I want to thank you for the help i got, if it weren't for you we wouldn't have (almost!) fixed it.
« Last Edit: October 11, 2018, 08:48:20 AM by metallifan »

Offline Bassmann1968

  • Administrator
  • Hero Member
  • *****
  • Posts: 11851
Re: WTX 500 Help! High frequency Noise (9 Khz)
« Reply #5 on: October 11, 2018, 09:40:10 AM »
10k for Mids is ok.
Rock on!
Armin

PLEASE LIKE EDEN ON FACEBOOK!!!
No schematics available!!! Don't ask for it.
;)

WTP-Pre,1350,WTX260,WT300/400,WT1000,WTX1000N,NAV,WTDI
D112XLT,D210XST,D410XLT,D810RP,DNS410,DNS112