Mark lll hum diagnosis

[Heididog]

I hadn't paid much attention to the hum I was getting until a seasoned player told me it should not be making that noise - the amp should be dead quiet when sitting there and not playing. Got a Mark lll red stripe head and 1 x 12 cab. Went through the manual and did all the tube switching to no avail - all the knobs can be turned right down but when the master is turned up and the reverb on back is turned up, you get some hiss but a fairly juicy 60 cycle hum. This is with no input plugged in, also tried with reverb unplugged - no diff. (also with an input) Could this be capacitor or is this this normal to have a hum when things cranked? Right now my babys scattered in pieces on the table and my other babys getting pissed cus its Christmas dinner soon....help...

 

[Shep]

it could be cap's but mk III had this hum in it too.. i think it's normal..

 

[Boogiebabies]

If you are getting a hum with nothing plugged in the input, the shorting jack is bent allowing your preamp to be on with no guitar plugged in. In this case you would hear hum and funny noises if you turned the knobs because the preamp is picking up tube noise and whatever else that may be floating around. Looking at the input jack, there is a small tab that touches the large ring portion of the jack. You need to insert a cable half way in and push the shorting tab towards the long ring connector to short the preamp when you pull the cable out. After this is repaired, you may want to then look at the far right side of the preamp board.
In front of the third, or closest 30uf/500V blue filter cap are two 100 Ohm (Brown/Black/Brown/Gold) resistors. With a meter and the amp off, these need to test in parallel at 50 ohms. If these are bad, the hum may be coming from the heater supply for the power tubes. From there, I would clean the tube sockets and pot's with contact cleaner to rule out any other culprits. The worst case is an open filter cap, but you did not mention that the amp lacked power or sounded bad.

 

[Heididog]

Thanks, I'll check that out and report back. The hum doesn't vary - it is becomes audible with the master and the reverb pots at about halfways each and gets to where you could hear it across the room with both pots turned up. The amp sounds good when played through - I don't have the experience to know if it should be sounding better i.e. tired tubes etc but it can still knock Christmas ornaments off the tree and remove one of the cats nine lives.

 

[Heididog]

The jack checks out - the shorting tab is working as it should when the 1/4 inch not plugged in. Couldn't find which resistors you meant - got the 3 caps stacked beside each other on the board but when you say "in front" do you mean on the board? There are 2 1 watt (I think - about a 1/4 inch dia and 3/4 long) resistors on the board beside the caps where the leads come out - they are brown/black/red/silver and there are also ones soldered onto the tube sockets also1 watt. There is a yellow/purple/brown/silver soldered onto a pin in the 6L6 socket and a1 watt red/purple/red/silver onto a pin for the EL34. There are also some 1/4 watts on the sockets and also on the board but nothing that is black/white/brown/gold. (I should also download a chart that shows the colour value so I don't sound like a goof - knew it many years ago in high school but that was when Marconi was just a boy) Also grabbed a can of contact cleaner when I went to town so will try that tomorrow before turkey dinner. Not sure if it means anything but still got the hum when I pulled the EL34's out and it was on simul=class sojust the 6L6s were in. Anything else I could try to isolate?

Thanks
Neil

 

[Blueracer]

This is a bit of a long shot but something easy to try. There is a ground switch on the back somewhere close to ac plug. It has three positions. Normally it is in the center off position. Try the other two positions and see if either position helps. If it does we can take it from there to help. If not then it is just something that you tried that didn't work.

 

[Heididog]

Yup - had tried that - doesn't make any difference.

 

[Boogiebabies]

I meant Brown/Black/Brown/Gold. There is a 680 Ohm beside each one and four 1N4007 diodes just above them. Bottom right corner of the circuit boards.

 

[Heididog]

Found em - thanks. Yes they check out. Since the hum is affected by the volume control, I'm thinking it is being introduced in the preamp I am starting to suspect it may be from one of the 4 high voltage capacitors. They are 20 years old - maybe some AC ripple getting through? I have found some really crappy schematics on tubefreak. http://www.tubefreak.com/mk3-1.gif
http://www.tubefreak.com/mk3-2.gif
Anyone know of some better ones out there? I might leave a message at the factory to also see if on the right track.

 

[Boogiebabies]

The screen grid resistors on the power tubes would be 2.7K for the outer and 470 Ohm on the inner. I would try to clean the pots, tube sockets and spray the chassis grounds before I dove in for a filter cap job. I would also check the four 1N4007 diodes for continuity, these are the ones above the two 100 Ohm resistors. Also, check that the two thick green wires on the left rear of the circuit board are soldered well. I'm just trying to rule out heater noise before you have to drop money on diagnosing coupling caps, cathode caps or filter caps. The four diodes are used as a rectifier bridge to convert the AC heater supply to DC for the V1. If something is wrong with the V1 heater supply, it amplifies the noise through the rest of the amp.

The clearest and most accurate schematics can be bought from Musicparts.com. The ones Mesa may send you are basically the same junk from Tube Freak and are missing components as well.