Sovek Mig 50 making static noise

[Ghostplanet]

Hello, this is my first post. I have a sovtek mig 50, aka tube midget. It is making a lot of static noise. The noise is present when the guitar is plugged in or unplugged. The static does make the guitar signal cut in and out. It tends to be worse as I increase the gain. I did the tap test on the tubes to check if they have gone monophonic, there was no excessive noise. Also the power tubes do not seem to be glowing abnormally bright. When I bought the amp a number of years ago the seller mentioned that the caps were going bad. He did not specify which cap he thought were going bad. I have posted a link to a video I made of the noise. It was with my cell phone, so I hope the quality is enough to help. I have built a number of effects pedals and have done some minor repairs on amps in the past. I would love to save a few dollars and fix the amp myself. Thanks for any help.

https://www.youtube.com/watch?v=6EY8...ugIQhJ85Kwg_JQ

Wes

[nsubulysses]

Hi Wes. Welcome to the forum The link to your youtube video doesn't work but here are a few easy things you could try. I just worked on a MIG50H last night that was noisy and cutting out.

Clean all the pots and jacks with contact cleaner (jacks) and Deoxit (pots)

Clean all the tube sockets with contact cleaner. Dusty dirty tube sockets can be noisy

Pull the phase inverter tube (the preamp tube farthest away from the input jack) and see if the noise persists. If it does, the noise is coming from the output section. When you pull the phase inverter you are breaking the signal chain right at the beginning of the power amp. So if you pull it and the noise is gone you know it is coming from before that point, i.e. the preamp section

Doing these things might eliminate the noise, or if not it will at least rule some things out and get you headed in the right direction.

[Ghostplanet]

I just pulled the phase inverter tube and the noise went away. So i've narrowed it down to the preamp section. Thanks for the help.

[Chuck H]

FYI, any caps in the preamp aren't prone to age failure. The seller was almost certainly referring to the power supply filter capacitors which do degrade with age. I only mention it because it's VERY common for a novice to make the jump from this point and just replace all the preamp capacitors figuring they're likely to fix the problem and upgrade the caps at the same time. What happens more often is that the problem isn't fixed and is now compounded by a new problem, like a cold solder joint or miswire, that occurred during the restoration attempt. At that point the problem becomes much harder to diagnose because multiple symptoms of different problems exist. Many an amp in this position has ended up collecting dust.

My vote is for dirty tube sockets and/or oxidized tube pins. As suggested, get some contact cleaner and some potentiometer cleaner, follow the directions on the label and work over all the jacks, switches, pots, tube pins and tube socket pin slots.

P.S. If the seller told you caps were in need of replacement years ago then the condition has gotten worse. Even if you don't use the amp much. In fact, worse if you don't use the amp much as the caps in question fail with age and more quickly if the amp isn't in use. So, once the crackle and intermittent signal are corrected, pay attention to the amps noise level and clarity of notes in the upper registers. Excess hum and/or an out of tune ring modulator type tone accompanying some higher notes are both symptoms of power supply filter cap failure. If your amp doesn't exhibit any symptoms, don't worry about it until it does.

[Ghostplanet]

FYI, any caps in the preamp aren't prone to age failure. The seller was almost certainly referring to the power supply filter capacitors which do degrade with age. I only mention it because it's VERY common for a novice to make the jump from this point and just replace all the preamp capacitors figuring they're likely to fix the problem and upgrade the caps at the same time. What happens more often is that the problem isn't fixed and is now compounded by a new problem, like a cold solder joint or miswire, that occurred during the restoration attempt. At that point the problem becomes much harder to diagnose because multiple symptoms of different problems exist. Many an amp in this position has ended up collecting dust.

My vote is for dirty tube sockets and/or oxidized tube pins. As suggested, get some contact cleaner and some potentiometer cleaner, follow the directions on the label and work over all the jacks, switches, pots, tube pins and tube socket pin slots.

P.S. If the seller told you caps were in need of replacement years ago then the condition has gotten worse. Even if you don't use the amp much. In fact, worse if you don't use the amp much as the caps in question fail with age and more quickly if the amp isn't in use. So, once the crackle and intermittent signal are corrected, pay attention to the amps noise level and clarity of notes in the upper registers. Excess hum and/or an out of tune ring modulator type tone accompanying some higher notes are both symptoms of power supply filter cap failure. If your amp doesn't exhibit any symptoms, don't worry about it until it does.

I will try cleaning the sockets and pots next. If that doesn't remedy the problem I will move on to the power supply caps. Thanks so much for the guidance.

[Ghostplanet]

I think I may have discovered the problem. I was looking at the power supply caps to see if there was any physical damage. I noticed where one of the leads enters the board looks burnt. Could this be the problem? I attached a pic of the burn area. Its the cap on the left.

[Jazz P Bass]

That 'burnt mark' looks as if someone tried resoldering the joint.

Being a 'single sided board' means that the copper traces are on the other side.

After the socket cleaning make sure the preamp tube pins are straight.
Run them in & out of the socket a few times with some DeOxit wiped lightly on the pins.

The next item would be to measure the interstage coupling capacitors for any volts dc on the non plate side of the cap.
On most stages there should (ideally) be none.

By using a voltmeter, you can possibly track down the noisy circuit by looking for varying dc & or ac voltages where they should be steady.

[g1]

It's also possible there is a bad solder connection at that cap and it is arcing through the board. So to err on the side of caution you should at least inspect the solder joint on the other side of the board. I suppose if it is the filter/decoupler for the preamp stage, bad solder there may cause static type noises.

[Ghostplanet]

It's also possible there is a bad solder connection at that cap and it is arcing through the board. So to err on the side of caution you should at least inspect the solder joint on the other side of the board. I suppose if it is the filter/decoupler for the preamp stage, bad solder there may cause static type noises.

There is also a set of resistors in parallel near the caps that look scorched. Hummm?

[Chuck H]

That looks like it could be a shorted cap. That burned solder blob is a HV contact. Don't fire up the amp again until those caps are replaced. Some techs might advise to just replace the one bad cap. I happen to know those gray Illinois caps of that era are garbage and I've seen them fail many times in other amps. Just replace them all with a different brand.

It MAY be the problem, it may not. Probably is.

EDIT:corrected for polarity

[Ghostplanet]

That looks like it could be a shorted cap. That burned solder blob is a ground contact. Don't fire up the amp again until those caps are replaced. Some techs might advise to just replace the one bad cap. I happen to know those gray Illinois caps of that era are garbage and I've seen them fail many times in other amps. Just replace them all with a different brand.

It MAY be the problem, it may not. Probably is.

After checking the amp again I noticed there was a resistor near the power supply caps that was scorched. I checked the schematic and it looks like its the 470 ohm 5 watt power resistor. The plan is to buy new filtering caps and a new power resistor and see if that fixes the problem. Again thanks for all the help.

[Jazz P Bass]

Can you post the schematic that you have?

The one that I have does not show a 470 ohm/ 5 watt resistor on the power supply.

Resistors rarely fail by themselves.
Something downstream has aked for more current than the resistor can handle.
And at a 5 watt rating, I would want to know what is downstream.

[Ghostplanet]

Can you post the schematic that you have?

The one that I have does not show a 470 ohm/ 5 watt resistor on the power supply.

Resistors rarely fail by themselves.
Something downstream has aked for more current than the resistor can handle.
And at a 5 watt rating, I would want to know what is downstream.

Blue Guitar Schematics
If you go here it is the file labeled mig 50H.
Sorry I could not get my file to load.

[g1]

Here's the file. The 470R5W is between first and second filter nodes, so if that cap is one of the totem pair for the second node, a bad cap could explain the fried resistor.

[Ghostplanet]

Here's the file. The 470R5W is between first and second filter nodes, so if that cap is one of the totem pair for the second node, a bad cap could explain the fried resistor.
[ATTACH=CONFIG]31653[/ATTACH]

Seems quite logical Jim.