Ampeg V4 VT22 Loud Noise in Preamp

[misterc57]

This V4 has a rough looking board, see pics. Is that corrosion around many of the solder points?

The amp works well but suddenly emits a very loud buzz noise. Sometimes it can be controlled by the volume, and sometimes it is independent of the volume control.

Using a chopstick I can recreate the problem by touching the preamp board in the V1 and V2 area. I can make the buzz come on and off by chop sticking.

Any suggestions on how to cleanup the board and isolate the issue which I suspect is a bad connection?

Thank you

V1 is at the top of the first and third picture.. Note the large hole near the bottom of the board.

[g1]

Have you tried cleaning the V1 & V2 sockets? Also closely inspect all the board mounted socket solder connections as they often need resoldering.

[DrGonz78]

I try to get the leftover flux off the board with alcohol and a toothbrush. That’s what you referred to as corrosion I think. Then when you go hitting those solder joints it ends up cleaner.

[misterc57]

Yes flux (corrosion in my mind). Can flux be conductive?

I reflowed the 4 preamp tube pins on the board. Now I discovered a new problem. Very low output, and found only 19 VDC at pin 1 of the first two preamp tubes.

There is a 68K R that drops the supply to pin 1. After shutting down I read about 2M R on it. Now I am on the top side of the board and took another reading and it is about 68K. I powered up without preamp tubes installed and measure about 320 V at pins 1. However if I plug the tubes in I am back down to 19 V. I suspect the 68K R is failing when under a load?

I have swapped tubes around prior to checking this and had low output in all tests.

I would attach a schematic but have not found one that is legible yet.

[Helmholtz]

Flux is not normally conductive.

If the 68k connects to the plate (pin 1) it's not a supply dropper but the plate resistor.
In any case measure the voltage drop across that resistor.

If the input tube has a cathode bypass cap (at pin 3), disconnect it and see if voltages change.

Does the noise stop when you pull the input tube?

[misterc57]

Noise is gone (for now) but I need to check again at proper V.

There is about a 285 V drop across R13. See photo of schematic. No cathode bypass cap.

[Helmholtz]

Voltage drop across R13 should be only around 100V.
What is the cathode voltage (pin 3)?

Does the pin 1 voltage change when you vary the vol. pot VR 101?
Also check R14.

[misterc57]

R13 reads 60K 306 V one side, 16 V other side
R14 reads 3.4K only reading mV

V1 pin 1 : 19 V
V1 pin 3 : 114 mV

V1 pin 6 : 184 V
V1 pin 8 : 2 V

V2 pin 1 : 18.7 V
V2 pin 3 : 11 V

V2 pin 6 : 184 V
V2 pin 8 : 2 V

[misterc57]

VR101 has no change on V

[misterc57]

Replaced R13, no change

and again I am reading 2M R at each end of R13 on the bottom side of the board, and then a minute later I read 70K R

[Helmholtz]

Replaced R13, no change

and again I am reading 2M R at each end of R13 on the bottom side of the board, and then a minute later I read 70K R

Check for a broken board trace (often close to an eyelet).

[misterc57]

Check for a broken board trace (often close to an eyelet).

The reading is on the resistor leg itself (which I think would rule out the board), 306V on one side and now 6V on the other side.

Couple of better images of the schematic...

[misterc57]

and I have continuity from R13 to both V1 pin 1 and V2 pin 1.

[Helmholtz]

The reading is on the resistor leg itself (which I think would rule out the board), 306V on one side and now 6V on the other side.

Is the 2M reading measured on both resistor legs? Seems very unlikely with a new resistor provided your meter tips both made good contact.
A good resistor can't read higher when solderd in - except maybe if there's residual voltage in the circuit.

What are those voltages with V1 or V2 pulled?

[Helmholtz]

and I have continuity from R13 to both V1 pin 1 and V2 pin 1.

No wonder as both pins 1 are connected. The 68k is a shared plate resistor.