Sovtek Mig50 low power

[Zosomonstro]

I have a used Sovtek Mig 50 that had some scratchy pots. I sprayed them and that did not solve the problem, so I decided to replace the volume pots. That worked great. so I decided to replace all the pots. That's when the trouble started. The volume power, after a minute or so, just droped and now sounds distorted. Thought maybe power tubes blew. Changed them, no effect. Tried changing preamp tubes, no change. Tried reconnecting pots and volume would come back for a minute then drop out. Help! I am a novice and may have to have a pro check it out.
Thanks

[PerroGrande]

So, if I understand you correctly...

You had a working amp with a scratchy pot. You replaced this pot and the result was a working amp without the scratch.

You then replaced other pots and after a minute or so, the output from the amp went down in volume level and was distorted. You then swapped out a lot of tubes, etc, to no avail. Is this basically correct?

When you say you have tried "reconnecting the pots" I assume that this means that you re-installed the original pots. Is this assumption correct?

Are these pots you replaced on the front panel of the amp, or are some of them internal (such as a bias adjusting pot on a circuit board or something)?

Let's start with the basics first and then move from there. While there is no guarantee that pot replacement has anything to do with the other problem, it is the most recent event and should be eliminated first.

Have you verified:

1) The replacement pots are the correct value
2) The replacement pots are wired the same as the originals
3) You have the right value pots in the right places
4) Your solder joints are solid, making good contact, and not bridged with something else.
5) You haven't left something unplugged somewhere
6) You haven't accidentally pinched a wire to ground

Please don't think I'm trying to be insulting with these questions. I know they are basic things and you've probably done all this already. However, whenever I approach troubleshooting I always start with the most basic and work outward from there.

[Zosomonstro]

No insult taken at all. You gave me exactly what I needed, basic and to the point. You assumed everything correctly. All the pots are the front ones. No internal. And you caught one of my mistakes. All the replace pots were 1meg audio taper when I have now found out that I should have followed the schematic and replaced the Treble with a 250K the Middle with 25K Linear and the Presence with 5k linear. The Volumes are 1meg as well as the Bass.
They were all wired the same and the joints good. I am going to install the correct values and go from there. Again thanks for your awesome reply.

[PerroGrande]

Excellent! Glad to hear that you've isolated a very likely cause of the problem.

I can understand how a circuit expecting a 5k or 25k pot might get a little upset with a 1meg pot installed.

Let us know how it goes!

[Bruce / Mission Amps]

Keep in mind that when you replace the pots in these with real ones... the lugs are upside down from the circuit board and you might have the grounded end of the volume pots on the wrong side now.

[Zosomonstro]

Still trying

Thanks for all the info. I have now installed the correct value pots and as before the amp works for a minute, then after turning up, the volume drops out and is distorted. I'm not sure why it would work fine then drop out like that. Any more advice would be great. Oh, also the Bass control also does not seem to be working.
Thanks

[PerroGrande]

Questions --

1) If you keep the volume low, does it work and sound good for an extended period of time? I'm not sure if it is fading to quiet always, or only when loud.

2) Have you verified the orientation of the pots, as Bruce pointed out?

3) Do you have access to an audio probe? I looked at a Mig50 schematic and it doesn't appear to have sends or anything, so the only way to determine if the problem is in the pre or power stages will be via a probe.

4) I remember from the original that you swapped around a lot of tubes. Have you reverted to the originals, or are you still using the new ones?

[Zosomonstro]

Eureka!

I can't believe it was that simple! I checked all the pots as had been suggested and found them ok. Then after reading about some similar problems, I took a wooden chopstick turned on the amp with a steady input source and started poking around all solder joints. Bam! As soon as I hit the joint on the 8 ohm output jack, I almost blew a speaker. After examination, I found a small piece of solder shorting the jack also the joint might have been loose. So after resolder and cleaning the jack, amp is woking great! Now, only problem seems that the Bass control is not working. Checked pot ckecked joints. All other pots working fine. Hum? I'll keep at it. Thanks for all the great help!!

[PerroGrande]

Well good! Progress is being made...

I hate to sound like a broken record, but check the wiring on the Bass tone control pot.

If I'm reading the schematic correctly, the Bass control is a 1Meg Audio taper pot. However, it appears to be wired as an in-series variable resistor. In other words, the wiper lug would be wired with one of the two other leads (making it, in effect, a two-lead device). This is similar to the way the 5K Presence control is wired, I believe.

This is different than the way the treble, mid, and volume controls are wired. These are wired in "voltage divider" style, where all three leads connect independently.

[Zosomonstro]

I see what you mean, I checked the schematic also. I connected first one of the outer leads to the inner one, no effect. Then disconnected the first and then connected the other outer lead, no effect. Checked pot, it's ok. Could I have blown a cap? Will keep checking. Thanks!

[Zosomonstro]

Still trying, everything works great, except the bass control has no response. I have tried wiring it many different ways. I am now going to replace caps to see if I have a bad one. I did try connecting outside lugs to inner lug. No change. Thanks for help!

[Enzo]

When you checked the pot, I assume you measured from end to end and got whatever the value of the pots was. Did you measure resistance from the center leg - the wiper - to either end? Does it vary smoothly as the control is turned? And does it go from about zero or very low resistance all the way up to the pot value at the other end?

[Zosomonstro]

Hi Enzo, Yes I did check the pot. I also tried two other good pots.

[PerroGrande]

Well, there just isn't much to that tone control circuit other than pots and caps.

I did notice one thing from re-reading the postings and looking at the schematic...

You mentioned replacing the mid control with a 25K linear. The schematic I have shows this control as a 50K linear taper. The 1Meg Bass pot is in series with this 50K pot.

So -- check the wiring, orientation, and value of the MID pot as well...