Marshall vs232 makes a high pitched hum always

I would suspect a bad cable. Also, these symptoms show up if you mistakenly use a speaker cable (unsheided) as a guitar cable (shielded).

Does anything happen while you turn the knobs on the amp?

I had a similar problem with a yamaha amp. It turned out a couple of knobs went bad. Soldered new pots and it works fine for me now.

make sure the standby switch is operating correctly.Sounds like the power tube.

Hi,

This would only mean two things:
1. you have a bad capacitor in the circuit or the grounding on the circuit is broken and not filtering the obvious line noise from the circuit.

2. You have what is called, dirty power. That is when you have some kind of electrical device plugged that generates line noise which can be even a florescent light as they generate line noise.

Please take the following steps to narrow down the issue.

Take your setup and plug it in some where you know others can setup their rigs and it is using clean power. If, the humming goes away then it is dirty power where you originally setup.

If, it is still humming then you will need to take your equipment to a qualified marshall dealer have them look at it.

If this happens when guitar cable is disconnected, take unit in for repair. This should be easy to troubleshoot. Hint: demonstrate the problem in the shop. Things have a way of fixing themselves and then lots of time is wasted trying to recreate the problem.

DO NOT FACE THE GUITAR TOWARDS YOUR AMP. IT WILL MAKE IT SQUEAL.

I may be missing something but i cant see what amp you have? Is it a valve or solid state marshall?

If you have an instrument plugged into the amp, then you need to turn down the volume of the amp before turning on the amp. Otherwise you risk permanent damage to your speaker. Also, the instrument cord may be defective. The hot and ground wires may be shorted together. You need an ohm meter to test your cord. With the cord unplugged and detached, put the positive (red) lead of the ohm meter on the tip of the plug and the negative (black) lead on the sleeve of the plug. You should not have any reading at all. If you get a reading then you have a bad cord with a short.

Check that you have 15 volts across C19 and C20.

If either are bad, suspect ZD1 and ZD2 regulating Zener diodes.

Check the soldering and condition of the input jack as these are chronically bad due to damage of yanked cords.

The output transistors may have shorted and caused a resistor to smoke (literally!!). This should not be too expensive a repair for anyone that works on these amps. Figure on $20-$30 for parts.

Dan